The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twenty-first-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Here’s the press release, and an article in the Chandra Chronicles.)
A prerequisite to understanding the dark sector is to make sure we are on the right track. Can we be sure that we haven’t been fooled into believing in dark matter and dark energy? After all, we only infer their existence from detecting their gravitational fields; stronger-than-expected gravity in galaxies and clusters leads us to posit dark matter, while the acceleration of the universe (and the overall geometry of space) leads us to posit dark energy. Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einstein’s general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances?
A departure from general relativity on very large scales isn’t what one would expect on general principles. In most physical theories that we know and love, modifications are expected to arise on small scales (higher energies), while larger scales should behave themselves. But, we have to keep an open mind — in principle, it’s absolutely possible that gravity could be modified, and it’s worth taking seriously.
Furthermore, it would be really cool. Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. (And Vera Rubin agrees.) We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isn’t to express preferences, it’s to suggest hypotheses and then go out and test them.
The problem is, how do you test an idea as vague as “modifying general relativity”? You can imagine testing specific proposals for how gravity should be modified, like Milgrom’s MOND, but in more general terms we might worry that any observations could be explained by some modification of gravity.
But it’s not quite so bad — there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle. So if we imagine doing away with dark matter, we can safely predict that gravity always be pointing in the direction of the ordinary matter. That’s interesting but not immediately helpful, since it’s natural to expect that the ordinary matter and dark matter cluster in the same locations; even if there is dark matter, it’s no surprise to find the gravitational field pointing toward the visible matter as well.
What we really want is to take a big cluster of galaxies and simply sweep away all of the ordinary matter. Dark matter, by hypothesis, doesn’t interact directly with ordinary matter, so we can imagine moving the ordinary stuff while leaving the dark stuff behind. If we then check back and determine where the gravity is, it should be pointing either at the left-behind dark matter (if there is such a thing) or still at the ordinary matter (if not).
Happily, the universe has done exactly this for us. In the Bullet Cluster, more formally known as 1E 0657-56, we actually find two clusters of galaxies that have (relatively) recently passed right through each other. It turns out that the large majority (about 90%) of ordinary matter in a cluster is not in the galaxies themselves, but in hot X-ray emitting intergalactic gas. As the two clusters passed through each other, the hot gas in each smacked into the gas in the other, while the individual galaxies and the dark matter (presumed to be collisionless) passed right through. Here’s an mpeg animation of what we think happened. As hinted at in last week’s NASA media advisory, astrophysicists led by Doug Clowe (Arizona) and Maxim Markevitch (CfA) have now compared images of the gas obtained by the Chandra X-ray telescope to “maps” of the gravitational field deduced from weak lensing observations. Their short paper is astro-ph/0608407, and a longer one on lensing is astro-ph/0608408. And the answer is: there’s definitely dark matter there!
Despite the super-secret embargoed nature of this result, enough hints were given in the media advisory and elsewhere on the web that certain scientific sleuths were basically able to figure out what was going on. But they didn’t have access to the best part: pictures!
Here is 1E 0657-56 in all its glory, or at least some of it’s glory — this is the optical image, in which you can see the actual galaxies.
With some imagination it shouldn’t be too hard to make out the two separate concentrations of galaxies, a larger one on the left and a smaller one on the right. These are pretty clearly clusters, but you can take redshifts to verify that they’re all really at the same location in the universe, not just a random superposition of galaxies at very different distances. Even better, you can map out the gravitational fields of the clusters, using weak gravitational lensing. That is, you take very precise pictures of galaxies that are in the background of these clusters. The images of the background galaxies are gently distorted by the gravitational field of the clusters. The distortion is so gentle that you could never tell it was there if you only looked at one galaxy; but with more than a hundred galaxies, you begin to notice that the images are systematically aligned, characteristic of passing through a coherent gravitational lens. From these distortions it’s possible to work backwards and ask “what kind of mass concentration could have created such a gravitational lens?” Here’s the answer, superimposed on the optical image.
It’s about what you would expect: the dark matter is concentrated in the same regions as the galaxies themselves. But we can separately make X-ray observations to map out the hot gas, which constitutes most of the ordinary (baryonic) matter in the cluster. Here’s what we see.
This is why it’s the “Bullet” cluster — the bullet-shaped region on the right is a shock front. These two clusters have passed right through each other, creating an incredibly energetic collision between the gas in each of them. The fact that the “bullet” is so sharply defined indicates that the clusters are moving essentially perpendicular to our line of sight.
This collision has done exactly what we want — it’s swept out the ordinary matter from the clusters, displacing it with respect to the dark matter (and the galaxies, which act as collisionless particles for these purposes). You can see it directly by superimposing the weak-lensing map and the Chandra X-ray image.
Clicking on each of these images leads to a higher-resolution version. If you have a tabbed browser, the real fun is opening each of the images in a separate tab and clicking back and forth. The gravitational field, as reconstructed from lensing observations, is not pointing toward the ordinary matter. That’s exactly what you’d expect if you believed in dark matter, but makes no sense from the perspective of modified gravity. If these pictures don’t convince you that dark matter exists, I don’t know what will.
