Warm Dark Matter?

By cjohnson | February 7, 2006 9:35 pm

Spotted on P.P.Cook’s Tangent Space (via Uncertain Principles): A link to a BBC News article entitled “Dark matter comes out of the cold”.

Apparently a team of Astronomers at Cambridge’s Insitute of Astronomy have reported on a set of observations of 12 dwarf galaxies in our local cluster which apparently tell us a wealth of new information about Dark Matter and its properties. In particular, they claim to be able to put a lower limit on the effective temperature (average speed) of the dark matter particles, and…. it is not cold, but warm!

If true (I suspect that it is still preliminary…I don’t think there’s a paper yet), it will be very interesting to see how it fits with everything else we’ve come to expect from other studies of dark matter (currently expected to be most likely cold), including, for example, the rather intricate simulation work that various groups do (see e.g. a post I did here), including work by our very own Risa (see links to various talks here, and more information here).

So does the new data make it too hot? Or just right? Inquiring blonde girls (and bears) want to know.

One to watch….


  • http://history-science.blogspot.com Doran

    The first thought that comes to my mind, with dark matter proposed at 10^4 K is whether this stuff radiates and what that radiation is. Chad Orzel remarks that its on the order of the temperature of the sun whose radiation we are all familiar with (except those stuck in basement labs). So can we expect a new type of radiation?

  • Kea

    Er…how come nobody mentioned the possibility of NO dark matter?

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Er…because of many different data sets.

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Kea.. I’m not sure what you mean….. this possibility is mentioned a lot…..but it is just simply not consistent with everything else we know. We’ve discussed this a lot on the blog… have a look through the archives, and some of the background links I supplied in the post, near the end. See Vera Rubin’s Scientific American article, for example, here as a pdf file: http://kicp.uchicago.edu/~risa/compton/Rubin98.pdf

    There is always the possibility that we’ve missed some huge idea that could surprise us all, but it would have to overturn so many interconnected parts of physics, that people are not sitting waiting for such a surprise, but rather going ahead with the business of trying to understand what we can infer from indirect measurements, etc….



  • Kea

    Hi Clifford

    OK, so people like the Conformal Gravity crowd haven’t analysed the WMAP data yet (there aren’t so many of them, after all) but AFAIK, despite the conventional wisdom, I believe the question is open, no?

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Hi Kea, I think the way Clifford put it in his previous email is the right way to think about it. If you are referring to C^2 gravity, then my understanding is that this idea fails, for example, to agree with nucleosynthesis constraints. If another model then I’m not sure, but I don’t know of a viable alternative right now (doesn’t mean there isn’t one).

  • http://eskesthai.blogspot.com/2006/02/evidence-for-extra-dimensions-and.html Plato

    I think there are two points to consider here and I am having some confusion with them. I see one that may be appropriate to “warm dark matter” in the form of strangelets? How these are produced and how one might see it in ways that Cliffords sees?

    What are the rules here in regards to temperature variation from the cosmic background? Would one find this relation to CFT on the boundary, appropriate as to the conditions, of the blackhole state, applicable, to warm dark matter?

    Just one of those nagging itches and how it is related I am not sure :( Is it okay to think out loud here? Please forgive my ignorance.

  • http://thebumblebeeblog.blogspot.com/ Poppycock

    I read about this in the Guardian, and was hoping someone would comment on it here. In this article they describe the dark matter as coming in “1,000 light-year blocks”. What does this mean? I was totally baffled! I also second Doran’s question about the radiation. The Guardian article just says it won’t be “normal radiation”, which didn’t help much.

    And cvj, I think you are right that there is no paper yet, but they are going to publish in Nature.

  • Edd

    Poppycock: I had a bit of a chuckle about the ‘1000 light year bricks’ – I don’t think that’s what’s being suggested at all, although the caveats about it being unpublished apply to my comment as much as anywhere else! That article does rather give the impression of supergalactic structure being based on some kind of invisible Lego. I think it’s a journalistic misunderstanding or unfortunate misphrasing.

    Dark matter doesn’t interact electromagnetically, so it won’t radiate electromagnetically. Possibly we don’t know enough to rule out other kinds of radiation, but I am guessing that if it does radiate at all then it isn’t in any currently detectable manner.

    On a more general note, it seems that talking about warm and cold dark matter isn’t very helpful. The labels cold, warm and hot dark matter aren’t giving us enough distinction. I’ve already heard this stuff referred to as ‘lukewarm’ and ‘tepid’ in conversation.

  • http://countiblis.blogspot.com Count Iblis

    Clifford, that’s exciting news for these type of models :)

    However, if you calculate the mass by assuming a velocity dispersion of 100 km/s (not sure if that’s correct for the galaxies they’ve looked at), you find that the mass is about 15 electron masses. That seems to be consistent with MeV Dark Matter

  • http://bostonalarm.com yanniru

    Does anyone one hear known the expected temperature of solar axions? It seems this layman that the 10,000 K temperature is consistent with axionic dark matter radiated from the stars in dwarf galaxies.

  • http://countiblis.blogspot.com Count Iblis

    Hmmm, the article mentions 9 km/s for the speed. I don’t see the 10^4 K mentioned anywhere. But assuming both figures are correct, then the mass would be about 1 GeV. This would be consistent with this and some other models of strongly interacting dark matter, see e.g. here.

