Dark Matter Detected, or Not? Live Blogging the Seminar

By JoAnne Hewett | December 17, 2009 1:22 pm

1:10 PM:  As previously reported by Sean, the physics community has been all atwitter recently with rumors that the CDMS collaboration has observed dark matter events.  Personally, I have heard rumors that they have either 0, 1, 3, or 4 signal events above the 0.5 events expected from background, with the number being dependent on who is propagating the rumor.  The journal Nature has denied that a paper will be published, but everyone seems to agree that a CDMS paper will appear on the arXiv tonight.

And now, finally, the day of reckoning has arrived.  Members of the CDMS collaboration are giving at least 2 simultaneous seminars this afternoon (SLAC and Fermilab).  2:00 PM  PST.  The excitement in the air is palpable.  Not much work is being done – everyone is pretty much talking in the hallways, trying to pass the time until 2:00.  David E. Kaplan is here from Johns Hopkins University to film the event for a documentary he is making.  Rumors are spreading that at least 3 groups of theorists have papers ready to submit to the ArXiv after the seminars are concluded.  The era of data has begun!

While we wait, let’s recall how CDMS detects dark matter.  The Cryogenic Dark Matter Search consists of Ge detectors, situated deep underground in the Soudan mine in northern Minnesota.  The depth is required to reduce background from cosmic rays.  As the earth travels through the sky, it presumably sweeps through clumps of dark matter which subsequently scatters with the particles in the Ge.  The scattering is expected to be elastic, meaning the initial and final scattering particles are the same.  The  detector measures the ionization and phonons from these interactions.  Background events mainly create electron recoils, and so a nuclear recoil gives a good DM signal.  The energy of this recoil gives information about the mass of the DM particle and the number of events compared to the total amount of data taken gives the interaction rate.

1:30:   So what does a theorist do while waiting for the seminar?  A quick calculation, of course!  What else?  The energy of the highest observed recoil event, with an assumed DM velocity distribution,
gives a bound on the mass of the DM particle.    Using some DM velocity assumptions from 0912.2358, one could do a quick and dirty calculation!  My student Randy did so, and here’s the plot (vertical axis is WIMP mass in units of GeV and horizontal axis is the recoil energy (in GeV):

cdms1

Taking this to the talk, and then can immediately read off the WIMP mass bound!  Wanna bet how many theory papers appearing tonight have this plot?

1:45:  In my seat – front and center.  The auditorium is packed – SRO.  Folks from UC Santa Cruz and Berkeley are here.  The speaker is Jodi Cooley from Sounthern Medthodist University and is wired and ready to go.

1:54: JoAnne beat me to it, but I’ll join in on the live blogging with the view from the back of the Auditorium! –Risa

1:55:  Learned that there’s a group of theorists at the KITP in Santa Barbara watching this event over skype.

1:58: You can watch it yourself — Live Streaming Video starting at 2 PM:

http://www-group.slac.stanford.edu/kipac/cdms_live.html

– Risa

2:00:  A hush just came over the room…JoA

2:01:  Jodi is showing her outline – starts with an intro, then describes the detector, then gives the results, and then discusses future plans for the collaboration.   My colleague sitting next to me just leaned over and whispered “It looks like a signal talk.” - JoA

2;05: liveblogging appears to be deleting some of our comments, so you gotta pay close attention! –Risa

2:12: Quick comment on JoAnne’s nice plot while we are still in the intro. It’s important to remember that the actual value of the WIMP mass depends, as JoAnne says, on “some
DM velocity assumptions”. The paper she cites is eminently reasonable, but the values are still wildly uncertain (see e.g. arXiv:0906.5361), so if we do actually have a dark matter detection, understanding the expected density and velocity distribution of dark matter in our own galaxy becomes much more important. –Risa

2:15:  slide 14 discusses the direct detection event rate.  Expected signal event rate is < 0.01 events/kg-day.  kg-day is the unit used by the experiments to measure their sensitivity – kg denotes the size of the detector, and day is the number of days the detector was taking data.  -JoA

2:20:  Each detector consists of 230 g Ge and 100 g Si.  There are 30 detectors stacked into 5 towers.  -JoA

2:23 Experiment is designed to have ~0.5 background events.
– Risa

2:24 1/2 mile Underground in the Soudan mine. Note that this is in Minnesota, not the Sudan. –Risa

