The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twenty-first-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Here’s the press release, and an article in the Chandra Chronicles.)
A prerequisite to understanding the dark sector is to make sure we are on the right track. Can we be sure that we haven’t been fooled into believing in dark matter and dark energy? After all, we only infer their existence from detecting their gravitational fields; stronger-than-expected gravity in galaxies and clusters leads us to posit dark matter, while the acceleration of the universe (and the overall geometry of space) leads us to posit dark energy. Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einstein’s general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances?
A departure from general relativity on very large scales isn’t what one would expect on general principles. In most physical theories that we know and love, modifications are expected to arise on small scales (higher energies), while larger scales should behave themselves. But, we have to keep an open mind — in principle, it’s absolutely possible that gravity could be modified, and it’s worth taking seriously.
Furthermore, it would be really cool. Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. (And Vera Rubin agrees.) We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isn’t to express preferences, it’s to suggest hypotheses and then go out and test them.
The problem is, how do you test an idea as vague as “modifying general relativity”? You can imagine testing specific proposals for how gravity should be modified, like Milgrom’s MOND, but in more general terms we might worry that any observations could be explained by some modification of gravity.
But it’s not quite so bad — there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle. So if we imagine doing away with dark matter, we can safely predict that gravity always be pointing in the direction of the ordinary matter. That’s interesting but not immediately helpful, since it’s natural to expect that the ordinary matter and dark matter cluster in the same locations; even if there is dark matter, it’s no surprise to find the gravitational field pointing toward the visible matter as well.
What we really want is to take a big cluster of galaxies and simply sweep away all of the ordinary matter. Dark matter, by hypothesis, doesn’t interact directly with ordinary matter, so we can imagine moving the ordinary stuff while leaving the dark stuff behind. If we then check back and determine where the gravity is, it should be pointing either at the left-behind dark matter (if there is such a thing) or still at the ordinary matter (if not).
Happily, the universe has done exactly this for us. In the Bullet Cluster, more formally known as 1E 0657-56, we actually find two clusters of galaxies that have (relatively) recently passed right through each other. It turns out that the large majority (about 90%) of ordinary matter in a cluster is not in the galaxies themselves, but in hot X-ray emitting intergalactic gas. As the two clusters passed through each other, the hot gas in each smacked into the gas in the other, while the individual galaxies and the dark matter (presumed to be collisionless) passed right through. Here’s an mpeg animation of what we think happened. As hinted at in last week’s NASA media advisory, astrophysicists led by Doug Clowe (Arizona) and Maxim Markevitch (CfA) have now compared images of the gas obtained by the Chandra X-ray telescope to “maps” of the gravitational field deduced from weak lensing observations. Their short paper is astro-ph/0608407, and a longer one on lensing is astro-ph/0608408. And the answer is: there’s definitely dark matter there!
Despite the super-secret embargoed nature of this result, enough hints were given in the media advisory and elsewhere on the web that certain scientific sleuths were basically able to figure out what was going on. But they didn’t have access to the best part: pictures!
Here is 1E 0657-56 in all its glory, or at least some of it’s glory — this is the optical image, in which you can see the actual galaxies.
With some imagination it shouldn’t be too hard to make out the two separate concentrations of galaxies, a larger one on the left and a smaller one on the right. These are pretty clearly clusters, but you can take redshifts to verify that they’re all really at the same location in the universe, not just a random superposition of galaxies at very different distances. Even better, you can map out the gravitational fields of the clusters, using weak gravitational lensing. That is, you take very precise pictures of galaxies that are in the background of these clusters. The images of the background galaxies are gently distorted by the gravitational field of the clusters. The distortion is so gentle that you could never tell it was there if you only looked at one galaxy; but with more than a hundred galaxies, you begin to notice that the images are systematically aligned, characteristic of passing through a coherent gravitational lens. From these distortions it’s possible to work backwards and ask “what kind of mass concentration could have created such a gravitational lens?” Here’s the answer, superimposed on the optical image.
It’s about what you would expect: the dark matter is concentrated in the same regions as the galaxies themselves. But we can separately make X-ray observations to map out the hot gas, which constitutes most of the ordinary (baryonic) matter in the cluster. Here’s what we see.
This is why it’s the “Bullet” cluster — the bullet-shaped region on the right is a shock front. These two clusters have passed right through each other, creating an incredibly energetic collision between the gas in each of them. The fact that the “bullet” is so sharply defined indicates that the clusters are moving essentially perpendicular to our line of sight.
This collision has done exactly what we want — it’s swept out the ordinary matter from the clusters, displacing it with respect to the dark matter (and the galaxies, which act as collisionless particles for these purposes). You can see it directly by superimposing the weak-lensing map and the Chandra X-ray image.
Clicking on each of these images leads to a higher-resolution version. If you have a tabbed browser, the real fun is opening each of the images in a separate tab and clicking back and forth. The gravitational field, as reconstructed from lensing observations, is not pointing toward the ordinary matter. That’s exactly what you’d expect if you believed in dark matter, but makes no sense from the perspective of modified gravity. If these pictures don’t convince you that dark matter exists, I don’t know what will.
So is this the long-anticipated (in certain circles) end of MOND? What need do we have for modified gravity if there clearly is dark matter? Truth is, it was already very difficult to explain the dynamics of clusters (as opposed to individual galaxies) in terms of MOND without invoking anything but ordinary matter. Even MOND partisans generally agree that some form of dark matter is necessary to account for cluster dynamics and cosmology. It’s certainly conceivable that we are faced with both modified gravity and dark matter. If the dark matter is sufficiently “warm,” it might fail to accumulate in galaxies, but still be important for clusters. Needless to say, the picture begins to become somewhat baroque and unattractive. But the point is not whether or not MOND remains interesting; after all, someone else might come up with a different theory of modified gravity tomorrow that can fit both galaxies and clusters. The point is that, independently of any specific model of modified gravity, we now know that there definitely is dark matter out there. It will always be possible that some sort of modification of gravity lurks just below our threshold of detection; but now we have established beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics.
That’s huge news for physicists. Theorists now know what to think about (particle-physics models of dark matter) and experimentalists know what to look for (direct and indirect detection of dark matter particles, production of dark matter candidates at accelerators). The dark matter isn’t just ordinary matter that’s not shining; limits from primordial nucleosynthesis and the cosmic microwave background imply a strict upper bound on the amount of ordinary matter, and it’s not nearly enough to account for all the matter we need. This new result doesn’t tell us which particle the new dark matter is, but it confirms that there is such a particle. We’re definitely making progress on the crucial project of understanding the inventory of the universe.
What about dark energy? The characteristic features of dark energy are that it is smooth (spread evenly throughout space) and persistent (evolving slowly, if at all, with time). In particular, dark energy doesn’t accumulate in dense regions such as galaxies or clusters — it’s the same everywhere. So these observations don’t tell us anything directly about the nature of the 70% of the universe that is purportedly in this ultra-exotic component. In fact we know rather less about dark energy than we do about dark matter, so we have more freedom to speculate. It’s still quite possible that the acceleration of the universe can be explained by modifying gravity rather than invoking a mysterious new dark component. One of our next tasks, then, is obviously to come up with experiments that might distinguish between dark energy and modified gravity — and some of us are doing our best. Stay tuned, as darkness gradually encroaches upon our universe, and Einstein continues to have the last laugh.
August 21st, 2006 at 12:32 pm
Incredible stuff!
As always, explained very intelligibly.
Do you think this will be the final work on MOND?
NM
August 21st, 2006 at 12:44 pm
[...] Sean Carroll has a post at Cosmic Variance on some fascinating cosmological results which seem (Sean’s take is obviously rather more authoritative than mine) to demonstrate the existence of dark matter as comprehensively as one could reasonably hope for. posted on Monday, August 21st, 2006 at 12:44 pm Post a comment [...]
August 21st, 2006 at 12:55 pm
So here’s something that’s been bothering me for literally years: if dark matter is dark because it doesn’t interact with “normal” matter, does it interact with itself? If so, using what forces? If not, how does it ever clump? Wouldn’t it need some repulsive — or at least scattering — force to keep the constituents of a clump from simply passing “through each other” as they collapse? Or would a clump be truly dynamic, like a three-dimensional fountain? Or is this all left as an exercise for the reader?
Thanks!
JD
August 21st, 2006 at 12:56 pm
Bravo, Sean! Thanks again for another excellent post.
August 21st, 2006 at 12:58 pm
JD– the dark matter probably interacts with itself, but only weakly (thus “weakly interacting massive particles”). And, of course, through gravity. It’s the gravitational force that causes it to clump together, and the accumulated effect of small perturbations causes the distribution to “relax” rather than just having the particles zip through and out the other side.
August 21st, 2006 at 1:24 pm
[...] Your mission: Go to the press release website for more information, and lovely pictures. Read Sean’s post about it on Cosmic Variance. Sit back and think about it….. Then celebrate! [...]
August 21st, 2006 at 2:20 pm
That’s a very nice post!
With all the extensions, my Firefox browser is a bit slow. I think I’ll use an “empty” profile to do that tabs thingy.
August 21st, 2006 at 2:30 pm
Sean:
If dark matter exceeds visible matter by a factor of 3-5 why doesn’t the dark matter clump due to gravity to form large structures such as stars and galaxies? BTW I realy enjoyed reading your review paper on the cosmological constant a few years back. As a layman, but retired engineer very much interested in theoritical physics, I am interested in the latest ideas. Regarding the CC: if the QFT (and ST) prediction of the CC value is off by 60 to 120 orders of magnitude what does this say about our view of the validity of these theoritical ideas vis-a-vis the vaccuum energy density?
Really enjoy your blog.
August 21st, 2006 at 2:48 pm
Hey Sean,
What proportion of matter in the cluster is in the stars and the like versus in the gas?
Thanx.
August 21st, 2006 at 2:55 pm
Aaron– it’s about 90% gas, depending on the details.
Cecil– the dark matter does clump into galaxies and clusters. Even into star-sized things, just not nearly as efficiently as ordinary matter, which can collide and dissipate its energy. But for galaxies and clusters, most of the matter is dark, not ordinary.
About the cosmological constant (which is the same idea as “vacuum energy,” just with a different name), it’s clear there’s something important we don’t know. Wish I could tell you what it is.
August 21st, 2006 at 3:10 pm
[...] Sean Carroll at Cosmic Variance has a very nice explanation of a new result that shows that dark matter must exist and that all of the evidence for dark matter cannot be explained away by modifying Einstein’s general relativity. This shows one of the things I really like about blogs. I am in a related field, but I have very little expertise at astronomy. This explanation goes well beyond what I could in the popular press without being a report for experts only. [...]
August 21st, 2006 at 3:22 pm
1E 0657-56
As predicted, the NASA telecon on Chandra results was on followup studies of the “bullet cluster”. Press release is here. Lots of pretty pictures….
August 21st, 2006 at 3:29 pm
MOND is dead. Long live MOND!
August 21st, 2006 at 3:49 pm
Sean, any news on constraints on DM-DM cross section as a function of the assumed DM particle mass?
August 21st, 2006 at 3:58 pm
Hi Sean,
great post, thanks for the explanation!
the dark matter does clump into galaxies and clusters. Even into star-sized things, just not nearly as efficiently as ordinary matter, which can collide and dissipate its energy.
How do we know dark matter clumps into star-sized things? Is there evidence for that?
Best, B.
August 21st, 2006 at 4:01 pm
Count Iblis, the actual papers did calculate a limit on the DM-DM scattering cross-section, but I don’t know the number; you should have a look at the papers directly.
B, the DM should clump on all scales, no reason why not. But we don’t have any direct evidence (and a one-solar-mass DM overdensity would be much more diffuse than a real star). Furthemore, on those scales you have to worry about being disrupted by tides etc — so I really don’t know, again you’d have to ask an expert.
August 21st, 2006 at 4:06 pm
Sweeeet! Loving the third picture.
