Welcome back everyone. Last time we talked about the stage, space-time, in which the drama of cosmology occurs and now we have to think about the actors that occur on that stage. What do we pour into that empty universe? Well, of course, as we've seen it's not completely empty because of the quantum fields, but what occurs inside the space-time of, that is the stage of cosmology. And what physicists have come up with through enormous amount of work is something called, The Standard Model of Particle Physics. And this is all that work that goes on in those giant particle accelerators. Physicists coming to understand what the most basic constituents of matter are, that's the work that they've done and this is what we call the standard model. And so, the standard model remarkably tells us about all the kinds of matter that there are in the universe, except for the dark matter because we still don't know what that is, but all the kinds of the normal matter that we're used to and what they found is, there's basically two classes of matter. There are what are called fermions and bosons. Fermions are what matter is composed of and the bosons, bosons are really the mediators of the forces. And as we have figured out from our studies of physics is there's basically four kinds of forces in the universe. There is electromagnetism, gravity, the strong nuclear force, and the weak nuclear force. And the bosons are actually particles that jump back and forth between the fermions allowing those particles to feel the forces. What the standard model does with extraordinary mathematical rigor is provide the details about the fundamental particles and their interactions. Now, gravity really is not part of, we don't really have a firm understanding of exactly how gravity works in the same way as we have for the other ones because we're still trying to understand a quantum theory of gravity, but first, certainly, for the other three forces the work in the particle accelerators has given us enormous details. All right. Let's think about the fermions for a minute. The most basic types of fermions are what are called the quarks and leptons. So, those are the two different possibilities. And so the leptons are things like electrons. They are particles that they all have charge and they're very light. The quarks are more massive particles and the interesting thing about the quarks, the quarks are what go into things like protons and neutrons. And so, the quarks have charge also, but they are basically--, the interesting thing about quarks is you'll never find in the universe now quarks on their own. Quarks are always bound together into something like a proton or a neutron. And now the important thing to understand about these particles, and there's a whole host of--, right now I've only spoken about the electrons and the quarks which make up the protons and neutrons, but actually there's a whole zoo of particles that can be built out of quarks and there are other particles that also fit into the leptons, things like neutrinos which we've talked about before. But the important thing to understand is that each one of these particles feels some subset of the four forces. They don't all necessarily feel all the possible forces. Everything feels gravity but then the rest of the particles, if you don't have a charge for example, if you're not electrically charged, you don't feel the electromagnetic force, okay? On this slide behind me, you see a diagram that shows you all the different possible combinations which particles feel which forces. And the interesting thing, one fascinating thing about the study of the standard model, is we've come to recognize that it is at least possible to have a whole set of particles that would feel none of the forces that we experience. People call this, the shadow sector of particle physics. No one has ever discovered a shadow particle or evidence for a shadow particle but it's interesting because our formulation, our understanding, of matter and forces now is that if you don't exchange a boson that has associated with a force, then you don't know that other particle is there. You can imagine a whole set of particles that responded to a different kind of electromagnetism or a different kind of strong nuclear force such that you could have a person built out of that kind of matter, standing right next to you and you would have no idea that they were there because you couldn't interact with them at all. They would be like a ghost in some sense. They could sit in the chair with you and because none of your matter interact with their matter, you wouldn't be able to see them or feel their presence at all. That's an interesting sort of theoretical possibility that comes out of things. Okay. We've seen there is matter, the leptons and the quarks, and there are forces which come from the bosons. We also have to understand there is anti-matter. Anti-matter are basically elementary fermions that have the opposite charge of the normal matter. An anti-electron is a particle of matter that has a positive electrical charge rather than negative electrical charge. And the interesting about anti-matter is if you take matter and anti-matter and bring them together they annihilate each other in a burst of energy. An electron and anti-electron, what's called the positron, if they come together they will disappear in a burst of light, essentially. Now, one interesting question that astronomers really haven't been able to answer or physicists haven't been able to answer is, why does the current universe made only of matter? What happened to all the anti-matter? Because most of the laws of physics, or the laws of physics do imply that you should have equal amounts of matter and anti-matter so that's still an open mystery. Okay. We've gone through basically the kinds of stuff that there is in the universe and the forces between them and now we can move on in our story of cosmology. We've talked about fermions and bosons and some of you may have heard about the Higgs boson, you may be wondering like, "Wait a minute, why is it that boson associated with a force?" The Higgs field is very interesting and it was proposed as a mechanism to understand why particles have the property of mass or why they respond in a way that is similar to mass. And so, the Higgs field and the Higgs boson is actually a unique and very interesting quantum field that fills the universe. And what it does, it's partly at least responsible for giving particles inertia, giving particles resistance to changing their state of motion. And it comes down to essentially, you can think of it is almost like a fog that exists throughout all of space-time. And the more mass a particle has it's really responsible to sort of--, the Higgs boson is what gives it that ability to respond, to resist motion. It's almost like gathering a lot of people together in a crowd, in a party, like you're a very popular person and the more people that would gather towards you, the harder would be to move through the party. The Higgs field is what sort of gives particles with more mass that resistance to changes in motion. The Higgs field and the Higgs particle which really actually came from six scientists including Dick Hagen here at the University of Rochester. They all published at the same time and it was just sort of unfortunate history that only one guy's name got associated with it. But all six scientists published their results at the same time in 1964 and it was only recently in 2013 or 2012 that the Higgs was, we could say that the Higgs was actually discovered. That shows you how hard it was to actually build a machine that could actually find this particle. And the important thing to understand is that the Higgs is what gives mass to quarks and electrons, the protons, when you get to the level of protons actually the mass is coming from other proxies but these elementary particles the Higgs, is the Higgs field and the Higgs boson, are what's responsible for generating mass for those. Now, there's one very important thing to understand about the standard model which is why we're not done, why physicists aren't like, "Okay, hey, we've got the standard model, everything is great." Is that the standard model has a number of constants that you have to go out and measure them and then you put them into the theory. It just like Newton's theory of gravity. There's that big number G in it and that tells you how strong actually gravity is. You may have a theory of gravity but if you want to know actually the actual strength of it you have to go out and measure this number and put it in. The standard model has 26 of these numbers and what it turns out is that, these numbers, the universe and particularly life seems to be very finely tuned to the value of these numbers. For example, if you were to change any of them by just even a little tiny bit, you may say what's called the fine structure constant which is associated with electromagnetism. If you were to just increase that by one in a million or decrease it by one in a million, what you find is that you wouldn't, for example, be able to get biomolecules to form and so life wouldn't be possible. If you changed gravity by just a little bit, you would either, not either, everything would clump together too fast and stars would only live for a million years or stars would never form. There's this problem that physicists call, the Problem of Fine Tuning, which is that the universe seems to require very careful adjustment of these constants or else we would have ended up with a very different universe than we see today. And so that's really where people are trying to--, it's one of the frontiers, we're trying to understand some natural way for these numbers to occur or as we'll see some people come to think about the idea of the multiverse so that there's many universes with many different values. And we just happen to live in the universe that has the right values for life. But this problem of fine tuning is one that I particularly think is very interesting and very important and may lead us in new directions. Okay. Let's go on.