Video Transcript: The Expanding Universe

Now, it's worth pointing out though, that the universe has always been a part of the discussion. I mean, the universe meaning every physical thing that exists everything our whole universe. That's been a part of the discussion since those earliest days since Aristotle. Our model of the universe was that we had the Earth at the centre and the star and the sun and the moon, the planets and the stars and that was our universe. But with each new discovery, our universe has grown more sophisticated. It's grown larger. It's grown in so many different ways. And as galaxies now are a part of our vocabulary, we can get a richer understanding of just how amazing this universe really is. 

And that's what we're going to do in this last Unit of our course. Very cool stuff. I'm very excited to share it with you. And the story in many ways starts back with the Doppler effect. So we talked about the Doppler effect before and how that can be used to measure the speed of an object and we started by thinking about sound and how the pitch of sound changes when an object goes past you. We saw how that connects to light. And that's the first picture in our gallery. Here is showing how when a star or any object is moving away from you, it's color will look a little bit redder, and when it's moving toward you, it's color will look a little bit bluer. 

And that's an important tool that we use not only for studying stars and for studying galaxies, but it's a really important tool for understanding the universe as well. I think I mentioned this guy Vesto Slipher, not someone you hear a lot about. In fact, in my entire education in astronomy, I never really heard his name. But he was one of the first people to take spectrum of these Galaxy nebulas, before they even really knew they were galaxies. It's a spectrum of them. And what he found was that those spectrum those lines in the spectrum were shifted so much that these galaxies must be moving really, really fast. And it was Hubble Edwin Hubble, you can see him in the second picture in our gallery who really took many different people's ideas. Right. He took the idea that Henrietta Leavitt had discovered, the Cepheid variable stars can be used to measure the distances of galaxies. He took this idea that Vesto Slipher had discovered that these galaxies seem to be moving away from us at very great speeds. And he said, I want to try to measure the speed of these galaxies, and how fast they're, how fast they're moving, their speed and how far away they are, and see if I can map out the motion of galaxies around us. We could try to figure out how are things changing? Maybe some objects are moving away, some are moving toward maybe they're swirling in some way. Let's try to understand the motion of these galaxies. 

And there's a general pattern that was observed. This is the third picture in the gallery shows that pattern. So certainly at the bottom of this, of this chart, okay. So this is showing the spectrum of various objects. So if you start with just a single star and you look at it spectrum, we saw real pictures this is kind of an animated picture of that. If you look at the spectrum of a single star, it'll look something like that bottom spectrum, you'll see the full rainbow because the star is hot, hot enough and it actually gives a thermal spectrum of the rainbow. And then you'll see these absorption lines and remember this is where that light is being absorbed by the actually the atmosphere of the star. Okay, so these are the same as like those emission lines. These are specific lines. And what we would expect to see if the star is moving toward or away from us is that those lines would be shifted to the left or to the right. 

So when you look at a galaxy, a galaxy is more or less made up of just a lot of stars. And so the spectrum of a whole galaxy will look approximately like the spectrum of the star. What Hubble noticed was that for really large galaxies, that spectrum looked a lot like a star, but it was shifted a little bit to the red, which would suggest that that galaxy is moving away from us at a relatively small speed. Now when I say small, it's actually insanely fast. But on the scale of the universe, it was relatively small. 

And then if you look at smaller galaxies, and galaxies that look smaller on the scale, this is the thing about this would mean if a galaxy looks a little bit smaller, it could be that it's actually a smaller galaxy or it could be that it's the same kind of galaxy that's just further away. And this is where that classification, Hubble's classification could come in handy. Because if you see two galaxies that look basically like they're the same shape, right? Maybe they're both spirals, they both have the same kind of spiral structure, then it's not unreasonable, in fact, I'd say it's reasonable to assume that they're probably the same size roughly. All right. So if they're actually the same size, but one looks smaller, it must be further away. And in fact, what Hubble saw was that those smaller galaxies objects that those galaxies that just look smaller on the picture, their spectra are shifted even further to the red, which suggests, you're measuring the speed with that shift. So the further away this galaxy is, the faster it's moving away. 

And if you look at even smaller galaxies, they're shifting even more, so they're moving even faster. Now, the trick here is that Hubble was able to use not just the size of the galaxies, but he could actually use measured distances of the galaxies so you don't just have to assume that they're the same size, and that the smaller ones are just further but he could use Cepheid variable stars to literally measure the distance to the galaxy. And then you can say, well, how does the speed of the galaxies moving relate to how far away it is? 

And this is what Hubble discovered. We call it Hubble's law. And it's this graph, which is the distance to a galaxy versus how fast that galaxy is moving. And one of the things you'll notice about this graph is that all of the speeds of these galaxies show that the galaxies are moving away from us. They are all moving away from us. There's only one or two that move towards us, but they're local galaxies that are kind of connected by gravity to us. All the galaxies we observe in the sky are moving away from us and we're moving away from them. That's an interesting thing to know. 

The other really interesting thing is that the further I mean, it's a very nice fit here with more or less a straight line. The further away the galaxy is, the faster it's moving away from us. This is interesting. Okay, so something's far away. It's moving faster. And what this implies is that there is some expansion going on. It's not that we're at the center and all the galaxies are flying away from us, but it's that we're all in space, and space itself is expanding. And that can account for exactly this kind of motion. 

And one way to think about that is, is thinking about living on the surface of the earth. And if the Earth were being blown up bigger and bigger like a balloon you can imagine Okay, so from where I live, from here to Detroit, it's about 100 miles, let's say. If the Earth got twice as big, it would be 200 miles. From here to LA it's a 1000 miles. If you had twice as big, it would be 2000 miles. So now let's compare. Let's see the comparison. In that one hour, let's say that it took the earth to get way bigger. The distance between me and Detroit only grew by 100 miles, that's 100 miles per hour. That's how fast Detroit was moving away from me or I was moving away from Detroit. Whereas for LA, it moved a 1000 miles away. It moved 1000 miles in that hour, a much greater speed. But really, the expansion was a constant. Everything in the whole earth expanded at the same rate, but something as far away as it moved away faster, just because it was farther away. It's the nature of expansion. 

A way that's often illustrated is what's sometimes called like a raisin bread analogy. So if you have a loaf of raisin bread, you're going to bake and you have raisins that are all equally spread. In this in this case, this is the fifth picture in the gallery. Each of these raisins it starts off as one centimeter apart. All right, and we are our galaxy is one of those raisins. Well, sometime later, let's say this loaf of raisin bread gets three times bigger. So let's maybe what one hour later, should we say. Okay, well the closest galaxy arrays into us went from one centimeter to three centimeters, okay, so it went to centimeters per hour that was it speed of recession. How fast it moved away from us. The farthest one went from three centimeters away to nine centimeters away, it went six centimeters per second. Right. So it's the same idea. So this is sort of a three dimensional model. Right? 

So everything we see in space and these galaxies and things within our galaxy are staying near us. But the other galaxies are flying away at tremendous speeds. The further away they are, the faster they're going. Even if we look to the very edge of the universe. They're so far away. They're moving extremely fast away from us. This implies that our universe is expanding. That was an enormous discovery. I mean, it could have been anything. If you think about Hubble was looking at this event that galaxies were all orbiting around some other center. Maybe there was a center of the galaxy, some gravitational center that all the galaxies were orbiting around and could have been discovered that just like our Milky Way, we found that all of these stars are moving around the center. But that's not the way our universe is organized. Our universe is organized that everything is moving apart. 

And as we'll see as we get into the next lesson, that implies some very interesting things about how our universe began, and how our universe continues to change. Okay, we'll see you next time.