Welcome back everyone. So, you know, we've talked about the constituence of the universe in what's called the standard model of particle physics. But of course dark matter is not there. And we've already seen that dark matter constitutes by far the largest fraction of matter in the universe. So what do we know about dark matter? Well, the one fun thing to remember is of course that the only way we know about dark matter is through its gravitational interaction. So let's just talk about gravity for a moment. We do not have a theory of gravity on a fundamental level. We do not have a quantum theory of gravity. We don't understand sort of the particle nature of gravity yet. So that is really something that fundamental. We, we were, there's still a major unanswered question about gravity. But we do know enough certainly to understand what things about its relationship via or you know, in, in lieu of our, what we're trying to understand about dark matter. So gravity is the weakest force. And if you don't believe me, just take a balloon, rub it against your shirt, and stick it to your hair. Right What that means is that, that the gravity, the electromagnetic interaction, which is keeping the balloon stuck to your hair, is stronger than the gravitational force of the entire earth. So it's an incredibly weak force but it's an incredibly powerful force if you get enough matter together. It's only attractive, which is interesting, so we know that with electromagnetism you can either have repulsive or attractive forces, but for gravity it's always attraction. It's really essentially the shape of space-time, we've also learned that from general relativity. And what we you know, the assumption is, is that it couples to dark matter. Right? It must couple to dark matter because we see the luminous matter, our kind of stuff, being pushed and pulled, or being pulled around because of this invisible dark matter. So when people began to ask the question, well what is the dark matter, the first thing they thought, oh it must be like, maybe, black holes or, or you know chunks of rock that are so dark that you can't see them. So the idea of what were called machos massive, compact halo objects. And these we're thinking that, what people were thinking was that they were things like black holes or neutron stars, cold neutron stars, that were floating around in the hado, halos of the gal, of galaxies. And you know, after doing a lot of work, and looking for these things, they pretty conclusively showed that they weren't there. So whatever the dark matter was, was not the something like black holes or, or ordinary matter that was just in some form that couldn't be seen. So what that meant also is that the dark matter does not interact with the electromagnetic field or the strong nuclear force either. So what that left then, people thinking about was the weak nuclear force, and that led to what was called WIMPS, weakly interacting massive particles. So we had machos, and WIMPS, which you know, made everybody laugh for a while and may have been sort of a stupid idea but you know [LAUGH] coming up with a, a you know, good names for things sometimes is important for you know, for, for getting them looked at. So the WIMPS is really the basic idea that we have now, that, that, that, these particles that if they exist, they only interact with the weak nuclear force. And so now as people begin to look for, directly, do direct measurements, for are looking for dark matter particles, we're really concentrating on detectors that can focus on weak interactions. So there's a number of these pro experiments being done, oftentimes in mines deep under the Earth where you can screen out cosmic rays so you could look directly for the dark matter. And so far none have been found but we'll see what happens. I mean, that's something that's going to be interesting to see over the, the next ten, 20 years. There's an interesting question about the Higgs Field, because the Higgs Field only acts with mass, interacts with massive particles. So since dark matter must have mass, does it also interact with the Higgs field, and that's an open question. We don't know the answer to it yet. One thing we do understand, though, by doing simulations of the history of the universe and looking at, say, something like the number of galaxies and the type of galaxies and the distribution of galaxies is we can tell whether or not from those simulations, the dark matter was hot, meaning that it was moving at very high speeds, almost relativistic speeds. Or whether it was cold, meaning it moves at speeds that, the kind of speeds that we see for normal, you know for planets and st, stars moving through space. And what we found is that those simulations showed a very clear distinction between relativistic dark matter and slow, more slowly-moving, or cold dark matter. And it really gave us a clear indication that the dark matter has to be non-relativistic. So right now when we think about dark matter, we think about that it's, it's cold, that it's cold, dark matter is the basic model for the universe. We don't know what it's made of, but we do have a pretty clear understanding that it has to be moving pretty slow. okay. So now that we understand or, understand what we don't understand about the dark matter, let's move on.
