Tuesday, August 15, 2006

NASA Announces Dark Matter Discovery

NASA Announces Dark Matter Discovery

Astronomers who used NASA's Chandra X-ray Observatory will host a media teleconference at 1 p.m. EDT Monday, Aug. 21, to announce how dark and normal matter have been forced apart in an extraordinarily energetic collision.

Shortly before the start of the briefing, images and graphics about the research will be posted at:


Briefing participants:

  • Maxim Markevitch, astrophysicist, Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.
  • Doug Clowe, postdoctoral fellow, University of Arizona, Tucson, Ariz.
  • Sean Carroll, assistant professor of physics, University of Chicago, Ill.

A video file about the discovery will air on NASA TV at noon, Aug. 21. Audio of the event will be streamed live on the Web at:


Note added Aug. 16th: See also the discussion at Lubos' blog.

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  1. Could this be about a collision between galaxies that created a dark matter galaxy? Or a collision between a galaxy and a dark matter galaxy? There's some evidence for dark matter galaxies, right? And Maxim Markevitch, one of the guys at the briefing, works on galaxy clusters and collisions.

  2. Okay... I did a little nosing around while you were probably asleep (one of the benefits of living in Shanghai).

    Since one of the folks at the teleconference is Maxim Markevitch, I bet this is about the Bullet Cluster, 1E0657-56. He talked about this at a conference last November.

    The Bullet Cluster consists of colliding galaxies where the gas crashed to a halt and got really hot, while the dark matter kept going straight through! Markevitch has used the Chandra observatory to see the X-rays from the hot gas. People can also see the dark matter because its gravity bends the light from galaxies further back. And, they're not in the same place!

    Also last year, people found signs of a galaxy almost entirely of dark matter.

    So, dark matter looks ever more real, leaving the question: what is it?

  3. The Bullet Cluster consists of colliding galaxies where the gas crashed to a halt and got really hot, while the dark matter kept going straight through!

    So stipulated. Where is the complimentary bleb of dark matter on the other side? View the collision center-of-mass. Matter interacts in all sorts of ways; dark matter only has gravitation. The two matter collision partners squish, slow, and heat. Their dark matter clouds are non-interactive and keep traveling, passing through the mess and each other, progessively slowing on either side from increasing gravitational potential. It should look like a dark matter dumbbell not a morningstar.

  4. Doesn't give off light, only detected by gravitation...those should be big clues what it is.

  5. If it takes large energies to decouple dark matter from visible matter then why is dark matter decoupled in our own solar system? Where is the stuff?

    Could the displaced lensing be due to a large gravitational wave shed by the collision?

    How much energy did the visible matter have going into the collision?

    How much is now visible as heat/x-rays?

  6. Dear Bee,

    thank you for hinting at this announcement

    and John:

    thank you very much for giving these details about the Bullet cluster!

    If it takes large energies to decouple dark matter from visible matter...

    eeh, which energy scale is involved here? I guess it is not a scale of high energy physics, so, speaking of extraordinarily energetic collisions might be misleading. Most probably it means that there is a very high impact velocity for galaxy collsions involved.

    What are the typical velocity scales in such collisions? For the Bullet cluster, the collision velocity is roughly 5.000 km/sec or 1/60 c, that's a lot - see
    Markevitch et al., AJ 567:L27-L31. It corresponds to a kinetic energy of roughly 130 keV for a hydrogen atom - OK, that's a lot again for atomic physics, but not on the scale of high energy physics...

    why is dark matter decoupled in our own solar system? Where is the stuff?

    Is it not thought to be extremely homogeneous on the scale of solar systems, or even the typical distance between stars within galaxies?

    Best, Stefan.

  7. Cool, thanks for the answers. But I have one more nagging question.

    If dark matter is locally homogeneous what is keeping it so evenly distributed? Why hasn't it undergone gravitational collapse? Now if there is a pressure keeping dark matter homogeneous in our own galaxy wouldn't that add to (?off diagonal?) elements of the stress-energy tensor, and thus have a gravitational contribution?

    This is exciting stuff!

