Within the first tiny fraction of a second after the Big Bang, the Universe blew up. At least that’s the basic idea behind the theory known as inflation.
While the Universe is expanding today, its growth rate is relatively slow. If inflation is correct, however, things began with a lot more oomph: everything we see today went from a tiny bubble to a substantial size in less time than our most precise clocks can measure. Thanks to its ability to describe a lot of the appearance of the modern Universe, the theory of inflation has joined the Big Bang as part of the most widely accepted way scientists think about the origin of the cosmos.
Inflation made the news last week when researchers announced they had found the best evidence yet for its existence. If these results hold up—and they may very well—then we will have learned something very important about the early cosmos.
So what is inflation?
I am writing this article partly aboard airplanes en route between South Dakota, Texas, and Richmond, Virginia. The weather in all three places is about the same today, which is slightly odd given their different local climates. But it’s also a completely insignificant coincidence. If you compared temperature and precipitation on random dates throughout the year, you wouldn’t find many patterns beyond inanities like “Hey, it’s colder in winter”.
Now imagine if the weather in the three states was nearly identical all the time, despite having more than half of the continental United States between them. You’d be right in suspecting something weird is going on: three widely separated places on Earth can’t and won’t have the same weather.
That’s still less dramatic than the coincidence we see in the early Universe. If you compare any two points on the night sky, their temperature as measured in microwave light is identical to a few millionths of a degree. That light, known as the cosmic microwave background, comes to us from nearly the beginning of the Universe, so it has been traveling for 13.8 billion years. Even with the expansion of the cosmos, two points on opposite sides of the sky were never in the same place, yet they have the same temperature… assuming the current rate of the expansion of the Universe has been roughly the same since the beginning.
But maybe it hasn’t. The cosmic temperature coincidence (which would be a great band name), along with several other annoying aspects of the Universe, led a group of researchers to propose the theory of inflation. In brief, right after the Big Bang, the Universe was a chaotic froth of quantum particles, all banging into each other at high energy. Two bubbles less than a millimeter apart might have radically different temperature, density, and other important properties.
One quantum ingredient—the “inflation field”—in the primordial froth caused the contents of the shaken soda can of the cosmos to expand at a mind-boggling rate. Each tiny bubble expanded in size by a factor of 100 trillion trillion: 1026 in scientific notation, or 100,000,000,000,000,000,000,000,000. It’s a ridiculous number no matter how you write it, akin to my seatmate on this airplane suddenly moving while I lurch the opposite way until a whole galaxy separates us. And inflation was as sudden as it was huge: the whole process began and ended while the cosmos was far less than one second old.
But now we see why the whole observable Universe is nearly the same temperature: our cosmos was one of those primordial bubbles that expanded. Because everything in that bubble was more or less the same temperature, the cosmos we see is nearly the same everywhere we look. Fluctuations inside the bubble also had their effect: they led to galaxies, stars, planets, and physicists who think about inflation while flying on airplanes.
One predicted side effect of inflation is primordial gravitational waves: twisty ripples in the structure of the Universe. These can’t be detected directly by any existing experiment, but they have an effect on any light passing through them, much like water ripples do. Many current experiments are trying to measure that secondary effect.
That’s why last week’s announcement from the muscularly named BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) project was such a big deal. Researchers working with this telescope measured what could be those primordial gravitational waves, which in turn could be produced by inflation—a piece of evidence much stronger than temperature coincidences.
As Sir Mix-a-Lot might say, there’s a big “but” here. The basic idea of inflation is simple and elegant; turning it into a workable theory is more complicated. Additionally, if inflation happened, it was so early on that we can only ever see indirect evidence for it. As a result, researchers have proposed a lot of different variations, each designed to produce various effects or to make inflation mesh with other theories. So, we have theories with more than one inflation field, others with names like “eternal inflation” or “chaotic inflation,” and many excessively complicated models.
To complicate matters, inflation isn’t the only theory that could explain the temperature of the early cosmos; some alternatives can even make primordial gravitational waves. But that’s the way of science: researchers propose ideas to match what observations show, but the ultimate judge is the Universe itself. If a theory works, it stays; if it doesn’t, it must be refined or abandoned. It’s an exciting time, and whatever we discover—inflation or not—will tell us a lot about our Universe’s earliest moments.