 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about dark energy and inflation and finish up our discussion on how the universe was formed. So what is the universe made of? Well, we've looked a little bit about this before. Only about one percent of the universe is visible matter that we've studied. So less than one percent of that is stars, which we see as stars in galaxies. That's the ordinary matter. About four percent hydrogen and helium. So this is the ordinary matter that we see, which constitutes about five percent of the material in the universe. But one percent of it is in the visible form that we can see. Dark matter makes up about twenty-seven percent of the universe and dark energy sixty-eight percent of the universe. So what we've studied in astronomy and what we've looked at over the course of these lectures, the planets, the stars, the nebulae, the galaxies, is only a tiny portion of the content of the universe. So we've only looked at that very little portion. Now how has our understanding of this changed over time? Well, here are a couple of charts showing what we looked at from the 1970s through 1990s. And our understanding of the composition of the universe has changed over the decades. Of course the universe's composition has not changed, we're just starting to learn more about it. And that ordinary visible matter has become less and less important over time. So here we see the 1970s, we had our ordinary visible matter, and we had some kind of dark or missing matter that we didn't know about. That was ordinary material, much like what everything else was made up of. By the 80s we did have that ordinary dark matter, and we had some ordinary visible matter is now a much smaller portion, an exotic dark matter, some kind of unusual particle made up a large portion of the universe. By the 1990s we learned about dark energy, and dark energy now consumes most of this chart, and we see only a little bit, tiny bit left for ordinary visible matter. Now what is the dark matter? Well, it cannot be ordinary matter, we've been able to rule that out because the deuterium abundance limits what the amount of ordinary matter could be. That tells us about what formed in the early history of the universe during the Big Bang, and that means no more than 5% of the matter can be the ordinary type of matter. So what is it? Well, we looked at previously, weakly interacting massive particles, or so-called wimps. They are very hard to detect because they interact through the weak nuclear force. So we don't see them otherwise, except through that and gravity, they don't emit any electromagnetic light that we can see. They've been proposed to exist, and we are still looking to try to detect some type of massive particle that exists out there. Dark matter is needed. We have to have it. It is a requirement in order to form galaxies and structures on the time scales that they formed in our universe. We know that dark matter does not interact, except through the weak nuclear force and gravity. So it could have started to form structures before ordinary matter. That allows us to form these structures. So it would start to form some gravity wells, and the ordinary matter would then gather into these gravity wells and form the structures that we see that would form the filaments and voids that we see today. So let's look at a brief history of the universe here, and we start off with quantum fluctuations. That's the big bang. What happened early on? Maybe you tend to put in there some kind of quantum fluctuations within the very tiniest scales that actually could have helped with the creation of the universe. Then inflation kicked in and caused the universe to expand very rapidly in a tiny fraction of a second, going from atomic size to universe size in just a very short period of time. Matter started to form and nuclear fusion now begins at just a fraction of a second, and by three to four minutes the nuclear fusion has ended, and the cosmic background radiation is produced and released at about 400,000 years, and then there's the dark ages where not much goes on, and then the universe, stars and galaxies begin to form, and then at the very end dark energy accelerates this expansion. So what was a slower expansion is now going at a far more rapid rate. Now with any model there are going to be problems. So what are some of the problems with the big bang model? Well we're going to look at two of these. One of these is the flatness problem. That is the density equals the critical density. Why does the universe look so flat? If it did not, nothing would have formed. No structures would have formed. So in order for us to be here we had to have been incredibly close to a value of one for this variable. So incredibly close to one, to many decimal places, otherwise the universe would have either immediately collapsed or would have immediately accelerated out to nothing. So this is a difficulty as to why. Why is the universe so flat? The horizon problem is in the uniformity of the background radiation. Why is it so uniform no matter where you look in the universe? And that is a difficulty because you have areas that have never been in contact, but they have the same temperatures. So it would take time. There's no reason that they would know what the other temperatures were because they are so far apart. But we have to look at some way that they may have been able to be in contact in the past. So what do we use to solve this? Well we do what we call the inflationary hypothesis, which says that in a fraction of an instant of time the universe grew by a factor of 10 to the 50th power. So it went from being atomic size, subatomic size, to all of a sudden being standard size, galaxy size, universe sized here. So the standard model, which would trace back at the red line, wouldn't account for things. It wouldn't work because it wouldn't get things small enough at a quick enough time period. This very small era of inflation that could have taken place could have explained then the two problems that we had and can solve these two problems with the flatness and the horizon problem. But by zipping up in that tiny fraction of time, the universe expanded. And what we're seeing now, let's take a look at a couple examples here. What are we looking at for the image here? Why does it look so flat? Why are the distant regions are in