2:23:  Slide 24:  they can select or tune the background level as desired by their cuts.  They choose 0.5 background events in the signal box as a target.  The depth is at 6060 mwe (meters water equivalent) – JoA

2:27:  About to open the signal box… – JoA

2:30: November 5, 2009 opened the box in a big conference call. –Risa

2:31:  Measured surface event background in 0.6 plus/minus 0.1 (stat).  They use 3 techniques to determine this.  Slide 30.  Neutron bckgrnd is 0.04 plus 0.04 minus 0.03.  Radiogenic bckgrnd in 0.03-0.06 events. – JoA

2:32: Experiment has 194.1 kg-days WIMP equivalent @ 60 GeV/c^2 –Risa

2:33:  THE NUMBER IS TWO!!!! – JoA

2:34: I consistently heard 3 events, so interesting comment on the rumor mill — Risa

2:35: The two events happened at different times, different months, different detectors. — Risa

2:38: Were in inner region of the detector where there is better background rejection. — Risa

2:40 Probability of observing these two events is 23%. Clearly not the solid detection we were all hoping for! –Risa

2:41:  Refined calculation of the surfact background including timing,  post unblinding leakage estimate is now 0.8 plus/minus 0.1 (stat) plus/minus 0.2 (sys)  -JoA

2;42:  The 23% probability of observing the 2 events includes the post-unblinding leakage.  -JoA

2:45  Summarizing now:  cross section limit is < 3.8 x 10^{-44} for WIMP mass of 70 GeV at 90% CL.  -JoA

2:47:  More towers of detectors have recently been installed, with more to be installed in 2010.  -JoA

2:48:  arXiv number is 0912.3320  -JoA

2:50: Bottom line: “The results cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject the possibility that either event is signal.” –Risa

2:51: Question about other unanalyzed data. Answer: next step is really SuperCDMS. — Risa

2:52: The events took place on 8/5/07 and 10/27/07. –Risa

2:55: Very solid, careful work, a new upper limit, and a new thing to add to “hints of dark matter”. But could easily be a statistical fluctuation. Going to be a bit longer until something solid. — Risa

First event was detected on 10/27/07 with recoil energy of roughly 12 keV, and the second was on 08/05/07 with roughly 15 keV.   A 3rd event lies just outside their box with recoil of 12 keV.  -JoA

2:59:  The crowd is filing out amid much discussion, while Dimopoulos and crowd are being interviewed.  There will be much discussion all afternoon! – JoA

3:01:  Given the above figure, if these events are interpreted as signal, the lower bound on the WIMP mass for these recoil energies is roughly 2 GeV.  Not that constraining!  -JoA

3:18: Summary in CDMS’s own words here (pdf). -Sean

4:00: Corrected figure now posted. :) -JoA

6:40: Can’t find the paper on the arXiv and 0912.3320 is a nice paper (on DM) written by my good friend and collaborator Hooman Davoudiasl, but has nothing to do with CDMS…

Jodi told me it was submitted at 1, just before the talk, which means it won’t be out until the next round (Sunday night, I think). –Risa

CATEGORIZED UNDER: Science
  • Stuart Coleman

    I worked with Jodi Cooley while at Stanford (she was a post-doc while I was there, before she moved to SMU) so I’m very excited to see this talk. It’s just too bad I’m not still at Stanford or I’d go up to SLAC to see it live (also if I were still at Stanford and working for CDMS I’d probably have known the result weeks ago, but that’s neither here nor there).

    At any rate, go CDMS!

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Thanks for the live-blogging.

  • onymous

    Your plot seems to say that CDMS can’t see WIMPs heavier than 1 GeV. Or maybe I can’t read your axis labels correctly.

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  • Sili

    The livestream is most frustrating. Random snippets of sound and then nothing for ages.

    Ah – there was a “Welcome, everybody”.

    ::bites nails::

    ETA: Thank Higgs! The other stream is smooth.

    ETA2:

    My colleague sitting next to me just leaned over and whispered ”It looks like a signal talk.”

    YAY!

  • Brian Mingus

    this webcast is choppy :(

    Spoiler alert: they found evidence for dark matter!

  • Amos Zeeberg (Discover Web Editor)

    Hey, JoAnne and Risa, don’t blog in the same post at the same time! You could bulldoze each other’s edits. Sounds like you might’ve already…

    Unf I don’t think there’s a way around this. Sorry for the trouble. Maybe someone post comments instead?

