MIT-The Second Law and Cosmology

In spite of its old age, the Second Law of Thermodynamics “is alive and kicking,” says 
Max Tegmark, stimulating research on “really, really big puzzles.” In Tegmark’s case, “big” encompasses the cosmos, and investigating the entropy of the universe offers one path into understanding “how we started out.”

Tegmark frames his talk with paradoxical questions: Why is entropy so low, and why is entropy so high? The first question is “crucial to understanding the arrow of time,” and involves the microscopic definition of entropy. 13.7 billion years after the Big Bang, entropy in the observable universe is in “the ballpark of 10⁸⁹ bits — crudely speaking, a google.” This is much lower than the theoretical limit to how much entropy our cosmos could contain. Also, Tegmark wonders, why has our solar system ended up so far from thermal equilibrium, since when the universe was younger, the temperature was almost the same everywhere?

It turns out that in cosmology, unlike classical physics, atoms start out at uniform density and end up, abetted by gravity, “clumpy,” with gas getting denser and forming stars. Tegmark shows a supercomputer simulation of this process, which depicts the evolution of a universe with galaxies and solar systems like our own. Different temperatures in the universe aren’t due to magic, he says, just Einstein’s theory of gravity and basic gas physics.

But, Tegmark ponders, why was the universe uniform in the beginning? One “crazy sounding answer” involves inflation. A tiny region of space much smaller than an atom, which is very uniform and very dense, begins to expand exponentially, until it makes up all space in our known universe. It gets weirder. Tegmark invokes inflation to explain not only the low entropy of the cosmos, but its high entropy as well. That same 10⁸⁹bits can also be viewed as “such a big number that it suggests…that we’re in some kind of multiverse, or some much larger reality than what we can observe.” The initial conditions that make up these 10 to the 89th bits “just tell us where in space we live, our address in space.” We should call the Big Bang “not the beginning but the end of inflation in this part of space. … If we zoom out in the universe, we should expect to see much more entropy.” If you don’t get this intuitively, that’s OK, Tegmark reassures us, but “if we categorically reject ideas in science just because they feel crazy, we will probably reject whatever the correct theory is, too.”

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