  8. Dear John:

    Thanks a lot for the info, that is really interesting! So it seems that via weak lensing they have experimental evidence for dark matter which has separated from the cluster. I was wondering yesterday what exactly they mean with 'discovery', i.e. can they say more about the dark matter? How it clusters, how it clumps, something about its equation of state?

    One way or the other, I thought that would almost certainly rule out MOND.



    PS: I just found that on slide 25 there is a note vs. MOND.

  9. Exaclty! Where's the friction? Something has to allow dark matter to slow down enough to gravitational condense. Dark matter must interact with itself with more than just gravity.

  10. For more info on the Bullet Cluster - and a nice picture! - try week238 of This Week's Finds. I'm updating it and fixing mistakes as I find them.

    Al: I got the impression that there are two blobs of dark matter, one for each cluster. But, I haven't looked at the gravitational microlensing data - it may be in Markevitch's talk, available via week238.

    Bee: Yes, MOND is not looking so good. And yes, I hope this press conference will actually tell us something new about dark matter.

    Insightaction writes: If it takes large energies to decouple dark matter from visible matter then why is dark matter decoupled in our own solar system? Where is the stuff?

    As I'll explain, this stuff about "taking high energies to decouple dark matter from normal matter" is more confusing than helpful. Don't expect clarity from a mere "media advisory".

    Most people theorize that dark matter doesn't clump up too much. The idea is that it interacts very weakly with itself and ordinary matter - except via gravity. Given this, and given its low density, it would be very hard to detect in our Solar System. About the only thing we'd notice is its gravitational pull, which becomes noticeable only on length scales comparable to the size of a galaxy, or larger.

    People have searched for dark matter here on Earth. Some even claim to have found it, but their findings have not been replicated.

    Stefan writes: eeh, which energy scale is involved here? I guess it is not a scale of high energy physics, so, speaking of extraordinarily energetic collisions might be misleading. Most probably it means that there is a very high impact velocity for galaxy collsions involved.

    Yeah, this business has nothing to do with high-energy physics in the traditional sense - just smacking galactic clusters at each other, moving around 4,500 kilometers per second. A lot of energy, but not much energy per particle.

    The gas in the Bullet Cluster is very hot by normal standards: about 160 million degrees. But that's just 14 keV per particle. So, it's not much energy per particle - not compared to what we call high-energy physics these days. And, the high temperature is just a side-effect of the gas getting stopped dead in its tracks, while the dark matter keeps sailing along.

  11. MonD was already more or less ruled out by Low surface brightness galaxies, so no big surprise.

    People claim the relativistic variant (TeVeS) improves things, but probably not enough to save the theory.

    Either way I don't see how they can explain this observation away.

    (otoh the Pioneer anomaly will likely come back in fashion for 'speculators')

  12. Oaky I want have to apologize for the confusing questions. When I asked about high energies, I wasn't necessarily refering to particle physics; what I meant was that it seems that our galaxy hasn't collided with another galaxy to have a dark matter over shoot occur. But what I was really trying to tease at was much trickier, estimates of local distributions of dark matter.


    So from my cursory understanding of large N gravitation simulations a small number of particles cool a large subset into stable orbits by taking away momentum. Unfortunatelly this process can occur nearly indefinitely. Further condenstation is only balanced by another force like ion repulsion, neutron degeneracy, molecular forces, etc...


    So hazarding a guess on dark matter densities, pressures, and temperatures what is the best estimate for the scale of the force that keeps dark matter apart?

  13. Dear John:

    thanks a lot for compiling all these information!

    Ok, a hydrogen atom, or a proton, with a velocity of 4.500 km/s has a kinetic energy of 100 keV, so if the intergalactic hydrogen clouds of the colliding galaxy clusters interact, it makes perfect sense to me to find parts of the gas thermalized at 10 keV. A handfull of collisions per particle can be enough to go from free streaming to partial thermalization, even if the mean free path is most probaly enormously long...


    force that keeps dark matter apart?

    Aeh, good question. What actually is it that prevents dark matter from gravitational collaps? Are there degenerate dark matter stars? Dark matter black holes?

    BTW, Christine has raised another good question in the post on her blog: How can one make sure that the overshooting dark matter is at the same distance as the visible gas in the bullet cluster? Is it possible to extract the redshift of the lensing system in these gravitational lensing measurements?

