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.
Comments
197 responses to “Dark Matter Exists”
ok, I’ll take a stab it DV –
1) The x-ray gas does have a few emission lines (highly ionized iron mostly) which you can get redshifts for, but this is not normally done (only case I think of is a z=1 cluster from some ASCA spectra, but that might have turned out to be wrong). So at some level we are taking on faith that the gas and the galaxies are in the same place. That said, we do not have any observations of cluster-sized groups of galaxies without the hot x-ray gas, and no observations of x-ray gas clouds with those sizes and temperatures that aren’t in the same place on the sky as a cluster of galaxies, so if you want to try to say they’re different objects, you have a huge cosmic conspiracy to explain away.
2) The merger is mostly in the plane of the sky, there is a reference in the paper to some spectra taken by another group showing that the velocity difference (redshift converted to velocity assuming no distance-based redshift) is about 600 km/s while the velocity of the merger is measured from the X-ray at ~4700 km/s. The velocity from the X-ray measurement is based on the supersonic bow shock, and can be measured in a variety of ways, all which agree within errors: change in temperature across the shock, change in gas density across the shock, opening angle of the shock front.
3) Because dark matter does not give off light and doesn’t interact (much) with other dark matter particles and with baryons except through gravity, it will fall into potential wells but doesn’t have a way of slowing itself down. Baryons by contrast will fall in, collide with each other to thermalize, and then emit light to cool down. In doing so they can collapse still further, allowing them to form smaller structures like stars, etc. So while dark matter is dominant mass component in the galaxy as a whole, locally around stars the baryons have much higher densities. There probably are some dark matter particles trapped within the sun’s potential, but the baryons vastly outnumber (and out mass) them.
4) The galaxies and the dark matter are affected by tidal forces during the merger, which will slow everything down (which is why these events end in the 2 clusters merging into 1 – they lose enough velocity during the initial collision to be gravitationall bound, so they will eventually stop moving away and start moving back toward each other again, then lose more velocity during the second collisions, etc). The self-gravity of the galaxies is large enough to prevent the tidal forces from ripping them apart, but likely some of the smaller galaxies will have lost some stars (although in a counter-intuitive way, the faster the collision the smaller the loss of gas and stars will be, because there is less time for the tidal forces to affect the galaxies).
#176: Skeptic23, thank you so much!! Wow, this is so damn cool, and you guys all deserve the Nobel Price for your knowledge! I’m completely satisfied, I’m not ‘Skeptic’, I’m a true Dark Matter Believer now!! 🙂
To summarize in one sentence: We know that large clouds of x-ray gas and galaxies cluster always travel together, so the clusters and the gas must be in the same location, and the great traces we see after the massive gas-clouds-collision gives us ‘bulletproof’ evidence for direction and velocity of the x-ray gas, and hence the galaxies clusters, AND the gravitational lensing we clearly can see are not were it should be – if we don’t calculate with a large amount of Dark Matter, then all pieces fall in place.
Beautiful!!
Thanks again.
[…] So for my first proper post I would like to talk about a recent development in science that I have read a few articles about recently: the finding of concrete proof for the existence of Dark Matter. For those who don’t know, the concept of Dark Matter arose in response to a long-standing cosmological problem. Essentially, the large star clusters known as galaxies do not seem from observation to contain enough mass to produce the gravity required to keep the galaxies themselves bound together. In other words, the amount of observable matter in these galaxies is not nearly high enough to keep the galaxies from dissolving (as more mass creates higher gravitational pull). However, the fact that the galaxies do not in fact drift apart means that either gravity must work differently over long distances (alternative gravity theory) or that the galaxies are in fact filled with huge amounts of invisible matter (Dark Matter). Neither theory had ever had much evidence to support it, until very recently. By studying the gravitational interactions between stars in far off galaxies (using a technique called gravitational lensing) scientists believe that they have found proof of the existence of Dark Matter. A good explanation of the experiment can be found here. What is also interesting is the response issued by proponents of the special gravity theory, which can be viewed here. The technical details of it is still a bit beyond me, but its a very interesting topic that has been a mystery for a long time, and I have found it fascinating to read about over the past few weeks. […]
[…] å¬å®ŒæŠ¥å‘ŠåŽæƒ³èµ·å‰ä¸ä¹…在cv上看到的一篇è¯å®ždark matterå˜åœ¨çš„æ–‡ç« , 很值得一读. ä¸è¿‡çœ‹åŽæ„Ÿè§‰ç»™dark matter所赋予的性质太玄乎了, 很有点åƒå½”å¹´å‘现电ç£æ³¢æ—¶çš„以太… […]
[…] Dark Matter Exists […]
[…] – and finally, they say dark matter exists. cv 1, 2 Filed under: science — […]
[…] It’s common nowadays to throw theories such as Dark Matter and String Theory. However, this is a pretty interesting read about some pretty solid evidence on Dark Matter’s existance. […]
[…] Smolin does mention a number of experimental results that have already been obtained, but none of them is naturally explained by LQG any more than by string theory, and most of them are, to be blunt, likely to go away. He mentions the claimed observation that the fine-structure constant is varying with time (against which more precise data has already been obtained), certain large-angle anomalies in the cosmic microwave background anisotropy, and the possibility of large-scale modifications of general relativity replacing dark matter. (Bad timing on that one.) I don’t know of any approach to quantum gravity that firmly predicts (or even better, predicted a head of time) that any of these should be true. That’s the least surprising thing in the world; gravity is a weak force, and most of the universe is in the regime where it is completely classical. […]
[…] Dark Matter Exists. […]
[…] And here it is. You can see the slides from the talk, as well as hear what I’m saying. I started somewhat lethargically, as it’s hard to switch gears quickly from one talk to another, but we built up some momentum by the end. I started quite broadly with the idea of different “gravitational degrees of freedom,” and worked my up to Bekenstein’s TeVeS model (a relativistic version of Milgrom’s MOND), explaining the empirical difficulties with clusters of galaxies, the cosmic microwave background, and most recently the Bullet Cluster. We can’t say that the idea is ruled out, but the evidence that dark matter of some sort exists is overwhelming, which removes much of the motivation for modifying gravity. […]
what is the speed of dark?
