COSMOS Reveals the Cosmos

By Sean Carroll | January 8, 2007 1:20 pm

The internet works so that we don’t have to! This week is the big annual meeting of the American Astronomical Society in Seattle, so expect to see a series of astro-news stories pop up all through the week. The first one concerns a new result from the Cosmological Evolution Survey (COSMOS) — they’ve used weak lensing to reconstruct a three-dimensional image of where the dark matter is. Here is an image from the Nature paper by Richard Massey et al. (subscription required).

COSMOS dark matter map

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!

Fortunately, I don’t have to go into details about the result, as others already have. Phil, Clifford, Rob, Angela, and Steinn have all blogged about the finding. (We’re all on a first-name basis around here.) Steinn’s post is, admittedly, pretty consise, but he wins points for breaking an even better story — Google is joining the Large Synoptic Survey Telescope consortium! Rob is even live-blogging the entire meeting, which is an heroic undertaking. (Yes, it’s true that he did bump into me up in Seattle, but I’m not there for the meeting! In fact I’m already back in LA. There are reasons to visit Seattle other than the AAS.)

Ah, I remember the good old days of ’04, when there wasn’t any competition out there in the cosmo-blogging world. Our internet is all grown up now. Sadly, Michael Bérubé is retiring from the game, which will leave the blogosphere a much poorer place. Read Sunday’s Credo for an example. Without his inspiration, I certainly wouldn’t be doing this myself.

Anyway — the COSMOS project is well worth being wowed by in its own right. It’s an ambitious undertaking; they take a two-square-degree field of the sky and beat on it with every telescope they can find — in optical, infrared, ultraviolet, X-rays, and radio waves. More than half a dozen ground-based telescopes, as well as five satellites (the Hubble Space Telescope, Spitzer infrared observatory, XMM and Chandra for X-rays, and GALEX for the ultraviolet), are joined in the effort. Here’s the abstract from one of their recent summary papers:

The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) with large-scale structure (LSS) over the redshift range z < 0.5 to 6. The survey includes multi-wavelength imaging and spectroscopy from X-ray to radio wavelengths covering a 2 square deg area, including HST imaging. Given the very high sensitivity and resolution of these datasets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS datasets, and summarize the science goals.

This new dark matter map is just the beginning of fun stuff to emerge from this collaboration — stay tuned!

Update: There I go again.

“I like to think of visible matter as the olive in the martini of dark matter,” said Sean Carroll, a theoretical physicist at Caltech.

I love my job.

CATEGORIZED UNDER: Science
  • http://www.pieterkok.com/index.html PK

    The Bad Astronomer shows a different image where you can see lumping occur in most recent times (at least, it is somewhat suggestive). I don’t see that here, do you have an explanation for that?

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

    I’m not really sure what the differences are between the two images. Phil showed the one from the press release, mine was swiped from the Nature paper.

  • Doug

    So, does this mean we’ll get fewer emails from electric and superconducting universe believers now Sean? At least for a few weeks I hope.

  • http://backreaction.blogspot.com/ B

    I have a question about the analysis that might sound a bit weird, but it’s been puzzling me. How do they set a ‘zero’ to the distortions? I mean, they can find out delta rho, but how do they set rho = 0? Do they just define the minimum of rho to be zero? I’ll give you an analogy: take a plane slice of glass. It doesn’t cause any lensing, but the density isn’t zero. How do they get rid of this normalization? Also, a void in a non-zero density ( negative delta rho) acts as a diffraction lens, no? Do you know if anything like this is observed? Best,

    B.

  • Jeff

    I’m up here (AAS) now and was just chatting with one of Massey’s co-authors. Everyone’s excited by the paper, of course. Here’s to making that excitement turn into funded, dedicated experimental missions!

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

    Doug, sorry — emails from crackpots are not a conserved quantity. They can be produced without limit.

    B, what gravitational lensing really measures is not rho, but the Newtonian gravitational potential Phi. (Related to rho, of course, by Poisson’s equation.) That’s good, because fluctuations in Phi can be small (“gravity is weak”) even when delta rho is large compared to the average value of rho. And yes, there certainly can be divergence of geodesics in underdense regions. But the overall average value of rho doesn’t really matter very much — if you added a smooth component, for example, it would change the overall curvature and expansion rate of the universe (just as dark energy does in fact do), but it wouldn’t affect the weak lensing maps. You’re only sensitive to the amount of clumping matter.

  • http://combingthesphere.blogspot.com Daryl McCullough

    Wow, did I ever have the wrong impression about Dark Matter. I just thought it was a bookkeeping thing—to fit the observed behavior of galaxies, there must be more mass than is accounted for in visible stars (or whatever the argument is). I had no idea that it was possible to actually figure out where the missing matter has to be.

