Dark Matter Exists
Sean at 11:52 am, August 21st, 2006The 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.
Incredible stuff!
As always, explained very intelligibly.
Do you think this will be the final work on MOND?
NM
[...] Sean Carroll has a post at Cosmic Variance on some fascinating cosmological results which seem (Sean’s take is obviously rather more authoritative than mine) to demonstrate the existence of dark matter as comprehensively as one could reasonably hope for. posted on Monday, August 21st, 2006 at 12:44 pm Post a comment [...]
So here’s something that’s been bothering me for literally years: if dark matter is dark because it doesn’t interact with “normal” matter, does it interact with itself? If so, using what forces? If not, how does it ever clump? Wouldn’t it need some repulsive — or at least scattering — force to keep the constituents of a clump from simply passing “through each other” as they collapse? Or would a clump be truly dynamic, like a three-dimensional fountain? Or is this all left as an exercise for the reader?
Thanks!
JD
Bravo, Sean! Thanks again for another excellent post.
JD– the dark matter probably interacts with itself, but only weakly (thus “weakly interacting massive particles”). And, of course, through gravity. It’s the gravitational force that causes it to clump together, and the accumulated effect of small perturbations causes the distribution to “relax” rather than just having the particles zip through and out the other side.
[...] Your mission: Go to the press release website for more information, and lovely pictures. Read Sean’s post about it on Cosmic Variance. Sit back and think about it….. Then celebrate! [...]
That’s a very nice post!
With all the extensions, my Firefox browser is a bit slow. I think I’ll use an “empty” profile to do that tabs thingy.
Sean:
If dark matter exceeds visible matter by a factor of 3-5 why doesn’t the dark matter clump due to gravity to form large structures such as stars and galaxies? BTW I realy enjoyed reading your review paper on the cosmological constant a few years back. As a layman, but retired engineer very much interested in theoritical physics, I am interested in the latest ideas. Regarding the CC: if the QFT (and ST) prediction of the CC value is off by 60 to 120 orders of magnitude what does this say about our view of the validity of these theoritical ideas vis-a-vis the vaccuum energy density?
Really enjoy your blog.
Hey Sean,
What proportion of matter in the cluster is in the stars and the like versus in the gas?
Thanx.
Aaron– it’s about 90% gas, depending on the details.
Cecil– the dark matter does clump into galaxies and clusters. Even into star-sized things, just not nearly as efficiently as ordinary matter, which can collide and dissipate its energy. But for galaxies and clusters, most of the matter is dark, not ordinary.
About the cosmological constant (which is the same idea as “vacuum energy,” just with a different name), it’s clear there’s something important we don’t know. Wish I could tell you what it is.
[...] Sean Carroll at Cosmic Variance has a very nice explanation of a new result that shows that dark matter must exist and that all of the evidence for dark matter cannot be explained away by modifying Einstein’s general relativity. This shows one of the things I really like about blogs. I am in a related field, but I have very little expertise at astronomy. This explanation goes well beyond what I could in the popular press without being a report for experts only. [...]
1E 0657-56
As predicted, the NASA telecon on Chandra results was on followup studies of the “bullet cluster”. Press release is here. Lots of pretty pictures….
MOND is dead. Long live MOND!
Sean, any news on constraints on DM-DM cross section as a function of the assumed DM particle mass?
Hi Sean,
great post, thanks for the explanation!
the dark matter does clump into galaxies and clusters. Even into star-sized things, just not nearly as efficiently as ordinary matter, which can collide and dissipate its energy.
How do we know dark matter clumps into star-sized things? Is there evidence for that?
Best, B.
Count Iblis, the actual papers did calculate a limit on the DM-DM scattering cross-section, but I don’t know the number; you should have a look at the papers directly.
B, the DM should clump on all scales, no reason why not. But we don’t have any direct evidence (and a one-solar-mass DM overdensity would be much more diffuse than a real star). Furthemore, on those scales you have to worry about being disrupted by tides etc — so I really don’t know, again you’d have to ask an expert.
Sweeeet! Loving the third picture.
I do have a question about something you mentioned:
What’s the connection between scale and energy? Are you just saying that at smaller scales, forces are stronger, and potential energies are higher, or is there something more going on?
The pictures are beautiful, but I couldn’t find a link on the Chandra site to any paper.