and does it matter?
If the relative spins of entangled pairs can change instantaneously ie. travel faster than light speeed(or so some bloke down the local told me) then how do we know that the very edge of the universe which maybe did not coallesce and clump together for the supposed time scientists allow but sped apart after the supposed hyper inflation has pulled space-time apart at faster than light speeds and therefore the majority of the universe is and always be unobservable to us.If dark matter can overcome this gravitational connundrum could it not be that this could account for the missing mass? non-scientist. Perhaps the bloke down the pub had already had far too much Guiness
This post has less to do with the specific observation being discussed and more to do with the issue of when, if at all, dark matter will be found. Ultimately, in my opinion, we will have to directly detect this stuff before we can be dead sure it is exists. Sean and friends, I have been wondering the following:
1). How soon is a WIMP detection predicted to take place? I have heard that a detection may occur during the 2007 CDMSII and Xenon10 runs, as these two experiments are moving into the “favored region” (x-section 10^-44 cm2) of the WIMP parameter space!
2). What will happen if the particles are not found even with the planned 1-ton direct detectors and not produced by the LHC?
3). Will a lack of a detection by 2020 be enough to get the cosmology community to move away from the dark matter particle hypothesis?
4). A lot of pundits say that it is a bad sign for the dark matter idea that the particles have not turned up yet. I am skeptical of these pundits; it took a lot of innovation and huge detectors to find the neutrinos. Does anyone know how long it was from when neutrinos were predicted to exist to when they were actually found?
[…] informacje “po ludzku” (aczkolwiek w jÄ™zyku angielskim) można znaleźć tutaj, natomiast dla chÄ™tnych (i majÄ…cych sporÄ… wiedzÄ™) jest dostÄ™pny oficjalny tekst (nie wiem jak przetÅ‚umaczyć “paper” z angielskiego w tym kontekÅ›cie) napisany przez ludzi którzy wykazali siÄ™ wiedzÄ… i pracÄ… potrzebnÄ… by tÄ™ ciemnÄ… materiÄ™ pokazać (dla pewnych “wartoÅ›ci” sÅ‚owa “pokazać”). […]
[…] Dark Matter Exists […]
Ooo very intresting and all, but theres so much to read and such complicated words, i just need a definition 😀 Good stuff though
[…] It is, needless to say, really cool. The image itself is not where the real science lies, of course; it’s spatially distorted, and very hard to show error bars in a 3-d plot. But there is definitely important science lurking in the details; for example, they seem to find dark-matter concentrations with little or no ordinary matter in the same place. It’ll take some work to figure out whether this is easily compatible with the theoretical models (one could imagine dissipative effects clearing baryons out of a region, leaving dark matter behind, in a mini-version of the Bullet Cluster), or whether we’re going to be challenged. Fun either way! […]
From the planetary velocities around the sun and the stellar velocities around the centre of our galaxy, there does not seem to be any dark matter in our solar system or galaxy. Why is this?
Correction 0f wrong email address on entry 194.
[…] Meanwhile, the important story this week is the discovery of direct evidence for Dark Matter. Here is something fundamentally new that expands our understanding the universe and opens up whole areas for research. The news stories dutifully reprinted the press release but the lack of in depth stories was telling. The news media could have written articles that helped educate people about what the discovery means, how it ties in to the Big Bang, Dark Energy, the modified gravity hypothesis, and so on. If any of the big boys were carrying these stories, I missed them. […]
[…] Cosmic Variance on dark matter. […]