In this lecture, I'm going to tell you about the history of our universe. How the universe was born in the big bang 14 billion years ago, and how we know that that really happened, that it is true. What are the measurements that we make in our universe today in order to know that the universe was created in a big bang 14 billion years ago? Okay, I'm going to try to explain to you something about our universe that seems absurd. Our universe has a beginning and that a beginning was very extreme. Ten to the minus 35 seconds after the beginning of the universe, about 14 billion years ago, so let me say that again. Ten to the minus 35 seconds, that's 0 point and then 34 zeroes and then a one, as you see here. Seconds after the Big Bang. All the galaxies in the universe that we see now, all 100 billion galaxies each of which made up of 100 billion stars were in volume as big as a grapefruit. Isn't that crazy? That can't be true can it? But it is true, and I am going to show you that it's true. We can measure this. We have evidence for it. The first evidence for it came in the 1920s when American astronomers started to find that our Milky Way galaxy was not the only galaxy in the universe, but that there were other island universes, as they called them, that were made up of 100 billion stars like our Milky Way, and they were very far away. Not only were they very far away, but they were also flying away from us. And the further away the galaxy was, the faster it was flying away from us. And that law, is very precise. It's not just the further, the faster, it's twice as far, twice as fast. And that's called Hubble's law, after the American astronomer, Edwin Hubble Was one of the astronomers who discovered this. So lets imagine if galaxies are flying away from us it means they were closer to us yesterday and the year before yesterday. And it means they will be further away Tomorrow. So, the universe is getting bigger, it's expanding. And if it's expanding with a certain speed, you can calculate how longer it goes started expanding. If you measure how far away a galaxy is and then you measure what speed its going at, then you can calculate how long it took to get there. And if you do that, you find a number that is about 13.7 billion years, give or take a few percepts. All measurements have uncertainty, so I'll say 14 billion years from now. That's the first evidence that the universe had a small beginning. But how small was it when it began? Did it begin expanding when it was as big as the Earth or as big as the sun, or did it begin expanding when it was even smaller? That also we can work out. Imagine we run the movie backwards, and all the galaxies are coming closer and closer to each other as we go further in the past. An at some point they all merged together and they became one big clump of hot gas. And still it's shrinking if we move the movie backwards. Well, at some point the gas got so hot that it becomes ionized. That the atoms collide so quickly with each other, that the electrons are cut loose and we get what is called a plasma. And a plasma makes a lot of heat and light but it traps it. Now let's go back we now have a hot universe that makes a lot of radiation because it's hot. Everything that is hot glows as you know. And now, it expands and then it becomes transparent because when it expands again the electrons reunite with the nuclei and you get neutral atoms like in the air of this room. And neutral atoms may control transparant gas. So then, all this radiation just flies away and it flies through the entire universe and we can still see it today. And so, the universe has become a thousand times bigger since then. So, where it was 3,000 degrees when it became neutral, It is now a thousand times bigger and a thousand times cooler and it's only three degrees above absolute 0. And at that temperature it glows in microwaves. And those microwaves we can capture with radio telescopes. And we see it coming from all around us, the entire sky is glowing in microwaves, and it's very uniform. It's the same temperature everywhere. Except if you look at this map, you see tiny little clumps. Some are red. That means they're a bit hotter. Some are blue. That means they're a bit colder. And those little clamps indicates the beginnings of things bein g formed in our universe that have structure like galaxies and stars. So the universe isn't completely smooth and because it's not completely smooth gravity can pile up material together and make galaxies. Those are the galaxies that we now live in. With the stars that our earth turns around, and our earth itself. So in the very early universe, 300,000 years after the Big Bang, you can already see the seeds of galaxies. The next evidence takes us all the way back to one second after the beginning. If we go even further back to the beginning, the universe wasn't just 3000 degrees, but it was 100 million degrees. And 100 million degrees is so hot, that the same thing happens to the nuclei of the atoms that happens in the interior of our sun. Material burns in nuclear fusion in the interior of our sun, because hydrogen atoms bump into each other so fast that they stick together and make helium nuclei. And that we can see because if we look in our universe now, we see that the universe is not only hydrogen, but one in ten items in the universe are helium. And if you calculate how many atoms should be helium from calculating from the conditions of the big bang how much material will become helium, you find exactly one in ten atoms as predicted. So even just one second after the Big Bang, it still fits. And so, we know that the universe must have been that small and that hot. So we can play the movie all the way back to one second after the Big Bang. And that's how we know that the Big Bang really happened. So, if I summarize, there are at least three reasons from which we know for a fact that the