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Entropy and Our Cosmic Telephone Number

How can the cosmos have the entropy we see today if the temperature was almost exactly the same everywhere after the Big Bang? The answer may lie in our “cosmic telephone number”.

When it comes to big question, there aren’t much bigger than this: what is the entropy of the Universe, the amount of disorder or “mixedupness” in the cosmos?

Max Tegmark at the Massachusetts Institute of Technology in Cambridge tackles exactly this question in a talk he gave at an MIT Symposium, called “Meeting the Entropy Challenge”, the transcript of which is now on the arXiv. A video of the talk is below.

On the one had is the question: why is the entropy of the universe so low?

“When the universe was much younger, the temperature was almost exactly the same everywhere. We started out 400,000 years after our big bang in a situation where the density was almost perfectly uniform throughout our observable universe, and the temperature was almost exactly the same everywhere to within a part in 10^5,” says Tegmark

“So how could you take something with almost the same temperature everywhere and then make something really hot here and something else really cold? How did that happen?”

The answer, he says, is in two parts. First inflation, this is the mysterious process in which the universe increased its volume by many orders of magnitude in just a fraction of a second. In the process, this magnified any tiny fluctuations that may have existed before inflation. And second gravity, which magnifies any clumpiness by pulling clumps together to form bigger clumps. This eventually forms galaxies, stars, planets and living things like us.

But Tegmark goes on to say that the process of inflation implies that other parts of the Universe that are unobservable to us may have grown from much more uniform parts of the early universe and so be hugely uniform now. And that recent results in string theory hint that the laws of physics may be quite different on those regions.

So the amount of entropy in our part of the Universe is, in a sense, part of the identity of our region. It is a set of coordinates that tell us what part of the universe we are in. “Like a cosmic telephone number” is how Tegmark describes it.

That’s an extraordinary idea. But then cosmology is filled with fantastic thinkers with crazy ideas and always has been. If you get a chance to watch the talk, it’s easier to digest than the transcript but both are recommended.

Ref: arxiv.org/abs/0904.3931: The Second Law and Cosmology

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