"Forget it. I didn't have that thing inside me where I wanted to smash against somebody and watch them break. I was too sensitive for that and disliked being that sensitive." -Josh Brolin
How do you figure out what something is made of? You take it apart -- cracking it open if necessary -- and look inside. This is something we've been doing since... well, since before our ancestors were even human.
For most things here on Earth, that's easy enough. If you wanted to go down to the smallest scales possible, however, it becomes harder and harder to "crack" those things open. To look at very small microscopic structures, you need something like a focused beam of high-energy electrons to probe them.
To examine even smaller scales, to the sub-molecular, sub-atomic, and even sub-nuclear scale, you have to go to progressively more energetic particle accelerators, colliders and detectors to get there.
By knowing what went in and examining what comes out, we can reconstruct to an extraordinary level of accuracy exactly what took place at every step of the way. At the end of the day, we learn not only what goes on inside of these subatomic particles, but what the fundamental particles that make up what we know as "matter" are, as well as the interactions that govern them.
But what about the largest bound structures in the Universe? Bigger than planets, stars, solar systems, star clusters or even entire galaxies, I'm talking about clusters of galaxies, where anywhere from a few to thousands of large galaxies are clumped together through the irresistible force of gravity. What are those made out of?
You might naïvely think that they're made out of stars; and of course that is a part of the story. It's the only part of the story you see, but it is by no means the entire story. Because we know how stars work -- from the inside out -- when we measure the total amount of light across a slew of different wavelengths of a galaxy, we can figure out how much mass is in those galaxies in the form of stars. We'll call that number, the mass of the stars in a cluster of galaxies, M*.
But there's another way to measure mass, and that's by looking at gravity. We have a bunch of different ways to measure gravitational mass of a cluster of galaxies, ranging from inferred mass via gravitational dynamics (from looking at the velocity of individual member galaxies) to gravitational lensing, where background light is distorted due to the presence of intervening mass. That will give you the total amount of gravitational mass in the cluster: we'll call it MG.
Well, what do you suppose happens when we compare M* to MG? If stars were responsible for all the mass of these clusters, these two numbers would be equal for each and every cluster. But not only are they not, they're not even close! In fact, in pretty much all clusters we have good measurements of, MG is about 50 times larger than M*.
What's even more spectacular is that, according to Big Bang Nucleosythnesis -- the measured vs. predicted abundances of Hydrogen and Helium in the young Universe -- the total amount of protons, neutrons, and electrons (i.e., all the normal matter in the Universe) is only about 7 or 8 times larger than M* is measured to be. In other words, not only is most of the matter in the Universe not in stars, it's not even the same type of matter we know exists!
And the gravitational measurements don't lie; they're a direct consequence of Einstein's theory of relativity. Here's how it works.
Image credit: Mike Hudson. Research page available at http://mhvm.uwaterloo.ca/.
The nearby galaxies/cluster causes the light from all objects behind it to be bent and distorted in a very specific (and mass-dependent) way, known as shearing. When we look at a cluster of galaxies, or any massive object with light sources behind it, we can infer both the magnitude and distribution of the mass in a very straightforward fashion by making this measurement.
One very famous example -- in a picture you might not recognize -- is shown below.
You can clearly see the two big clusters of galaxies (left and right), and the mass reconstruction (in contours) surrounding them. To no one's surprise, the mass follows the two clusters of galaxies, and that's where the light is bent.
But what this image doesn't show is that these two clusters have just recently (in cosmic terms, anyway) collided, and what happens when two clusters collide? Well, remember, they're about 15% gas and 2% stars, and the rest this ill-understood "dark matter." Just like two rounds of bird shot fired simultaneously at one another, the stars themselves will mostly pass through one another unhindered, but the gas should splat together like two globs of pancake batter, shown in pink, below.
The dark matter, shown in blue in the simulation above, should pass right through everything, staying linked to the stars that passed right through as well. The wonderful thing is that when two globs of gas smack together at these speeds, they heat up sufficiently to emit X-rays, something the Chandra X-ray telescope can detect! When we combine the actual measured data of this cluster -- known as the Bullet Cluster -- from Chandra, Hubble, and gravitational lensing, here's what we find.
Image credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.