So is this the long-anticipated (in certain circles) end of MOND? What need do we have for modified gravity if there clearly is dark matter? Truth is, it was already very difficult to explain the dynamics of clusters (as opposed to individual galaxies) in terms of MOND without invoking anything but ordinary matter. Even MOND partisans generally agree that some form of dark matter is necessary to account for cluster dynamics and cosmology. It’s certainly conceivable that we are faced with both modified gravity and dark matter. If the dark matter is sufficiently “warm,” it might fail to accumulate in galaxies, but still be important for clusters. Needless to say, the picture begins to become somewhat baroque and unattractive. But the point is not whether or not MOND remains interesting; after all, someone else might come up with a different theory of modified gravity tomorrow that can fit both galaxies and clusters. The point is that, independently of any specific model of modified gravity, we now know that there definitely is dark matter out there. It will always be possible that some sort of modification of gravity lurks just below our threshold of detection; but now we have established beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics.
That’s huge news for physicists. Theorists now know what to think about (particle-physics models of dark matter) and experimentalists know what to look for (direct and indirect detection of dark matter particles, production of dark matter candidates at accelerators). The dark matter isn’t just ordinary matter that’s not shining; limits from primordial nucleosynthesis and the cosmic microwave background imply a strict upper bound on the amount of ordinary matter, and it’s not nearly enough to account for all the matter we need. This new result doesn’t tell us which particle the new dark matter is, but it confirms that there is such a particle. We’re definitely making progress on the crucial project of understanding the inventory of the universe.
What about dark energy? The characteristic features of dark energy are that it is smooth (spread evenly throughout space) and persistent (evolving slowly, if at all, with time). In particular, dark energy doesn’t accumulate in dense regions such as galaxies or clusters — it’s the same everywhere. So these observations don’t tell us anything directly about the nature of the 70% of the universe that is purportedly in this ultra-exotic component. In fact we know rather less about dark energy than we do about dark matter, so we have more freedom to speculate. It’s still quite possible that the acceleration of the universe can be explained by modifying gravity rather than invoking a mysterious new dark component. One of our next tasks, then, is obviously to come up with experiments that might distinguish between dark energy and modified gravity — and some of us are doing our best. Stay tuned, as darkness gradually encroaches upon our universe, and Einstein continues to have the last laugh.
Excellent post, I also like all the comments. I have one question about the possible nature of DM:
If dark matter as far as we know interacts only with gravity, would not dark matter clump together and form not starts but black holes ? The only thing stopping normal stars into becoming black holes are the other 3 forces (I think). So why does it clump together but is evenly spread across the galaxy cluster?
Thanks for any replies.
Tom asked (#124):
could it be that non-baryonic dark matter and baryonic matter attract eachother, but non-baryonic dark matter repels itself?
This would mean dark matter would cluster around galaxies, but would never collapse on itself (ie into dark stars or dark planets).
There are actually proposals for “self-interacting” dark matter, which would do something like that. Simulations of galaxy formation using “standard” (non-interacting) dark matter appeared to produce very dense dark-matter cores at the centers of galaxies, and observations of gas motions in dwarf galaxies indicated there wasn’t as much dark matter in their centers as predicted by the simulations. So if dark matter were self-interacting, there would be lots of collisions in the cores, which would tend to keep the cores from being as dense.
I think the current thinking is that the earlier dark-matter simulations (or the measurements and extrapolations made from them) may have over-estimated the core densities; more accurate simulations and measurements of the simulations may have cores that aren’t as dense, which mean you don’t need to postulate self-interacting dark matter after all.
In a comment (number 82) I said, about the mpeg animation at
http://chandra.harvard.edu/photo/2006/1e0657/media/bullet.mpg
that
at 2 seconds into the animation, the collision has just started with the two clusters just coming into contact at a point, and the red hot gas Ordinary Matter is no longer evenly dispersed, but has migrated to two crescents opposite the contact point.
Ann Miller said (in comment number 121) that she “… see[s] the same thing. …”.
Peter Erwin said (in comment number 125) “… the dark matter blobs (blue) are moving faster and leaving some of the gas behind … it’s really just an animated cartoon … all sorts of fine details will be missing or wrong …”.
After thinking about it, I think that the early part of the animation is qualitatively incorrect,
and that the wrong part is more important than just a “fine detail”,
and
that a correct animation sequence would look somewhat like the image I put on the web at
http://idisk.mac.com/sm17h-Public/animbull.jpg
in which
purple is a combination of Dark Matter and Hot Gas,
blue is Dark Matter, and
red is collision/shockwave concentrated Hot Gas.
The image is very crude but I hope it makes the idea clear.
If I happen to be correct that the early part of the mpeg animation is qualitatively wrong, then I wonder how it got made that way. Was there a failure of communication between astrophysicists and PR animation illustrators ?
Since the animation is an important part of the media message sent to the public by the “media advisory” and “media teleconference” it seems to me to be important that it be qualitatively correct.
Tony Smith
http://www.valdostamuseum.org/hamsmith/
In an earlier paper based on the same observations,
astro-ph/0309303,
they give an upper bound on DM-DM cross section ,
sigma/m
Great article – very well written!