I do have a question about something you mentioned:
What’s the connection between scale and energy? Are you just saying that at smaller scales, forces are stronger, and potential energies are higher, or is there something more going on?
August 21st, 2006 at 4:07 pm
The pictures are beautiful, but I couldn’t find a link on the Chandra site to any paper.
Several questions: how close are the redshifts of the clusters? The mass determination presumably was done with weak lensing since no arcs are visible. How many background galaxies were there for each cluster. The key result is that the mass distributions are offset from the gas. So the centers of the mass distributions must have been taken as free parameters in the lensing fits. What were the other free parameters in the fits? The key evidence if you think about it is that the lensing distribution is not circularly symmetric around the center of mass. [Recall that Bekenstein's theory was constructed to get lensing right, so we expect lensing even far from the mass distribution.] So, how exactly did they do the fits? Was their mass model robust enough to allow for a circularly symmetric profile? And if so, with what confidence is such a profile ruled out?
I know this sounds like raining on a parade, but, as perhaps Eugene was suggesting, dark matter theories themselves have died many deaths. And some of the most dramatic have been delivered with pictures [think of the early APM data of the galaxy distribution or the Moore et al. simulations of the number of satellites expected in a galaxy].
Bottom line for me: Dark matter theories are way ahead of Mond and its extensions, and this latest result is yet another hurdle. But I still think it is interesting to explore the alternatives.
August 21st, 2006 at 4:14 pm
A wealth of links, four nice pictures to look at closely, and a very clear explanation. This is what blogs are for!
MOND is dead! MOND will stay dead! (probably)
August 21st, 2006 at 4:34 pm
Scott, man, such a wet blanket. The paper is here. It’s a Letter, so I don’t think they go into much detail about your questions, which are good ones.
But I don’t think it’s quite right to imply that Bekenstein’s theory could match the lensing observations. His theory is supposed to get the magnitude of the lensing right in a circularly-symmetric profile, but I don’t see how it could give rise to two big blobs of gravitational field offset from the mass distribution. Unless there was substantial energy density in the independent excitations of the new Bekenstein fields themselves — in which case it would really just be an especially messy and unmotivated version of dark matter.
Also: even in Bekenstein’s theory it’s necessary to have dark matter (e.g. neutrinos) to simultaneously fit galaxies and clusters. The point of this new result is not “MOND can’t do it alone,” since we already knew that. It’s “no modified-gravity theory can do it alone, we need dark matter.”
Aaron, in quantum mechanics, higher energies correspond to shorter distances. So as we probe smaller and smaller scales, new high-energy phenomena come into play. Nothing analogous usually occurs when we go to larger scales, but it’s good to be open-minded.
August 21st, 2006 at 4:35 pm
Sean, I’ll have a look.
Scott the papers will appear 2½ hours from now on arXiv:
http://arxiv.org/abs/astro-ph/0608407
http://arxiv.org/abs/astro-ph/0608408
The first one can already be downloaded via the link Sean gave:
http://chandra.harvard.edu/photo/2006/1e0657/media/paper.pdf
I guess the DM-DM scattering cross-section is on the other paper.
August 21st, 2006 at 5:01 pm
[...] Sean Carroll at Cosmic Variance ; Sean is intimately involved with this work, and his discussion of it is well worth reading. [...]
August 21st, 2006 at 5:05 pm
Singing Vikings
On the physics front, everything is now abuzz with news that the Chandra X-ray observatory has made the most direct observation of dark matter so far. Read more about it [...]
August 21st, 2006 at 5:09 pm
Very nicely written and explained.
A question: My (basic) understanding of gravitational lensing is that you measure the amount (in degrees) that angle is bent, and you infer how much mass is causing the bending. Stronger masses bend light more. But the relationship between “angle of bending” and “mass” is governed by an application of general relativity. If MOND were true, would this not imply that a smaller amount of matter (or dark matter) would be able to bend light more strongly than predicted by general relativity if it passed by at a greater distance to the matter causing the bending?
August 21st, 2006 at 5:31 pm
Acolyte, it’s not just the angle; it’s the direction in which the light is bent. The important thing here is that the gravitational field is pointing somewhere other than where the ordinary matter is located.
August 21st, 2006 at 5:33 pm
Awesome pictures Sean!
My favorite is the superimposed weak-lensing/Chandra X-ray image. I’m trying to understand the bullet-shaped shock front. Is the bullet-shape the result of ordinary matter being gravitationally “dragged” by the dark matter cluster?
August 21st, 2006 at 5:39 pm
Yes, that would be one prediction, but it is not really relevant to this result, since perhaps 90% of the ordinary mass is in the “pink” regions in the picture above, yet the lensing is happening in the blue regions which thus have only 10% of the mass. it does not matter if there were different lensing rules, assuming they were still symmetric under the centers of mass which seems likely. Any lensing caused by regular matter would be more in the pink regions. Since it isn’t there must be something in the blue regions, something we can’t see, causing the lensing…
August 21st, 2006 at 5:42 pm
Shouldn’t the dark matter distribution of each galaxy be tear-drop shaped or otherwise distorted because of the gravitational interaction with the other galaxy? Or is it a result that two spherical distributions of matter that pass through each other remain spherical?
August 21st, 2006 at 6:02 pm
Hello Sean, Can I use some of the images of this post in my blog?
August 21st, 2006 at 6:16 pm
Luis, sure, just link to the NASA site linked above.
Arun, if you look closely, the DM distribution is distorted. However, the precision of the technique isn’t sufficient to nail that down with a great deal of confidence, is my bet.
August 21st, 2006 at 6:19 pm
Acolyte, it’s not just the angle; it’s the direction in which the light is bent. The important thing here is that the gravitational field is pointing somewhere other than where the ordinary matter is located.
Ok, I realize that the gravitational field doesn’t seem to be focused on where the visible matter is.
I don’t know much about the mathematics of gravitation, so here is a different question: is the solution unique? That is, is the only explanation for the observed bending pattern the presence of M amount of dark matter in the location shown. Or, is this hypothesis one of a family of possible solutions to the problem.
As an analogy, suppose someone throws a ball at me, straight to my left eye, but something deflects the ball so that it hits me on the right corner of my lip. So, the ball is deflected by a a certain amount in the bottom-right direction. But, this could have happened either because someone nudged the ball by a little amount shortly after it was launched, or by a greater amount as it was getting closer to me.
So, the question is: how certain are we that the thing that did the nudging is right there close to the galaxies and not somewhere else? Is it perhaps that there is a unique solution to the bending of light from all the background galaxies, so that it could only be there?
Another question: could the gravitational field be strong in the wrong place for a different reason, say, the presence of a black hole that has sucked in all the matter in its vicinity? Or can this be ruled out?
August 21st, 2006 at 6:22 pm
Sean, have you seen this paper? Can you comment?
August 21st, 2006 at 6:37 pm
Hi Sean,
The DM should clump on all scales, no reason why not. But we don’t have any direct evidence
Okay, thats also my understanding of the situation. I was just puzzled by your statement that it does clump even into star-sized things.
Thanks for the clarification.
Best, B.
August 21st, 2006 at 6:39 pm
Sean,
Great post, pics, links and animations.
One hopelessly anthropomophic question: About the X-rays from that 160 million degree plasma – would they fry any life on the trillions or so planets in the galaxies involved in the collision, or is the intensity too low?
August 21st, 2006 at 6:52 pm
Over the last several days, I created a video from a physically motivated simulation of the Bullet Cluster. Last week Maruša asked me if I could make an animation, and I agreed. I thought it would be best to run an actual simulation rather than guessing at the collision’s dynamics. I used Enzo for the calculation. This was used in the Stanford/SLAC press release. Take a look…
Bullet Cluster Visualization
August 21st, 2006 at 7:47 pm
Since you guys ignore the EM force which is 10^39 greater than G I think you guys should wake up! This animation shows exactly what you would expect from an EM Field interaction and the X-Rays would be the natural conclusion. This isn’t Proof of Dark Energy or Dark Matter. It is just the illogical conclusions of people who keep refusing to see the obvious electromagnetic interactions of the Universe. It qualifies more as Garbage In and Garbage Out of a computer simulation than the realization that standard Maxwell interactions predict this and nothing else really does. Of course Red Shifts will not be velocity either. They are simple optical tuning effects in space. That will crank out all of the nonsense of a failing theory.
August 21st, 2006 at 8:55 pm
Paul: right on! When will these people realize that there are four simultaneous 24-hour days in a single rotation of the earth? Creation occurs via opposites. Singularity is the death math of religious/academic Godism. Earth Opposites should split apart – and cascade molten lava upon God Worshippers, for they are the evil on Earth.
August 21st, 2006 at 9:05 pm
Dark Matter Exists | Cosmic Variance
Dark Matter Exists | Cosmic Variance
This is a really impressive article about how the existance of dark matter has now been proven. Infact if your brain parses the pictures and explanations better than mine you will even see it for yourself. Excuse me…
August 21st, 2006 at 9:11 pm
[proud parent voice] Our son finally made it! He got quoted on the BBC website! [wipes tears away]
Fantastic post that made sense to this layman. Great news; it really is an exciting time for your field, isn’t it?
August 21st, 2006 at 9:16 pm
Sean,
Informative (and entertaining) as always. You know I still pull out those GR notes of yours from time to time.
CapitalistImperialistPig,
You’re right about the intensity. The IntraCluster Medium is an interesting assembly of matter, because it is so low in density, perhaps one lonely particle per 1000 cubic centimeters, if my memory from grad school is right. It is hot, in large part, because it can not cool itself effectively because interactions between the widely spaced particles are rare. The typical cooling time is longer than the age of the universe, except near the centers of massive clusters. So the actual rate at which energy is radiated away (or at planets) is low.
August 21st, 2006 at 9:42 pm
[...] From Cosmic Variance: The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twentyfirst-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Here’s the press release, and an article in the Chandra Chronicles.) [...]
August 21st, 2006 at 9:52 pm
70% of the universe is ultraexotic?
Isn’t that an oxymoron?
Add why do I live in the boring 30%?
August 21st, 2006 at 10:11 pm
[...] Dark matter exists!. In another example of science blogs as news sources, this story quotes Sean from Cosmic Variance. He writes about the discovery in this post. For the hardcore, the paper is now in the astro-ph archive, in multiple formats. When, oh when, will biologists get a preprint archive? [...]
August 21st, 2006 at 10:21 pm
I’m not a physicist (stopped taking physics once I got to college and realized that I was calculus-impaired), so this is probably a rather ignorant question, as it is likely either physically impossible or so obvious that it’s already been considered:
If it is known to be non-baryonic, is it possible for dark matter to consist of some sort of large clump of massive bosons functioning in some unknown manner?
Or would it be possible for supermassive black holes at the center of these galaxies to pass by or through each other in some manner, or maybe even slingshot around each other, with the “matter” of the galaxies either being ejected or being slowed down by various collisions and interactions, and what we’re seeing are two semi-naked black holes (which cause the gravitational lensing and would comprise the majority of the mass), flying away from each other, eventually pulling apart the cloud of matter from the collision behind them into the two red gas clumps we see in the picture?
Granted it wouldn’t explain any of the other measurements showing that that there couldn’t have been enough matter in the universe to account for its effects. Like I said, probably very ignorant, but when I think of something large and massive and invisible, that would have to come to mind.
August 21st, 2006 at 10:59 pm
Hi Scott and Sean,
Well, a cursory reading on the Angus et. al. (2006) paper cited by the Letter is rather interesting. In that paper, they show that you can actually construct multi-center mass distributions, with the lensing maps not projected onto the centers, in the TeVes framework. This has something to do with the complicated direct vector field interaction with metric.
In Angus et. al., they claim that for certain classes of models of TeVeS, Sean is indeed right that there exist still a baryon discrepancy (i.e. they can’t get enough lensing). However, they only consider some classes of models, and they have not fully explored all the parameter space of TeVeS (e.g. the case where the \Chi field is important).
