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Skip to 0 minutes and 31 secondsAt the beginning of this third week, let us see what we have learned about gravity so far. I said earlier that gravity is the engine of the Universe. So that means that the gravitational force is the fundamental force which is responsible for the expansion of the Universe. So let me explain a little more what this means. Our first experience of gravity is the attractive force between two masses. We have seen what has led from Galileo to Newton and to Einstein, to Einstein's equations, which are a novel way of seeing the gravitational force. Now of course, Einstein's equations include this attractive force between two masses. But there is much more to Einstein's equations than just this attractive force.

Skip to 1 minute and 28 secondsOne consequence, as we have seen, is the fact that there are solutions to the evolution of the Universe, solutions of the Einstein's equations, which correspond to an expanding Universe. So that means that the expansion of Universe can be derived from Einstein's equations. And so it's a consequence of the theory of gravity. And so it is in this sense that I say that gravity is the engine of the Universe, is responsible for the expansion of the Universe. Now what is this expansion? We have seen that distant galaxies from us seem to recede at velocity which is all the greater the further the galaxy is, the more distant the galaxy is.

Skip to 2 minutes and 15 secondsAnd so this is due to the fact that it's really the texture of space time between us and the distant galaxy, which is expanding. And it is this effect that is responsible for the Hubble's law, which says that the velocity of recession of the distant galaxy is proportional to its distance. Now, an expanding Universe is cooling down and is getting less and less dense. So when we get back in time, to the origin of the Universe, when we get to a universe which is more dense and also hotter, more energetic Universe, and so the primordial Universe is an energetic version of our universe where all structures have been deleted.

Skip to 3 minutes and 7 secondsAnd it can be considered as a very dense soup of elementary particles.

Skip to 3 minutes and 19 secondsThis week we will study the very early Universe. Right after the Big Bang, the Universe was opaque. That means that light was emitted, but was immediately trapped by the elementary particles, and could not propagate through the Universe. Some 380,000 years after the Big Bang, one believes that the Universe underwent a transformation and became transparent to light. And so this means that the light emitted at that time is still travelling through the Universe at present times. And so one goal of cosmology is to study this very early light with instruments like this one, which is a Planck satellite, which was launched in 2009, to have very detailed studies of this first light in the Universe.

Skip to 4 minutes and 13 secondsSo we'll try to understand the origin of this mechanism, the origin of this first light, and how one detects it. And we will welcome George Smoot, who was awarded the Nobel Prize for discovering tiny fluctuations in this light, fluctuations that give us very precious information about the dynamics of the Universe at those very early times.

Where do we stand?

It is time to summarize what we have learned so far about gravity, and to realize what it means to say that “gravity is the engine of the Universe”. This week, we will be interested in what is sometimes known as the first light in the Universe, and George Smoot will explain how he discovered tiny fluctuations in this light which carry key information on the first instants after the Big Bang. (4:46)

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This video is from the free online course:

Gravity! From the Big Bang to Black Holes

Paris Diderot

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    Galileo and the falling bodies

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    First encounter with relativity

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

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