    And such intricate shapes. Very interesting.

  • http://mollishka.blogspot.com mollishka

    B: yes, a negative Δρ would in fact act as a “diffraction” lens, but it would be very difficult to actually observe … instead of making background objects brighter, it would make them dimmer (and thus more difficult to detect). Lensing due to clumping of matter is a lot easier to detect because matter can clump in a small region such that it is a lot denser than the surrounding area—but the average density of the universe is low enough that it is practically impossible for a small region to be so devoid of matter that it has a detectable lensing signature. Lensing by very large regions devoid of matter—called voids—has been discussed, but the basic consensus is that it will be very very difficult to measure. See, for example, astro-ph/9811458.

  • http://backreaction.blogspot.com/ B

    Hi Sean, Hi mollishka,

    How would one decide whether one has an overdense ‘dark matter’ region or a void region in a sourrounding of ‘dark’ negative rho (don’t tell me there’s no matter with negative rho, I know that). Wouldn’t both act similarly?

    Btw, Sean, thanks for nominating me for the Science Anthology :-) , I only found out today, that’s nice.

    Best,

    B.

  • http://www.infradig.com absolutely

    The BA says this:

    The left image, in red, shows the location of normal matter on the sky. The right image, in blue, is where the dark matter sits. They match! It’s been theorized for some time that as the Universe cooled after the Big Bang, dark matter formed long filaments many millions of light years across. As that happened, normal matter would be attracted to it gravitationally. Today, the normal matter should trace the location of the dark matter. Now we can see this is true!

    You say this:

    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!

    There seems an interesting dichotomy there?

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

    Well, most of the dark matter concentrations are indeed found where the ordinary matter is. But (apparently) some of the smaller ones are not. It’s worth both celebrating the general success of the model, and keeping an eye on possible discrepancies. (Crossing your fingers, hoping with better data that they turn into real discrepancies, and we can learn something.)

    B, I think that yes, negative energies could give rise to a simliar dispersive effect as underdense regions. If you found that they only way to fit the data was to invoke negative energies — well, that would be interesting.

    How do we know that the Sun doesn’t have zero mass, but all of empty space around it has a uniform negative mass density? (Think about this long enough and you will end up rediscovering the Jeans swindle.)

    My pleasure on the Science Blogging Anthology!

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  • http://www.badastronomy.com Phil Plait, aka The Bad Astronomer

    Yeah, I wanted to talk about the detailed discrepencies between the two images, but my post was already too long. :-)

    Sean, I am really sorry I missed you. I suspect Jennifer can fill you in.

  • kapakapa

    Incredible visualization, but pardon the total amateur image I get from this graphic.

    Am I the only one who sees an ensconced cosmic-hippo happily surrounded by a jugon, a flying pig, a manta ray and even a floating peace of chocolate?

    I’ll get me a coat.

  • http://quasar9.blogspot.com/ Quasar9

    Hi Sean, would that be “shaken not stirred”

  • Paul Valletta

    Quote:”Dark matter does not reflect or emit detectable light, yet it accounts for most of the mass in the Universe.” if DM did actually reflect or emit light, would not this fact then cause our night cosmic skies to be a total ‘White’ background?..the light emmitted from just our Galaxy would shield all of the Universe’s distant Galaxies from even the most powerful Telescopes, Hubble included.

    It may be simply a logic fact that we need DM and it’s lensing properties in order to factor where our Galaxy ends, and other Galaxies begin?

  • http://quasar9.blogspot.com/ Quasar9

    If it is gravity that is clumping DM
    then where is the centre of gravity
    And if Visible matter is clumped DM
    doesn’t that make DM very fine space dust
    =============================
    If dark energy repels matter (visible & dark)
    could that cause reverse gravitational lensing
    =============================

  • Steve

    This seems the best place to ask: do these results necessarily mean the death of MOND theory? (If it does, I would feel quite sorry for those who have worked on such an alternative idea for years, only to have it trashed by new evidence.)

  • ovidio

    I think that B’s first question (comment #4) is about the sheet-mass degeneracy. Does anyone want to answer that question?

  • http://backreaction.blogspot.com/ B

    Hi Sean,

    I think that yes, negative energies could give rise to a simliar dispersive effect as underdense regions. If you found that they only way to fit the data was to invoke negative energies — well, that would be interesting.

    Thanks. Nah, I’m not fitting any data (so far), just wanted to make sure.

