Cosmology in Ten Minutes

by Danielle Fong

Recently, unusual features of the cosmic microwave background, a ‘snapshot’ of the early universe, have raised issues with our understanding of the Big Bang. A Caltech team has shown how we might fix our theories. They suggest that there might have been an asymmetry in the energy that once powered the big bang. If this is correct, anomalies in the CMB may be traces of structure from a time before our explosive beginnings.

True to form, when a discussion appeared on Hacker News I rushed to comment, and this article erupted from that attempt. The current scientific understanding of our cosmic origins is a mystery to the public at large, but it was only after I noticed the bewilderment of my fellow hackers that I realized how poor a job we scientists have done in conveying the motivation behind our discoveries.

This article represents an attempt to replace that sense of bewilderment with that of wonder. I want more than to explain what cosmologists believe. I want give people a deep sense of why we believe it, of how we’ve come to our current understanding, and of why we care.

Look close, and it seems the universe is lopsided.

The cosmic microwave background (CMB) is like a snapshot of the early universe. It was once all hot plasma, gas so hot that the atoms inside it were broken up. Because it was hot, it emitted light. Because it was dense, it was opaque: the light emitted couldn’t just pass through, instead it had to bounce around. But once cool enough, the universe became transparent: all the light could now travel freely. It was as if the photographic shutter of the universe was lifted.

The Cosmic Microwave Background Radiation (from WMAP). False Color/ The light from this moment became the cosmic microwave background radiation. Because the universe seemed to have cooled at almost exactly the same time everywhere, the CMB is, unlike almost everything else in astronomy1, a picture of the entire universe at almost exactly the same moment in time. It is the best picture we have of the structure of the early universe.

The universe appears to have expanded evenly since then. We know it’s expanding now. Light is like a wave. Since the speed of light is constant, an illuminated object moving towards us has its wave crests squish together, turning bluer, and an object moving away from us has the distance between crests expand, turning more red. This is called a red shift. Since he knew the colors of certain celestial objects, Edwin Hubble was able to observe that the further something is from us, the more red-shifted its light, and therefore the faster it is speeding away.

Since we know that the early universe was hot, dense and small, and we know now that it’s cooler, sparse, big, and expanding, we can reasonably deduce that, long ago, there was a Big Bang. The universe exploded.

The Crab Nebula Strikingly, the CMB is almost the same everywhere you look. There are minor fluctuations, but even they seem to have the same distribution everywhere. The CMB, our best picture of the early universe, is extraordinarily smooth. It is one of the smoothest things ever observed in nature. This might not seem like a mystery. You might imagine that anything expanding, hot and dense would look roughly the same in all directions. It needn’t. Nebulae are formed by exploding stars, and they aren’t particularly smooth. In fact, in nature, it would seem, more often than not, that explosions are messy.

In 1981, Alan Guth suggested what might be called a ‘recipe for a universe': inflation theory. Until then, nobody had come up with any good ideas for why the universe was so smooth and even. It is as if God2 had pressed the entire universe with a cosmic clothes iron.

Guth said, suppose you started with pretty much any initial universe. Suppose you also had an extremely strong, extremely smooth field of energy. If this field started dumping energy into the rest of the universe, it would also evenly expand space itself.3 The universe would undergo a period of exponential expansion — inflation — having the effect of flattening and smoothing the rest of the universe. Inflation is God’s clothing iron.

A flat, smooth universe isn’t the only thing that inflation predicted. For example, at small physical scales, quantum mechanical fluctuations persist. During inflation these fluctuations are blown up as well, and these would seed, almost entirely, the cosmological structure of the universe. We see these fluctuations in the CMB. According to inflation, they are tiny quantum fluctuations blown up to a cosmic scale. They are, quite literally, the ancestors of our galaxies.

It wasn’t just that there were fluctuations. Inflation theory predicted a very specific distribution and type4. When people finally had the technological capability to check, that’s just what they found. The universe appeared, at a cosmic scale, astonishingly consistent with this simple theory. Yet recently our observational capacities have improved. A CMB survey called WMAP has uncovered several surprising and unexplained features, not all of which fit well with the our previous inflation theories.

If you divided the sky in half by tracing the orbit of the earth around the sun5, and compared, in each half, the size of big fluctuations, those between 3 and 5 degrees wide, you would come to the conclusion that one side has fluctuations outweighing the other by an alarmingly large amount. One side of the universe is bumpier than the other. Moreover, the difference is larger than would be accounted for by randomness, at least 99 times out of 100.6

This asymmetry looks real. It has been checked against every known experimental error and background effect astrophysicists have been able to think of. And if it is real, our previous inflation theories, with one field of energy to inflate the early universe, won’t work. They can’t account for this anomaly.

The authors Erickcek, Kamionkowski, and Carroll don’t merely point out this problem. They posit a solution. They describe another inflation model, consistent with our new observations. They suggest the universe had not one, but many fields of universe inflating energy. There’s just one problem. At least one of these fields needs to be asymmetric.

Where could such an asymmetry come from? It is possible that we’ll never know. Cosmology offers us the hope of uncovering consistent, compelling stories of our origins. Thousands of independent observations fit neatly in cosmology’s book. But while we may discover a few lost pages from our first chapters, we may never know all reasons why our book was written in the first place.7

Nevertheless, the authors make an exciting point. Wherever the asymmetry in the inflation field came from, it must have existed before inflation. It must have existed before the big bang. We had once imagined that time before our explosive beginnings would forever remain a mystery. Yet hidden in the CMB are hints of times earlier still. In this wonderful piece of work, the authors carefully consider what anomalies in the CMB could mean. And in the process, they may have discovered a way to look farther into the past than ever before.

Notes:

[1] – Since light moves at a finite speed, when we see something far away, we’re seeing light emitted in the past. What we see of something a light-year away is (at least) one year old.

[2] – I mean ‘God’ here as in a figure of speech. Feel free to substitute ‘Mother Nature’, ‘Allah’, or the ‘Flying Spaghetti Monster’ while reading.

[3] – What does it mean, exactly, for energy to expand space itself? It’s roughly analogous to blowing up a balloon. We know that the gravity of the universe, just like the elastic outside walls of a balloon, pull its contents inward. In a balloon, air pressure pushes against that inward force of the walls. During cosmic inflation, the inflationary force pushes against gravity. There’s one important difference though. We don’t actually know what the inflationary force is. Air blows up our balloons, but we have few clues as to what blew up the universe.

[4] – The quantum fluctuations predicted by inflation follow a nearly-scale-invariant random Gaussian distribution. These fluctuations show up in the CMB, and for the most part follow these predictions pretty closely.

[5] – The line dividing the two halves of the sky here is called the ecliptic.

[6] – Formally, this statement is true at at least the 99% confidence level.

[7] – There are some questions forever beyond our grasp. Even if we knew from where the Big Bang had come, we could always probe further, and ask where that came from.