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.
The content was fascinating, but what impressed me more was
Sean’s writing style. I felt the same sense of scientific wonder as I got when reading a Carl Sagan book for the first time as a teenager.
Ok – just one question on the lensing – can someone post a picture which shows the galaxies that are being ‘lensed’. Or a picture which false-colors galaxies based on their red-shifts, instead their normal colors.
Eugene said ” TeVeS, on the other hand, may still be alive but is getting as baroque as the multiparameters that we need to cook up for DM to make it all fit.”
Isn’t that a fancy way of saying “TeVeS is dead”?
JoAnne: I sincerely hope that all your fears come true. This whole “discovery striptease” that NASA ran was undignified idiocy of the first order, and they should thank whoever leaked the announcement. Look at what happened with WMAP — all that secrecy leading up to an announcement that amounted to nothing much at all. They ended up looking foolish. I hope that LHC will be one long leakfest.
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I am a software engineer by profession and its been like 8 years since I touched a book on physics but this post on something as abstract as dark matter and dark energy – I could almost understand!!!
Fantastic job!Thank you!
Nice explanation. Still blatantly rubbish, but a nice explanation. This dark matter nonsense is just Vulcan all over again. The problem with inferring the existance of something based on its effect on something else is that it assumes that the hole in your current understanding is only big enough for one explanation.
In other words, the dark matter camp is trying to assert that if there is something a MOND can’t explain then it proves their theory must be correct. It does not.
It is totally inconceivable that something as important to the phyiscal characteristics of the universe as dark matter is supposed to be has not turned up here on Earth, or even in the nearby part of the universe. How is it formed? It it outweighs normal matter then it must be relatively easy to make, yet no trace of it has ever popped out of any experiment at CERN et al. Why?
Dark matter is just plain silly and, like Vulcan before it, will ultimately turn out to be a patch over a fundamental gap that we do not yet even realise is there in our knowledge. A new force? A modification of the way the known forces interact under some circumstances? Who knows. But just postulating magic pixie dust, no matter in what quantities, is not in the long run going to be a viable explanation.
The problem with inferring the existance of something based on its effect on something else
That would be how we infer the existence of everything.
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Why cant the gravity in the blue areas simply be caused by the galaxies in respective group? Why does the blue area have to contain any dark matter? (the galaxies also act as collisionless particles) Why cant this simply be a collision between two clusters where the galaxies passed each other without collisions but the interstellar hot gasses collided and resulted in the formation above?
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Could the anomalous lensing be explained by gravity travelling at the speed of light, thus acting as though it was lagging behind? This would be consistant with there being no such dark matter. I’ve seen possibly explainations of “dark matter” in galaxies by simply using GR properly.
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“The problem with inferring the existance of something based on its effect on something else
That would be how we infer the existence of everything.”
Which is why we often make mistakes. The problem is that, in the immortal words of Donald Rumsfeld, if you don’t know what it is that you don’t know then you can not simply say that Theory A is the correct one because you have shown Theory B to be incorrect.
Pointing at odd movements of bodies and saying that it implies the existance of a mass you can’t see is fair enough most of the time but when it implies something this outlandish it is more likely that there is a whole new facet of physical law to be uncovered.
Dark matter theory tries to fiddle the subject by introducing a wonderous magic substance which basically only interacts with gravity in the normal way. The problem is that gravity is so weak that huge amounts of this magical material are required to explain the observations. It seems far less likely that this is the correct solution than that some force (by which I mean a physical consequence of normal matter, time and space) is appearing which hitherto has been hidden not by its enormity (which is the supposed case for DM) but by its subtlty.
Again, Vulcan shows the way: the strange movement of Mercury was not caused by something which was huge and strangely impossible to see, it was caused by a small relativistic effect which was hidden in normal situations by the very fact of its weakness.
When people start saying that an effect is caused by something 3 times the mass of the visible universe but which has escaped notice for the whole of history, is it really unreasonable to doubt them? I don’t think so.
Dark matter is a fudge; a placeholder until we come up with something sensible. Anyone looking to explain the galactic movements using existing laws and forces without modification (which is effectively what DM is trying to do) is on a hiding to nothing in the long run.
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The problem is with the statement…
The problem is that the nucleosynthesis and CMB arguments are based on arguments that assume one is observing a “dead” universe. This is because it is much simpler for physicists and astronomers to develop theories and match data with those theories if one asserts that the universe still is, as it once was, “dead”. If instead one assumes that the universe may now be quite “alive”, then the “dark matter” may simply be advanced civilizations that have adopted the form of Matrioshka Brains. No new mysterious particles are required if you simply turn the dead or alive coin over.
Robert Bradbury
1. Wikipedia: Matrioshka Brain
I am going to sit and read the papers, probably this weekend on my flight to DC, but I find these results a little hard to accept without reading the paper. Sean, I think you did a great job, but with a degree in physics, I am looking for some “numbers.” I do have to say my biggest issue with the results from the pictures is that there is no shock wave in the dark matter. Even if dark matter is weakly interacting with itself, I would still expect to see some sort of shock wave at these energies and speeds. It would be nice to see the normal matter having some sort of effect on the dark matter as well, which I can’t pick up from the pictures, but hope are still present. Great site, great article, great thinking, great discussion.
From the article:
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.
Question:
How do we know that? This is the basic argument on which the separation of Dark/ordinary matter is done using the images
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Unlike Sean, I am not too upset to see life get significantly harder for MOND enthusiasts — if anything MOND’s primary purpose in life has been to explain just how hard it is to “explain” dark matter by modifying gravity.
The irony is that while the original argument in favor of MOND was one of simplicity (look, I can explain all these rotation curves with just one parameter!) i morphed into something horribly baroque. Not only do you want to explain galaxy rotation curves, but you also need to get the acoustic peaks in the CMB, and weak lensing — since in conventional cosmology, the standard explanations of these observations all rely on the presence of dark matter So far as I can see, attempts to implement MOND in metric theories of gravity require a different component for each of these three effects, where dark matter explains them all in a unified fashion. Moreover, MOND requires Lorentz symmetry to be broken in some way (as Sean will know well) whereas dark matter just needs a particle we haven’t seen yet, with a mass and cross-section that is not wildly at odds with our expectations for TeV scale physics.
I know Occam’s Razor isn’t any substitute for experiment, but for my money a MOND-driven universe is far more preposterous than one with where Omega_baryon is smaller than Omega_matter 🙂
On the other hand, Dark energy is something that could very well be understood in terms of a “modified” gravity. And, unlike dark matter, there is no “simple” explanation for the accelerating universe — a cosmological constant is just one number (assuming the dark energy is not dynamic), but understanding it is bound to involve a major advance in theoretical physics.
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is it possible that the Xrays somehow took longer to get to us than the gravity,
(because of differences in the *refractive index* between the Xrays and gravity
waves) and this explains the transposed centers of material. If the collision is relatively fast and energetic, couldnt this also explain the images ?
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