  • Paul Valletta

    I believe the article is the handwaving to this Horizon programme on Wed 8th;http://www.bbc.co.uk/sn/tvradio/programmes/horizon/index.shtml

    “On next: Most of Our Universe Is Missing
    The world’s leading cosmologists seek the Universe’s most elusive secret.”

    I have been waiting for this episode, so hopefully I will shed some light (pun intentional!) sometime tomorrow.

  • Paul Valletta

    !should of course be Thursday 9th Feb!

  • http://eskesthai.blogspot.com/2006/02/evidence-for-extra-dimensions-and.html Plato

    I’m sorry I put this post in the wrong thread and mixed up “Clifford’s quote” from that thread and placed it here.:)Yikes!

  • http://www.anthropic-principle.ORG island

    I’m sorry I put this post in the wrong thread and mixed up “Clifford’s quote” from that thread and placed it here.:)Yikes!

    Are you sure this is the right blog?… 😉

  • neal

    I’m trying to figure out from the BBC article what exactly has been done here. It looks like they’ve measured the phase-space density in the cores of some dwarf spheroidal galaxies, and then applied Liouville’s theorem to say something about the initial phase space density (and hence the DM temperature).

    If this is the case, then one immediate objection is that you only measure the coarse-grained density, which is NOT conserved, but gives a lower bound on the (conserved) fine-grained density.

    Also I believe that in simulated DM halos, the regions of high phase-space density are in the subhalos — the central core of a halo generally has pretty low phase-space density. Of course, you don’t probe the subhalos because they have no stars in them. So any estimate of the phase-space density based on the central stellar velocity dispersion is again bound to biased low. I don’t see how they can get anything besides an upper bound on the DM temperature.

    Of course, they might have done something entirely different than what I’m guessing, I suppose we’ll have to wait for the paper to come out…

  • http://eskesthai.blogspot.com/2006/02/evidence-for-extra-dimensions-and.html Plato

    oh yes island:)Read now…as the music plays.

  • Kea

    “…then my understanding is that this idea fails, for example, to agree with nucleosynthesis constraints.”

    Mannheim in a 1999 paper addresses the original 1993 Knox and Kosowsky issue with nucleosynthesis. Yes, it appears to be a problem, but not necessarily unresolvable.

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

    I think it’s just too difficult at this point to figure out what they’ve done, since there is a BBC article but no real paper. One thing to keep in mind is that “warm” dark matter often refers specifically to dark matter that was marginally relativistic when it first stopped interacting with other particles and dropped out of equilibrium — it doesn’t refer to the temperature today. In fact, you’d expect that the temperature of warm DM today would be similar to that of the CMB, if it weren’t for evolution later on (like grouping into halos etc.).

    The basic point seems to be that the DM is less concentrated — more spread out — in these galaxies than you might have thought it should be. There are other indications that dark matter doesn’t clump that effectively on small scales (a lack of cusps at the center of galaxies, a lack of small satellite galaxies), but those are messy because ordinary matter is also playing a role. If it’s true that these systems have much more DM than ordinary matter, it might make them a useful testing ground for these questions.

  • Jay

    There’s a conference proceeding from the group on astro-ph today:


    Basically, they are arguing that all of the Milky Way dwarf spheroidals have masses ~ 4 x 10^7 M_sun. The masses come from a straightforward application of the Jeans theorem, with the usual (and perhaps unjustified) assumptions like isotropy. Much of the conclusion is based upon velocity dispersion measurements in the outer parts of the galaxies from small numbers of stars.

  • Shantanu

    I am getting lost with so many recent papers/candidates on variants of
    variants of dark matter such as
    darm matter with early decays : astro-ph/0507300, dark matter with scalar interactions hep-th/0407097, Dark matter with non-zero pressues, astro-ph/0512213,
    dark matter with inelastic couplings, astro-ph/0208403 , non-point dark matter hep-ph/0203179,
    self-interacting dark matter astro-ph/9909386 and probably many more which I may missed.
    (Inspite of all this there is no laboratory evidence for any of these)

    Can someone(Sean, Risa, Mark) summarize briefly as to what is the motivation for adding
    these exotic properties to cold dark matter particles. Is it to address the cold dark matter crisis
    at smaller scales(which I thought from one of the ary posts on this blog) is not as severe as thought before?
    Of course there are tons of candidates even for vanialla cold dark matter, though the WIMP
    and axion are the most popular ones.

  • Aaron Bergman

    What I know on the subject comes from this review on Paul Steinhardt’s webpage.

  • http://eskesthai.blogspot.com/2006/02/evidence-for-extra-dimensions-and.html Plato
  • http://countiblis.blogspot.com Count Iblis


    Can someone(Sean, Risa, Mark) summarize briefly as to what is the motivation for adding
    these exotic properties to cold dark matter particles.

    It’s all explained in this paper

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

    Shantanu, I don’t know if you can call it a “crisis” (although people do), but there are several issues with CDM on small scales. In various ways, we don’t observe as much small-scale clumpiness as you might expect. Here’s a good review:


    It might turn out that none of them is serious, and messy astrophysics can fix them all — or not.

  • Neal

    I can’t speak for the other models, but inelastic dark matter is for explaining the DAMA/CDMS conflict, not for astrophysics. I don’t think it really effects structure as it’s still pretty non-interacting…

  • http://countiblis.blogspot.com Count Iblis

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