    Best, stefan

  14. BTW, about MOND:

    I am just now a little bit puzzled: What is stopped in the collision of the bullet cluster is the intergalactic gas between the galaxies in the colliding clusters, right? You see this gas in the Chandra photos, you do not see the actual stars that make up the galaxies in 10 keV X ray?

    So, the galaxies themselves are collsionsless - you just do not see them on the Chandra photo. That is what I make of the following statement in the Possible Future Measurements section of the Markevitch et al. 2002 Astrophysical Journal paper. They write (my emphasis):

    There is a clear offset between the centroid of the bullet subcluster's galaxies and its gas. If one measures the location of the subcluster's dark matter density peak (e.g., from weak lensing or detailed modeling of the gas distribution), one may determine whether the dark matter is collisionless, as are the galaxies, or whether it experiences an analog of ram pressure, as does the gas.

    The important thing here: Collisionless dark matter would behave as the galaxies. It would not be stopped in the collision. Only the gas is stopped.

    Now, I do not know exaclty how big the contribution of the gas to the overall gravity of a galaxy is, but I would guess that since after the collision, galaxies and dark matter are still at the same place, the detection of collisionsless dark matter would not necessarily imply the end of MOND - at least not more so than the existence of weak lensing, or of the dark ghost galaxy.

    Best, stefan

  15. I'm no physicist, so this may be a stupid question, but what would be the observable difference between dark matter and a black hole?


  16. Stefan, you make an interesting point. Also you raise the question of what fraction of the mass of a cluster is in the IGM ("intergalactic medium" the gas in between the galaxies). I believe I read somewhere an estimate that a large part like 3/4 of the baryonic matter in a cluster is estimated to be in the IGM. But I don't trust this estimate, or my memory of it.

    Anyway I think your point is that THE STARS, THE VISIBLE GALAXIES AND THE DARK MATTER of each cluster are still together. Only the intergalactic gas has been stripped out by the collision.

    In a different picture, with an optical image superimposed, we should be able to see the individual galaxies of each cluster, and they should be centered where the DM is----in two separate clumps. (because presumably galaxies almost never collide, even when clusters pass thru each other)

    so to complete the refutation of MOND, we need a piece of information that says that the intergalactic gas in a cluster is a significant part (like 3/4) of the total baryonic mass of the cluster

    thanks for the prompt, Stefan. I didn't get that from anything I read

  17. Stefan what are your thoughts about the optical HST photo on page 23?

  18. Rich,

    Dark matter is called "dark" because we can't see it, i.e. it's not bright like stars are, or other things we can see with telescopes.

    A black hole is called "black" because it's a region where so much mass is condensed that the escape velocity is greater than the speed of light. Since nothing can travel faster than the speed of light that means that nothing which falls in can get out again, including light.

    So despite the similar colours, the two things aren't closely related. It may be possible to make a black hole out of dark matter, just as it is with normal matter, and that would look very much like a "normal" black hole. By definition, you can't really tell what's inside a black hole.

    By hypothesis, you also can't tell that dark matter is there except by its gravitational effects, but there's no particular evidence that it's condensed enough for those gravitional effects to be those of a black hole. They look more like the gravitational effects of a dilute, weakly interactive dust cloud. Ghosts you can't see or feel except for the effect of their aggregate mass over galactic scales.

    Hope that helps!

  19. Hi who, insightaction:

    I guess you have already read the very illuminating explanations by Sean at Cosmic Variance.

    So, indeed, galaxies and their stars travel together with the dark matter as deduced from weak lensing, and the shocked gas stays behind.

    The big point I have missed ist that the gas makes up ~80% of the baryonic mass of the clusters, so it is the main component of ordinary matter!

    Hi rillian - thank you for the explanations!

    Best, stefan

  20. rillian,

    Thanks for the explanation. I understand a bit more now anyway.


  21. Stefan, thank you very much.

    It took me awhile to figure out the necessity of the lengthy preamble concerning simulating hydrogen x-ray sources. But now I see that it is absolutely necessary in the arguing that 80% of the baryonic matter is hydrogen gas.


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