    Actually it should work if you reload the post-editor *immediately* before making changes [pasted from outside doc] and then immediately save changes.

  • Stephen

    I’ve seen the experiment whilst down in the mine for MINOS. I hope there’s a slide of their He3 tank…

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Watching the FNAL talk — I can’t believe people are interrupting to ask questions rather than letting the speaker get to the punchline.

  • Matt

    Whenever they talk about Soudan, I think of the Sedan test.

  • Michael

    Yay science!

  • Kyle

    What does it all mean?

  • Amos Zeeberg (Discover Web Editor)

    I think the upshot is: 2 events [observed] >> .5 plus/minus .1 [background]

    Means very high likelihood that there was a real WIMP signal.

  • Aaron Sheldon

    Given a Poisson process with an event rate of 1, there is a 26% chance of observing 2 events, due to chance alone.

  • charly

    @Sean: You need to make sure assumptions are reasonable before looking at the punchline!

  • Stuart Coleman

    The webcast froze for me right on the “Opening the Box” slide. What terrible timing.

  • Sili

    GAH! Damn you, Realplayer! All that introduction, and then you conk out before the reveal?!

    Is the streaming delayed? I certainly didn’t notice anything sounding like an actual result unlike our lovely, gracious hosts.

    Thank you!

  • Kyle

    Can someone explain exactly what this all means? Or point me to somewhere where it dumbs it down a bit?

  • Ben Martin

    Interesting results, to be sure, but there’s a distinct feeling of being underwhelmed by the very modest strength of the result.

  • Aaron Sheldon

    If they had the same event rate (2:1, signal: expected background), but collected 10 times as much data, that is observing 20 events, then it would be statistically significant to the degree accepted in the physics community, with a Poisson probability of 0.3%.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Kyle, have a look at the CDMS homepage, and their public explanation page.

    The punchline is that these guys have been trying for a while to directly detect the dark matter in a deep underground laboratory. They’ve seen a couple of events that look provocative, but those very well could be just an ordinary background (23% chance). So it’s intriguing, but we don’t know yet.

  • Eric

    In what units is the cross-section limit <3 x 10^{-44} given?

  • Amos Zeeberg (Discover Web Editor)

    @Kyle: Here’s some background on this experiment in accessible Discover-talk. Fascinating stuff.
    http://discovermagazine.com/2007/may/mining-for-dark-matter

  • http://scienceblogs.com/catdynamics Steinn Sigurdsson

    so bit over 2 sigma, about what might be expected given the constraints

    thanks for liveblogging it – had people stopping me in the corridor asking if I had heard what the result was

  • onymous

    Eric: cm^2.

    Steinn: over 2 sigma? That’s not how I interpret “23% chance”….

  • Sili

    The events took place on 8/5/07 and 10/27/07

    Seriously?

    Didn’t the datacollection last until March this year?
    OK, September 2008, but still. That looks oddly clustered.

    I know, gut feelings about statistics are wrong, but … I don’t like that.

  • http://scienceblogs.com/catdynamics Steinn Sigurdsson

    @onymous – well, do you believe their pre-estimated mean of 0.5 or their post-mean of 0.8?
    And don’t forget you can’t have negative events, so this is a one sided distribution

    you can play the confidence intervals, but it is not 1 sigma, and it is not 3 sigma
    so it is 2 sigma – and over 2 sigma with the 0.5 mean and under 2 sigm with the 0.8 mean

    it is two events, at some level that can be overanalyzed….

  • Joshua

    So does this mean that there is _only_ a 23% chance that the result is due to background? That sounds like a 1.3-sigma finding.

  • Kevin

    The 2:00 hush was really strange.

  • Dave English

    From what I’ve read, everyone is looking for very small dark matter, could it be in larger masses? What creates Dark Meteors as seen by many who look for meteors. I can say not all of them are birds or insects as two Dark Meteors I saw in the late ’90s in different years both exploded into a very round and dim light, one was almost as big as the Moon. This was confirmation to me that Dark Meteors are real as there has been some controversy about them. Both were observed with 10X50 binoculars.

  • charly

    The blogosphere hyped it, and the blogosphere was underwhelmed.

