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.
What if dark matter doesn’t exist at all?
why can’t these gravitational lensing effects and speeds of rotation of different parts of galaxies be explained by something else?
MOND seems to imply that gravity is the same everywhere in the universe but just acts differently for very low accelerations.
the universe is very large, and very old… what is stoping forces from being variable, either in time, or space?
it could be that gravity is stronger in some regions of the universe and weaker in others. Or maybe gravity has been changing throughout the history of the universe. it may have started off stronger and has gradually become weaker. this could explain why the old light we are viewing from distant galaxies appears to need a better (different) gravitational explanation than what works here on earth.
My interest in theoretical physics is purly amature, so please help me understand if/why a spatially variable or temporarly variable theory of gravity (or both) can’t be the answer.
Michael Morrison, I’m also an amateur in science. I think the reason for scientist to avoid variable laws of nature is that it would make science so much harder. You could hardly call it a law if there has to be special exceptions for every observation…
One of the great goals for physics today is to combine the laws of big objects (planets/stars/galaxies) with the laws of small objects (electrons, quantum particles). The main problem is that current law of gravity (Einstein’s general relativity) doesn’t work on the very small scale.
To visualize this: Think of your car as the law of gravity. When you drive on the highway everything works smooth and perfectly normal. And then if you make a turn on to a tiny country road, your car is mysteriously transformed to a crazy donkey that kicks and bites you. This can’t be right; you must buy a new car! (Or maybe redefine the definition of a road ๐
Currently there are three candidates for the Dark Matter:
1) Brown Dwarfs, nicknamed MACHOs (MAssive Compact Halo Objects).
2) Supermassive Black Holes.
3) Non-baryonic matter, called WIMPs (Weakly Interacting Massive Particles).
More info:
http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
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I am actually somewhat puzzled why all astronomers are so confident that they know the mass of the ordinary matter (i.e. the mass of stars) in a galaxy so exactly. The ‘known’ figures are largely based on the apparent luminosity of stars and the (more or less empirical) mass-luminosity relationship. It is obvious that any errors in the latter will have a crucial influence: according to the mass-luminosity relationship, a star with half the mass has only 1/10 of the luminosity, so with 10 times as many stars of half the mass, one would have the same overall brightness but 5 times the overall mass. Looking at http://planetquest.jpl.nasa.gov/SIM/science_henry.pdf , one finds indeed that the luminosities for stars less than 1 solar mass are uncertain by about 2-3 magnitudes (i.e. up to about a factor 10). It is quite remarkable that the mass luminosity relationship, which a) is quite uncertain for low mass stars, b) obtained only in the solar neighbourhood and c) obtained only from double stars, is applied to all stars in our or other galaxies regardless. I don’t therefore think that the observations justify the conclusion of dark matter here. There might be much more mass in the form of ordinary stars than thought.
With regard to the ‘dark matter’ conclusions based on the observations of the motion of gas (rather than stars) in galaxies, see also my webpage Galactic Rotation Curves and the Dark Matter Myth.
Thomas
Thomas Smid, on your website you also mistrusts Curved Space and General Relativity among a lot of other scientific achievements. Isn’t that pretty bold…? I mean, for General Relativity and Curved Space we have darn good physical proof.
I’m no physicist, but is anybody taking into account the gravitation fields created by all the energy in those galaxy clusters? e=mc^2 says that energy has mass equivalent, where mass = energy / (speed of light)^2. Even if you just consider the potential energy bound up in the mutual attraction between the individual stars, that is a LOT of energy. Would the mass of all that energy be enough to account for the gravitational effects we’re seeing?
On the contrary, it is bold by astronomers to arrive at the ‘dark matter’ conclusion considering the uncertainties I addressed in my post above.
With regard to Curved Space and General Relativity: I not only mistrust these, but I know that these are flawed concepts. This can be said solely on the basis of conceptual theoretical consistency (as explained in more details on my web-pages about Cosmology and Relativity (see under General Relativity)). The bending of light that you mentioned above can therefore not prove a curved space. It could at best indicate an effect of gravity on light, but, given the electromagnetic nature of light, it is much more reasonable to assume that it is caused by electric rather than gravitational fields (see the page Plasma Theory of ‘Gravitational Lensing’ of Light on my other site plasmaphysics.org.uk).