The Letter mentioned two problems with such a construction : that the ‘multicenter’ maps that Angus et. al. constructed has all the centers lined up, and that for the Angus model to work there must exist a small central mass concentration (between the two subclusters). Neither is true observationally : the small central peak is consistent with standard Newtonian prediction and the peaks do not line up.
The bottom line is : TeVeS does not necessarily predict a lensing profile that scales simply (this will be ruled out by the many observations of colliding clusters, including this one). Indeed, Angus et. al.’s paper was to demonstrate this fact, making the point that TeVeS does not have to reduce to MOND. However, there exist plenty of parameter space to be explored. TeVeS is quite, to put it mildly, messy.
MOND probably dead as a empirical law. TeVeS, on the other hand, may still be alive but is getting as baroque as the multiparameters that we need to cook up for DM to make it all fit.
August 21st, 2006 at 11:06 pm
Actually, after hitting “submit”, I think I take back my statement about MOND is dead as an empirical law. MOND is very much alive as a empirical law that “predicts” the rotation curves. But it does not say anything about lensing. Any relativistic extenstion to MOND that predicts a simple scaling of lensing with baryonic mass distiribution is dead, but no such theory exist. (TeVeS does not predict a simple scaling in multicenter models.)
August 21st, 2006 at 11:09 pm
I see over and over stated that “Dark matter clumps to galaxys and clusters”… but here’s a turn of thought.
Why not have it be that matter tends to clump to regions where dark matter has clumped. Maybe it’s all chicken-and-egg thinking, but why lock ourselfs into thinking that the dark matter is running around looking for normal matter to hang out with? Bottom line, maybe normal matter is following it around.
Regardless though, great reading. My hat is off to the people that make expanding our knowlage on the extream edge possible.
August 21st, 2006 at 11:32 pm
Why has no scientist tried to prove the Einstein-aether theory, because in that theory free energy is possible. And capitalism wont allow free energy to be sold, because there isn’t enough money in that.
On a side note, dark matter is in pop culture alot, and we want to be able to keep using the word “Dark Matter” so we try to prove it exists, because it sound cool.
August 21st, 2006 at 11:40 pm
Great post Sean! I am very relieved to be rid of MOND for good and for all.
I am a bit confused about the process of releasing the results….NASA had the big press conference today, and the papers are just now appearing. Yet (as I was reminded today) Michael Peskin showed the pictures in his lecture at the SLAC Summer Institute. Obviously he had “hot” copies because part of the analysis was done here at SLAC at the Kavli Institute.
Gosh, this brings home my biggest fear of how the release of data is going to be handled at the LHC…
PS: I tried hard to link to Michael’s SSI talk,
August 22nd, 2006 at 12:10 am
[...] Cosmic Variance has a very readable article here that explains the results in a little more depth [...]
August 22nd, 2006 at 1:03 am
The content was fascinating, but what impressed me more was
Sean’s writing style. I felt the same sense of scientific wonder as I got when reading a Carl Sagan book for the first time as a teenager.
August 22nd, 2006 at 1:16 am
Ok – just one question on the lensing – can someone post a picture which shows the galaxies that are being ‘lensed’. Or a picture which false-colors galaxies based on their red-shifts, instead their normal colors.
August 22nd, 2006 at 1:42 am
Eugene said ” TeVeS, on the other hand, may still be alive but is getting as baroque as the multiparameters that we need to cook up for DM to make it all fit.”
Isn’t that a fancy way of saying “TeVeS is dead”?
JoAnne: I sincerely hope that all your fears come true. This whole “discovery striptease” that NASA ran was undignified idiocy of the first order, and they should thank whoever leaked the announcement. Look at what happened with WMAP — all that secrecy leading up to an announcement that amounted to nothing much at all. They ended up looking foolish. I hope that LHC will be one long leakfest.
August 22nd, 2006 at 2:02 am
[...] Check it out. Send To Bookmark Sharing ServicesThese icons link to social bookmarking sites where readers can share and discover new web pages. [...]
August 22nd, 2006 at 3:00 am
I am a software engineer by profession and its been like 8 years since I touched a book on physics but this post on something as abstract as dark matter and dark energy – I could almost understand!!!
Fantastic job!Thank you!
August 22nd, 2006 at 3:39 am
Nice explanation. Still blatantly rubbish, but a nice explanation. This dark matter nonsense is just Vulcan all over again. The problem with inferring the existance of something based on its effect on something else is that it assumes that the hole in your current understanding is only big enough for one explanation.
In other words, the dark matter camp is trying to assert that if there is something a MOND can’t explain then it proves their theory must be correct. It does not.
It is totally inconceivable that something as important to the phyiscal characteristics of the universe as dark matter is supposed to be has not turned up here on Earth, or even in the nearby part of the universe. How is it formed? It it outweighs normal matter then it must be relatively easy to make, yet no trace of it has ever popped out of any experiment at CERN et al. Why?
Dark matter is just plain silly and, like Vulcan before it, will ultimately turn out to be a patch over a fundamental gap that we do not yet even realise is there in our knowledge. A new force? A modification of the way the known forces interact under some circumstances? Who knows. But just postulating magic pixie dust, no matter in what quantities, is not in the long run going to be a viable explanation.
August 22nd, 2006 at 5:06 am
The problem with inferring the existance of something based on its effect on something else
That would be how we infer the existence of everything.
August 22nd, 2006 at 5:06 am
[...] Cosmic Variance 有个很好的帖å，解释了这个事情。 [...]
August 22nd, 2006 at 5:22 am
[...] Cosmic Variance explain it all with much more detail. [...]
August 22nd, 2006 at 5:53 am
One Meeleeon Visitors
(Picture me dressed as Dr. Evil, with my pinky in the corner of my mouth.) I’ve been watching our visitor meter over the last week or so, waiting to post and thank our readers when Cosmic Variance’s one millionth visitor (to the main page…
August 22nd, 2006 at 5:54 am
Why cant the gravity in the blue areas simply be caused by the galaxies in respective group? Why does the blue area have to contain any dark matter? (the galaxies also act as collisionless particles) Why cant this simply be a collision between two clusters where the galaxies passed each other without collisions but the interstellar hot gasses collided and resulted in the formation above?
August 22nd, 2006 at 5:55 am
[...] Sean « Dark Matter Exists [...]
August 22nd, 2006 at 5:58 am
Could the anomalous lensing be explained by gravity travelling at the speed of light, thus acting as though it was lagging behind? This would be consistant with there being no such dark matter. I’ve seen possibly explainations of “dark matter” in galaxies by simply using GR properly.
August 22nd, 2006 at 6:16 am
[...] Una buena explicación asequible la podéis encontrar en este post de Cosmic Variance. Si os peleáis con el inglés, hay un pequeño resumen en castellano en Astroseti. [...]
August 22nd, 2006 at 6:18 am
“The problem with inferring the existance of something based on its effect on something else
That would be how we infer the existence of everything.”
Which is why we often make mistakes. The problem is that, in the immortal words of Donald Rumsfeld, if you don’t know what it is that you don’t know then you can not simply say that Theory A is the correct one because you have shown Theory B to be incorrect.
Pointing at odd movements of bodies and saying that it implies the existance of a mass you can’t see is fair enough most of the time but when it implies something this outlandish it is more likely that there is a whole new facet of physical law to be uncovered.
Dark matter theory tries to fiddle the subject by introducing a wonderous magic substance which basically only interacts with gravity in the normal way. The problem is that gravity is so weak that huge amounts of this magical material are required to explain the observations. It seems far less likely that this is the correct solution than that some force (by which I mean a physical consequence of normal matter, time and space) is appearing which hitherto has been hidden not by its enormity (which is the supposed case for DM) but by its subtlty.
Again, Vulcan shows the way: the strange movement of Mercury was not caused by something which was huge and strangely impossible to see, it was caused by a small relativistic effect which was hidden in normal situations by the very fact of its weakness.
When people start saying that an effect is caused by something 3 times the mass of the visible universe but which has escaped notice for the whole of history, is it really unreasonable to doubt them? I don’t think so.
Dark matter is a fudge; a placeholder until we come up with something sensible. Anyone looking to explain the galactic movements using existing laws and forces without modification (which is effectively what DM is trying to do) is on a hiding to nothing in the long run.
August 22nd, 2006 at 7:05 am
Dark Matter
August 22nd, 2006 at 7:06 am
The problem is with the statement…
The problem is that the nucleosynthesis and CMB arguments are based on arguments that assume one is observing a “dead” universe. This is because it is much simpler for physicists and astronomers to develop theories and match data with those theories if one asserts that the universe still is, as it once was, “dead”. If instead one assumes that the universe may now be quite “alive”, then the “dark matter” may simply be advanced civilizations that have adopted the form of Matrioshka Brains. No new mysterious particles are required if you simply turn the dead or alive coin over.
Robert Bradbury
1. Wikipedia: Matrioshka Brain
August 22nd, 2006 at 7:33 am
I am going to sit and read the papers, probably this weekend on my flight to DC, but I find these results a little hard to accept without reading the paper. Sean, I think you did a great job, but with a degree in physics, I am looking for some “numbers.” I do have to say my biggest issue with the results from the pictures is that there is no shock wave in the dark matter. Even if dark matter is weakly interacting with itself, I would still expect to see some sort of shock wave at these energies and speeds. It would be nice to see the normal matter having some sort of effect on the dark matter as well, which I can’t pick up from the pictures, but hope are still present. Great site, great article, great thinking, great discussion.
August 22nd, 2006 at 7:40 am
From the article:
It turns out that the large majority (about 90%) of ordinary matter in a cluster is not in the galaxies themselves, but in hot X-ray emitting intergalactic gas.
Question:
How do we know that? This is the basic argument on which the separation of Dark/ordinary matter is done using the images
August 22nd, 2006 at 7:40 am
It’s really out there!!
Exactly what you’ve read, there’s no way dark matter is not there
!
More on the empiric proof of the existence of Dark Matter is explained by Sean, and in a lot of places. Ah, and of course, in the paper to be published in ApJL.
August 22nd, 2006 at 7:50 am
Unlike Sean, I am not too upset to see life get significantly harder for MOND enthusiasts — if anything MOND’s primary purpose in life has been to explain just how hard it is to “explain” dark matter by modifying gravity.
The irony is that while the original argument in favor of MOND was one of simplicity (look, I can explain all these rotation curves with just one parameter!) i morphed into something horribly baroque. Not only do you want to explain galaxy rotation curves, but you also need to get the acoustic peaks in the CMB, and weak lensing — since in conventional cosmology, the standard explanations of these observations all rely on the presence of dark matter So far as I can see, attempts to implement MOND in metric theories of gravity require a different component for each of these three effects, where dark matter explains them all in a unified fashion. Moreover, MOND requires Lorentz symmetry to be broken in some way (as Sean will know well) whereas dark matter just needs a particle we haven’t seen yet, with a mass and cross-section that is not wildly at odds with our expectations for TeV scale physics.
I know Occam’s Razor isn’t any substitute for experiment, but for my money a MOND-driven universe is far more preposterous than one with where Omega_baryon is smaller than Omega_matter
On the other hand, Dark energy is something that could very well be understood in terms of a “modified” gravity. And, unlike dark matter, there is no “simple” explanation for the accelerating universe — a cosmological constant is just one number (assuming the dark energy is not dynamic), but understanding it is bound to involve a major advance in theoretical physics.
August 22nd, 2006 at 7:54 am
[...] You can see the complete notice here: http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/ [...]
August 22nd, 2006 at 8:23 am
is it possible that the Xrays somehow took longer to get to us than the gravity,
(because of differences in the *refractive index* between the Xrays and gravity
waves) and this explains the transposed centers of material. If the collision is relatively fast and energetic, couldnt this also explain the images ?
