How to Deduce the History of the Universe | Astrophysicist Alex Filippenko

Okay, so, with today's big telescopes, we know the current rate at which the Universe is expanding. It's just some value. But the— that's not the end of the story, and this goes all the way back to Newton. Newton supposedly saw an apple— come on, computer. There we go. Newton supposedly saw an apple fall from the apple tree and wondered whether the reason it falls is related in some way to the orbit of the Moon around the Earth. What's the relationship? around the Earth. And indeed, they are related through gravity and the laws of motion. And, you know, so everything acts upon everything else. And if you toss the apple up, okay, the gravitational attraction of the Earth slows it down in its upward journey. You can't give a talk about gravity without using the proverbial Newtonian apple, right? So here it is. All right. So, and eventually it stops and then comes back down. Now, if the Universe is really dense, Then all bits of the universe will be pulling on all other bits a lot and slowing down the expansion. All right, so the expansion should be slowing down, and maybe if it's dense enough, it'll someday stop and then implode on itself. It'll collapse. So in that case, in a dense universe, you start with a Big Bang and end with a Big Crunch, or you could say Big Bang Gnab Gib, which is Big Bang backwards, right? So that's one possible fate of the universe to. recollapse someday. And so, although we see an expanding Universe— so if you were to live billions of years, and you're lying on your back, and you're looking at all these galaxies moving away and getting fainter, you'd say, that's good, that's good. And then you'd notice something a bit strange, and right around now you're getting kind of nervous, and then, ah, you know, goodbye, cruel world, right? The Universe implodes in on itself, becomes this hot, compressed Big Crunch. All right? So that's one possible fate For the Universe. But there's an alternative. Had I eaten my Wheaties this morning, and if there weren't the technical difficulty of a ceiling here, I could, in principle, heave this apple at a speed greater than or equal to Earth's escape speed. In that case, the apple would continue to slow down, but it would never come back. Okay? It would either never come to a stop, or even if it does come to a stop, that would be at time equals infinity, but it would never come back. So, that's like what a rocket does, right? Rockets are launched at a speed of around 7 miles a second, so that they can get away from Earth's gravity. And they keep on slowing down, but they don't stop. And in a similar way, then, if the density of the Universe is low, then the current expansion will be forever slowing down, but it'll never turn back around. And so, this would be an eternal Universe, That keeps getting less dense, darker, colder with time. Not more dense, more compressed, brighter, and hotter, like the Big Crunch. So, that would be a very different fate. Okay? Well, we would like to know the fate of the Universe. What will happen to the expansion? A way of doing so is to monitor the history of the expansion. Let me go back to this analogy. Again, it's really useful to think of analogies. If I measure the speed of the apple at many times during its motion, and I find that it's been slowing down a lot, then I can calculate that it'll someday reverse its motion. If it hasn't been slowing down a lot, then it'll keep on going away from the Earth forever. So, in a similar way, if we examine the past history of expansion, we could predict the future. The way we examine the past history is to look back in time, and the way we do that is to look at progressively greater distances. Because light doesn't travel infinitely fast. It travels about a foot per billionth of a second. Alright? So, what's your name? You asked that good question. Oliver. Pardon? Oliver. Oliver. Okay. I'm seeing Oliver as he was about 12 billionths of a second ago. He may not even exist anymore. Oh, he does. Okay. But he need not have. You see the Sun as it was a little over 8 minutes ago, because it takes 8 minutes, 8 and a third minutes, to traverse 93 million miles. The light does. The typical stars you see in the sky are some tens, hundreds, or thousands of light-years away. So you're seeing them as they were tens, hundreds, or thousands of years ago. So if you look at galaxies that are, say, a billion light-years away, and 4 billion light-years away, and maybe that little blob is 9 billion light-years away, You're seeing them as they were 1, 4, 9 billion years ago. And encoded in their light is information about the expansion of the Universe as it was 1, 4, 9 billion years ago. So, you can look back in time by looking at progressively greater distances. All right? But you need to know the distances of these galaxies. You know, this— is this galaxy 4 billion light-years away, or pi billion light-years, or 2. What is it? So, the way we determine distances of galaxies is by finding stars within them whose properties we recognize. So, let's say, in this galaxy here, there's that star right there. Let's call it Tim, just for kicks, okay? Because I just saw Tim walk in from my peripheral vision. And let's say you study Tim, and you know that Tim is just like this star, Betelgeuse. In the left shoulder of the great constellation Orion, the great hunter Orion. So Betelgeuse is a luminous, powerful, magnificent star. You might think all the stars are the same, but they're not. They come in different categories, okay? So this is an amazing star. And you can see how bright it looks, and we know its distance, so we can figure out how powerful it really is. And if Tim is exactly the same type of star, then you can figure out How far away Tim must be in order to look as faint as he does. All right? And that's, then, the distance of the galaxy. And you might do this for a bunch of stars to gain confidence that you got the distance correctly. All right? You use this technique all the time when you judge the distance of an oncoming car at night. You've calibrated how bright the headlights are of a car of known distance. Like, 6 feet away, ooh, that's a pretty bright headlight, right? Then, you look at the headlights of more distant cars, and they look fainter. And so, you determine the distance, almost intuitively, almost instinctively, right? I mean, if you're not very good at doing this, then you shouldn't be driving at night, right? So, cars, stars, it's the same sort of thing. Find a type of star whose true power you know, look at how bright it appears to be, and thus, determine the distance, all right?