Thomas
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Ok Thomas, I’ll bite:
The X-ray emitting gas clouds in these clusters are really fully ionized plasmas. They have baryonic masses 10 times higher than the galaxies, and because they’re fully ionized I would expect they’d cause lensing effects as well (and probably be the dominant source in galaxy clusters) if the lensing was caused by electric rather than gravitational effects. So why when the plasma is seperated out from the galaxies does the lensing effect stay with the galaxies?
Also, regarding the stellar masses – look up the micro-lensing results (the MACHO project, EROS, etc) which exclude sub-solar mass objects from making up a significant fraction of the mass of the galaxy. Even if you were to use your physics of electric rather than gravitational effects for the lensing, you could still compare the rate at which microlensing occurs toward the galactic center to away from it to see that there is not a population of low mass objects filling the disk or the halo.
Skeptic23,
As I mentioned already further above (post 156), the mass of the stellar matter in galaxies may be underestimated considering the uncertainties associated with the mass-luminosity relationship used to derive stellar masses. Also the mass of the hot gas may be overestimated as its determination also may involve questionable assumptions.
With regard to your ‘Macho’ argument: I was not suggesting that there is a substantial amount of invisible objects, but simply that, due the usual mass luminosity relationship being incorrect, the mass of a star with a given luminosity is being underestimated (especially for low luminosity stars).
Thomas
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The problem with your ‘electromagnetic gravity’ is that if you let a peanut fly by the sun, it will also bend over. As far as I know peanuts are not electromagnetic, nor iron-based. But if you really have solid proof that Einstein was actually wrong, I strongly recommend a scientific publication. The smell of Mr. Nobel is substantial.
#158. Walter Brameld IV,
I was just talking to Twostein III and he explained the relation between energy and mass for me. Your hypothesis is appealing, but most probably wrong according to Twostein III. Here it goes: Mass = energy – same thing different form. But, the (very strange) thing is – energy in its form doesn’t have any mass. (or maybe that goes without saying?) If energy had mass it would not be energy – it would be mass (mamma mia I’m going crazy here!).
Try weighing one pound, or one kilogram of radio waves, or one kilogram of light waves – how much is that? The trick to have light travel by 299.792.458 m/s is not to give it any mass. If light had mass it would never reach the maximum (speed of light) since the acceleration would make it heavier and heavier and heavier. Acceleration = gravity (on mass), and anyone with a decent car can feel that acceleration/gravity, on the way to work (one late morning).
This of course is only a theory, created by Twostein’s father Einstein, but it’s the best we have and no one has for a 100 years proved it wrong (to make Thomas Smid calm ๐).
Regarding posts 14 and 16 in this thread:
The new papers don’t discuss the scattering cross section, but Markevitch, et al (astro-ph/0309303, ApJ 606, 819(2004)) obtain a limit from the older data:
σ/m 2/g
This is obtained by requiring that the typical DM particle not have scattered as it passed through the cluster.
What does this tell us about the properties of the DM? Consider two cases:
1:The DM is “rocks” (including stars and “dust”). By this, I mean matter whose density is about 1 g/cm3 and whose scattering is approximately given by its geometrical cross section.
The Sun has: m = 2×1033 g and R = 7×1010 cm. Thus, σ/m ~ 10-11 cm2/g. So, stars will pass right through, as is seen in the images.
For rocks, σ/m is inversely proportional to R, so DM rocks are constrained to be larger than a few centimeters.
2:The DM is elementary particles of mass about 100 proton massess (in the range favored by supersymmetry theorists). That is, m = 2×10-22 g. This implies σ -22 cm2. This upper limit is huge, much larger than any observed scattering cross section of elementary particles. The data is more than ten orders of magnitude away from testing the most popular DM models.
Sorry for the garble. The cross section limit is: σ/m < 1 cm2/g
Jon, thanks!
This means that the limit from the bullet cluster is not competitive with other limits from simulations of galactic DM which is about a factor of 10 lower.
Hey Sean,
Are you sure this proves that there is dark matter there?
Could it not be the halos of whatever fields mediate the modified gravity in a modified gravity theory? In other words, in a modified gravity theory there is some field that modifies the geodesics around galaxies/clusters.
What happens to that field when two galaxies or clusters collide?
I don’t think we know, but it is certainly possible that the “field halos” pass
through each other much like dark matter. Indeed, in a sense perhaps they ARE dark matter, but not made of particles.
