String Theory and Cosmology

Here, finally, is the promised second installment on the subject of string theory and cosmology.

In the previous post, I tried to give a relatively quick overview of what "string theory" is, more or less.  Basically, it's a proposed re-envisioning of the fundamental structure of matter that would help to solve some of the outstanding questions in particle physics.  So, what does this have to do with cosmology - the study of the physics and history of the universe as a whole?

If you're familiar with the idea of the Big Bang, you know that the early universe was a hot and dense place. So hot, and so dense, that the sorts of objects we see in the universe today (stars, galaxies, and ourselves) could not have existed.  The whole universe was filled with a sea of interacting particles and radiation, for the first few hundred thousand years of its existence.   As you imagine 'running the movie backwards' and ask about the earlier and earlier moments after the Big Bang, you have to imagine the universe getting hotter and hotter, and denser and denser, the closer you get to that initial moment.

There are limits to what telescopes, looking out farther and farther, can tell us about the hot, dense early universe.  Instead, we turn to high-energy particle accelerators here on earth.  By smashing particles into each other at higher and higher energies, we are able to momentarily mimic the conditions of the earliest universe.  The hope is that as we learn more about the behavior and structure of matter at high energies, we will be able to understand what must have happened in the very earliest universe.  This is why there are deep ties between cosmology and particle physics:  to understand the early universe, we need to understand matter in extreme conditions.   These extreme conditions are where current particle physics theory starts to break down:  we don't know what mathematical formulas or physical models to apply for particles and their interactions at the highest energies that are relevant in upcoming accelerator experiments and in the Big Bang.

String theory definitely is one theory that could fit the bill.  If it turns out to be true, then many physicists hope it will help to answer some outstanding puzzles in cosmology theory too.  One of the biggest of these is the mystery of "inflation", which is a proposed period of rapid expansion in the first 10^-36 seconds. While we have a number of indirect lines of evidence that inflation occurred, existing particle physics theory doesn't have a way to explain why it might have happened.  Perhaps string theory will provide an answer?

So, to sum up:  the primary reason that string theory is relevant for cosmology is that it could provide a more complete and fundamental description of matter applicable even to the extreme conditions of the Big Bang.  A deeper theory of particle physics is definitely one of the major pieces needed for better understanding the history of the universe.  There are other (somewhat more exotic) ways that string theory either might impact cosmological phenomena, or even suggest a different philosophical viewpoint on the nature of physical law (what I'm alluding to here is the so-called anthropic principle, which would take another blog post to describe).  However, the main connection between string theory and cosmology is through the mysterious physics of the hot, dense early universe.