August 22nd, 2006 at 8:36 am
[...] Well, holy shiat… They’ve actually done it. NASA has found DIRECT proof of dark matter!! I must admit, i was skeptical of dark matter/energy. It always seemed like a convienient gapfill for not-quite-perfect theories and observations. But, dammit here it is! I wonder if its as fun as this though? [...]
August 22nd, 2006 at 8:42 am
[...] The nice people over at Cosmic Variance have reported that Dark Matter Exists! [...]
August 22nd, 2006 at 8:45 am
Is it possible that super-super massive black holes exist at the center of galaxy clusters? I think (I could be wrong) it’s proven that most if not all galaxies contain a supermassive black hole at their center. Why not galaxy clusters? Couldn’t this at least contribute to the shift in the measured center of mass that’s being attributed to dark matter? Yes, I know I’m reaching, I just can’t stomach dark matter. A magic substance that is ignores all forces other than gravity and refuses to show up in particle accelerators, yet is 5x more common than regular matter.
August 22nd, 2006 at 9:43 am
Hi,
Some comments from a regular lurker:
1- The Bulletcluster definitely does NOT rule out MOND, contrary to what some people are claiming now. As Sean mentions in his post, MOND needs (predicts
) dark matter at clustergalactic scales and to get the CMB right with Bekenstein´s TeVeS theory one still needs some form of dark matter like neutrino´s for instance, that would only cluster at clustergalactic scale. Why bother then about MOND when it still needs dark matter? The point is that MOND, a simple one parameter modification of Newton´s law, explains the rotation curves of all kinds of galaxies with different morphologies very well. From the dark matter perspective this regularity is quite surprising, since structure formation is very much a stochastic process. All the explanations for this regularity within the dark matter paradigm that I have seen so far are not really convincing. To explain the Tully-Fisher law for instance, that describes a certain correlation between the luminous matter of a galaxy and the asymptotic velocity of its rotation curve one needs to invoke different fine-tuned processes for different kinds of galaxies.
It is quite strange (again from the CDM perspective) that MOND has not been ruled out yet. To do so one would only need to find some galaxies, where MOND predicts way too much modification (invoking dark matter will not help you there). But I guess that galaxies with way too little modification will also mean the final blow to MOND, since this would mean that MOND also needs dark matter at the galactic scale.
2- Let´s assume that the reconstruction of the gravitational field from the weak lensing is done properly and that we really observe an offset between the gravitational field and the luminous matter distribution. Does this then prove the existence of dark matter and rule out modified gravity at that scale. I am not so sure. In particular I don´t agree with the statement that “there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle.” I would expect this feature to hold in a spherical symmetric situation but not necessarily in a dynamical situation as two colliding clusters. Suppose that the modified gravitational field does not propagate with the speed of light, but way slower, as happens for instance in the ghostcondensate model of Arkani-Hamed et al. In that case the gravitational field would need some time to catch up with the source and maybe this is what we observe? But Ok, I agree that the dark matter explanation seems to be simpler
.
Great pictures by the way!
August 22nd, 2006 at 9:48 am
Some overall consistency checks on this phenomenon should be performed:
1) The two clusters would show up as outliers from the “fundamental plane” relation of clusters in the space of 2-component virial theorem parameters [Dantas et al. ApJ Letters, 528, L5, 2000, stro-ph/9910541];
2) N-body gravitational simulations of collisions of 2 (or 3)-component cluster models (barionic+DM; or + gas) with appropriate masses and orbital configurations should be performed in order to check the overall consistency of the dynamical timescales.
3) It would be valuable to perform similar N-body simulations without DM, with gravity law modified to include MOND terms, and other mixed simulations with DM and MOND together in order to verify whether the phenomenon can still be reproduced.
Christine
August 22nd, 2006 at 9:55 am
Wow. I wanna be a theoretical quantum-dynamicist! First I can learn how to spell it!
But first I have to finish my first career and get the Kids through college. Oh well.
So if there is dark matter way out there billions of light-parsecs away, what about our local dark matter in our galaxy? It is a wee bit closer, so what makes it unsuitable for investigation?
If the discovery confirms that there is that much dark matter, is our universe then no longer eternally exploding but will collapse in a little while, say, 50-100 billion years?
August 22nd, 2006 at 10:04 am
Is it just me or has this entire story taken on the feel of “Astroid to Destroy Earth in 12 Days.” Instead of being a story about strong evidence for dark matter it is being reported in the general media, and even here as absolute proof. I guess I’m not allowed to comment on the validity of science, living in Kansas.
August 22nd, 2006 at 10:29 am
[...] so, along with the 12 planets debacle, ther is some pretty big dark matter news. Here’s a very thorough explanation for the “proof” of the exsistence of dark matter. [...]
August 22nd, 2006 at 10:33 am
Sean, you refer to “… an mpeg animation of what we think happened …”
available on the web at
http://chandra.harvard.edu/photo/2006/1e0657/media/bullet.mpg
It seems to me to show, a various times t in seconds of the mpeg:
t = 0
two spheres, separated in space,
a small one on the left (call it A)
and a large one on the right (call it B)
with
blue gravitating Dark Matter evenly dispersed throughout A and B
and
red hot gas Ordinary Matter also evenly dispersed throughout A and B.
t = 2 seconds
A and B have just begun to collide, and are in contact at a point,
with blue gravitating Dark Matter still evenly dispersed in A and B.
However,
red hot gas Ordinary Matter is no longer evenly dispersed,
but has migrated in A and B to crescents opposite the contact point.
What is the mechanism that causes that migration of the hot gas?
Tony Smith
http://www.valdostamuseum.org/hamsmith/
August 22nd, 2006 at 10:55 am
[...] Veja mais também no Chandra Chronicles e em Cosmic Variance. Atualizaremos este post com notícias em português sobre a importantíssima descoberta assim que surgirem. [...]
August 22nd, 2006 at 11:07 am
Christine Dantas has some interesting technical comments, which for some reason her browser would not let her post here.
You can find these comments posted here instead.
August 22nd, 2006 at 11:13 am
Sorry folks, in our attempts to keep the site running we ran into a commenting snafu, but it’s in the process of being fixed now.
August 22nd, 2006 at 11:14 am
Hi,
It seems now my post has been uploaded. Thanks!
Christine
August 22nd, 2006 at 11:20 am
[...] En Cosmic Variance hay un interesante análisis de las implicaciones de este suceso. [...]
August 22nd, 2006 at 11:45 am
Dark Matter is Real!
Holy cow. I think this is perhaps one of the biggest discoveries in astronomy since extra-solar planets were discovered.
I won’t say much about this, because Cosmic Variance did such a wonderful job that I’m just going to send you there to …
August 22nd, 2006 at 12:42 pm
[...] Dark Matter Exists | Cosmic Variance Holy cow. This is one of the biggest days in cosmology. Actual proof of Dark Matter. This means that only about 5% of our known universe is made up of what we call normal matter! Whoa. Anyway, Einstein’s General Relativity doesn’t work too well as you move out to encompase the entire universe. So now that we can directly observe Dark Matter and its interactions, we can develop new theories that will help refine General Relativity and our understanding of the universe. I smell a Nobel Prize in Physics in the workings. __________________ Harken: Seems odd you’d name your ship after a battle you were on the wrong side of. Mal: May have been the losing side. Still not convinced it was the wrong one. Brown Coats Unite! |Firefly|Soundtrack|Serenity|Soundtrack| [...]
August 22nd, 2006 at 12:53 pm
Jack Said
Only to a String Theorist
where beauty is paramount!
After all, there is no accounting for taste.
August 22nd, 2006 at 12:54 pm
How is the dark matter subject of this post different from essentially invisible neutrinos passing through planets like Earth?
How does the visible gas collision differ from Cherenkov radiation?
August 22nd, 2006 at 1:48 pm
Sean:
Does the inter-galactic gas of a “typical” cluster emit X-rays? If not then how is this gas detected? I guess I am still a little confused about the dynamics of the collision. Is there an estimate of the percentage of the inter-galactic gas that is swept from the cluster? The pictures suggest a very large percentage only if the gas emits X-rays all the time. If a large percentage has been swept out would this not affect the gravitation field of the cluster after the collision and thus cause the galaxies to may be move apart. Just wondering.
August 22nd, 2006 at 2:11 pm
[...] Dark Matter, Conclusive proof that dark matter exists, warp drive here i come [...]
August 22nd, 2006 at 3:57 pm
Hi
this is a message from the Cosmic AAA – Paris Agency
Why does nobody point out the fact that the whole thing is still thought from the now very unquestionned Big-Bang point of view ? The whole thing is stuck in a mass of the universe theory that absolutely wants a total mass to be. What if there was no such thing as a dynamic equilibrium of a so-called mass of the universe ? It’s not only to be thought as a local problem but also as a global question concerning a universe that might be driven by different currents of energy of which the Big-Bang effect is only a local wave in which we are but a molecule.
August 22nd, 2006 at 4:59 pm
If dark matter is weakly interacting, one would expect that the material takes a very long time to thermalize — especially if the only available form of radiant energy is gravitational. It would seem that we could address this question by observing the relative distributions of visible and dark matter in the lensing regions. Specifically, after yanking a nontrivial fraction of the mass of the cluster out (the gas), the remaining visible and dark matter will expand slightly, relaxing out into the slightly shallower gravitational well. Same for the dark matter. The rate of expansion should depend on their “temperatures”. In fact, the diffusivity should scale as T^(3/2) M^(-1/2) P^(-1) d^(-2), where T is the temperature, M is the RMS mass of the particles, P is their pressure and d is their (effective collision) diameter. Note that for the visible matter and the typically proposed dark matter, P ~ 0, and d ~ 0, so both diffusivities should be huge. We may be looking too late after the event.
August 22nd, 2006 at 6:06 pm
Wow, Dark Matter exists!
Sean Carroll hits a home run in explaining some recent science about showing the existence of Dark Matter. I initially had used the word prove, but in science you really never prove anything; you instead add evidence in favor of
August 22nd, 2006 at 7:06 pm
If MOND modifies gravity, how would that affect gravitational lensing?
Would that change the weak lensing results for the Bullet Cluster?
August 22nd, 2006 at 7:09 pm
I thought that higher energies corresponded to shorter distances in QM because of diffraction — to probe shorter distances, you need particles with smaller wavelengths to create a sharp image. But the de Broglie wavelengths of the things astronomers study must be unimaginably small; how could diffraction effects be relevant? I can’t imagine objects the size of galaxies exhibiting quantum interference…
What’s the catch?
August 22nd, 2006 at 9:44 pm
fun bits from the web
Some interesting bits I’ve found surfing lately:
*here is anamusing bit of comic commentary during my standard surfing the other day. The author makes a very funny/good point as well as addressing one of my current annoyances with comics (the wierd ph…
August 22nd, 2006 at 9:54 pm
[...] • Chandra Press Release (includes nifty visualizations and animations) • NASA Press Release • Dark Matter Exists – Outstanding article by Sean Carroll at Cosmic Variance • Colliding Clusters Shed Light on Dark Matter – Scientific American • Colossal Cosmic Collision Reveals Mysterious Dark Matter – Space.com • Rob Knop at Galactic Interactions Filed under: Astronomy, Science, Space | [...]
August 23rd, 2006 at 2:10 am
How do we know that these galaxy clusters are not accompanied by huge numbers of black holes, invisible to us, and other ordinary matter that we can’t see from here, causing the observed lensing?
August 23rd, 2006 at 3:54 am
Roger (#76):
In many clusters, there is an extra-large, extra-massive galaxy sitting at the approximate center of the cluster (often what’s called a cD galaxy = central dominant galaxy). Given that such galaxies probably have their own supermassive black holes, that’s the closest you get to “a supermassive black hole at the center of a cluster.”