So, I would agree that this proves that there is something there other than the baryons, but that may or may not be conventional dark matter.
Glenn
A peanut, unlike light, has mass and will obviously be subject to gravity.
Thomas
Hi Glenn–
What the observations demonstrate is that there are some independently propagating degrees of freedom, not merely tied to the ordinary matter, that are sourcing a substantial gravitational field. To me, that’s more or less the definition of “dark matter.” Of course they may have all sorts of exotic properties that are worth investigating.
Regarding Jon Thaler’s comments:
There was a proposal a few years back (from Spergal and Steinhardt) that the problems with CDM (too much substructure, cuspy cores that have never been observed, etc) could be solved using strongly-interacting dark matter. The proposed theoretically allowable range was from 0.5 cm^2/g (below this there weren’t enough ineractions to help) to 5 cm^2/g. So an upper limit of 1 cm^2/g is cutting out a large part of this range.
While there were other observations which could produce lower upper limits (the one that comes to mind is an observation of an elliptically shaped strong lensing system by Miralda-Escude, don’t know offhand which galactic limits you are referring to) they all had certain assumption regarding lack of substructure along the line of sight, etc. The bullet cluster observations have the advantage that you know exactly what happened, so those upper limits can be made without caveats.
#142: I’m not a scientist, or even really scientifically inclined. But I found the three questions interesting: they’re some of the few challenge-questions in the thread that are both comprehensable and persuasive to a layman. Thank you for those, I’ll be puzzling over them for some time!
For the second question, though, I _think_ the answer is something like “the clusters themselves are on the same plane (the same distance). In their current form they do not match any morphology, and _do_ look rather like two vehicles after a crash: disturbed exactly as if there was an impact/pass-through.” That argument is probablistic. But you have things here, moving at different rates, so the probabilities are, ummm, astronomical. ๐
#142: I’m not a scientist, or even scientifically inclined. But I found the three questions interesting follow-up on a great article: they’re some of the few challenge-questions in the thread that are both comprehensable and persuasive to a layman. Thank you for those, I’ll be puzzling over them for some time!
For the second question, though, I _think_ the answer is something like “the clusters themselves are on the same plane (the same distance). In their current form they do not match any morphology,” — or whatever the technical wording is for ‘galaxies do not form like this’ –” and they _do_ look rather like two vehicles after a crash: disturbed exactly as if there was an impact/pass-through.” That argument is probablistic. But you have massive collections here, parts moving at different rates (eg, the strange occurance of gas seperated from the galaxies), so the probabilities are, ummm, astronomical. ๐
#171: I apologize. I had forgotten about the Strongly interacting DM proposal (astro-ph/990938). A more recent paper (astro-ph/000634) by Wandelt, et al. (including Spergel & Steinhart) concludes:
"The favored dark matter candidates, axions and neutralinos, are effectively collisionless and, hence, are in some considerable jeopardy. The Spergel-Steinhardt proposal has stimulated the interesting possibility that dark matter consists of particles that interact through the strong force with ordinary matter. Our reevaluation of constraints leads us to conclude that the exotic hadron possibility is now ruled out for a substantial range of masses near 1 GeV and cross-sections near 10รขหโ24 cm2, eliminating some of the most attractive possibilities. At the same time, the re-evaluation has re-opened a region encompassing larger masses and cross-sections previously thought to be ruled out."
This is based on the lack of observed cusps in DM halos. (I have been told that this is a controversial result, but I’m not an expert.)
#173: Thanks Robbie! (My face is turning red; the big tribute should of course go to Sean and Cosmic Variance, but thanks anyway!)
Your answer for the second question is probably correct. But I’m not completely satisfied, Sean wrote: “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.”
If you look at this high-resolution (1.18 MB) on Visible-Light and X-Ray Composite Image of Galaxy Cluster 1E 0657-556 you see no ‘crash-morphology’ in the cluster formation or any single galaxy – as Sean pointed out; (the galaxies) passed right through. On the other hand when galaxies do get close we can see a dramatic ‘crash-morphology’ as in this picture.
Maybe your answer rise a new question:
4) Why isn’t the two galaxy clusters more affected by mutual gravitation (or maybe they are to a pro)?
It would be very interesting if Sean (or any other professional) would answer the questions in post #142, but he is probably a very busy man. In the meantime, I will also puzzle along! ๐