  • Mandeep

    Holy crap — the detections were in 2007, and it took them this long to analyze and get sure of the systematics??! (Makes me feel a lot better about some 2007/2008 cluster data *i’m* still analyzing!) I’m actually not criticizing — i know how long it can take to do the right painstaking work on anything, esp. anything *this* significant, i’m just noting it. Yeah, i’m not gonna get *too* hyped up yet, 2 events, 0.5 expected bkgd, 23% probability etc. — but i know that CDMS and the Xenon expts are at the forefront of this, and you can *bet* that this is going to stir up the direct detection expt’l and theoretical fields like nothing in recent times! Would be amazing if this is the real deal.. gonna be an exciting few months ahead for all the direct detection folks!!

  • Aaron Sheldon

    Sorry Joshua a 23% chance of observing at least 2 background events is not the probability that the events are due to background. To find that you need to apply Bayes’ Theorem to invert the conditional probabilities, and to do that you need to estimate the expected event rate due to dark matter, and on top of that you need to estimate a prior probability that dark matter is observable…

    Applying Bayes’ theorem is easy to compute mathematically but hard to justify physically.

  • Ellipsis

    The road to hell may be paved with 3 sigma bumps, but the road to insanity is surely paved with ~1.2 sigma fluctuations.

  • onymous

    Still no paper on the arxiv? Odd…

  • Brian Too

    Bah! Two events? TWO?? That’s hardly anything at all…

    However the good news is that this is some type of direct detection method. The DM/DE field has been plagued with “indirectly, we infer the presence of an unknown something, due to our inability to explain the measured results… Because we cannot directly detect the unknown something, we call it Dark (meaning undetectable) and that explains everything”. No. No it does not. Nice try though!

    DM/DE is speculative at best until some direct detection exists. Otherwise other explanations can serve just as well.

  • http://www.shaky.com Timon of Athens

    “The results cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject the possibility that either event is signal.”

    And for this they had simultaneous talks, pre-announced arxiv papers, etc etc etc? Give me a break. Won’t it be just great when *everyone* starts arranging simultaneous talks, a program of carefully orchestrated leaks, pre-announced arxiv papers etc for *their* boring non-results? What a ridiculous and pathetic performance. And the saddest thing is that this is precisely what most of us were expecting.

    By the way, next month I will be submitting an absolutely sensational paper to the arxiv, in which I will announce a first-principles computation of the electron mass correct to 11 decimal places, based on my recent discovery that all compactifications of E8 x E8 heterotic string theory are internally inconsistent except for *one*. Well, ok, the paper will not quite do that. But there will definitely be a paper. And a talk.

  • Brian137

    Thanks, JoAnne and Risa. I read your post slowly, savoring the suspense. It was better than being there because I got to eat a little snack (cherries from Chile) while I read. And the ending was very realistic – I could really relate..

  • Disappointed

    Totally agree with Timon of Athens! Why all that noise? That’s ridiculous…

  • Disappointed

    Not to mention that the paper is not on arXiv…

  • Low Math, Meekly Interacting

    So…what’s the straight dope on this? We have an estimation that the chance of the result being noise is in the neighborhood of 25%. An educated guess, then, that the odds against this small signal being totally spurious are a tad better than even.

    I’m not knocking anyone for a null finding, and I admire the courage it takes to work your ass off and then meticulously expand on “I got nothin’” under the withering scrutiny of your peers.

    But to my untrained eye, this looks like a hard-earned, extremely high-quality nothing. Like I said, I respect the big fat zero, I truly do. But what’s with the buildup? Was this just the blogospheric echo-chamber resonating out of control, or what?

  • Disappointed

    Exactly! I TOTALLY respect a fat zero, but please keep it down…

  • King Cynic

    As far as I can see the buildup was not coming from CDMS directly. I blame teh internets.

  • Tod R. Lauer

    Sorry, but I’m surprised that there is no interpretive context for this (I presume that this will be in the paper, and it may have been in the part of the talk that dropped out over the internet). Given, a presumed local density of the DM halo, a velocity distribution, detector properties, etc. is a positive detection of 2 events interesting and in what way? 2 events is highly interesting if you expect say 20 events. It’s most likely to be background if the expectation is say 0.2. It’s completely ambiguous if all serious candidates would give results close to that seen. I would have really liked some sort of guide here.

    But regardless, this result, even with the high background probability, is a valid observation. History is rich with high quality upper-limits, null detections, and even weak measures that were the first steps to better measures later on. Indeed for those who have waited decades to figure out what DM is, the CDMS work says that the end of the dark age may be near.