contact? What about the horizon problem? Well now we know that these regions that are now distant were actually in contact prior to inflation occurring. So inflation re-anked them apart and shoved them at great distances across the universe, but they used to be in contact, and therefore they would know and would all be the same temperature. With the flatness problem, we are only seeing a small portion of the inflated universe. So we use the example of the ant on the balloon here. When the balloon is not blown up very much, the ant can see curvature around either side. However, when the ant is on a much more inflated balloon, then everything looks flat. And if you could imagine this balloon being inflated 10 times, 50 times more, the ant would only be seeing a very tiny portion of the entire universe. We are not seeing the whole universe, therefore, when we look out into space. We're only seeing that tiny portion around us that is our part of the universe. Now what could have caused the inflation? Well let's take a look at, first of all, let's take a look at what we have for the four forces of nature, because this is one of the things that we think could have happened to cause the inflation. The four forces of nature are gravity, which is the weakest. The electromagnetic force, the strong nuclear force, and the weak nuclear force. So those are the three, and they will unify under conditions of high temperature and pressure. And yes, we've been able to unify these. In supercolliders, we can unify the electromagnetic and the weak force. They become one and the same at high temperatures. So we have what we call the electro-weak force. If we could get to even higher values, we theory say that we can include the strong force in this as well. And we then have just two forces, one that combines the strong weak and electromagnetic and gravity. But gravity splits out here. This is way beyond our ability to create these kinds of temperatures to really be able to see and test these other versions. So we can get up to the level for electro-weak. The other ones are really beyond what we can do at this point. But perhaps one thing that is thought is that when gravity separated out, when these two forces then diverged, that that could have caused this massive inflation of the universe. So our current theories can unify all of the forces except for gravity. Now, what do we get when we unify these? Well, we have, again, that we call the grand unified theories, which looks at the forces are really just different manifestations of the same force. And one way of thinking about this is what is used as string theory. In this, points of matter are instead replaced by one-dimensional strings. So the one-dimensional strings down here then can become other things. They exist in an 11-dimensional space. So we don't use all of the, we only see a few of those dimensions. They exist in a much higher dimensional space. And the way these strings vibrate and move will then make mean that they can appear in our universe as matter or light or gravity. And they make up everything in the universe so that everything is just a different manifestation of something else. So the string then, depending on how it vibrates, may become one of the quarks within one of the protons within the nucleus. Another spring vibrating in a different way may become an electron. And though that will then be part of an atom, which in this case we see comes up as part of a diamond. So it's all going down to even deeper levels within the subatomic sphere. Now how do we get all of the forces together? And that's where a lot of trouble exists. General relativity and quantum mechanics are difficult to combine. In fact, they're incompatible. General relativity describes things exactly. Quantum mechanics is based on probabilities. So things have probabilities of doing something. It's not precise in that sense. So what we do as we look at here is trying to get some kind of theory of quantum gravity. And that's where we have not gotten yet. And that would combine quantum field theory with general relativity to really be able to then combine them together and better understand what might have happened in that very early history of the universe. String theory attempts to resolve this, but we're not there yet. What is the answer? I can't tell you. We're not there. This is current research. Maybe someday we will have the answer. Could it be in a decade or a century or a millennium? How long will it take us to be able to better understand how to merge quantum mechanics and general relativity? So the last thing we want to talk about here is what we call the anthropic principle, which really suggests that we are really just a lucky accident. The anthropic principle says that physical laws are what they are because we wouldn't be here otherwise. Could there be other universes with different physical laws? Well, there could be. But if it is a universe where gravity is repulsive instead of attractive, then no material will ever combine together. You might have atoms that could combine under other forces if they work as they do in this universe, but they would not be able to form larger objects as we do under gravity. So what are physical laws like? There could be completely different physical laws. So, they could collapse immediately, or they could expand too fast for any kind of structures to form. And you hear about this sometimes. We talk about the multiverse. Could there be other universes out there? Now, of course, could there be other universes with similar physical laws where life could be possible? Well, that's his possibility as well. However, the whole thing is conjecture since we know of no way to ever visit these universes. So, let's go ahead and finish up with our summary. And what we looked at is that only a tiny fraction of the universe is the ordinary matter that we have studied in most of this course. The standard Big Bang model has difficulty explaining the horizon and the flatness problems. The inflationary model helps us to resolve these issues. So put the inflationary model with the Big Bang, and we're able to resolve these issues. And unified theories continue work on trying to combine the four forces of nature. So that concludes this lecture on dark energy and inflation. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone. And I will see you in class.