Big Bang?

We have come so far in the last 100 years, and so has our picture of the Universe. From an island galaxy ruled by Newton's gravity and classical electromagnetism, we've come through the discovery of general relativity, the expanding Universe, the need for dark matter, the big bang, the synthesis of all the elements in the Universe, and, for good measure, we walked on the Moon. By the 1970s, we had a fabulous picture of the History of the Universe.

There's just one (huge) problem: What caused the Big Bang? We know the laws of gravity and quantum mechanics, and we know that the Universe is finite in age, expanding, cooling, and bathed in the afterglow of the Big Bang. As far as we could tell, galaxies and clusters of galaxies looked exactly as they should, and the only cosmological problem left was the one of the dark matter holding clusters and galaxies together.

But a closer look revealed a number of problems. First off, this "leftover glow" from the Big Bang was the same exact temperature everywhere. Why? Why would this be the case? After all, if you look in one direction, you find a temperature of 2.725 Kelvin, and it comes from a distance of around 46 billion light-years away. But in the opposite direction, 46 billion light-years the other way, the temperature is also 2.725 Kelvin. How could this be, if these two things never touched each other? It takes time for temperatures to even out; this is why the people in the back seat of your car always complain about a lack of air conditioning in the summer! Even today, we know that the temperature difference in any two parts of the sky is only a few hundred thousandths of a degree:

So, that's the first problem. Why is the Universe the same temperature everywhere?

But that's not the only problem. If you take a look outside, the Earth looks pretty flat to you, doesn't it. We know it's a sphere, but the reason it looks flat to us is because we can only see a tiny area of it. What about the Universe? Well, we can imagine three possible "shapes" for the Universe: flat, sphere-like, or saddle-like:

What we observe is not only that the Universe is flat, but it's so flat that, back in the early stages of the big bang, it had to be flat to 1 part in 10^51! This is so unlikely, it would be like throwing a dart at the entire Earth and hitting the correct atom.

Furthermore, there were other problems as well, such as:

• What provided the tiny, gravitational imperfections that allowed stars, galaxies, and clusters of galaxies to form?
• Why, if the Universe was so hot early on, are there no stable relics (like magnetic monopoles, for example) left over? And finally,
• How did we wind up with a Universe that was hot, dense, and expanding in the first place?

There were a number of very smart people working on these problems, many of whom made great contributions. But it was a (then) young MIT physicist who figured it out:

The theory of Cosmological Inflation was put forth by Alan Guth in late 1979, and by time the paper was published in early 1980, practically every cosmologist on the planet was working on it.

Here's what Guth's inflation says. Start with a completely random Universe. Maybe some parts are expanding, maybe some parts are contracting, maybe some parts are hot, maybe some parts are cold. But in one (perhaps miniscule) location, you get the right conditions for inflation. What inflation does is it takes this one tiny region of space, and inflates it, like a high pressure hose inflating an infinitely stretchable balloon. Regardless of what the Universe looked like before inflation, after only a tiny fraction of a second of inflation, the Universe will be stretched flat, will be empty, expanding exponentially fast, and will be unstable.

The exponential expansion solves most of the above problems. Things can be the same temperature everywhere because the tiny region where inflation started -- that gives rise to our Universe -- could easily have been uniform enough to give us the same temperature everywhere in the Universe. The Universe is flat, because inflation stretched it so that it appears flat. (Take a look at this balloon from the ant's perspective if you don't believe it.)

And, the tiny little imperfections that give rise to stars, galaxies, and clusters can be created in a very clever way. Empty space, according to the laws of quantum mechanics, isn't so empty. Tiny little pairs of particles and antiparticles, waves and anti-waves, are popping in and out of existence all the time. But in an exponentially expanding Universe, the space between them gets stretched so far that they can't find each other again to annihilate, and this creates slight differences in densities that persist to this day.

But perhaps the most remarkable thing about inflation is that it's unstable! This exponentially expanding space is full of this mysterious energy, but since E=mc^2, we can use this energy to make matter! And that's precisely what happens. This unstable energy converts into photons, quarks, electrons, neutrinos, and all the types of matter and antimatter that are physically possible. At the end of Inflation, this gives us a Universe that is:

1. roughly the same temperature everywhere,
2. necessarily flat (or indiscernible from flat),
3. devoid of any crazy stuff that may have existed before inflation,
4. seeded with tiny differences in densities on all scales, and
5. hot, dense, full of matter, and expanding!

And that describes the Big Bang, as we need it to be, almost exactly.

Inflation is a rich area, and Alan Guth wasn't the only one working on it, but he was the first and only one to articulate how inflation solves all of these problems. I've had the privilege to meet Alan Guth, and he's very congenial and humble, if just slightly socially awkward. I've also had a chance to meet other important people who've worked on inflation, such as Andrei Linde, Alexei Starobinski, and Paul Steinhardt. They're not humble, and make overt glory-grabs when it comes to taking the credit for inflation. Make no mistake about it: this is Guth's idea and Guth's alone. Watson didn't invent the telephone, Hilbert didn't invent General Relativity, and for this idea, Guth will surely win a Nobel Prize. It makes me feel dirty to realize that he's going to wind up sharing it with some of the more political (and less scientifically deserving) people above.

But from me, Alan Guth gets the accolade he deserves: the inventor of inflation, the most important scientist of his decade, and the glory of figuring out what must have caused the Big Bang!