“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.
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.
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.