universe started 14 billion years ago in a Big Bang. First of all, we see that all the galaxies are flying away from us. The universe is expanding and that means that it was smaller in the past. And if we calculate how fast it's expanding, we find it has to be 13.7 billion years old, give or take a few percent. Secondly, we can find out from the microwave background glow, that the universe was very hot 300,000 years after the big bang, and all that radiation is still with us. And thirdly, we see that one in ten atoms in the universe, is helium. And helium, you can only make, if the universe is so hot and dense, like in the interior of stars, so that one in ten atoms can fuse to helium. That's the third reason why we know the universe started in a hot big bang 13.7 billion years ago. Now, as an after thought there is a fourth way of knowing that the universe had a beginning and it's very easy to think but very hard to think about. Again, it seems a little absurd. We know the universe had a beginning because it's dark at night. This is an argument that was invented for the first time by the people like over some German Astronomers in around 1800. And it goes as follows, before Edwin Hubble discover that we live in an expanding universe. We thought that the universe was infinitely big and infinitely old and it had no beginning. And then said let's think about what that means. If the universe is infinitely big and infantely old it means you can look as far you like. Everywhere in any direction of the sky you can look as far as you like. Now, what is that mean? That means that eventually if the universe is the same everywhere, anywhere you look your line of sight will hit a star. So that means that the night sky doesn't consist of a lot of dark with some stars in it, but it means that every point in the sky that you look at is a star. And that means that every point in the sky is as bright as the surface of a star. Which means it's as bright as surface of our Sun. And it would hurt your eyes as much. As it hurts you now to look into the sun during the day, to look into any part of the night sky. So that means that the night sky would be even much brighter than the sun. And of course the day sky, too, because in the day sky there are stars, as well, except normally you don't see them. Because the sun is so bright. So if the universe is infinitely old and infinitely big, it can't be dark at night. It would always be very light. So how does he solve big by giving the universe a beginning? Well, If the universe had a beginning then it is now only a finite age and we know what that is for our universe it's 14 billion years. That means you can't look infinitely far away, you can only look 14 billion light years away. And if you go 14 billion light-years away in our universe, you haven't seen a star in every direction yet because the universe is quite empty. Stars are fairly rare because the universe is extremely big. So that's the explanation. The universe had a beginning. It's only 14 billion years old. Therefore, the light of stars that are further then 14 billion light-years away from us has not reached us yet and we have not seen them yet. And that is why tonight sky is dark. Because there was a beginning. Because there was a Big Bang. As another afterthought you might think: well that guy is awfully certain of himself, right. He knows the universe started as a big bang, so does that now mean we know everything? And the answer is no, certainly not. For example, I told you about the hydrogen and the helium that was made in the big bang, but if you look at it, it turns out only four percent of what is in our universe Is the stuff that you and I are made of, and the stuff that stars and planets are made of, atoms and nuclei. 96% of the universe is made of things that we have no idea what it is. We call them dark matter, which is some unknown stuff that feels gravity and attracts things. And then there's dark energy that does the opposite, it pushes things away from each other. But the universe is 25% dark matter and 70% dark energy and we have no idea what dark matter is. And we are no idea what dark energy is. We just know that it must be there. Otherwise, the universe would behave differently. Also when you create this enormous ball of heat, normally what our current understanding of particle physics would say is that in the end out of that you just make pure energy. When everything has started expanding for a while you make particles and anti-particles and complicated stuff. They all come together again and you only have pure energy and radiation. So why is there matter in our universe? Why is there not just energy? That's another question we don't understand. And then the other thing, I told you what happens from 10 to the minus 35 seconds after the Big Bang until now. I didn't tell you how you make a Big Bang. That we don't know. All the way at the beginning, less than ten to the minus 35 seconds after the Big Bang our current theories of nature just break down. We don't know how nature works. If it's so hot and dense as right in the Big Bang. So that means also that we don't understand how you make a Big Bang. But Mother Nature knows, because she did it at least once, our universe exists. And then the next question is, if you can do it once, why don't you do it twice, or three times, or four times? Is there only one universe? Or are there many universes? Again, we don't know. Logically, it seems perfectly sensible that anything the laws of nature can do, they can do more than once. So if there is one universe, there could also be a million universes. But we don't know. So there are plenty of Nobel Prizes still to be won. What is dark matter? What is dark energy? Why is there matter in the universe? How do galaxies form from a uniform shoop? How do you make a Big Bang? Is there more than one universe? That's half a dozen noble prizes for you.
