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.
Is it possible that super-super massive black holes exist at the center of galaxy clusters? I think (I could be wrong) it’s proven that most if not all galaxies contain a supermassive black hole at their center. Why not galaxy clusters? Couldn’t this at least contribute to the shift in the measured center of mass that’s being attributed to dark matter? Yes, I know I’m reaching, I just can’t stomach dark matter. A magic substance that is ignores all forces other than gravity and refuses to show up in particle accelerators, yet is 5x more common than regular matter.
Hi,
Some comments from a regular lurker:
1- The Bulletcluster definitely does NOT rule out MOND, contrary to what some people are claiming now. As Sean mentions in his post, MOND needs (predicts 😉 ) dark matter at clustergalactic scales and to get the CMB right with Bekenstein´s TeVeS theory one still needs some form of dark matter like neutrino´s for instance, that would only cluster at clustergalactic scale. Why bother then about MOND when it still needs dark matter? The point is that MOND, a simple one parameter modification of Newton´s law, explains the rotation curves of all kinds of galaxies with different morphologies very well. From the dark matter perspective this regularity is quite surprising, since structure formation is very much a stochastic process. All the explanations for this regularity within the dark matter paradigm that I have seen so far are not really convincing. To explain the Tully-Fisher law for instance, that describes a certain correlation between the luminous matter of a galaxy and the asymptotic velocity of its rotation curve one needs to invoke different fine-tuned processes for different kinds of galaxies.
It is quite strange (again from the CDM perspective) that MOND has not been ruled out yet. To do so one would only need to find some galaxies, where MOND predicts way too much modification (invoking dark matter will not help you there). But I guess that galaxies with way too little modification will also mean the final blow to MOND, since this would mean that MOND also needs dark matter at the galactic scale.
2- Let´s assume that the reconstruction of the gravitational field from the weak lensing is done properly and that we really observe an offset between the gravitational field and the luminous matter distribution. Does this then prove the existence of dark matter and rule out modified gravity at that scale. I am not so sure. In particular I don´t agree with the statement that “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.” I would expect this feature to hold in a spherical symmetric situation but not necessarily in a dynamical situation as two colliding clusters. Suppose that the modified gravitational field does not propagate with the speed of light, but way slower, as happens for instance in the ghostcondensate model of Arkani-Hamed et al. In that case the gravitational field would need some time to catch up with the source and maybe this is what we observe? But Ok, I agree that the dark matter explanation seems to be simpler ;).
Great pictures by the way!
Some overall consistency checks on this phenomenon should be performed:
1) The two clusters would show up as outliers from the “fundamental plane” relation of clusters in the space of 2-component virial theorem parameters [Dantas et al. ApJ Letters, 528, L5, 2000, stro-ph/9910541];
2) N-body gravitational simulations of collisions of 2 (or 3)-component cluster models (barionic+DM; or + gas) with appropriate masses and orbital configurations should be performed in order to check the overall consistency of the dynamical timescales.
3) It would be valuable to perform similar N-body simulations without DM, with gravity law modified to include MOND terms, and other mixed simulations with DM and MOND together in order to verify whether the phenomenon can still be reproduced.
Christine
Wow. I wanna be a theoretical quantum-dynamicist! First I can learn how to spell it!
But first I have to finish my first career and get the Kids through college. Oh well.
So if there is dark matter way out there billions of light-parsecs away, what about our local dark matter in our galaxy? It is a wee bit closer, so what makes it unsuitable for investigation?
If the discovery confirms that there is that much dark matter, is our universe then no longer eternally exploding but will collapse in a little while, say, 50-100 billion years?
Is it just me or has this entire story taken on the feel of “Astroid to Destroy Earth in 12 Days.” Instead of being a story about strong evidence for dark matter it is being reported in the general media, and even here as absolute proof. I guess I’m not allowed to comment on the validity of science, living in Kansas.
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Sean, you refer to “… an mpeg animation of what we think happened …”
available on the web at
http://chandra.harvard.edu/photo/2006/1e0657/media/bullet.mpg
It seems to me to show, a various times t in seconds of the mpeg:
t = 0
two spheres, separated in space,
a small one on the left (call it A)
and a large one on the right (call it B)
with
blue gravitating Dark Matter evenly dispersed throughout A and B
and
red hot gas Ordinary Matter also evenly dispersed throughout A and B.
t = 2 seconds
A and B have just begun to collide, and are in contact at a point,
with blue gravitating Dark Matter still evenly dispersed in A and B.
However,
red hot gas Ordinary Matter is no longer evenly dispersed,
but has migrated in A and B to crescents opposite the contact point.
What is the mechanism that causes that migration of the hot gas?
Tony Smith
http://www.valdostamuseum.org/hamsmith/
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Christine Dantas has some interesting technical comments, which for some reason her browser would not let her post here.
You can find these comments posted here instead.
Sorry folks, in our attempts to keep the site running we ran into a commenting snafu, but it’s in the process of being fixed now.
Hi,
It seems now my post has been uploaded. Thanks!
Christine
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Jack Said
Only to a String Theorist 🙂 where beauty is paramount!
After all, there is no accounting for taste.
How is the dark matter subject of this post different from essentially invisible neutrinos passing through planets like Earth?
How does the visible gas collision differ from Cherenkov radiation?
Sean:
Does the inter-galactic gas of a “typical” cluster emit X-rays? If not then how is this gas detected? I guess I am still a little confused about the dynamics of the collision. Is there an estimate of the percentage of the inter-galactic gas that is swept from the cluster? The pictures suggest a very large percentage only if the gas emits X-rays all the time. If a large percentage has been swept out would this not affect the gravitation field of the cluster after the collision and thus cause the galaxies to may be move apart. Just wondering.
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Hi
this is a message from the Cosmic AAA – Paris Agency
Why does nobody point out the fact that the whole thing is still thought from the now very unquestionned Big-Bang point of view ? The whole thing is stuck in a mass of the universe theory that absolutely wants a total mass to be. What if there was no such thing as a dynamic equilibrium of a so-called mass of the universe ? It’s not only to be thought as a local problem but also as a global question concerning a universe that might be driven by different currents of energy of which the Big-Bang effect is only a local wave in which we are but a molecule.
If dark matter is weakly interacting, one would expect that the material takes a very long time to thermalize — especially if the only available form of radiant energy is gravitational. It would seem that we could address this question by observing the relative distributions of visible and dark matter in the lensing regions. Specifically, after yanking a nontrivial fraction of the mass of the cluster out (the gas), the remaining visible and dark matter will expand slightly, relaxing out into the slightly shallower gravitational well. Same for the dark matter. The rate of expansion should depend on their “temperatures”. In fact, the diffusivity should scale as T^(3/2) M^(-1/2) P^(-1) d^(-2), where T is the temperature, M is the RMS mass of the particles, P is their pressure and d is their (effective collision) diameter. Note that for the visible matter and the typically proposed dark matter, P ~ 0, and d ~ 0, so both diffusivities should be huge. We may be looking too late after the event.
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If MOND modifies gravity, how would that affect gravitational lensing?
Would that change the weak lensing results for the Bullet Cluster?
I thought that higher energies corresponded to shorter distances in QM because of diffraction — to probe shorter distances, you need particles with smaller wavelengths to create a sharp image. But the de Broglie wavelengths of the things astronomers study must be unimaginably small; how could diffraction effects be relevant? I can’t imagine objects the size of galaxies exhibiting quantum interference…
What’s the catch?
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