So there is dark matter, because stars can't be responsible for the mass, and the gas (which is most of the normal mass) isn't where most of the gravitation is!
But one cluster isn't proof; this is science, and that needs to be repeated! So I can point to another cluster in a different stage of collision and merger: the Musket Ball Cluster!
Good separation of X-ray emitting gas and stars+dark matter; we're good here. But what about a much more complicated collision? Let's take a look at Abell 520, known colloquially as the Train-Wreck Cluster.
This might actually have a problem! It's hard to tell from the image above, but if we took a close look at the optical, lensing, and X-ray components separately, what we find is that the luminous matter (stars) exists in clumps, the X-rays bunch in the center, but the gravitational lensing occurs both around the stars -- where we expect it -- and also around the X-rays, where we don't expect it!
This, if true, would pose a big problem for standard theories of dark matter! When I wrote on this before, I noted that this was a real puzzle that needed serious attention paid to it.
“This result is a puzzle,” said astronomer James Jee of the University of California, Davis, leader of the Hubble study. “Dark matter is not behaving as predicted, and it’s not obviously clear what is going on. Theories of galaxy formation and dark matter must explain what we are seeing.”
Luckily for us, serious attention was paid to it, by a team led by Douglas Clowe, one of the experts on mass reconstruction and weak lensing. Results?
The new lensing results (with new composite telescope data) find almost the exact same results for the mass distribution as the old results. There's just one big difference, and this difference is incredibly meaningful.
From D. Clowe et al. (bold emphasis mine):
We do not detect the previously claimed "dark core" that contains excess mass with no significant galaxy overdensity at the location of the X-ray plasma. This peak has been suggested to be indicative of a large self-interaction cross-section for dark matter (at least ~5σ larger than the upper limit of 0.7 cm2 g–1 determined by observations of the Bullet Cluster). We find no such indication and instead find that the mass distribution of A520, after subtraction of the X-ray plasma mass, is in good agreement with the luminosity distribution of the cluster galaxies. We conclude that A520 shows no evidence to contradict the collisionless dark matter scenario.
Like I've told you every time we've looked in depth at any perceived problem with the standard model of dark matter cosmology, Dark Matter wins because it works!
There is no more prolific atom-smasher (and dark matter smasher) than the Universe itself. If we want to learn what it's doing, all we have to do is look, and listen to what it tells us about itself. And what it's telling us -- at every turn -- is that dark matter is real, it's there, and it's everywhere.
Most excellent article, as usual. The new results on Abell 520 are especially interesting, considering that it was the poster child for alternatives to dark matter.
Do you have a reference for the CL0024 mass distribution plot? Your caption says it's from LSST, but since they don't have a telescope or a camera yet, I'm not sure where they got their data.
From the D. Clowe article quote:
We conclude that A520 shows no evidence to contradict the collisionless dark matter scenario.
If no tendency to collide has been observed by Dark Matter, is it possible that DM particles are surrounded by a repulsive field of some kind?
One question though... If dark matter consists of particles, they should hold to the same laws of quantum mechanics as every other particle too. Ok, they do not interact with the strong or electro-magnetic force, but they have mass. So it should be possible to create dark matter particles in collision experiments, unless they are really heavy. If the particles are really heavy and our energy is not enough to produce them, they might still be produced in other collisions, for example in AGN jets with gas clouds. So the jets should loose energy faster than expected at very high energy levels because dark matter particles are produced which can not be detected. Is something like that observed somewhere?
dark matter always wins, because you want it to win. Physicists need to re-learn lessons of too many adjustable parameters and too few measurements.
"dark matter always wins, because you want it to win."
You're the one claiming that.
Got any evidence, or is it all projection?
The cosmological evidence for dark matter is strong. The reasoning from Big-Bang nucleosynthesis is persuasive. The physical evidence from particle investigation is non-existent.
Where this sits on the scale of "extraordinary claims require extraordinary evidence" - difficult to say.
New theory: dark matter is composed of large, human+ size particles called "gods". They are drifting around waiting for some sentient creature to believe in them.
The dark matter observations are correct.
But until there is an accepted theory; no one knows how to interpret the dark matter observations.
Is it A, B, or C?
