One of the things that’s got to be frustrating about astronomy is just how little astronomers have to work with. They can’t walk up to a star and stick a thermometer in it or weigh it on a scale. They can’t even go around a star and look at it from a different angle. They can’t go anywhere the Earth doesn’t want to go, and the instruments on space probes don’t go very far or very fast. They can’t collect matter samples from distant stars and planets because matter, traveling at less than the speed of light, hasn’t had nearly enough time to get here. That leaves them with pretty much nothing but light. Okay, electromagnetic radiation of all frequencies, but it’s still just photons. Basically, all they can do is stand in one spot and watch.
And what’s amazing is that they keep coming up with ways of teasing unbelievable amounts of information out of the light that reaches us. They can see what its frequency distribution is, what spectral lines have been added or removed, which tells them what atoms and molecules are involved, and also whether that matter’s moving toward or away from us, and how fast. And a million other bits of information beyond that.
To illustrate, Sean Carroll (no, not
Sean Carroll the biologist,
Sean Carroll the cosmologist)
explains how scientists recently demonstrated that dark matter really exists.
the whole thing, because it’s clearly explained, with cool pictures.
In a nutshell, though, it’s an example of what I was talking about above, of teasing out all sorts of information out of light.
The idea is that dark matter supposedly affects gravity, so presumably all you need to do is find a big lump of dark matter and see if gravity “points” at it. Unfortunately, dark matter tends to be clustered in the same places as ordinary matter, so it’s hard to tell whether gravity is being affected by ordinary or dark matter.
So what you need is to find a way to take away all of the ordinary matter from a clump of dark matter, and see whether the gravity “points” toward the regular matter or the dark matter.
And as it happens, the universe has provided us with a place where this has happened. Actually, this is astronomers’ ace in the hole: they can’t set up their own experiments, like slam two galaxies into each other to see what happens. Fortunately, in a universe as mindbogglingly huge as ours, you can often find some place where the “experiment” has happened naturally, if you look hard enough. And that’s what happened here: they found two clusters of galaxies that ran into each other — I’ll pause here to let you appreciate the scope of that event — that stripped both of them of most of their ordinary matter (interstellar gas).
So how can they tell where the gravity wells are? Again, by studying light. By examining the background galaxies, i.e., the ones behind the two clusters that ran into each other, they were able to detect systematic distortion that indicates gravitational lensing, i.e., light rays being bent by gravity.
And lo and behold, they found that the gravity was following not the regular matter, but where the dark matter would be if it existed. So evidently it does.
Now I just need someone to explain to me what dark matter is, given that apparently it’s not just ordinary atoms, rocks, gas, etc. And what about dark energy, which I gather is even harder to detect? Is quantum involved?