But these supermassive black holes really aren’t all that massive, relative to the galaxies and clusters. Central black holes in galaxies have masses that are generally less than 1% of the visible mass of the whole galaxy (that is, the mass of all the stars in the galaxy). The hot intergalactic gas (that’s emitting the X-rays) is, in turn, about 5 times the mass of all the stars in the cluster’s galaxies — and yet the estimated dark matter in a cluster is about 5-10 times the mass of the stars + hot gas. So the “supermassive” black holes are a tiny, tiny fraction of the total.
August 23rd, 2006 at 7:33 am
Maybe this is the first direct evidence of mirror matter. For mirror matter theory see
Mirror Matter and
The Mirror Matter Webpage . If mirror dark matter really exits, it might be even technologically useful:
.
August 23rd, 2006 at 8:23 am
Which is why we often make mistakes. The problem is that, in the immortal words of Donald Rumsfeld, if you don’t know what it is that you don’t know then you can not simply say that Theory A is the correct one because you have shown Theory B to be incorrect.
Good job no one is doing that, then. What they’re actually doing is taking the dark matter model, working out what ought to be observed if it is true, and comparing to reality. You know…science.
Dark matter theory tries to fiddle the subject by introducing a wonderous magic substance which basically only interacts with gravity in the normal way. The problem is that gravity is so weak that huge amounts of this magical material are required to explain the observations. It seems far less likely that this is the correct solution than that some force (by which I mean a physical consequence of normal matter, time and space) is appearing which hitherto has been hidden not by its enormity (which is the supposed case for DM) but by its subtlty.
So…new physics via new particles = magic. New physics via new/modified forces = good science.
Well, feel free to use your personal incredulity as a compass to reality if you wish. I’ll stick with that ’science’ thing; seems to work better.
August 23rd, 2006 at 10:43 am
Good job no one is doing that, then. What they’re actually doing is taking the dark matter model, working out what ought to be observed if it is true, and comparing to reality. You know…science.
While I don’t disagree with you necessarily, simply shouting ‘observations!’ doesn’t actually prove a theory to be more or less valid. DM theory does have a lot of evidence that supports it under our current knowledge, but still has areas where it doesn’t seem to work or at least doesn’t work as should be expected, especially at smaller scales. Also, MOND does appear to fit in many situations… this instance not being one of them of course, which illustrates that it certainly isn’t a complete theory, but not necessarily that it is entirely wrong.
I personally feel that DM theory is a much better fit to what we’ve observed of our universe than most other theories so far brought forward, but I’m more than happy to keep an open mind and be cautious of seemingly complete answers. Until Lavoisier showed the relationship of oxygen in regards to combustion, Phlogiston theory was considered to be fairly sound, was well accepted, and matched well with what science at that point could readily observe.
August 23rd, 2006 at 11:44 am
[...] Not being an astrophysicist, I don’t have anything clever to say about the recent discovery that seems to have located a nice big chunk of dark matter, but I really didn’t want to post any more pictures of wildlife. So here’s a picture of superheated gasses! [...]
August 23rd, 2006 at 1:30 pm
Aaron F: features on very short distances are relevant to larger scales only if you can construct some information transfer between the small systems and the larger systems. In other words, a change in the state of the small system must induce a change in the state of the large system. However, as you point out, in order to probe (or if you like “interrogate”) the small scale system, you need particles with a short enough wavelength to scatter off the small system. Hence the larger energy.
You can call it diffraction (which is correct) but that masks the more profound point of how variations on small enough distances become irrelevant to larger systems. In particular, we can keep coherence in quantum systems (a big deal in quantum information theory), even though we necessarily trace out (= ignore) all effects of string theory (ignoring one of two interacting systems and looking only at the other typically destroys coherence).
August 23rd, 2006 at 3:36 pm
Dear Richard C,
These observations are inconsistent with MOND, and there’s no way to make MOND compatible with them. In contrast, other observations, like the rotation curves of galaxies, can be explained by dark matter and MOND. However, these can only be explained with dark matter. You don’t have to keep an open mind about it. The location of the mass is derived from the lensing maps. The mass is clearly not located where the dominant source of baryonic (not dark) matter is located (this is the X-ray emitting gas). There is no way to make a theory where gravity follows baryonic matter compatible with these observations.
August 23rd, 2006 at 5:24 pm
This would be a big discovery had it not been based on photomanipulation. Normally I would be eager to embrace such a revolutionary finding but Im afraid I just dont buy it.
I was disappointed to find this article much more personal and informal than I am used to. It read more like a teenager’s diary than a scientific examination. Unfortunately in cosmology, pictures arent enough no matter how much they’ve been photoshopped. Applying filters and other types of detection does not explain something that requires heaviy modification of relativity and I find no justification for that here.
Some galaxies have higher than expected gravity…. ok, have you considered our measurements could be wrong? Perhaps there is some phenomenon at work which has yet to be discovered. Suggesting we make major modification to relativity just so one theory will fit instead of another is ludicrous and decidededly NOT GOOD SCIENCE.
Ive seen nothing to prove or disprove the existence of dark matter or dark energy other than the fact that some people choose to use those terms as a scapegoat for something nobody yet fully understands. From what I can see, it is still pseudoscience.
August 23rd, 2006 at 6:13 pm
Christopher — your post reveals some rather severe misunderstanding on your part. First and foremost, the result is most certainly not “based on photomanipulation.” The images present in Sean’s excellent post and in the Chandra press releases are a representation of the data and a representation of the information the data convey, but they are most definitely not the sum and total of the data. You can be assured that observational astronomy does not consist of applying filters in Photoshop. The sophisticated technique used to determine the dark matter distribution actually depends on relativity, and the dark matter hypothesis does not demand modification to relativity. That’s actually why many people prefer the idea of dark matter, since it represents an unknown quantity as opposed to a massive misunderstanding of gravity. I am also a bit confused by your criticism of the ApJ letter; as Sean pointed out, it is a Letter and therefore leaves some potential questions unanswered, but I suspect that you are actual criticizing the press release and not the Clowe et al. paper. The press release is in fact informal, as that is its function.
Finally, it is fundamentally incorrect to equate “science in progress” with “pseudoscience.” In fact, I would argue that it is the “in progress” that actually defines the scientific method. All science, astronomy and cosmology — here, not only the question of dark matter but the perhaps less exotic study of cluster dynamics — involves the comparison of a body of ideas with a body of evidence. Remaining open to the possibility that new data will contradict the prevailing idea (or “the more natural idea” or the most attractive idea) is an important part of what you call “not good science.”
August 23rd, 2006 at 7:26 pm
[...] Original post by Sean and software by Elliott Back [...]
August 24th, 2006 at 12:33 am
What would the consequence be for “civilizations” that may exist within the both Galactic Groups in question?
Would they detect their Galaxies as having a Dark Matter Halo, or would they be more inclined to percieve a Positron Halo around their respective Galactic homes?
There is another problem with the images and data, one can produce a perfect match for the Galaxies to have really just RE-BOUNDED away from each other after impact?
Like two billiard balls colliding, unless you follow both balls for a great length of time (before impact-just after impact ) you can have no way of telling if there was any “passing_through” interaction.
As we are at a far away location, our observation and data would fit Dark Matter with a “rebounded” effect just as an infered “follow-through” effect.
August 24th, 2006 at 4:05 am
I give an interesting model of dark energy in my new book Quantu Motion. Further references can be found in my website http://www.quantummotion.org/
August 24th, 2006 at 8:14 am
[...] Dark Matter – Proven to exist? Dark Matter Exists | Cosmic Variance posted this you know where… figured I’d drop it in here too. The problem of Dark Matter has been at the heart of modern cosmology since the mid 1960’s. Vera Rubin was one of the pioneers in realizing that the universe was made up of much more matter than we could see and account for. I find it amazing that people can live in a universe this rich with mystery and immense complexity (enough for thousands of towering intellectuals to try and comprehend) and when told there may be no creator, wonder, "Is this then all that there is?" What more could one want? You see that people are actively trying to modify General Relativity, one of the great pillars of modern physics. And with evidence and observation, they will do so if needed and it will be accepted and taught. How anyone can call active science a Religion is beyond me. [...]
August 24th, 2006 at 1:06 pm
Electomagnetic radiation does’nt work, so we have to look at the affects of the matter through gases and gravition? It should be detected direclty, like all matter (neutrinos).
Neutinos are contained in all matter.
August 24th, 2006 at 5:22 pm
[...] Friday afternoon I’ll be on NPR’s Science Friday to talk about the recent dark matter results. Nothing that regular readers haven’t heard already, I suspect. [...]
August 24th, 2006 at 5:25 pm
[...] Update 8/24/06: For a more detailed analysis of this story, see this Cosmic Variance post, and check out as well Anthony Kendall’s fine article here. [...]
August 24th, 2006 at 6:29 pm
thanks for the black holes
August 24th, 2006 at 6:33 pm
Do you know that this universe name is DARN and that our twin universe name is DAL??? And that one cosmos has 8 universes and that they work in pares? and that a Transfinite creates 108 cosmos??????????
August 24th, 2006 at 6:54 pm
[...] [...]
August 24th, 2006 at 7:46 pm
Great post, great comments.
How about Tony’s question? I see the same thing.
August 24th, 2006 at 7:55 pm
[...] Dark matter has been basically proven to exist; click here for a clear and detailed exposition of the evidence. [...]
August 24th, 2006 at 10:28 pm
[...] Oh, you miss Pluto, do you? Oh, forget about that crusty old dwarf-planet. We’ve got something much better for you: The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twenty-first-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Here’s the press release, and an article in the Chandra Chronicles.) [...]
August 25th, 2006 at 5:51 am
*newbee question*
could it be that non-baryonic dark matter and baryonic matter attract eachother, but non-baryonic dark matter repels itself?
This would mean dark matter would cluster around galaxies, but would never collapse on itself (ie into dark stars or dark planets).
August 25th, 2006 at 5:58 am
Tony Smith (#82) and ann miller (#121):
There’s a partial explanation of the animation here, in the captions for the various images and animations (caption 5, down near the bottom of the page).
What you’re seeing in the first few seconds of the animation isn’t the gas “migrating” to one side or the other; the dark matter blobs (blue) are moving faster and leaving some of the gas behind (if you look carefully, you’ll see that the gas is moving in the same direction, just not as fast as the dark matter).
Most importantly, the beginning of the caption calls the animation “an artist’s representation of the huge collision in the bullet cluster” (emphasis mine). To my eyes, it’s clearly not a genuine (physics-based) simulation. In fact, if you scrub back and forth, you’ll see that the last four seconds of the main animation are just two unchanging composite objects (left-hand DM + distorted gas and right-hand DM + distorted gas) sliding apart.
So it’s really just an animated cartoon, so to speak, of the underlying physical processes, and all sorts of fine details will be missing or wrong.
August 25th, 2006 at 6:22 am
Excellent post, I also like all the comments. I have one question about the possible nature of DM:
If dark matter as far as we know interacts only with gravity, would not dark matter clump together and form not starts but black holes ? The only thing stopping normal stars into becoming black holes are the other 3 forces (I think). So why does it clump together but is evenly spread across the galaxy cluster?
Thanks for any replies.
August 25th, 2006 at 7:03 am
Tom asked (#124):
could it be that non-baryonic dark matter and baryonic matter attract eachother, but non-baryonic dark matter repels itself?
This would mean dark matter would cluster around galaxies, but would never collapse on itself (ie into dark stars or dark planets).
There are actually proposals for “self-interacting” dark matter, which would do something like that. Simulations of galaxy formation using “standard” (non-interacting) dark matter appeared to produce very dense dark-matter cores at the centers of galaxies, and observations of gas motions in dwarf galaxies indicated there wasn’t as much dark matter in their centers as predicted by the simulations. So if dark matter were self-interacting, there would be lots of collisions in the cores, which would tend to keep the cores from being as dense.
