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.
How do we know that these galaxy clusters are not accompanied by huge numbers of black holes, invisible to us, and other ordinary matter that we can’t see from here, causing the observed lensing?
Roger (#76):
In many clusters, there is an extra-large, extra-massive galaxy sitting at the approximate center of the cluster (often what’s called a cD galaxy = central dominant galaxy). Given that such galaxies probably have their own supermassive black holes, that’s the closest you get to “a supermassive black hole at the center of a cluster.”
But these supermassive black holes really aren’t all that massive, relative to the galaxies and clusters. Central black holes in galaxies have masses that are generally less than 1% of the visible mass of the whole galaxy (that is, the mass of all the stars in the galaxy). The hot intergalactic gas (that’s emitting the X-rays) is, in turn, about 5 times the mass of all the stars in the cluster’s galaxies — and yet the estimated dark matter in a cluster is about 5-10 times the mass of the stars + hot gas. So the “supermassive” black holes are a tiny, tiny fraction of the total.
Maybe this is the first direct evidence of mirror matter. For mirror matter theory see
Mirror Matter and
The Mirror Matter Webpage . If mirror dark matter really exits, it might be even technologically useful:
.
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.
Good job no one is doing that, then. What they’re actually doing is taking the dark matter model, working out what ought to be observed if it is true, and comparing to reality. You know…science.
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.
So…new physics via new particles = magic. New physics via new/modified forces = good science.
Well, feel free to use your personal incredulity as a compass to reality if you wish. I’ll stick with that ‘science’ thing; seems to work better.
Good job no one is doing that, then. What they’re actually doing is taking the dark matter model, working out what ought to be observed if it is true, and comparing to reality. You know…science.
While I don’t disagree with you necessarily, simply shouting ‘observations!’ doesn’t actually prove a theory to be more or less valid. DM theory does have a lot of evidence that supports it under our current knowledge, but still has areas where it doesn’t seem to work or at least doesn’t work as should be expected, especially at smaller scales. Also, MOND does appear to fit in many situations… this instance not being one of them of course, which illustrates that it certainly isn’t a complete theory, but not necessarily that it is entirely wrong.
I personally feel that DM theory is a much better fit to what we’ve observed of our universe than most other theories so far brought forward, but I’m more than happy to keep an open mind and be cautious of seemingly complete answers. Until Lavoisier showed the relationship of oxygen in regards to combustion, Phlogiston theory was considered to be fairly sound, was well accepted, and matched well with what science at that point could readily observe.
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Aaron F: features on very short distances are relevant to larger scales only if you can construct some information transfer between the small systems and the larger systems. In other words, a change in the state of the small system must induce a change in the state of the large system. However, as you point out, in order to probe (or if you like “interrogate”) the small scale system, you need particles with a short enough wavelength to scatter off the small system. Hence the larger energy.
You can call it diffraction (which is correct) but that masks the more profound point of how variations on small enough distances become irrelevant to larger systems. In particular, we can keep coherence in quantum systems (a big deal in quantum information theory), even though we necessarily trace out (= ignore) all effects of string theory (ignoring one of two interacting systems and looking only at the other typically destroys coherence).
Dear Richard C,
These observations are inconsistent with MOND, and there’s no way to make MOND compatible with them. In contrast, other observations, like the rotation curves of galaxies, can be explained by dark matter and MOND. However, these can only be explained with dark matter. You don’t have to keep an open mind about it. The location of the mass is derived from the lensing maps. The mass is clearly not located where the dominant source of baryonic (not dark) matter is located (this is the X-ray emitting gas). There is no way to make a theory where gravity follows baryonic matter compatible with these observations.
This would be a big discovery had it not been based on photomanipulation. Normally I would be eager to embrace such a revolutionary finding but Im afraid I just dont buy it.
I was disappointed to find this article much more personal and informal than I am used to. It read more like a teenager’s diary than a scientific examination. Unfortunately in cosmology, pictures arent enough no matter how much they’ve been photoshopped. Applying filters and other types of detection does not explain something that requires heaviy modification of relativity and I find no justification for that here.
