(This is Part 1 of a series on primordial black holes!)
Do I really need to go over the evidence for dark matter again?
Okay, fine, for those of you in the back who weren’t paying attention the first time. If you’re already down with the dark matter, then you can go ahead and skip to the next section. If you have no idea what I’m talking about, then check this out.
Our evidence for the existence of dark matter goes back almost an entire century. In the 1930’s Swiss-American astronomer Fritz Zwicky – who had a quant habit of getting into verbal altercations with pretty much everyone, especially including his superiors – was studying the Coma cluster. This is a cluster of galaxies sitting over 300 light-years away from Earth.
Now after taking a bunch of measurements he discovered that the galaxies within the Coma cluster were moving far too quickly. As in, when you add up all the masses of all the galaxies, there should be a maximum speed limit driven by their mutual gravitational pull. But the galaxies were moving much faster than that limit, so the cluster should have dissolved billions of years ago.
But there it was, just…existing.
He published his study but never returned to it, probably because he was busy getting into more fights.
Forty years later, Vera Rubin was studying the Andromeda Galaxy, mostly because her colleagues wouldn’t let a woman do “real” cosmology. She found that it was spinning way too fast. Again, adding up all the visible matter gives us an estimate of the total gravitational strength within the galaxy, which sets an upper limit to how quickly things can spin around. Once again, she found that stars were orbiting much, much faster than they should be, and that Andromeda should’ve spun itself apart eons ago.
This isn’t just some small effect that you can pretend to ignore by arguing about measurement error or uncertainties. We’re talking 5-10x more gravity than can be accounted for by measuring all the hot, glowy stuff in anything bigger than a galaxy.
Unlike Zwicky, Rubin would keep at it, spending almost all of the 1970’s publishing paper after paper to show that this was the real deal, observable in every galaxy she studied. But since Zwicky discovered this effect first way back when, his name got attached to it: “dunkle materie” in German (yes I’m aware that I’m butchering the pronunciation), or dark matter in English.
But that’s all ancient history. You want more evidence for dark matter? You’ve got it. When we use gravitational lensing to weigh galaxy clusters (avoiding the need to count all the hot glowy things) we get the same answer: there’s a hidden source of gravity. The most famous example of this is the Bullet Cluster, a recently merged galaxy cluster, which shows us where a) all the galaxies are, b) where all the hot cluster has is, and c) through gravitational lensing where all the mass is. None of these line up, because when the two clusters merged together the galaxies sailed on by each other (plenty of space in a cluster to do that), the gas got all tangled up, and the dark matter just did its thing.
And when we look to the cosmic microwave background, the only way it can have the properties that it does is for some sort of invisible form of matter to be running around the early universe. You need pockets of matter that don’t get dissolved by the intense radiation of the young cosmos. If you take away the dark matter, the entire map of the cosmic microwave background looks totally different.
And finally, when we look at how large structures in the universe evolve, they do it way too quickly: there has to be extra sources of gravity to pull everything together in time. In fact, you can’t even build a Milky Way galaxy in time for it to be…the Milky Way galaxy. We wouldn’t even exist if it weren’t for dark matter pooling together and forming the seeds of galaxies while regular matter was too hot and bothered to pay attention.
Everywhere we look, gravity at large scales is acting differently than we expect.
Maybe we’re getting our theory of gravity wrong. That’s fine, been there, done that. But every attempt to modify gravity to explain one or some of these observations ends up unable to explain the rest. No matter what, and we’ve tried really hard, if you change gravity then you STILL end up needing some sort of dark matter to explain the wealth of observations we have in front of us.
So…what is it? Enter Stephen Hawking, stage right.
To be continued…