I think the current thinking is that the earlier dark-matter simulations (or the measurements and extrapolations made from them) may have over-estimated the core densities; more accurate simulations and measurements of the simulations may have cores that aren’t as dense, which mean you don’t need to postulate self-interacting dark matter after all.
August 25th, 2006 at 11:08 am
In a comment (number 82) I said, about the mpeg animation at
http://chandra.harvard.edu/photo/2006/1e0657/media/bullet.mpg
that
at 2 seconds into the animation, the collision has just started with the two clusters just coming into contact at a point, and the red hot gas Ordinary Matter is no longer evenly dispersed, but has migrated to two crescents opposite the contact point.
Ann Miller said (in comment number 121) that she “… see[s] the same thing. …”.
Peter Erwin said (in comment number 125) “… the dark matter blobs (blue) are moving faster and leaving some of the gas behind … it’s really just an animated cartoon … all sorts of fine details will be missing or wrong …”.
After thinking about it, I think that the early part of the animation is qualitatively incorrect,
and that the wrong part is more important than just a “fine detail”,
and
that a correct animation sequence would look somewhat like the image I put on the web at
http://idisk.mac.com/sm17h-Public/animbull.jpg
in which
purple is a combination of Dark Matter and Hot Gas,
blue is Dark Matter, and
red is collision/shockwave concentrated Hot Gas.
The image is very crude but I hope it makes the idea clear.
If I happen to be correct that the early part of the mpeg animation is qualitatively wrong, then I wonder how it got made that way. Was there a failure of communication between astrophysicists and PR animation illustrators ?
Since the animation is an important part of the media message sent to the public by the “media advisory” and “media teleconference” it seems to me to be important that it be qualitatively correct.
Tony Smith
http://www.valdostamuseum.org/hamsmith/
August 25th, 2006 at 2:05 pm
In an earlier paper based on the same observations,
astro-ph/0309303,
they give an upper bound on DM-DM cross section ,
sigma/m
August 25th, 2006 at 2:08 pm
Great article – very well written!
I have a question relating to the MoND theory, though. I only recently learned about MoND and TeVeS, and though I can’t say that I am a supporter of either theory I am suprised by the number people jumping up to dance on MoND’s grave. The fact that MoND can apparently be applied to a large number of galaxies with reasonable accuracy strikes me as being very interesting, regardless of the underlying reasons.
The theory may not be correct, but the fact that the MoND theory can describe galaxies is fascinating. If the rotational effects are caused entirely by gravity as we know it (and thus matter that is dark to us), why would it accumulate in a way to create such a consistant result? What underlying mechanic is working to produce such consistancy? What happens next to galaxies/regions where some/most/all of their DM has been separated from their Light Matter? How will we recognise post-collision regions in the sky, and how will they behave? Will the galaxies still have/eventually return to MoND-esque rotation curves?
Again – the theory may not be correct, but I still can’t understand why so many people, here and elsewhere, are singing “Ding-Dong! The MoND is Dead!” The sentiment strikes me as being an unreasonably negative emotional reaction, when we should be getting excited about the new possibilities, new questions, and new directions to look for answers.
August 25th, 2006 at 2:34 pm
In an earlier paper based on the same observations,
astro-ph/0309303,
they give an upper bound on DM-DM cross section ,
sigma/m < 1 cm^2 gm^(-1)
which rules out a large parameter space of Strongly Interacting Dark matter theories.
August 25th, 2006 at 9:26 pm
[...] To illustrate, Sean Carroll (no, not Sean Carroll the biologist, Sean Carroll the cosmologist) explains how scientists recently demonstrated that dark matter really exists. [...]
August 25th, 2006 at 10:43 pm
[...] Well. We’ve got dark matter. How ’bout that? Actually this is really cool because this is the first time direct observation has been accomplished! [...]
August 26th, 2006 at 5:39 am
[...] Here’s an article by Sean Carroll on CosmicVariance about it, and for those who don’t like reading, there’s an audio segment on Science Friday where Sean talks as a guest. He also shortly talks about the much more mysterious Dark Energy. [...]
August 27th, 2006 at 8:20 am
la materia oscura
El otro dÃa mencioné en Zooglea el comentario de Sean Carroll en Cosmic Variance acerca de la “fama mediática” del asunto Plutón frente a la poca importancia relativa que se ha dado al descubrimiento de la “materia oscura”, teorizada, pr…
August 27th, 2006 at 8:59 am
[...] Wie viele Objekte schwirren in unserem Sonnensystem eigentlich rum? Kollege Hoopy Froop machte mich heute auf einen Artikel aufmerksam, dass Dunkle Materie tatsächlich existiert. Wo ist die Grenze? Was finden wir noch alles? Können wir überhaupt alles über unser Sonnensystem, über unser Universum herausfinden? [...]
August 27th, 2006 at 8:40 pm
Sean [#10]:
The cosmological constant could simply be the signature or mark of the Grand Designer, who does not change and for whom the idea of “vacuum energy’ might be apt.
August 27th, 2006 at 9:40 pm
Subhendra
In astro-ph/0309303 they mention lower bounds on sigma/m from rotation curve studies, such as sigma/m > 0.5 cm^2 gm^-1, but there seem to be many conflicting claims about this number. Is there any consensus here that would pin down sigma/m to a small but positive value?
August 27th, 2006 at 9:53 pm
Identifying Dark Matter
Last week’s dramatic evidence in favor of particulate dark matter, and weighing against modifications to gravity as an explanation for the dynamics of galaxy clusters is another terrific result of observational cosmology. Equally important howev…
August 27th, 2006 at 9:57 pm
[...] Last week’s dramatic evidence in favor of particulate dark matter, and weighing against modifications to gravity, as an explanation for the dynamics of galaxy clusters is another terrific result of observational cosmology. Equally important however, are the implications of these observations, at some of the largest scales in the universe, for the physics of the unimaginably small – particle physics. [...]
August 27th, 2006 at 10:08 pm
Perhaps Dr. Murrays original paper on non linear gravity can shed some light on the gravitational question and the accelerating universe. Click Here.
August 28th, 2006 at 7:30 pm
Hi Sean,
Exiting news and great writing, thanks!
I figure my knowledge in the ‘deep filed’ of astronomy and physics compared to you guys, is not even on the same scale as DM + DE compared to ordinary matter, i.e. 4-5%. Even so, I take the liberty to play ‘The Devils Advocacy’ and post some ‘exasperating’ questions:
1) Is it possible to measure redshifts on X-ray observations from hot gas? If not – how can we be sure that the intergalactic gas is on the same distance (location) as the galaxy cluster?
2) I guess you can’t use redshifts to measure distance/speed in the perpendicular (x) direction? If so – how can we be sure that the galaxy cluster is really moving in away from each other in that direction?
3) There are 5 times more dark matter than ordinary matter in the universe, and dark matter gives an explanation why stars revolve around the center of galaxies at a constant speed, at different distances from the center; Why doesn’t our solar system rotate in the same way, due to dark matter (DM and OM party together, so it must be all around us)?
Finally, here’s a beautiful picture of the history of our fantastic universe which also include the mysterious dark energy:
http://map.gsfc.nasa.gov/m_ig/060915/CMB_Timeline75.jpg
Greetings from Dark Vader
August 29th, 2006 at 12:13 am
[...] OK, most school starts next week so I think the (relative) summer hiatus of BT is over. As the science contributer I ought mention the biggest news of the week. No, not that Pluto was demoted from planet to dwarf planet. Not even that the Poincare Conjecture proof was confirmed. No, the big news by my book was the result that dark matter exists. (I’d originally written “proof”, but there’s still a bit of wiggle room for MOND — basically an alternative to dark matter) For those really interested check out John Baez’ post about dark matter. Yeah, it’s a bit more technical but has cool pictures. Also check out Cosmic Variance’s follow up which has some interesting thoughts. Also check out this primer on Dark Matter if you’re not quite sure what it is. [...]
August 29th, 2006 at 12:21 am
[...] The past two weeks were historical for the field of theoretical physics. We are a step closer to understanding the nature of the dark components of the universe. Sean has written perhaps the most comprehensive, easy to read and heavily linked post on the topic of Dark Matter. Read and soak! Politics, sound waves and CSNY fans collide and create mystical psycho-acoustics. Jim finds himself mesmerized and tells us about his experiences with photons and pressure waves. [...]
August 29th, 2006 at 5:36 am
…sorry for the humorous misspellings in my last post, but it was 02:30 in the night, and this is not my native language, life is tough…
August 30th, 2006 at 2:32 pm
Dear Sean,
we met at lunch at Saclay about a little more than a year ago, after your talk at SPhT. We talked a bit about blogging and blogging scientists.
By that time I was a postdoc there. Now I am back in Lisbon, where I am from, and this Summer I am doing a little internship in the portuguese newspaper Público, as a scientific journalist.
Last week was pretty busy. I wrote about things like the Poincaré conjecture, and also of course about dark matter. This post of yours was a very useful source of information. Congratulations and keep up the good work.
Best,
Filipe.
August 31st, 2006 at 12:40 pm
Hi,
I find your conclusion somewhat premature. If the observations would indicate that the lensing is neither associated with the galaxy clusters themselves nor with the hot has, now that would be a proof of dark matter. I mention this because I have suggested on my webpage Plasma Theory of ‘Gravitational Lensing’ of Light that the lensing of light is actually due to the fact that stars and galaxies should be electrically charged, which might lead to a lensing effect. So the observed lensing could be due to the visible matter after all.
Thomas
August 31st, 2006 at 1:18 pm
Sorry about the re-post, but I noticed a mistake in my first post above:
I think the conclusions drawn in the article are somewhat premature. If the observations would indicate that the lensing is neither associated with the galaxy clusters themselves nor with the hot has, now that would be a proof of dark matter. I mention this because I have suggested on my webpage Plasma Theory of ‘Gravitational Lensing’ of Light that the lensing of light is actually due to the fact that stars (and galaxies) should be electrically charged. So the observed lensing might be due to ordinary matter after all.
Anyway, I don’t think that one can say that ‘Dark Matter’ is proven unless one can identify it positively (i.e. unless it is not ‘dark’ anymore).
Thomas
September 2nd, 2006 at 12:50 pm
Why are these galantic clusters so dark? Only by Xray we can see them. These hot gases might be like exhaust ones.
The spherical harmonics are the angular portion of the solution to Laplace’s equation in spherical coordinates where azimuthal symmetry is not present. And there are three types of galaxies.
*elliptical galaxy e.g. NGC4881 Three Dimension GM(
September 3rd, 2006 at 12:15 am
[...] – Dark Matter exists:Â After much speculation, scientists now have conclusive proof of dark matter. [...]
September 4th, 2006 at 12:01 am
What if dark matter doesn’t exist at all?
why can’t these gravitational lensing effects and speeds of rotation of different parts of galaxies be explained by something else?
MOND seems to imply that gravity is the same everywhere in the universe but just acts differently for very low accelerations.
the universe is very large, and very old… what is stoping forces from being variable, either in time, or space?
it could be that gravity is stronger in some regions of the universe and weaker in others. Or maybe gravity has been changing throughout the history of the universe. it may have started off stronger and has gradually become weaker. this could explain why the old light we are viewing from distant galaxies appears to need a better (different) gravitational explanation than what works here on earth.
My interest in theoretical physics is purly amature, so please help me understand if/why a spatially variable or temporarly variable theory of gravity (or both) can’t be the answer.
September 6th, 2006 at 8:41 am
Michael Morrison, I’m also an amateur in science. I think the reason for scientist to avoid variable laws of nature is that it would make science so much harder. You could hardly call it a law if there has to be special exceptions for every observation…
One of the great goals for physics today is to combine the laws of big objects (planets/stars/galaxies) with the laws of small objects (electrons, quantum particles). The main problem is that current law of gravity (Einstein’s general relativity) doesn’t work on the very small scale.
