“The Universe is made mostly of dark matter and dark energy, and we don’t know what either of them is.” –Saul Perlmutter
When I was starting out as a graduate student, one of the most exciting (and daunting) tasks facing me was to piece together a scientifically accurate and useful picture of the Universe, including its composition, structure, and history. (And I owe a huge shout-out to my PhD advisor, who helped me immeasurably in that task.)
The big question facing me, as far as I was concerned, was deciding whose ideas were right, and which were the ones I should spend my time and energy pursuing?
Make no mistake about it, there are lots of good ideas out there. But having (or exploring) a good idea is not the same as good science. For a good idea to make the leap to being a scientifically good idea, it needs to do both of the following things:
- It needs to explain already observed phenomena at least as well as the presently accepted and used ideas.
- It needs to make new predictions. A new prediction either explains other, hitherto unaccounted-for observations or gives you something unexpected to look for.
A classic example is Einstein’s theory of gravity, general relativity. The big idea was that matter and energy curved spacetime, and that this curved spacetime was the cause of all the effects we attribute to gravitational force.
But it didn’t just explain all of the things that the old theory, Newton’s gravity, explained. It also predicted an anomaly in Mercury’s orbit, which had been observed but was hitherto unexplained. But additionally, it also made a brand new prediction: that near very massive objects, starlight would appear to bend!
Einstein completed his theory in 1915, and fortuitously, there was a total solar eclipse coming up in 1919, where this new prediction could be tested! You can read some details about this observational test of general relativity, or you can just read the New York Times’ headlines reporting the results.
That was an easy one. And although general relativity is now popularly accepted, it took an awfully long time before that was the case. (Some of my older readers may even remember that!)
So I started learning. Looking at the predictions of different theories, looking at their ranges of validity and where they broke down, learning about the different observations and their uncertainties, and concluding what was probable, what was possible, and what was ruled out. And a cohesive picture started to come together. I learned how to measure the distances to different objects, both in and out of our galaxy.
The conclusion drawn from that? The more distant objects were, the faster they were receding away from us. This leads to Hubble expansion, and the picture that the Universe was expanding and cooling, and therefore was hotter in the past. What does that lead to?
The Big Bang! Although many alternatives are plausible based on Hubble expansion alone, the Big Bang was the theory that made the correct predictions for phenomena like the Cosmic Microwave Background and the abundances of the light elements, while the alternatives fell by the wayside.
But there were still some serious issues with the structures that formed in the Universe. On small scales, we measured how quickly individual galaxies rotated, and found that our predictions of how quickly they should rotate (line A, below) didn’t match up with how quickly they actually rotate (line B, below)!
Two ideas quickly emerged to deal with this problem, originally called the “missing mass problem”. One was Dark Matter, or the idea that there was more matter in the Universe than just protons, neutrons, and electrons could account for. And the other was to modify gravity, saying that the laws of gravity could be tweaked to account for these observations.
And neither one of these options appeared to be very satisfying to this young astrophysicist. (Which must have been maddening for his advisor!)
So we look for more evidence. These two ideas give different predictions for what happens on larger scales than galaxies. According to dark matter, there’s a diffuse, massive halo around every gravitationally bound structure, while according to modified versions of gravity, the laws only become different at very small accelerations.
Both versions provide an explanation for rotating galaxies, and to be completely honest, the modified gravity version is slightly better at that. But how do we decide which one’s right?
We have to make more predictions and more observations, and see which one works better. First off, we take a look at clusters of galaxies (above). Again, there’s a discrepancy between what happens with standard gravity and normal matter and what we observe; the galaxies fly around too quickly for how much normal matter is there.
Second, we can look at the large-scale galaxy distribution. How do these galaxies cluster? There seems to be not enough mass to produce the structures that we see, unless we either include dark matter or modify gravity.
What are the results?
Dark matter works great for both of these, while modified gravity doesn’t do a very good job with the clusters and fails abominably at reproducing the large-scale structure of the Universe. (If you want a little more detail, Silk damping is far more important in modified gravity than observations allow.)
There were also the anisotropies in the cosmic microwave background, which heavily favored dark matter and which modified gravity couldn’t explain, which was yet another piece of evidence pointing towards the same conclusion.
So I was in favor of dark matter, but I wasn’t entirely convinced. I wanted a “smoking gun” piece of evidence for dark matter. Something that was an entirely new prediction that we could look for — much like that 1919 eclipse was for general relativity — and decide whether dark matter predicts what we’re going to see.
Well, here’s one for you. What would happen if you took two of these clusters of galaxies and smashed them together?
The individual stars and galaxies should be like bullets fired towards one another: very small and very unlikely to hit one another. In other words, the light-emitting parts of the galaxies — the stars — should pass right through one another. But most of the normal matter — the gas and dust — should collide, smash together, and heat up.
According to a Universe without dark matter, most of the mass should be where these galaxy clusters smash together, while if these clusters did have dark matter halos, those halos, much like the individual stars, should pass through one another undisturbed.
Well, I had the good fortune of being in graduate school at the same place where this guy was working at the time. He was part of a group that examined the cluster above, known as the Bullet Cluster. These are two clusters of galaxies that have just recently collided into one another!
You can look at the gas in them, too, by looking in X-ray light. Where are the X-rays? If these clusters just recently collided, they ought to be in between these two clusters. Let’s take a look (in pink), and see what we get.
That’s just what we expect. But now we have to ask, where’s the gravity coming from? If there’s no dark matter, the gravity should line up with the gas. But if there is dark matter, the gravity should line up with the individual stars. Let’s take a look at what the mass — due to observations of gravitational lensing (a verified prediction of general relativity) — is telling us.
Not entirely convinced? Let’s overlay all three images atop one another and see what we get.
That was the last thing I was waiting for: a new prediction made by dark matter, where the observations gave me exactly what the predictions told me they should.
In other words, before the collision, gravity, stars, and gas all lined up. After the collision, the gas (which is where most of the normal matter is) doesn’t line up with the stars. But gravity does. This only works if there’s some extra type of matter that doesn’t smash together and collide like normal matter (i.e., protons, neutrons, and electrons) does.
And that’s what convinced me that dark matter’s a far better explanation than anything else out there for what’s going on, gravitationally, in the Universe.