    You are mentioning the Jeans inst. to say a uniform gas wouldn’t stay uniform? My argument doesn’t rely on the negative density to be uniformly distributed (I just used that for an easy example). In fact, I’d want it to clump like ordinary matter. As mentioned above, such a distribution when left alone would make a diffraction lensing which is hard to detect, but in some configurations it would act just like a gravitational lens.

    Hi ovido,

    I think that B’s first question (comment #4) is about the sheet-mass degeneracy. Does anyone want to answer that question?

    Never heard of the sheet-mass degeneracy, but if anyone wants to explain it to me – go ahead!

    Best,

    B.

  • http://eskesthai.blogspot.com/2007/01/hubble-maps-cosmic-web-of-clumpy-dark.html Plato

    More on name

    Diagram of the Lagrange Point gravitational forces associated with the Sun-Earth system.

    The example of “how one may see” is further expounded upon to show how dark matter and dark energy are in action as a 90% aspect of the cosmos, while the remaining 10% is a discrete measure of what is cosmologically matter orientated. We don’t loose sight of these relationships, but are helped to further develop them in terms of this gravitational relationship.

  • citrine

    kapakapa on Jan 9th, 2007 at 5:25 am
    Am I the only one who sees an ensconced cosmic-hippo happily surrounded by a jugon, a flying pig, a manta ray and even a floating peace of chocolate?

    ******************************************************
    kapakapa

    You are not alone. The entire picture looks to me like a cluster of flying pigs. (“We’ll find dark matter when pigs fly?”)

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

    Steve, there’s more about that here and here. MOND is not dead, but it’s increasingly unattractive; for example, even its supporters admit that it only does away with a bit of the dark matter, not with most of it.

  • Cynthia

    Sean, you say:

    “…most of the dark matter concentrations are indeed found where ordinary matter is. But (apparently) some of the smaller ones [concentrations] are not.”

    Let me pose two questions regarding this statement:
    1) What is the current estimated breakdown between “clothed” dark matter (meaning dark matter draped in ordinary matter) and “naked” dark matter (meaning dark matter not draped in ordinary matter)?
    2) From looking at this 3-D image, how’s one to distinguish between what clumpy shapes/colors represent “clothed” dark matter, and–on the other hand–what clumpy shapes/colors represent “naked” dark matter? In other words, is the difference between “clothed” and “naked” expressed in , for instance, contrasting colors and/or contrasting shapes?

    By the way, I’m aware of the evolution of time (the cosmic arrow of time, so to speak) within this 3-D image. To me, however, this 3-D image just doesn’t seem to convey this issue of “clothed” versus “naked” dark matter…

  • http://www.allysonbeatrice.com/blog/ Allyson

    So when are you coming to give a talk at JPL about all this stuff? With slides! And perhaps puppets!

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

    Cynthia, you’ll have to look into the papers themselves — this image by itself doesn’t have that information. Don’t ask me, I’m just the blogger.

    Allyson, I’m happy to come to JPL to give a talk. But not about this! See above.

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  • kapakapa

    22. citrine:

    I am afraid I brought this epoch making news down to street level. My apology to scientists.

    “We’ll find dark matter when pigs fly?”

    As DM gets clumpier and collapses more, will they someday find literally pitch dark black holes/clusters?? A total amateur ponders, probably a totally dumb question.

    Cheers.

  • Shantanu

    Hi
    Sean (and Steve),
    I agree with what you said about MOND. However unless and until dark matter is
    discovered(or produced) in a laboratory based experiment, people are still going to be
    skeptical. Also the results from Bullet cluster do not tell us anything about the mass of the dark matter particle or even about the number density of dark matter particles.

    Also I have come across papers by Peebles, Damour, Visser (and perhaps others)
    which talk about dark matter with scalar interactions, or dark matter with non-zero pressures
    and many other exotic models for dark matter.

    If this is really true, then of course the distinction between dark matter and modified gravity
    becomes much more blurred and then whether you call it dark matter or modified gravity
    or both is only a pedantic question. Do you agree?
    Thanks

  • Chris W.

    Mostly off-topic, but not entirely (via N.E.W.):

    The Lorne Trottier Public Science Symposium (link to poster)

    A COSMIC COINCIDENCE: WHY IS THE UNIVERSE JUST RIGHT FOR LIFE?

    January 25, 2007
    5:00 p.m. — 7:00 p.m.
    Stephen Leacock Building, McGill University
    (Montreal, Quebec, Canada)

    Panel: Paul Davies, George Efstathiou, David Gross, Leonard Susskind

    Moderator: Victoria Kaspi (McGill, Astrophysics)

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .

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