Several questions: how close are the redshifts of the clusters? The mass determination presumably was done with weak lensing since no arcs are visible. How many background galaxies were there for each cluster. The key result is that the mass distributions are offset from the gas. So the centers of the mass distributions must have been taken as free parameters in the lensing fits. What were the other free parameters in the fits? The key evidence if you think about it is that the lensing distribution is not circularly symmetric around the center of mass. [Recall that Bekenstein's theory was constructed to get lensing right, so we expect lensing even far from the mass distribution.] So, how exactly did they do the fits? Was their mass model robust enough to allow for a circularly symmetric profile? And if so, with what confidence is such a profile ruled out?
I know this sounds like raining on a parade, but, as perhaps Eugene was suggesting, dark matter theories themselves have died many deaths. And some of the most dramatic have been delivered with pictures [think of the early APM data of the galaxy distribution or the Moore et al. simulations of the number of satellites expected in a galaxy].
Bottom line for me: Dark matter theories are way ahead of Mond and its extensions, and this latest result is yet another hurdle. But I still think it is interesting to explore the alternatives.
A wealth of links, four nice pictures to look at closely, and a very clear explanation. This is what blogs are for!
MOND is dead! MOND will stay dead! (probably)
Scott, man, such a wet blanket. The paper is here. It’s a Letter, so I don’t think they go into much detail about your questions, which are good ones.
But I don’t think it’s quite right to imply that Bekenstein’s theory could match the lensing observations. His theory is supposed to get the magnitude of the lensing right in a circularly-symmetric profile, but I don’t see how it could give rise to two big blobs of gravitational field offset from the mass distribution. Unless there was substantial energy density in the independent excitations of the new Bekenstein fields themselves — in which case it would really just be an especially messy and unmotivated version of dark matter.
Also: even in Bekenstein’s theory it’s necessary to have dark matter (e.g. neutrinos) to simultaneously fit galaxies and clusters. The point of this new result is not “MOND can’t do it alone,” since we already knew that. It’s “no modified-gravity theory can do it alone, we need dark matter.”
Aaron, in quantum mechanics, higher energies correspond to shorter distances. So as we probe smaller and smaller scales, new high-energy phenomena come into play. Nothing analogous usually occurs when we go to larger scales, but it’s good to be open-minded.
Sean, I’ll have a look.
Scott the papers will appear 2½ hours from now on arXiv:
http://arxiv.org/abs/astro-ph/0608407
http://arxiv.org/abs/astro-ph/0608408
The first one can already be downloaded via the link Sean gave:
http://chandra.harvard.edu/photo/2006/1e0657/media/paper.pdf
I guess the DM-DM scattering cross-section is on the other paper.
[...] Sean Carroll at Cosmic Variance ; Sean is intimately involved with this work, and his discussion of it is well worth reading. [...]
Singing Vikings
On the physics front, everything is now abuzz with news that the Chandra X-ray observatory has made the most direct observation of dark matter so far. Read more about it [...]
Very nicely written and explained.
A question: My (basic) understanding of gravitational lensing is that you measure the amount (in degrees) that angle is bent, and you infer how much mass is causing the bending. Stronger masses bend light more. But the relationship between “angle of bending” and “mass” is governed by an application of general relativity. If MOND were true, would this not imply that a smaller amount of matter (or dark matter) would be able to bend light more strongly than predicted by general relativity if it passed by at a greater distance to the matter causing the bending?
Acolyte, it’s not just the angle; it’s the direction in which the light is bent. The important thing here is that the gravitational field is pointing somewhere other than where the ordinary matter is located.
Awesome pictures Sean!
My favorite is the superimposed weak-lensing/Chandra X-ray image. I’m trying to understand the bullet-shaped shock front. Is the bullet-shape the result of ordinary matter being gravitationally “dragged” by the dark matter cluster?
Yes, that would be one prediction, but it is not really relevant to this result, since perhaps 90% of the ordinary mass is in the “pink” regions in the picture above, yet the lensing is happening in the blue regions which thus have only 10% of the mass. it does not matter if there were different lensing rules, assuming they were still symmetric under the centers of mass which seems likely. Any lensing caused by regular matter would be more in the pink regions. Since it isn’t there must be something in the blue regions, something we can’t see, causing the lensing…
Shouldn’t the dark matter distribution of each galaxy be tear-drop shaped or otherwise distorted because of the gravitational interaction with the other galaxy? Or is it a result that two spherical distributions of matter that pass through each other remain spherical?