To visualize this: Think of your car as the law of gravity. When you drive on the highway everything works smooth and perfectly normal. And then if you make a turn on to a tiny country road, your car is mysteriously transformed to a crazy donkey that kicks and bites you. This can’t be right; you must buy a new car! (Or maybe redefine the definition of a road
Currently there are three candidates for the Dark Matter:
1) Brown Dwarfs, nicknamed MACHOs (MAssive Compact Halo Objects).
2) Supermassive Black Holes.
3) Non-baryonic matter, called WIMPs (Weakly Interacting Massive Particles).
More info:
http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
September 6th, 2006 at 9:59 am
[...] Dark matter exists, which is really quite rad. [...]
September 7th, 2006 at 11:02 am
[...] Firstly it appears that they may finally have observational evidence of the presence of dark matter. There is a really good explaination of the results and some of the background of the science here. It also contains links to the academic paper and the NASA press release. Given that this has been such a bone of contention in modern physics it’s nice to see it starting to pan out one way or another. [...]
September 7th, 2006 at 1:15 pm
[...] I learned from New Scientist just now that various researchers working on modified gravity theories are casting doubt on the direct evidence of Dark Matter that was presented by Douglas Clowe and collaborators a few weeks ago. Recall an earlier post on it, here, and Sean Carroll’s post with more detail here. [...]
September 9th, 2006 at 7:22 am
I am actually somewhat puzzled why all astronomers are so confident that they know the mass of the ordinary matter (i.e. the mass of stars) in a galaxy so exactly. The ‘known’ figures are largely based on the apparent luminosity of stars and the (more or less empirical) mass-luminosity relationship. It is obvious that any errors in the latter will have a crucial influence: according to the mass-luminosity relationship, a star with half the mass has only 1/10 of the luminosity, so with 10 times as many stars of half the mass, one would have the same overall brightness but 5 times the overall mass. Looking at http://planetquest.jpl.nasa.gov/SIM/science_henry.pdf , one finds indeed that the luminosities for stars less than 1 solar mass are uncertain by about 2-3 magnitudes (i.e. up to about a factor 10). It is quite remarkable that the mass luminosity relationship, which a) is quite uncertain for low mass stars, b) obtained only in the solar neighbourhood and c) obtained only from double stars, is applied to all stars in our or other galaxies regardless. I don’t therefore think that the observations justify the conclusion of dark matter here. There might be much more mass in the form of ordinary stars than thought.
With regard to the ‘dark matter’ conclusions based on the observations of the motion of gas (rather than stars) in galaxies, see also my webpage Galactic Rotation Curves and the Dark Matter Myth.
Thomas
September 9th, 2006 at 9:18 am
Thomas Smid, on your website you also mistrusts Curved Space and General Relativity among a lot of other scientific achievements. Isn’t that pretty bold…? I mean, for General Relativity and Curved Space we have darn good physical proof.
September 9th, 2006 at 3:05 pm
I’m no physicist, but is anybody taking into account the gravitation fields created by all the energy in those galaxy clusters? e=mc^2 says that energy has mass equivalent, where mass = energy / (speed of light)^2. Even if you just consider the potential energy bound up in the mutual attraction between the individual stars, that is a LOT of energy. Would the mass of all that energy be enough to account for the gravitational effects we’re seeing?
September 10th, 2006 at 4:41 pm
On the contrary, it is bold by astronomers to arrive at the ‘dark matter’ conclusion considering the uncertainties I addressed in my post above.
With regard to Curved Space and General Relativity: I not only mistrust these, but I know that these are flawed concepts. This can be said solely on the basis of conceptual theoretical consistency (as explained in more details on my web-pages about Cosmology and Relativity (see under General Relativity)). The bending of light that you mentioned above can therefore not prove a curved space. It could at best indicate an effect of gravity on light, but, given the electromagnetic nature of light, it is much more reasonable to assume that it is caused by electric rather than gravitational fields (see the page Plasma Theory of ‘Gravitational Lensing’ of Light on my other site plasmaphysics.org.uk).
Thomas
September 10th, 2006 at 7:25 pm
[...] For a detailed explaination, see Cosmic Variance. [...]
September 10th, 2006 at 7:55 pm
Ok Thomas, I’ll bite:
The X-ray emitting gas clouds in these clusters are really fully ionized plasmas. They have baryonic masses 10 times higher than the galaxies, and because they’re fully ionized I would expect they’d cause lensing effects as well (and probably be the dominant source in galaxy clusters) if the lensing was caused by electric rather than gravitational effects. So why when the plasma is seperated out from the galaxies does the lensing effect stay with the galaxies?
Also, regarding the stellar masses – look up the micro-lensing results (the MACHO project, EROS, etc) which exclude sub-solar mass objects from making up a significant fraction of the mass of the galaxy. Even if you were to use your physics of electric rather than gravitational effects for the lensing, you could still compare the rate at which microlensing occurs toward the galactic center to away from it to see that there is not a population of low mass objects filling the disk or the halo.
September 11th, 2006 at 9:35 am
Skeptic23,
As I mentioned already further above (post 156), the mass of the stellar matter in galaxies may be underestimated considering the uncertainties associated with the mass-luminosity relationship used to derive stellar masses. Also the mass of the hot gas may be overestimated as its determination also may involve questionable assumptions.
With regard to your ‘Macho’ argument: I was not suggesting that there is a substantial amount of invisible objects, but simply that, due the usual mass luminosity relationship being incorrect, the mass of a star with a given luminosity is being underestimated (especially for low luminosity stars).
Thomas
September 11th, 2006 at 12:00 pm
.
The problem with your ‘electromagnetic gravity’ is that if you let a peanut fly by the sun, it will also bend over. As far as I know peanuts are not electromagnetic, nor iron-based. But if you really have solid proof that Einstein was actually wrong, I strongly recommend a scientific publication. The smell of Mr. Nobel is substantial.
September 11th, 2006 at 4:05 pm
#158. Walter Brameld IV,
I was just talking to Twostein III and he explained the relation between energy and mass for me. Your hypothesis is appealing, but most probably wrong according to Twostein III. Here it goes: Mass = energy – same thing different form. But, the (very strange) thing is – energy in its form doesn’t have any mass. (or maybe that goes without saying?) If energy had mass it would not be energy – it would be mass (mamma mia I’m going crazy here!).
Try weighing one pound, or one kilogram of radio waves, or one kilogram of light waves – how much is that? The trick to have light travel by 299.792.458 m/s is not to give it any mass. If light had mass it would never reach the maximum (speed of light) since the acceleration would make it heavier and heavier and heavier. Acceleration = gravity (on mass), and anyone with a decent car can feel that acceleration/gravity, on the way to work (one late morning).
This of course is only a theory, created by Twostein’s father Einstein, but it’s the best we have and no one has for a 100 years proved it wrong (to make Thomas Smid calm
).
September 13th, 2006 at 9:39 am
Regarding posts 14 and 16 in this thread:
The new papers don’t discuss the scattering cross section, but Markevitch, et al (astro-ph/0309303, ApJ 606, 819(2004)) obtain a limit from the older data:
σ/m 2/g
This is obtained by requiring that the typical DM particle not have scattered as it passed through the cluster.
What does this tell us about the properties of the DM? Consider two cases:
1:The DM is “rocks” (including stars and “dust”). By this, I mean matter whose density is about 1 g/cm3 and whose scattering is approximately given by its geometrical cross section.
The Sun has: m = 2×1033 g and R = 7×1010 cm. Thus, σ/m ~ 10-11 cm2/g. So, stars will pass right through, as is seen in the images.
For rocks, σ/m is inversely proportional to R, so DM rocks are constrained to be larger than a few centimeters.
2:The DM is elementary particles of mass about 100 proton massess (in the range favored by supersymmetry theorists). That is, m = 2×10-22 g. This implies σ -22 cm2. This upper limit is huge, much larger than any observed scattering cross section of elementary particles. The data is more than ten orders of magnitude away from testing the most popular DM models.
September 13th, 2006 at 9:42 am
Sorry for the garble. The cross section limit is: σ/m < 1 cm2/g
September 13th, 2006 at 10:28 am
Jon, thanks!
This means that the limit from the bullet cluster is not competitive with other limits from simulations of galactic DM which is about a factor of 10 lower.
September 13th, 2006 at 10:31 am
Hey Sean,
Are you sure this proves that there is dark matter there?
Could it not be the halos of whatever fields mediate the modified gravity in a modified gravity theory? In other words, in a modified gravity theory there is some field that modifies the geodesics around galaxies/clusters.
What happens to that field when two galaxies or clusters collide?
I don’t think we know, but it is certainly possible that the “field halos” pass
through each other much like dark matter. Indeed, in a sense perhaps they ARE dark matter, but not made of particles.
So, I would agree that this proves that there is something there other than the baryons, but that may or may not be conventional dark matter.
Glenn
September 13th, 2006 at 11:24 am
A peanut, unlike light, has mass and will obviously be subject to gravity.
Thomas
September 13th, 2006 at 11:33 am
Hi Glenn–
What the observations demonstrate is that there are some independently propagating degrees of freedom, not merely tied to the ordinary matter, that are sourcing a substantial gravitational field. To me, that’s more or less the definition of “dark matter.” Of course they may have all sorts of exotic properties that are worth investigating.
September 13th, 2006 at 5:23 pm
Regarding Jon Thaler’s comments:
There was a proposal a few years back (from Spergal and Steinhardt) that the problems with CDM (too much substructure, cuspy cores that have never been observed, etc) could be solved using strongly-interacting dark matter. The proposed theoretically allowable range was from 0.5 cm^2/g (below this there weren’t enough ineractions to help) to 5 cm^2/g. So an upper limit of 1 cm^2/g is cutting out a large part of this range.
While there were other observations which could produce lower upper limits (the one that comes to mind is an observation of an elliptically shaped strong lensing system by Miralda-Escude, don’t know offhand which galactic limits you are referring to) they all had certain assumption regarding lack of substructure along the line of sight, etc. The bullet cluster observations have the advantage that you know exactly what happened, so those upper limits can be made without caveats.
September 13th, 2006 at 6:55 pm
#142: I’m not a scientist, or even really scientifically inclined. But I found the three questions interesting: they’re some of the few challenge-questions in the thread that are both comprehensable and persuasive to a layman. Thank you for those, I’ll be puzzling over them for some time!
For the second question, though, I _think_ the answer is something like “the clusters themselves are on the same plane (the same distance). In their current form they do not match any morphology, and _do_ look rather like two vehicles after a crash: disturbed exactly as if there was an impact/pass-through.” That argument is probablistic. But you have things here, moving at different rates, so the probabilities are, ummm, astronomical.
September 13th, 2006 at 7:18 pm
#142: I’m not a scientist, or even scientifically inclined. But I found the three questions interesting follow-up on a great article: they’re some of the few challenge-questions in the thread that are both comprehensable and persuasive to a layman. Thank you for those, I’ll be puzzling over them for some time!
For the second question, though, I _think_ the answer is something like “the clusters themselves are on the same plane (the same distance). In their current form they do not match any morphology,” — or whatever the technical wording is for ‘galaxies do not form like this’ –” and they _do_ look rather like two vehicles after a crash: disturbed exactly as if there was an impact/pass-through.” That argument is probablistic. But you have massive collections here, parts moving at different rates (eg, the strange occurance of gas seperated from the galaxies), so the probabilities are, ummm, astronomical.
September 14th, 2006 at 12:35 am
#171: I apologize. I had forgotten about the Strongly interacting DM proposal (astro-ph/990938). A more recent paper (astro-ph/000634) by Wandelt, et al. (including Spergel & Steinhart) concludes:
"The favored dark matter candidates, axions and neutralinos, are effectively collisionless and, hence, are in some considerable jeopardy. The Spergel-Steinhardt proposal has stimulated the interesting possibility that dark matter consists of particles that interact through the strong force with ordinary matter. Our reevaluation of constraints leads us to conclude that the exotic hadron possibility is now ruled out for a substantial range of masses near 1 GeV and cross-sections near 10−24 cm2, eliminating some of the most attractive possibilities. At the same time, the re-evaluation has re-opened a region encompassing larger masses and cross-sections previously thought to be ruled out."
