Julien Lesgourgues – TRANSCRIPT
When I was a PhD student, which was actually not so long ago, we only had a vague idea about the history and the composition of our universe. There was still a lot of room for speculation on various ideas.
But since then, we, cosmologists, have experienced a burst of new observations and discoveries over the range of just 20 years. We now understand several details about the history of our universe over the past 13 billion years. And believe me, it has been terribly exciting to be part of a generation of physicists that, at a unique moment in the history of mankind, have been able for the first time, on a reliable basis, to understand what our universe looks like on very large scales. Humans have been making assumptions about that since ever, but the actual reality of our universe has been unveiled to us precisely over the past two decades. And for me, this is astonishing.
So let me illustrate more concretely what I mean when I say that we now understand our universe. For instance, these two pies show, at two very different moments, the cosmic recipe, that is, the composition of the universe in terms of different particles, like atoms, neutrinos, dark matter, et cetera. What is remarkable is that we are able to pinpoint the amount of each of these ingredients with a precision of a percent, despite the fact that most of these species cannot leave any track in our detectors, and despite the fact that the universe is, of course, far too big for sending detectors all around. This is a bit as if by observing a cake and not even tasting it, we could tell its recipe with percent precision.
Second example. This chronology shows that our universe over the past 13 billion years went through four different stages, each of them with extremely different properties. During the first stage, our universe was very dense, very homogeneous and in an ultrafast expansion. During the second stage, the whole universe was extremely hot and bright – a bit like the interior of a star, everywhere – and the expansion started to slow down. During the third stage, everything became cold and dark, and the first galaxies formed, separated by huge regions of empty space. Finally, during the fourth stage, the expansion of the universe started to accelerate again.
Amazingly, we have very strong proofs that each of these epochs really occurred, and we can even date them with high precision. So, the fascinating question that I want to address today is: but how could we understand all these details from our tiny planet? And how can we be so sure, while hundreds of generations before us have tried to reach this knowledge without being able to go beyond the level of assumptions or beliefs? The answer is connected to the word “spectrum.”
We understand all this because we have been able to measure the spectrum of the universe. A spectrum is a quantity used by scientists to describe anything in nature that vibrates, or fluctuates with time, or varies over space, like any sounds, or any lights, or any image. The spectrum is the decomposition of the sound, of the light, of the image into various frequencies or wavelengths.
So, for instance, any sound has its own spectrum. To show it, let me open on my laptop a sound analyzer – that’s it – and just blow this flute. (Flute sound) OK, that’s it. On the right, you see the spectrum of the sound that we just heard, and each music instrument has its own spectrum. Even two flutes or two violins can be distinguished through tiny differences in their spectrum, so the spectrum is really like the fingerprint of a given sound.
But what goes for a sound also goes for an image. Here is a famous painting, “The Garden of Earthly Delights.” This image can be decomposed into a sum of patterns of different frequencies, and the spectrum is just the amount of each frequency or wavelength. So, for instance, here is the spectrum of wavelengths in this particular image. Again, it’s like the fingerprint of the image.