  • http://eskesthai.blogspot.com/2009/12/supercdms-improvement-on-detection.html Plato

    More info on name

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Tod, what you’re asking for is not that easy to provide. You can phrase the result in terms of an upper limit on WIMP/nucleon cross section as a function of WIMP mass, given some assumptions about the DM distribution. But there are no precise predictions for where a realistic model should lie in that two-dimensional parameter space. A typical (for example) supersymmetric model has a jillion free parameters, all of which go into predicting the mass and cross-section. So what you get are regions where we could plausibly live, but those regions are pretty big.

    For some examples, go here:

    http://dendera.berkeley.edu/plotter/entryform.html

    and play around. You can find theories predicting all sorts of things (although not absolutely everything).

  • Brian137

    Again, my thanks to JoAnne, Risa, and Sean for the opportunity to read about the presentation of the results in, more or less, real time.

  • King Cynic

    Indeed. I was going nuts when the video feed froze just at the point of box opening. I only knew the last third of the talk through the blogging comments on this site!

  • Tod R. Lauer

    @Sean

    Thanks for the link. Having not followed this stuff at all closely, it’s nice to see even whopping limits. I guess the thing to do is to come back in a day or two when they have today’s CDMS stuff up…

  • Tim Tait

    Nice blogging.

    We only had it uninterrupted at the KITP because I was connected with Simona via skype!

  • Phil

    The great sadness of all is that, as usual, folks with zero to no familiarity with treatment of data are forgetting about something called the “null hypothesis”: it is clear in several of the plots shown today that no WIMP model can provide a better fit to these signals than background spillage into the “box”. Not like this will stop many theoreticians from foaming at the mouth. Physics is fast becoming two tribes, and one is the proverbial ship-o-fools. You know which one you belong to.

  • Darkman

    Just listened to the SLAC talk. Is `nucular’ the official CDMS pronunciation?

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  • http://www.twitter.com/complaintmetal Anto
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  • http://bruceleeeowe.wordpress.com/2009/12/04/what-does-solution-to-fermi-paradox-implies/ Bruceleeeowe

    Good to know. Keep your quest continued to adhered with dark matter, Sean!

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  • hackenkaus

    Gee, aren’t ya glad you had the film crews out catch the unveiling of a 1.5 sigma background event? They should have brought in Geraldo Rivera to open the signal box.

  • graviton383

    I have to admit to liking the comparison w/ Geraldo Rivera! But that’s just the way data-starved theorists are..hopefully something that will soon leave be only a (fond?) memory of this
    decade….

  • Ciaobella

    Your student Randy who? Let’s have a full attribution, yes?

  • Tim Tait

    To clarify something: The film crews were collecting footage for a documentary about particle physics and the LHC. While they were capturing the reaction of the physicists, they were not really there to record the result itself.

    And plenty of theorists understand statistics, it’s sad how people get satisfaction out of venting their unhappiness on the web.

  • Brian Mingus

    I created a plot of every dataset on that plotting site Sean sent out. Check out this nightmare of science: http://farm3.static.flickr.com/2803/4197084632_4e80dcb84b_o.png

  • charly

    I invite everybody to check out our new Dark Matter plotter website:

    http://dmtools.brown.edu/

    which is superseding the Berkeley site.

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  • John Duffield

    Can I politely point out that space has its vacuum energy. So E=mc² tells us it has a mass equivalence, just as the energy of a photon in a mirrored box will increase the mass of that system. And space is inhomogeneous, just like Einstein said:

    “According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν)..”

    That’s Einstein saying a gravitational field is inhomogeneous space, caused by the concentration of energy tied up as the matter of a planet. But that isn’t the only cause of inhomogeneous space, because space expands between the galaxies but not within, like the raisins-in-the-cake analogy. So galaxies will be surrounded by a shell of inhomogeneous space, hence flat galactic rotation curves. Now check out dark matter on wiki and note where it says:

    “The largest part of dark matter which does not interact with electromagnetic radiation is not only dark but also by definition utterly transparent; in recognition of this, it has been referred to as transparent matter by some astronomers”.

