Avignon Day 4: Dark Matter

By Sean Carroll | April 22, 2011 12:40 am

Yesterday’s talks were devoted to the idea of dark matter, which as you know is the hottest topic in cosmology these days, both theoretically and experimentally.

Eric Armengaud and Lars Bergstrom gave updates on the state of direct searches and indirect searches for dark matter, respectively. John March-Russell gave a theory talk about possible connections between dark matter and the baryon asymmetry. The density of dark matter and ordinary matter in the universe is the same, to within an order of magnitude, even though we usually think of them as arising from completely different mechanisms. That’s a coincidence that bugs some people, and the last couple of years have seen a boomlet of papers proposing models in which the two phenomena are actually connected. Tracy Slatyer gave an update on proposals for a new dark force coupled to dark matter, which could give rise to interesting signatures in both direct and indirect detection experiments.

This is science at its most intense. A big, looming mystery, a bounty of clever theoretical ideas, not nearly enough data to pinpoint the correct answer, but more than enough data to exclude or tightly constrain most of the ideas you might have. It wouldn’t be at all surprising if we finally discover the dark matter in the next few years; unfortunately, it wouldn’t really be surprising if it eluded detection for a very long time. If we knew the answers ahead of time, it wouldn’t be science (or nearly as much fun).

Today is our last day in Avignon, devoted to cosmic acceleration. My own talk later today is on “White and Dark Smokes in Cosmology.” (The title wasn’t my idea, but I couldn’t have done better, given the context.) It’s the last talk of the conference, so I’ll try to take a big-picture perspective and not sweat the technical details, but (following tradition) I will admit that it’s an excuse to talk about my own recent papers and ideas I think are interesting but haven’t written papers about. At least it should be short, which I understand is the primary criterion for a successful talk of this type.

Also, few people have strong feelings about non-gaussianities or neutrinos, but many people have strong feelings about reductionism. Quelle surprise!

  • B


    (Small typo: “it’s” should be “its”.)

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


  • Paul

    You’ll have to excuse me if the link is there and I’m missing it, but is the video available online?

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

      No videos, sorry. Slides are available.

  • http://www.damtp.cam.ac.uk/people/e.lim/ Eugene

    Cuz you can’t calculate reductionism, but you can calculate non-Gaussianities and neutrino abundances?

  • http://www.sunclipse.org Blake Stacey

    Ask people what they think about quantum gravity — or a theory of everything based on the Lie group E8 — and everyone has a strong opinion. But ask them what’ll happen when you shine light on a gadget with vanes that are black on one side and white and the other, in a perfect vacuum, and suddenly they’re completely quiet!

    John Baez

  • Trevor

    Sean, I was thinking about your slides on entropy, and maybe the answer is in your book (purchased, but unread as of yet!), but how does the entropy calculation depend on what you take as the “closed” system? That is, the horizon today is not the same as the horizon ever was, so the entropy within the horizon today may not necessarily be reflective of an ever-increasing process.

    As an example, assuming w=-1, in the far future we will be left in our “island Universe” Milkomeda (to borrow Loeb’s term for it), with scant evidence that the Universe was much larger than our patch of it. Someone doing the entropy calculation at that time would calculate a number reduced by a factor ~10^11 compared to what it would be in what we call the observable Universe today.

    So can we be sure that our Universe had low entropy at early times? Could we just not be counting it all?

  • Albert Zweistein

    Dark Matter? Moderately difficult problem.

    Some hints:

    Particle-LIKE? – Definitely
    Very dense? – You betcha
    Fit in a breadbox? – No way
    Problem with assumptions? – You wouldn’t believe

    Bonus hint: Ask yourself, what could produce the ARCADE-2 6x radio excess?

    Game on!
    Albert Z

  • Charon

    The Greenwood Space Travel Supply Co. (Seattle) sells dark matter in a can. You should recommend some of the experimentalists go get a sample there. (Or check to see if the can is labelled with ingredients.)

  • Sili

    But ask them what’ll happen when you shine light on a gadget with vanes that are black on one side and white and the other, in a perfect vacuum, and suddenly they’re completely quiet!

    Assuming friction can be ignored, the white sides will receive a net push forward, since they reflect light – admittedly diffusely, so the push won’t be as big as with mirrors.

    Of course, I’m a failed chemist, so what do I know.

    ETA: Not much, I guess, now that I’ve followed the link.

  • Matthew Saunders

    “Also, few people have strong feelings about non-gaussianities or neutrinos, but many people have strong feelings about reductionism.”

    Perhaps people are more interested in what they have emotional investment in? Perhaps more people (more than scientists) can have an emotional investment in ‘reductionism’ than the other two?

    (maybe it’s because Discover Mag is owned by a publisher that loves railroads? ;)

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  • m. bora cilek

    hi there everyone,
    finally I have solved the dark matter problem..this is no joke, believe me..
    just good-old (proven) laws of physics from Newton and Einstein..no new models, funny concepts or cumbersome mathematics…I will soon post an article in one of the e-print archives..the important part is only one or two paragraphs, rest will be references etc. as that is the norm for this kind of write-ups.. you will be amazed to see how long humanity have spent in the wrong direction..very embarrassing indeed..
    just note my name and wait for another 2-3 weeks..in the meantime I need your suggestions for alternative sites, to publish the article online simultaneously..

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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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