Some galaxies have higher than expected gravity…. ok, have you considered our measurements could be wrong? Perhaps there is some phenomenon at work which has yet to be discovered. Suggesting we make major modification to relativity just so one theory will fit instead of another is ludicrous and decidededly NOT GOOD SCIENCE.
Ive seen nothing to prove or disprove the existence of dark matter or dark energy other than the fact that some people choose to use those terms as a scapegoat for something nobody yet fully understands. From what I can see, it is still pseudoscience.
Christopher — your post reveals some rather severe misunderstanding on your part. First and foremost, the result is most certainly not “based on photomanipulation.” The images present in Sean’s excellent post and in the Chandra press releases are a representation of the data and a representation of the information the data convey, but they are most definitely not the sum and total of the data. You can be assured that observational astronomy does not consist of applying filters in Photoshop. The sophisticated technique used to determine the dark matter distribution actually depends on relativity, and the dark matter hypothesis does not demand modification to relativity. That’s actually why many people prefer the idea of dark matter, since it represents an unknown quantity as opposed to a massive misunderstanding of gravity. I am also a bit confused by your criticism of the ApJ letter; as Sean pointed out, it is a Letter and therefore leaves some potential questions unanswered, but I suspect that you are actual criticizing the press release and not the Clowe et al. paper. The press release is in fact informal, as that is its function.
Finally, it is fundamentally incorrect to equate “science in progress” with “pseudoscience.” In fact, I would argue that it is the “in progress” that actually defines the scientific method. All science, astronomy and cosmology — here, not only the question of dark matter but the perhaps less exotic study of cluster dynamics — involves the comparison of a body of ideas with a body of evidence. Remaining open to the possibility that new data will contradict the prevailing idea (or “the more natural idea” or the most attractive idea) is an important part of what you call “not good science.”
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What would the consequence be for “civilizations” that may exist within the both Galactic Groups in question?
Would they detect their Galaxies as having a Dark Matter Halo, or would they be more inclined to percieve a Positron Halo around their respective Galactic homes?
There is another problem with the images and data, one can produce a perfect match for the Galaxies to have really just RE-BOUNDED away from each other after impact?
Like two billiard balls colliding, unless you follow both balls for a great length of time (before impact-just after impact ) you can have no way of telling if there was any “passing_through” interaction.
As we are at a far away location, our observation and data would fit Dark Matter with a “rebounded” effect just as an infered “follow-through” effect.
I give an interesting model of dark energy in my new book Quantu Motion. Further references can be found in my website http://www.quantummotion.org/
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Electomagnetic radiation does’nt work, so we have to look at the affects of the matter through gases and gravition? It should be detected direclty, like all matter (neutrinos).
Neutinos are contained in all matter.
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thanks for the black holes
Do you know that this universe name is DARN and that our twin universe name is DAL??? And that one cosmos has 8 universes and that they work in pares? and that a Transfinite creates 108 cosmos??????????
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Great post, great comments.
How about Tony’s question? I see the same thing.
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*newbee question*
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).
Tony Smith (#82) and ann miller (#121):
There’s a partial explanation of the animation here, in the captions for the various images and animations (caption 5, down near the bottom of the page).
What you’re seeing in the first few seconds of the animation isn’t the gas “migrating” to one side or the other; the dark matter blobs (blue) are moving faster and leaving some of the gas behind (if you look carefully, you’ll see that the gas is moving in the same direction, just not as fast as the dark matter).
Most importantly, the beginning of the caption calls the animation “an artist’s representation of the huge collision in the bullet cluster” (emphasis mine). To my eyes, it’s clearly not a genuine (physics-based) simulation. In fact, if you scrub back and forth, you’ll see that the last four seconds of the main animation are just two unchanging composite objects (left-hand DM + distorted gas and right-hand DM + distorted gas) sliding apart.
So it’s really just an animated cartoon, so to speak, of the underlying physical processes, and all sorts of fine details will be missing or wrong.