This is based on the lack of observed cusps in DM halos. (I have been told that this is a controversial result, but I’m not an expert.)
September 14th, 2006 at 10:01 am
#173: Thanks Robbie! (My face is turning red; the big tribute should of course go to Sean and Cosmic Variance, but thanks anyway!)
Your answer for the second question is probably correct. But I’m not completely satisfied, Sean wrote: “As the two clusters passed through each other, the hot gas in each smacked into the gas in the other, while the individual galaxies and the dark matter (presumed to be collisionless) passed right through.”
If you look at this high-resolution (1.18 MB) on Visible-Light and X-Ray Composite Image of Galaxy Cluster 1E 0657-556 you see no ‘crash-morphology’ in the cluster formation or any single galaxy – as Sean pointed out; (the galaxies) passed right through. On the other hand when galaxies do get close we can see a dramatic ‘crash-morphology’ as in this picture.
Maybe your answer rise a new question:
4) Why isn’t the two galaxy clusters more affected by mutual gravitation (or maybe they are to a pro)?
It would be very interesting if Sean (or any other professional) would answer the questions in post #142, but he is probably a very busy man. In the meantime, I will also puzzle along!
September 14th, 2006 at 1:16 pm
ok, I’ll take a stab it DV -
1) The x-ray gas does have a few emission lines (highly ionized iron mostly) which you can get redshifts for, but this is not normally done (only case I think of is a z=1 cluster from some ASCA spectra, but that might have turned out to be wrong). So at some level we are taking on faith that the gas and the galaxies are in the same place. That said, we do not have any observations of cluster-sized groups of galaxies without the hot x-ray gas, and no observations of x-ray gas clouds with those sizes and temperatures that aren’t in the same place on the sky as a cluster of galaxies, so if you want to try to say they’re different objects, you have a huge cosmic conspiracy to explain away.
2) The merger is mostly in the plane of the sky, there is a reference in the paper to some spectra taken by another group showing that the velocity difference (redshift converted to velocity assuming no distance-based redshift) is about 600 km/s while the velocity of the merger is measured from the X-ray at ~4700 km/s. The velocity from the X-ray measurement is based on the supersonic bow shock, and can be measured in a variety of ways, all which agree within errors: change in temperature across the shock, change in gas density across the shock, opening angle of the shock front.
3) Because dark matter does not give off light and doesn’t interact (much) with other dark matter particles and with baryons except through gravity, it will fall into potential wells but doesn’t have a way of slowing itself down. Baryons by contrast will fall in, collide with each other to thermalize, and then emit light to cool down. In doing so they can collapse still further, allowing them to form smaller structures like stars, etc. So while dark matter is dominant mass component in the galaxy as a whole, locally around stars the baryons have much higher densities. There probably are some dark matter particles trapped within the sun’s potential, but the baryons vastly outnumber (and out mass) them.
4) The galaxies and the dark matter are affected by tidal forces during the merger, which will slow everything down (which is why these events end in the 2 clusters merging into 1 – they lose enough velocity during the initial collision to be gravitationall bound, so they will eventually stop moving away and start moving back toward each other again, then lose more velocity during the second collisions, etc). The self-gravity of the galaxies is large enough to prevent the tidal forces from ripping them apart, but likely some of the smaller galaxies will have lost some stars (although in a counter-intuitive way, the faster the collision the smaller the loss of gas and stars will be, because there is less time for the tidal forces to affect the galaxies).
September 14th, 2006 at 5:46 pm
#176: Skeptic23, thank you so much!! Wow, this is so damn cool, and you guys all deserve the Nobel Price for your knowledge! I’m completely satisfied, I’m not ‘Skeptic’, I’m a true Dark Matter Believer now!!
To summarize in one sentence: We know that large clouds of x-ray gas and galaxies cluster always travel together, so the clusters and the gas must be in the same location, and the great traces we see after the massive gas-clouds-collision gives us ‘bulletproof’ evidence for direction and velocity of the x-ray gas, and hence the galaxies clusters, AND the gravitational lensing we clearly can see are not were it should be – if we don’t calculate with a large amount of Dark Matter, then all pieces fall in place.
Beautiful!!
Thanks again.
September 23rd, 2006 at 9:29 pm
[...] So for my first proper post I would like to talk about a recent development in science that I have read a few articles about recently: the finding of concrete proof for the existence of Dark Matter. For those who don’t know, the concept of Dark Matter arose in response to a long-standing cosmological problem. Essentially, the large star clusters known as galaxies do not seem from observation to contain enough mass to produce the gravity required to keep the galaxies themselves bound together. In other words, the amount of observable matter in these galaxies is not nearly high enough to keep the galaxies from dissolving (as more mass creates higher gravitational pull). However, the fact that the galaxies do not in fact drift apart means that either gravity must work differently over long distances (alternative gravity theory) or that the galaxies are in fact filled with huge amounts of invisible matter (Dark Matter). Neither theory had ever had much evidence to support it, until very recently. By studying the gravitational interactions between stars in far off galaxies (using a technique called gravitational lensing) scientists believe that they have found proof of the existence of Dark Matter. A good explanation of the experiment can be found here. What is also interesting is the response issued by proponents of the special gravity theory, which can be viewed here. The technical details of it is still a bit beyond me, but its a very interesting topic that has been a mystery for a long time, and I have found it fascinating to read about over the past few weeks. [...]
September 26th, 2006 at 11:12 pm
[...] å¬å®ŒæŠ¥å‘ŠåŽæƒ³èµ·å‰ä¸ä¹…在cv上看到的一篇è¯å®ždark matterå˜åœ¨çš„æ–‡ç« , 很值得一读. ä¸è¿‡çœ‹åŽæ„Ÿè§‰ç»™dark matter所赋予的性质太玄乎了, 很有点åƒå½”å¹´å‘现电ç£æ³¢æ—¶çš„以太… [...]
September 28th, 2006 at 7:43 am
[...] Dark Matter Exists [...]
September 28th, 2006 at 10:19 pm
[...] – and finally, they say dark matter exists. cv 1, 2 Filed under: science — [...]
October 2nd, 2006 at 2:04 am
[...] It’s common nowadays to throw theories such as Dark Matter and String Theory. However, this is a pretty interesting read about some pretty solid evidence on Dark Matter’s existance. [...]
October 3rd, 2006 at 2:08 am
[...] Smolin does mention a number of experimental results that have already been obtained, but none of them is naturally explained by LQG any more than by string theory, and most of them are, to be blunt, likely to go away. He mentions the claimed observation that the fine-structure constant is varying with time (against which more precise data has already been obtained), certain large-angle anomalies in the cosmic microwave background anisotropy, and the possibility of large-scale modifications of general relativity replacing dark matter. (Bad timing on that one.) I don’t know of any approach to quantum gravity that firmly predicts (or even better, predicted a head of time) that any of these should be true. That’s the least surprising thing in the world; gravity is a weak force, and most of the universe is in the regime where it is completely classical. [...]
October 31st, 2006 at 9:38 am
[...] Dark Matter Exists. [...]
November 8th, 2006 at 8:41 am
[...] And here it is. You can see the slides from the talk, as well as hear what I’m saying. I started somewhat lethargically, as it’s hard to switch gears quickly from one talk to another, but we built up some momentum by the end. I started quite broadly with the idea of different “gravitational degrees of freedom,” and worked my up to Bekenstein’s TeVeS model (a relativistic version of Milgrom’s MOND), explaining the empirical difficulties with clusters of galaxies, the cosmic microwave background, and most recently the Bullet Cluster. We can’t say that the idea is ruled out, but the evidence that dark matter of some sort exists is overwhelming, which removes much of the motivation for modifying gravity. [...]
November 16th, 2006 at 11:55 am
what is the speed of dark?
November 16th, 2006 at 11:58 am
and does it matter?
November 16th, 2006 at 12:09 pm
If the relative spins of entangled pairs can change instantaneously ie. travel faster than light speeed(or so some bloke down the local told me) then how do we know that the very edge of the universe which maybe did not coallesce and clump together for the supposed time scientists allow but sped apart after the supposed hyper inflation has pulled space-time apart at faster than light speeds and therefore the majority of the universe is and always be unobservable to us.If dark matter can overcome this gravitational connundrum could it not be that this could account for the missing mass? non-scientist. Perhaps the bloke down the pub had already had far too much Guiness
November 17th, 2006 at 5:46 pm
This post has less to do with the specific observation being discussed and more to do with the issue of when, if at all, dark matter will be found. Ultimately, in my opinion, we will have to directly detect this stuff before we can be dead sure it is exists. Sean and friends, I have been wondering the following:
1). How soon is a WIMP detection predicted to take place? I have heard that a detection may occur during the 2007 CDMSII and Xenon10 runs, as these two experiments are moving into the “favored region” (x-section 10^-44 cm2) of the WIMP parameter space!
2). What will happen if the particles are not found even with the planned 1-ton direct detectors and not produced by the LHC?
3). Will a lack of a detection by 2020 be enough to get the cosmology community to move away from the dark matter particle hypothesis?
4). A lot of pundits say that it is a bad sign for the dark matter idea that the particles have not turned up yet. I am skeptical of these pundits; it took a lot of innovation and huge detectors to find the neutrinos. Does anyone know how long it was from when neutrinos were predicted to exist to when they were actually found?
November 18th, 2006 at 11:35 am
[...] informacje “po ludzku” (aczkolwiek w jÄ™zyku angielskim) można znaleźć tutaj, natomiast dla chÄ™tnych (i majÄ…cych sporÄ… wiedzÄ™) jest dostÄ™pny oficjalny tekst (nie wiem jak przetÅ‚umaczyć “paper” z angielskiego w tym kontekÅ›cie) napisany przez ludzi którzy wykazali siÄ™ wiedzÄ… i pracÄ… potrzebnÄ… by tÄ™ ciemnÄ… materiÄ™ pokazać (dla pewnych “wartoÅ›ci” sÅ‚owa “pokazać”). [...]
December 1st, 2006 at 10:02 pm
[...] Dark Matter Exists [...]
December 19th, 2006 at 11:52 am
Ooo very intresting and all, but theres so much to read and such complicated words, i just need a definition
Good stuff though
January 8th, 2007 at 1:20 pm
[...] It is, needless to say, really cool. The image itself is not where the real science lies, of course; it’s spatially distorted, and very hard to show error bars in a 3-d plot. But there is definitely important science lurking in the details; for example, they seem to find dark-matter concentrations with little or no ordinary matter in the same place. It’ll take some work to figure out whether this is easily compatible with the theoretical models (one could imagine dissipative effects clearing baryons out of a region, leaving dark matter behind, in a mini-version of the Bullet Cluster), or whether we’re going to be challenged. Fun either way! [...]
January 11th, 2007 at 12:24 pm
From the planetary velocities around the sun and the stellar velocities around the centre of our galaxy, there does not seem to be any dark matter in our solar system or galaxy. Why is this?
January 11th, 2007 at 12:28 pm
Correction 0f wrong email address on entry 194.
January 20th, 2007 at 11:55 am
[...] Meanwhile, the important story this week is the discovery of direct evidence for Dark Matter. Here is something fundamentally new that expands our understanding the universe and opens up whole areas for research. The news stories dutifully reprinted the press release but the lack of in depth stories was telling. The news media could have written articles that helped educate people about what the discovery means, how it ties in to the Big Bang, Dark Energy, the modified gravity hypothesis, and so on. If any of the big boys were carrying these stories, I missed them. [...]
January 26th, 2007 at 6:15 pm
[...] Cosmic Variance on dark matter. [...]