I have a question relating to the MoND theory, though. I only recently learned about MoND and TeVeS, and though I can’t say that I am a supporter of either theory I am suprised by the number people jumping up to dance on MoND’s grave. The fact that MoND can apparently be applied to a large number of galaxies with reasonable accuracy strikes me as being very interesting, regardless of the underlying reasons.
The theory may not be correct, but the fact that the MoND theory can describe galaxies is fascinating. If the rotational effects are caused entirely by gravity as we know it (and thus matter that is dark to us), why would it accumulate in a way to create such a consistant result? What underlying mechanic is working to produce such consistancy? What happens next to galaxies/regions where some/most/all of their DM has been separated from their Light Matter? How will we recognise post-collision regions in the sky, and how will they behave? Will the galaxies still have/eventually return to MoND-esque rotation curves?
Again – the theory may not be correct, but I still can’t understand why so many people, here and elsewhere, are singing “Ding-Dong! The MoND is Dead!” The sentiment strikes me as being an unreasonably negative emotional reaction, when we should be getting excited about the new possibilities, new questions, and new directions to look for answers.
In an earlier paper based on the same observations,
astro-ph/0309303,
they give an upper bound on DM-DM cross section ,
sigma/m < 1 cm^2 gm^(-1)
which rules out a large parameter space of Strongly Interacting Dark matter theories.
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Sean [#10]:
The cosmological constant could simply be the signature or mark of the Grand Designer, who does not change and for whom the idea of “vacuum energy’ might be apt.
Subhendra
In astro-ph/0309303 they mention lower bounds on sigma/m from rotation curve studies, such as sigma/m > 0.5 cm^2 gm^-1, but there seem to be many conflicting claims about this number. Is there any consensus here that would pin down sigma/m to a small but positive value?
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Perhaps Dr. Murrays original paper on non linear gravity can shed some light on the gravitational question and the accelerating universe. Click Here.
Hi Sean,
Exiting news and great writing, thanks!
I figure my knowledge in the ‘deep filed’ of astronomy and physics compared to you guys, is not even on the same scale as DM + DE compared to ordinary matter, i.e. 4-5%. Even so, I take the liberty to play ‘The Devils Advocacy’ and post some ‘exasperating’ questions:
1) Is it possible to measure redshifts on X-ray observations from hot gas? If not – how can we be sure that the intergalactic gas is on the same distance (location) as the galaxy cluster?
2) I guess you can’t use redshifts to measure distance/speed in the perpendicular (x) direction? If so – how can we be sure that the galaxy cluster is really moving in away from each other in that direction?
3) There are 5 times more dark matter than ordinary matter in the universe, and dark matter gives an explanation why stars revolve around the center of galaxies at a constant speed, at different distances from the center; Why doesn’t our solar system rotate in the same way, due to dark matter (DM and OM party together, so it must be all around us)?
Finally, here’s a beautiful picture of the history of our fantastic universe which also include the mysterious dark energy:
http://map.gsfc.nasa.gov/m_ig/060915/CMB_Timeline75.jpg
Greetings from Dark Vader
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…sorry for the humorous misspellings in my last post, but it was 02:30 in the night, and this is not my native language, life is tough… 🙂
Dear Sean,
we met at lunch at Saclay about a little more than a year ago, after your talk at SPhT. We talked a bit about blogging and blogging scientists.
By that time I was a postdoc there. Now I am back in Lisbon, where I am from, and this Summer I am doing a little internship in the portuguese newspaper Público, as a scientific journalist.
Last week was pretty busy. I wrote about things like the Poincaré conjecture, and also of course about dark matter. This post of yours was a very useful source of information. Congratulations and keep up the good work.
Best,
Filipe.
Hi,
I find your conclusion somewhat premature. If the observations would indicate that the lensing is neither associated with the galaxy clusters themselves nor with the hot has, now that would be a proof of dark matter. I mention this because I have suggested on my webpage Plasma Theory of ‘Gravitational Lensing’ of Light that the lensing of light is actually due to the fact that stars and galaxies should be electrically charged, which might lead to a lensing effect. So the observed lensing could be due to the visible matter after all.
Thomas
Sorry about the re-post, but I noticed a mistake in my first post above:
I think the conclusions drawn in the article are somewhat premature. If the observations would indicate that the lensing is neither associated with the galaxy clusters themselves nor with the hot has, now that would be a proof of dark matter. I mention this because I have suggested on my webpage Plasma Theory of ‘Gravitational Lensing’ of Light that the lensing of light is actually due to the fact that stars (and galaxies) should be electrically charged. So the observed lensing might be due to ordinary matter after all.
Anyway, I don’t think that one can say that ‘Dark Matter’ is proven unless one can identify it positively (i.e. unless it is not ‘dark’ anymore).
Thomas
Why are these galantic clusters so dark? Only by Xray we can see them. These hot gases might be like exhaust ones.
The spherical harmonics are the angular portion of the solution to Laplace’s equation in spherical coordinates where azimuthal symmetry is not present. And there are three types of galaxies.
*elliptical galaxy e.g. NGC4881 Three Dimension GM(
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