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Skip to 0 minutes and 17 secondsWe're trying to understand how gravity affects the Universe, how it evolved through space and time. And that's a long, complicated task. Fortunately, we have a tool that makes that much more straightforward. That the Universe is very big, and though the speed of light is extremely fast, 300,000 kilometres per second, the distances are so large that it takes light a very long time to get to us. So if we look out to different distances, we see the Universe at different epochs, different epochs in its existence. So we are here on the Earth, with our eyes and our instruments looking out.

Skip to 1 minute and 1 secondAnd if we look out how far is the moon, we see that it takes light about a second to get to us from the moon. So we take a picture and we look at it, we see the moon like a second ago. That's not so important. But we look at the sun, and we're seeing light that came to us eight minutes ago. Again, not too long. Although if we took a picture and waited eight minutes to develop, you would be upset. But eight minutes is not so much. It's relevant, though, when there's a big solar flare. You can get pictures of it. The light takes only eight minutes.

Skip to 1 minute and 37 secondsBut the flare can take a couple of days or more to get to you and affects your navigation, affects your communications, thousands of kinds of things you want to know. Those are relatively short distances. You go further, you go to Jupiter, and it takes almost an hour. It takes 40 something minutes for the light to get from Jupiter to here. So depending on where Jupiter is in its orbit, the time where we see the various Galilean satellites varies, just because of the light travel time. And so we can use that as a clock to see what's going on in the solar system. You go out much further, you get past hours, even to the month inside of the solar system.

Skip to 2 minutes and 16 secondsBut to get to the nearest stars, it takes light-- the very closest star, it takes light almost 4.4 years to get here. But the typical 15 nearest stars, it takes like 10 years. So you take a picture of those stars, it's not what they look like now, it's what they looked like 10 years ago. But if you take a typical star in our galaxy, the light takes anywhere from those 10 years up to almost 100,000 years, that is, 100 times 1,000 years for the light to get here. That's just our own galaxy. So when we take pictures of stars in our own galaxy, we're seeing them years ago, thousands of years ago, tens of thousands of years ago.

Skip to 3 minutes and 0 secondsWhat if we look to the nearest big galaxy, Andromeda galaxy? It takes light nearly 2 million years to get here from Andromeda. So if you were a big astronomer or a class studying a MOOC and they were showing you pictures of the earth, that were taken by the best telescope on Andromeda, what would you see? You would see no evidence of humans. Because that's when we think the first humans started to arise on the Earth. There would be no Great Wall. There would be no city lights. There would be nothings. That tells you some of the issues about looking for evidence of life on other planets.

Skip to 3 minutes and 35 secondsBut it tells you that even our nearest galaxy neighbours, are seeing us what we looked like two million years ago. And we're seeing them, what they looked like two million years ago. But communicating with them would take a very long time. But that's the nearest big galaxy. A typical galaxy, the light would take anywhere from those millions of years up to billions of years, few billions of years. And so for most of the first big cluster near us, it takes light some tens of millions of years to come to us. So we are not seeing those clusters the way they are right now.

Skip to 4 minutes and 13 secondsWe are seeing them the way they were millions of years ago, tens of millions of years ago, or 100 millions years ago, in the case of most, and some, a billion years ago. So as we look out from the Earth to bigger and bigger distances, the light is taking longer to get to us. If we look at the light now, it had to start out that much time earlier, that distance divided by the speed of light. So if we measure in units of light-years it tells us how far back in time we are going. We will see around us a set of sample of spheres that are samples of the Universe at different epochs, different times.

Skip to 4 minutes and 49 secondsAnd we can do that on the scale of a year, thousands of years, millions of years, billions of years. We can go out almost to 14 billion years. And at that point, because the Universe is expanding, it was much hotter and much brighter than it is today, and it becomes like looking at the surface of the sun. And we call that radiation, which we'll talk about later, the cosmic background radiation, because it surrounds us and that light is coming from everywhere. And shortly before that, we believe, is the beginning of our present Universe. So the big issue, the big thing that we're seeing is when we look around us, we see what we are seeing now.

Skip to 5 minutes and 31 secondsWe look further out, and we're seeing backwards in time, further and further out, we see backwards in time. So when we do surveys of the galaxies and of the objects that we see out in the Universe, we're seeing them in different epochs, just each corresponding to it.

Skip to 5 minutes and 49 secondsSo I have another slide which shows the cosmic sphere. We live in our galaxy. We're not in the centre of the galaxy. We're in the spiral arm. We consider spheres around us. We see what we think of as modern galaxies, big elliptical galaxies, big spiral galaxies. They're kind of yellow coloured, like the sun, for the most part. And they have a few other bright stars. But if we look out even further, we see the first primordial galaxies showing up, the smaller rarer galaxies that are undergoing starburst, a frenzy of activities. They show up in the white and the blue. Many of those will merge together and form the galaxies we see at the present time.

Skip to 6 minutes and 31 secondsSo the thing that we will see is that there is, we're thinking, we're making the maps of series of spheres around us, we see galaxies as they look back further and further in time, until we get to a point where there are no galaxies before any stars and galaxies were formed. We call those the Dark Ages. But that's a time when the structure is starting to form. And then, further than that, the Universe is too hot. It's like the sun. Nothing can be formed. And then the really interesting, high energy physics happens in the earlier phase and we have the beginning of time. And so the thing that's really relevant is that the Universe is actually very big.

Skip to 7 minutes and 14 secondsAnd what we will see is that there is an incredible number of galaxies out there. There are many billions of galaxies that we can see inside of spheres to which light can get to us. And there are many other things out there that we will find in our journey along the way.

Travel in the Universe with George Smoot

George Smoot takes you to a trip in space and time, away from the Earth to the outskirts of our visible universe, that is to the dawn of time (7:53)

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This video is from the free online course:

Gravity! From the Big Bang to Black Holes

Paris Diderot

Course highlights Get a taste of this course before you join:

  • Galileo and the falling bodies
    Galileo and the falling bodies

    Watch Pierre Binétruy explain how Galileo found the law of free, using a combination of real (inclined plane) and thought experiments (12 minutes)

  • First encounter with relativity
    First encounter with relativity

    Pierre Binétruy focuses on the notion of inertia, measured by mass, frame of reference and Galileo's principle of relativity