Preposterous Universe

Sunday, March 21, 2004
Inflation and the microwave background

Peter Woit has some questions for cosmologists. Too many good ones to answer in a brief comment, or even a single post, but let's tackle the first big one, which is fun to talk about in its own right.

The general question is "What can observations of the cosmic microwave background tell us about physics at very high energies?" Since there's no reason why non-experts shouldn't follow the discussion, I should explain briefly what the microwave background is and why "high energies" are interesting. It will take some time, but it's not useless for me, since I'm supposed to be writing some public-education web pages for the Kavli Institute for Cosmological Physics (formerly "Center for Cosmological Physics") here at Chicago, and I'll use this as a rough draft.

The cosmic microwave background (CMB) is the leftover radiation from the Big Bang. When the universe was much smaller it was much hotter, emitting blackbody radiation just like anything else, and we can detect that radiation today. The very early universe was so hot that it was opaque, since the electrons were ripped off of individual atoms and photons kept bumping into them. At 370,000 years after the Big Bang, the temperature had dipped below about 3,500 Kelvin, cool enough for electrons to recombine with nuclei to make atoms, and the universe suddenly became transparent. The radiation subsequently cooled to about 2.7 Kelvin, which is what we see today; it provides a snapshot of what the universe looked like when it was 370,000 years old (it's 13.7 billion years old right now, or about 100 billion dog years).

What it looked like was something extremely smooth; fluctuations in density from place to place were only about one part in 100,000. But we can detect these fluctuations; the image reproduced here is the famous map from the Wilkinson Microwave Anisotropy Probe satellite. Blue regions are slightly colder than average, red regions are slightly hotter. (The WMAP team actually knows physics much better than this color scheme would lead you to believe.)

There is a treasure trove of information contained in these fluctuations. In particular, statistical properties of the fluctuations depend on two things: the primordial perturbations from which they presumably arose, and the recipe of ingredients in our universe that controls the subsequent evolution of the perturbations between early times and now. Remarkably, an extremely simple specification of primordial perturbations works very well -- simply imagining that the perturbations are (on average) of equal strength at all distance scales. From this guess, and the observed fluctuations in the CMB sky, we can derive very tight constraints on interesting cosmological parameters, such as the amount of ordinary matter and dark matter in the universe. See Wayne Hu's tutorial for details.

But the simple guess for the form of the primordial perturbations is actually better than a guess -- it's a prediction of the inflationary universe scenario. Inflation is the idea that the extremely early universe underwent a period of accelerated expansion that stretched a tiny portion of space to the size of our entire observable universe. It was originally invented by Alan Guth and others to help explain precisely why the universe looks so smooth on large scales. See this intro by Ned Wright for more details.

Inflation came with an unexpected bonus. Try as it might, inflation can't make the universe perfectly smooth, simply due to the strictures of quantum mechanics. Heisenberg's uncertainty principle tells us that we can't specify the state of a system with perfect precision; there is always an irreducible jiggliness when we look at, for example, the position of an electron. But the principle holds as true for the entire universe as it does for an electron. So inflation makes the universe as smooth as it can (imagine removing the wrinkles form a sheet by stretching it at the edges), but there is some amount of fluctuation left over -- which, of course, precisely describes the universe we see. All of the stars, galaxies, and large-scale structure in our universe may have started as tiny quantum fluctuations in the primordial soup.

Okay, this has gone on a while already. Next time I'll be more quantitative about the perturbations, and talk about how they might reveal something about physics at very high energies.

Ideas on culture, science, politics.
Sean Carroll

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