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## Paris Diderot

Skip to 0 minutes and 14 secondsWe have just seen that the Universe consists of a very large number of galaxies. This Universe is in expansion. That means that galaxies apparently move away from us, as do the points which were marked on a balloon which I inflated. The stars of our galaxy are not receding from us, because they are attracted by the gravitational force exerted by the galaxy.

Skip to 0 minutes and 45 secondsThe velocity of light is finite. Since when does one know this fact? Well, there were speculations in the times of the ancient Greeks. Galileo even tried to measure the velocity of light by suddenly uncovering lanterns. But it is only in the 17th century with the Danish physicist, Romer, that there is the first try of measuring the velocity of light. Well, precise measurements were performed only in 1850 by two French physicists, Fizeau and Foucault, in a sort of competition of who would come first. And let me describe here the experiment performed by Fizeau.

Skip to 1 minute and 30 secondsFizeau was doing his experiment in the centre of Paris, in Montmartre, and sending a light beam to a place something like a little more than eight kilometres away, in Mont Valerien, where it was reflected and coming back. And Fizeau was using a cogwheel, that you see in the middle of this slide, which was turning at a very fast pace. And so the light would go from one cog on the way to Mont Valerien, on another cog on the way back. And by measuring the number of cogs in between, Fizeau could determine the time it took to go back and forth.

Skip to 2 minutes and 11 secondsWell, actually, the competition was won by Foucault, who had determined a few days earlier than Fizeau a velocity of light of 298,000 kilometres per second, pretty close to the precise value but we know now, which is 299,792.458 kilometres per second.

Skip to 2 minutes and 41 secondsLet us observe a sky on a beautiful starry night. You have all these luminous dots, which we call stars. But some of them are planets, so that means they don't emit light. They are just reflecting the light of the sun. Most of these dots are indeed stars. The stars of our own galaxy. And because the solar system is on the side of our galaxy, the Milky Way, there is a concentration in the certain direction, which is what we call precisely the Milky Way. And only a few of these luminous dots are not even stars but galaxies. And so we are observing the Universe today.

Skip to 3 minutes and 33 secondsI just said that when we observe the night sky, we see the Universe today. Well, that's was wrong. Actually, we see the Universe in the past. And the reason is the finiteness of the velocity of light. I have here this simple setup of two parallel mirrors with multiple reflections between the two mirrors in order to sort of give you some idea about this observation in the past. Now, because of the finiteness of the velocity of light, it takes a certain amount of time for the light to go back and forth between the two mirrors. And so that means that the reflection of myself in these mirrors is a reflection in the past.

Skip to 4 minutes and 20 secondsSo for an example, the fifth image of myself is slightly in the past, compared to where I am standing right now. And so for example, if you could see the reflection one kilometre from here, that would be slightly in the past. That would be 1/300,000 fraction of a second before the time I'm speaking.

Skip to 4 minutes and 53 secondsLet us return to our observation of the night sky. We see, for example, the Moon, but the light was emitted 1.28 seconds before. For the sun, the light was emitted 8.32 minutes before. Jupiter emitted this light 40 minutes before. Now, if you go to Alpha Centauri, which is the nearest system of stars, the light that we receive now was emitted by Alpha Centauri 4.2 years before. If you go to the nearest galaxy, the Andromeda galaxy, then the light was emitted by the galaxy 2.5 million years before.

Skip to 5 minutes and 44 secondsSo that means that, in a sense, we don't know what is the status of the Andromeda galaxy now, because the information that we get from this galaxy dates back to 2.5 million years ago. And vice versa. What people in Andromeda see of the Earth, as the status, the state of the Earth, 2.5 million years ago, roughly when mankind appeared. And so they have to bet that this civilisation that was starting has evolved into a fairly advanced civilisation in order to be able to receive that message. The messages they send today will reach us in 2.5 million years.

Skip to 6 minutes and 37 secondsSince I mentioned the distance between the Earth and the Moon, let me draw your attention to a small object that was left on the surface of the Moon by the Apollo missions. You see, at the centre of this picture, a small reflector, a small mirror, that was left there and which is being used to reflect a laser beam which is being sent from the surface of the Earth.

Skip to 7 minutes and 3 secondsYou have here a station in the US with a very powerful laser beam that is sent to the reflector, back to the Earth, and that allows to measure very precisely the distance between the Earth and the Moon, and also to see its variation over time, and that allows to have some precise measurement of some of the predictions of general relativity and that allows some stringent tests of general relativity.

Skip to 7 minutes and 41 secondsTo sum up, we have seen that the Universe is in expansion. And we have just discussed the finiteness of the velocity of light. This has some important consequences. One of them is that the further one observes, the earlier in time. And most importantly, one is not observing the whole Universe, just a slice of it in space and time.

# The history of the Universe unrolls in front of our eyes

A quick reminder on the finiteness of the velocity of light: how did physicist succeed in measuring this very large velocity (approximately 300 000 km/s)? This has far reaching consequences for our observation of the Universe: the light that we receive from a distant star or galaxy was emitted in the past (this video lasts 8:16 minutes).