    This isn’t to say WIMPS cannot exist, but dark matter is ubiquitous, and outweighs ordinary matter by a factor of five. So take a look at the clear night sky. Look at the gaps between the stars. That’s space. It’s dark, it is utterly transparent, it has a mass equivalence, and it is not homogeneous. And there’s a heck of a lot of it. Think it through. Then you’ll really have something to get excited about.

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  • http://www-thphys.physics.ox.ac.uk/people/RhysDavies/ Rhys

    The sequence of events over the last week or two has been interesting. The poor old CDMS collaboration did exactly what one should expect them to. They performed a very careful experiment and data analysis, and then released the results in the form of two talks (which were of course advertised in advance, like all research talks) and a paper. The subsequent hype (none of which seemed to originate from CDMS) and cascade of rumours reflected the excitement about the experiment, but now we are seeing irate comments on blogs from people furious about the fact it is a null result. And of course the crackpot crowd are continuing on their merry way, demonstrating that in reality they care as little about experimental physics as they do about theory.

    But despite the noise and misdirection from some corners, science marches inexorably on; I’m looking forward to more results from Xenon100 and SuperCDMS!

    John Duffield wrote:
    “Can I politely point out that space has its vacuum energy.”

    Of course you can. And then, being the expert on relativity that you are, you can tell us the form of the resulting stress-energy tensor, and explain why the resulting effects are nothing like those of dark matter.

  • Low Math, Meekly Interacting

    I wasn’t furious about anything, just sincerely mystified by the buildup to what turned out to be admirable but unspectacular results. What happened? Was it just data starvation? People hoping too much for the Red Ryder BB gun, when all that was under the tree was a very nice cardigan? I agree that the CDMS team appears quite blameless in all this (I’m not sure I buy the cynics’ speculations about deliberate hype), and I have, quite sincerely, a great deal of respect for those who honestly report a non-discovery. Too little of that happens in my line of work, so I really do put a lot of value in nothing. Just, you know, what the hell…?

  • http://www.shaky.com Timon of Athens

    Rhys said: “The poor old CDMS collaboration did exactly what one should expect them to. ”

    You got that part right.

  • R

    Today, there are two (or perhaps three) papers on hep-ph that take the two events seriously.

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  • John Duffield

    Rhys: the form of the stress-energy tensor around a star is a spherical shell of inhomogeneous space with diminishing energy density and vacuum impedance Z0 = √(μ0/ε0) reducing to a baseline level of interstellar space. Take a flat slice through the spherical shell and plot guv, and you’ve got an upside-down “witches hat” with a rounded top instead of a point. See http://en.wikipedia.org/wiki/Gravitational_potential for an image, and turn it the right way up to plot energy density. A gravitational field has its own energy, and since c = √(1/ε0μ0), the space where the gravitational field is detectable exhibits a local gradient in energy density causing curvilinear motion. This is akin to you swimming through a pool with a salinity gradient from left to right – you veer left. See “Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime” at http://www.iop.org/EJ/abstract/0256-307X/25/5/014.

    For a galaxy, there’s a wider distribution of baryonic matter so the hat is flatter, but it’s surrounded by extra brim of inhomogeneous space because inter-galactic space expands whilst the galaxy is gravitationally bound. The baseline level of intergalactic space is lower than that of interstellar space, hence gravity reaches out further than expected. Another way to think about it is this: if you could snap your fingers to make all the photons, neutrinos, electrons, protons, neutrons, etc of a galaxy disappear, to be left with just the space the galaxy was in, it would still exhibit a gravitational field because that space has a higher energy density than the surrounding space.

    One requires only a very basic understanding of the fundamentals of energy, mass, and particles to realise that a +1022keV photon is a pulse of extra stress-energy propagating through the stress-energy of space at c. Use pair production and the result is a 511keV electron and a 511keV positron. The extra stress-energy is still moving at c, hence spin, but it is no longer moving in aggregate with respect to you, and the result is mass. See “The nature of the electron” at http://arxiv.org/abs/physics/0512265. Likewise, the stress-energy of space isn’t moving with respect to you, and where this is higher than that baseline level, E=mc² says it has a mass equivalence. It’s cold, it’s dark, it’s transparent, it’s abundant, and with a mass equivalence, it is as close to matter as we need it to be – the contrast between space and matter fades away. Some might feel this poses a problem for the CDMS collaboration, but IMHO they’re the best people to take advantage of nonbaryonic dark matter that is not WIMPS or indeed any other particle. Out of adversity comes opportunity

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