“I’ve been noticing gravity since I was very young.” -Cameron Diaz
Yesterday, I told you about one of the simplest arguments for dark matter. We look out at the fluctuations in the microwave background on all the different angular scales we can measure — from about 0.2 degrees all the way up to the whole sky — and look at what the temperature fluctuations are doing.
We also look at the large-scale structure in the Universe, and try to correlate how mass clumps together.
We are only allowed — by the laws of physics — a few parameters to play with to try to fit this massive data set. We know the Hubble constant, the average CMB temperature, and the amount of primordial Helium in the Universe (these are the three cornerstones of the Big Bang, and are well measured by other means), so the only things we’re allowed to tweak is the “amount of stuff” in the Universe.
This stuff can be anything you can think of, and includes:
- normal matter (protons, neutrons, and electrons),
- photons (particles of light),
- dark matter (stuff that gravitationally works like matter, but doesn’t interact with photons or itself),
- vacuum energy / cosmological constant,
- exotic forms of matter (cosmic strings, domain walls, etc.), and
This isn’t an exhaustive list, but it may as well be. Remarkably, we can fit the data by tweaking these numbers, and we find that the Universe is made up of, more or less:
- 73% dark energy,
- 22% dark matter,
- 5% normal matter,
- 0.1% neutrinos,
- 0.01% photons,
- and a negligible amount of everything else.
(It is worth pointing out that part of how we measure neutrinos and photons is that things were very different in the past.)
So if the Universe has about five times as much dark matter as normal matter, where is it? Since gravitation is always attractive, it stands to reason that if we want to find dark matter, our best bet is to look at where the normal matter is. Moreover, we’d better look at where the most normal matter is.
And so your first guess would be to look at galaxy clusters; that’s where I’d look, too. (This one, above, is cluster Abell 1689.) Not surprisingly, observations of galaxy clusters were actually our first piece of evidence for dark matter, all the way back in 1933!
Now, we want to make sure we know where all the normal matter is, not just the stuff that our eyes catch. So we look in the infrared, and make sure we’re getting all of the cool gas and dust that gets heated only by reflected and absorbed starlight.
And although this image is rotated with respect to the other one, it’s pretty clear that they align very well.
What about very hot gas? That would be stuff that emits X-rays. Let’s take a look…
Well, the X-rays come from all over, but they’re centered on the largest, most massive galaxy, as well as a few other point sources.
Now, the big question is this: does gravity come exclusively from the places where this normal matter lives, or is there another source of gravity?
Well, let’s go over how we can figure this out.
One of the consequences of gravity is the phenomenon you see in the image above: gravitational lensing. When there’s a large amount of mass between us (the observer) and the object we’re looking at, that mass acts like a lens, distorting and bending the light coming from the image.
Here’s the important part: the way the light bends depends only on the total mass and its configuration. Unlike the other images I’ve shown you, it doesn’t care what the temperature is, and it doesn’t even care whether this is normal matter or dark matter. To a gravitational lens, mass is mass, period.
So take a closer look at Abell 1689. What’s going on deep inside that image?
Arcs, bent light paths, multiple images, distortions… oh my, this cluster is possibly the best one we’ve ever found in terms of the richness of lensing available.
It was only a matter of time before someone took advantage of this. The most detailed dark matter map of a galaxy cluster ever made — thanks to Dan Coe at JPL and his collaborators — is available right here.
As you can see, the dark matter often lines up with the normal matter, but not exclusively! And this is what we’d expect, because although they both experience the gravitational force, normal matter experiences collisions with other normal matter and with photons, but dark matter doesn’t.
Different forces means that, in a chaotic environment, dark matter and normal matter are likely to arrive in different locations. It’s only over the last few years that we’ve been able to differentiate where the dark matter is from the normal matter. And while it’s not as spectacular as it is in the Bullet Cluster,
It’s maybe even more important here. Because this isn’t some fantastic, rare collision of galaxy clusters happening here; this is just a run-of-the-mill galaxy cluster going about its daily affairs. And yet, dark matter distinguishes itself from the normal matter even in this cluster!
Yes, to those of you who are skeptical, I can’t prove that dark matter exists without discovering the particle responsible for it. But when I look at the entire suite of evidence, the evidence for the existence of dark matter is overwhelming. There’s still plenty we don’t understand and that we need to learn, but to go without any dark matter of any type? You have to throw out the entire Universe for that, and it just makes too much sense and works too well for me to do that without some really compelling refutation. (In my humble opinion.)
But this new data? It’s just more compelling evidence in favor of dark matter. And, if I do say so myself, an awfully pretty picture…