Hello Sean, Can I use some of the images of this post in my blog?
Luis, sure, just link to the NASA site linked above.
Arun, if you look closely, the DM distribution is distorted. However, the precision of the technique isn’t sufficient to nail that down with a great deal of confidence, is my bet.
Acolyte, it’s not just the angle; it’s the direction in which the light is bent. The important thing here is that the gravitational field is pointing somewhere other than where the ordinary matter is located.
Ok, I realize that the gravitational field doesn’t seem to be focused on where the visible matter is.
I don’t know much about the mathematics of gravitation, so here is a different question: is the solution unique? That is, is the only explanation for the observed bending pattern the presence of M amount of dark matter in the location shown. Or, is this hypothesis one of a family of possible solutions to the problem.
As an analogy, suppose someone throws a ball at me, straight to my left eye, but something deflects the ball so that it hits me on the right corner of my lip. So, the ball is deflected by a a certain amount in the bottom-right direction. But, this could have happened either because someone nudged the ball by a little amount shortly after it was launched, or by a greater amount as it was getting closer to me.
So, the question is: how certain are we that the thing that did the nudging is right there close to the galaxies and not somewhere else? Is it perhaps that there is a unique solution to the bending of light from all the background galaxies, so that it could only be there?
Another question: could the gravitational field be strong in the wrong place for a different reason, say, the presence of a black hole that has sucked in all the matter in its vicinity? Or can this be ruled out?
Sean, have you seen this paper? Can you comment?
Hi Sean,
The DM should clump on all scales, no reason why not. But we don’t have any direct evidence
Okay, thats also my understanding of the situation. I was just puzzled by your statement that it does clump even into star-sized things.
Thanks for the clarification.
Best, B.
Sean,
Great post, pics, links and animations.
One hopelessly anthropomophic question: About the X-rays from that 160 million degree plasma - would they fry any life on the trillions or so planets in the galaxies involved in the collision, or is the intensity too low?
Over the last several days, I created a video from a physically motivated simulation of the Bullet Cluster. Last week Maruša asked me if I could make an animation, and I agreed. I thought it would be best to run an actual simulation rather than guessing at the collision’s dynamics. I used Enzo for the calculation. This was used in the Stanford/SLAC press release. Take a look…
Bullet Cluster Visualization
Since you guys ignore the EM force which is 10^39 greater than G I think you guys should wake up! This animation shows exactly what you would expect from an EM Field interaction and the X-Rays would be the natural conclusion. This isn’t Proof of Dark Energy or Dark Matter. It is just the illogical conclusions of people who keep refusing to see the obvious electromagnetic interactions of the Universe. It qualifies more as Garbage In and Garbage Out of a computer simulation than the realization that standard Maxwell interactions predict this and nothing else really does. Of course Red Shifts will not be velocity either. They are simple optical tuning effects in space. That will crank out all of the nonsense of a failing theory.
Paul: right on! When will these people realize that there are four simultaneous 24-hour days in a single rotation of the earth? Creation occurs via opposites. Singularity is the death math of religious/academic Godism. Earth Opposites should split apart - and cascade molten lava upon God Worshippers, for they are the evil on Earth.
Dark Matter Exists | Cosmic Variance
Dark Matter Exists | Cosmic Variance
This is a really impressive article about how the existance of dark matter has now been proven. Infact if your brain parses the pictures and explanations better than mine you will even see it for yourself. Excuse me…
[proud parent voice] Our son finally made it! He got quoted on the BBC website! [wipes tears away]
Fantastic post that made sense to this layman. Great news; it really is an exciting time for your field, isn’t it?
Sean,
Informative (and entertaining) as always. You know I still pull out those GR notes of yours from time to time.
CapitalistImperialistPig,
You’re right about the intensity. The IntraCluster Medium is an interesting assembly of matter, because it is so low in density, perhaps one lonely particle per 1000 cubic centimeters, if my memory from grad school is right. It is hot, in large part, because it can not cool itself effectively because interactions between the widely spaced particles are rare. The typical cooling time is longer than the age of the universe, except near the centers of massive clusters. So the actual rate at which energy is radiated away (or at planets) is low.