The speculation is juicy; the evidence strong; but all answers are contended.
Nevertheless, excellent research.
i really wish the dark matter were replaced with "we have no friggin clue", because that, in essence is what is being said every time the discussion comes up.
copper knickers, "dark matter" IS "we don't have a frigging clue".
What clues they DO have are in there:
It's not luminous.
What, precisely, is your boggle?
@Semmel: You're quite right, and one of the three complementary approaches to identifying dark matter is accelerator-based (read, "LHC-based") production. The key parameter for this is the cross-section, that is, the rate at which a given collision (e.g., proton-proton) at a given energy can produce DM pairs.
It is straightforward (for a particle theorist) to derive a relationship for the production cross-section as a function of dark-matter particle energy. Detailed balance (just as in chemistry) can relate that to the cross-section for DM particles to annihilate one another.
...Which is another potential observational handle: observing particles produced with narrow energy bands from DM annihilation. The observation, or current lack thereof, of such narrow lines sets a limit on how large that cross-section can be.
As it turns out, the accelerator-based limits today are not competitive with the observational limits, for example from Fermi/GLAST.
That last picture of the bullet cluster, with explanation. I was just - WOW! Tried to explain my WOW-ing to my wife but gave up after a few and continued saying WOW
That means, if we observe any evidence for dark matter particles, its likely through gamma ray observations. Well.. hope we get it some time :)
I have 2 other questions related to dark matter that puzzle me since some time.
Can dark matter fall in black holes? Since dark matter does not interact with electromagnetic forces, it is not possible for them to heat up and loose their potential energy by radiation. So unless they have a direct hit on a black hole, they dont fall into it. That means, black holes consist of almost only 'normal' matter. Is that correct? If so, the relation of observable vs. dark matter changes over time (since black holes cant be observed directly). Is something like that detected?
The second question.. If dark matter is created after the big bang in the same way as all other particles, and they have to be created at very high energies, dark matter has to be formed before the other matter.. or in other words, dark matter synthesis must have stopped before the other matter was synthesized. So there should be a characteristic bump in the energy distribution of the particles in the microwave background radiation.
Semmel, anything falling in to a black hole has a very small target to hit. It REALLY DOES matter. Something not getting an orbit within the event horizon will just go into elliptical orbit.
Therefore if whatever DM is doesn't interact with the visible matter (which is why WIMPs are one commonly investigated DM) then it won't accrete in the disk and lose energy and fall in.
Od course it could be that DM does interact with matter normally and when it falls into the accretion disk, it becomes luminous matter, which takes it out of the definition of dark.
First I really appreciate you commenting out here; because I learn much from you. So thanks.
@Semmel (8 Dec 2012): WOW answered your first question spot-on; I don't have much to add. Dark matter must (again, just from detailed balance :-) interact with itself at some level, but the cross-section is tiny. Hence, the DM haloes around galaxies are virialized (the particles have some characteristic average velocity in random directions, like an ideal gas).
The CMB radiation is purely photons, not other particles. The latter interact, and therefore we don't see cosmological relics. However, the imprint of dark matter is all over the CMB! It reflects the large scale structure, which was seeded by clumps and filaments of dark matter.
See both this and the next article by Ethan, along with so many of his other discussions of the evidence we have for DM. Ethan explains this stuff almost infinitely better than I could (especially since I'm not an astrophysicist!).
Thank you very much all of you :)
"dark matter always wins, because you want it to win. Physicists need to re-learn lessons of too many adjustable parameters and too few measurements."
Well the great thing about science is that even when the practitioners "want something to win" -- which is often, but at least as often as wanting the mainstream view to win is wanting something contrary to current thinking -- the ultimate arbiter is the data.
And there's plenty of data that puts at least some serious constraints on the theory to limit "free parameters". That's why Abel 520 was considered such a problem -- the evidence it presented would have gone against the already-established parameters of Dark Matter as having a very tiny interaction cross-section.
But then a more detailed set of measurements -- gee, but I thought we didn't have any? -- shows that the cluster is not actually in conflict with other DM observations.
New data, not tweaking free parameters, caused Dark Matter to win. The exact opposite of your narrative. So who is it that wants a certain result and ignores data to get it?