[...] From Cosmic Variance: 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-twentyfirst-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.) [...]
70% of the universe is ultraexotic?
Isn’t that an oxymoron?
Add why do I live in the boring 30%?
[...] Dark matter exists!. In another example of science blogs as news sources, this story quotes Sean from Cosmic Variance. He writes about the discovery in this post. For the hardcore, the paper is now in the astro-ph archive, in multiple formats. When, oh when, will biologists get a preprint archive? [...]
I’m not a physicist (stopped taking physics once I got to college and realized that I was calculus-impaired), so this is probably a rather ignorant question, as it is likely either physically impossible or so obvious that it’s already been considered:
If it is known to be non-baryonic, is it possible for dark matter to consist of some sort of large clump of massive bosons functioning in some unknown manner?
Or would it be possible for supermassive black holes at the center of these galaxies to pass by or through each other in some manner, or maybe even slingshot around each other, with the “matter” of the galaxies either being ejected or being slowed down by various collisions and interactions, and what we’re seeing are two semi-naked black holes (which cause the gravitational lensing and would comprise the majority of the mass), flying away from each other, eventually pulling apart the cloud of matter from the collision behind them into the two red gas clumps we see in the picture?
Granted it wouldn’t explain any of the other measurements showing that that there couldn’t have been enough matter in the universe to account for its effects. Like I said, probably very ignorant, but when I think of something large and massive and invisible, that would have to come to mind.
Hi Scott and Sean,
Well, a cursory reading on the Angus et. al. (2006) paper cited by the Letter is rather interesting. In that paper, they show that you can actually construct multi-center mass distributions, with the lensing maps not projected onto the centers, in the TeVes framework. This has something to do with the complicated direct vector field interaction with metric.
In Angus et. al., they claim that for certain classes of models of TeVeS, Sean is indeed right that there exist still a baryon discrepancy (i.e. they can’t get enough lensing). However, they only consider some classes of models, and they have not fully explored all the parameter space of TeVeS (e.g. the case where the \Chi field is important).
The Letter mentioned two problems with such a construction : that the ‘multicenter’ maps that Angus et. al. constructed has all the centers lined up, and that for the Angus model to work there must exist a small central mass concentration (between the two subclusters). Neither is true observationally : the small central peak is consistent with standard Newtonian prediction and the peaks do not line up.
The bottom line is : TeVeS does not necessarily predict a lensing profile that scales simply (this will be ruled out by the many observations of colliding clusters, including this one). Indeed, Angus et. al.’s paper was to demonstrate this fact, making the point that TeVeS does not have to reduce to MOND. However, there exist plenty of parameter space to be explored. TeVeS is quite, to put it mildly, messy.
MOND probably dead as a empirical law. 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.
Actually, after hitting “submit”, I think I take back my statement about MOND is dead as an empirical law. MOND is very much alive as a empirical law that “predicts” the rotation curves. But it does not say anything about lensing. Any relativistic extenstion to MOND that predicts a simple scaling of lensing with baryonic mass distiribution is dead, but no such theory exist. (TeVeS does not predict a simple scaling in multicenter models.)
I see over and over stated that “Dark matter clumps to galaxys and clusters”… but here’s a turn of thought.
Why not have it be that matter tends to clump to regions where dark matter has clumped. Maybe it’s all chicken-and-egg thinking, but why lock ourselfs into thinking that the dark matter is running around looking for normal matter to hang out with? Bottom line, maybe normal matter is following it around.
Regardless though, great reading. My hat is off to the people that make expanding our knowlage on the extream edge possible.
Why has no scientist tried to prove the Einstein-aether theory, because in that theory free energy is possible. And capitalism wont allow free energy to be sold, because there isn’t enough money in that.
On a side note, dark matter is in pop culture alot, and we want to be able to keep using the word “Dark Matter” so we try to prove it exists, because it sound cool.
Great post Sean! I am very relieved to be rid of MOND for good and for all.
I am a bit confused about the process of releasing the results….NASA had the big press conference today, and the papers are just now appearing. Yet (as I was reminded today) Michael Peskin showed the pictures in his lecture at the SLAC Summer Institute. Obviously he had “hot” copies because part of the analysis was done here at SLAC at the Kavli Institute.
Gosh, this brings home my biggest fear of how the release of data is going to be handled at the LHC…
PS: I tried hard to link to Michael’s SSI talk,
[...] Cosmic Variance has a very readable article here that explains the results in a little more depth [...]
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.
[...] Cosmic Variance 有个很好的帖å，解释了这个事情。 [...]
[...] Cosmic Variance explain it all with much more detail. [...]
One Meeleeon Visitors
(Picture me dressed as Dr. Evil, with my pinky in the corner of my mouth.) I’ve been watching our visitor meter over the last week or so, waiting to post and thank our readers when Cosmic Variance’s one millionth visitor (to the main page…
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?
[...] Sean « Dark Matter Exists [...]
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.
[...] Una buena explicación asequible la podéis encontrar en este post de Cosmic Variance. Si os peleáis con el inglés, hay un pequeño resumen en castellano en Astroseti. [...]
“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.
Dark Matter
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
It’s really out there!!
Exactly what you’ve read, there’s no way dark matter is not there :-)!
More on the empiric proof of the existence of Dark Matter is explained by Sean, and in a lot of places. Ah, and of course, in the paper to be published in ApJL.
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.
[...] You can see the complete notice here: http://cosmicvariance.com/2006/08/21/dark-matter-exists/ [...]
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 ?
[...] Well, holy shiat… They’ve actually done it. NASA has found DIRECT proof of dark matter!! I must admit, i was skeptical of dark matter/energy. It always seemed like a convienient gapfill for not-quite-perfect theories and observations. But, dammit here it is! I wonder if its as fun as this though? [...]
[...] The nice people over at Cosmic Variance have reported that Dark Matter Exists! [...]
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.
[...] so, along with the 12 planets debacle, ther is some pretty big dark matter news. Here’s a very thorough explanation for the “proof” of the exsistence of dark matter. [...]
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/
[...] Veja mais também no Chandra Chronicles e em Cosmic Variance. Atualizaremos este post com notícias em português sobre a importantíssima descoberta assim que surgirem. [...]
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
[...] En Cosmic Variance hay un interesante análisis de las implicaciones de este suceso. [...]
Dark Matter is Real!
Holy cow. I think this is perhaps one of the biggest discoveries in astronomy since extra-solar planets were discovered.
I won’t say much about this, because Cosmic Variance did such a wonderful job that I’m just going to send you there to …
[...] Dark Matter Exists | Cosmic Variance Holy cow. This is one of the biggest days in cosmology. Actual proof of Dark Matter. This means that only about 5% of our known universe is made up of what we call normal matter! Whoa. Anyway, Einstein’s General Relativity doesn’t work too well as you move out to encompase the entire universe. So now that we can directly observe Dark Matter and its interactions, we can develop new theories that will help refine General Relativity and our understanding of the universe. I smell a Nobel Prize in Physics in the workings. __________________ Harken: Seems odd you’d name your ship after a battle you were on the wrong side of. Mal: May have been the losing side. Still not convinced it was the wrong one. Brown Coats Unite! |Firefly|Soundtrack|Serenity|Soundtrack| [...]
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.
[...] Dark Matter, Conclusive proof that dark matter exists, warp drive here i come [...]
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.
Wow, Dark Matter exists!
Sean Carroll hits a home run in explaining some recent science about showing the existence of Dark Matter. I initially had used the word prove, but in science you really never prove anything; you instead add evidence in favor of
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?
fun bits from the web
Some interesting bits I’ve found surfing lately:
*here is anamusing bit of comic commentary during my standard surfing the other day. The author makes a very funny/good point as well as addressing one of my current annoyances with comics (the wierd ph…
[...] • Chandra Press Release (includes nifty visualizations and animations) • NASA Press Release • Dark Matter Exists - Outstanding article by Sean Carroll at Cosmic Variance • Colliding Clusters Shed Light on Dark Matter - Scientific American • Colossal Cosmic Collision Reveals Mysterious Dark Matter - Space.com • Rob Knop at Galactic Interactions Filed under: Astronomy, Science, Space | [...]
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.
[...] Not being an astrophysicist, I don’t have anything clever to say about the recent discovery that seems to have located a nice big chunk of dark matter, but I really didn’t want to post any more pictures of wildlife. So here’s a picture of superheated gasses! [...]