When it comes to the Big Bang, you shouldn't believe everything you hear. In this week's "Ask a Physicist," we ask whether hyperinflation of the entire universe is sensible or just plain nuts.
Some time ago, I put a plea in a column for people to ask me questions about inflation. Lots of you took the bait, including Matt DiGioia who seems to relate an understanding of the universe with his personal well-being. However, I got so many interesting questions about other things, that like a puppy or a roomba, I was easily distracted. Finally, two months later, it's time to get down to bidness:
Can you break down inflation for us? How did it happen? Is it still going on? The more I understand about the beginnings of the Universe the better off I'll be.
Top image by Richard Ortolano/Shutterstock
Once, about 13.8 billion years ago, the universe began. For the first tiny fraction of a second, about 10^-44 or so, physics currently has no clue as to how everything worked. There's your daily dose of humility. After that, it started cranking along and expanding in a fairly ordinary way, except that everywhere you look there was insanely high densities and energies. Then, around 10^-36 seconds later something unexpected happened. In a tiny fraction of a second, the universe inflated to something like 10^100 times its original size and then just as suddenly continued with its ordinary expansion as though nothing had happened. This is the story of inflation, and from here on out, I'll try to explain why cosmologists think it's even a remotely reasonable idea.
Why is any of this a good idea?
Before getting into any details about how inflation happened, let me say a few words about why it would even occur to us to think about such a thing in the first place.
On large scales, the universe is very boring.
We now have a number of deep surveys of the night sky, including maps of over a hundred million galaxies. One of the most startling things in our observations is that statistically, and averaged over a reasonably big chunk of space, the universe is more or less uniform. We see about the same number of galaxies in the Northern hemisphere as in the Southern.
We see the same "wherever you go, there you are" attitude in the cosmic microwave background, as well. The CMB as the initiated call it, is the remnant radiation from the early universe, and it's coming at us from all directions. It's cold, only about 2.7 degrees above absolute zero, but in the early universe, it was blazing hot. Most importantly, though, it differs by less than a factor of 1/100,000th from point to point on the sky. It is incredibly uniform.
Without inflation, this presents a very serious problem: Why should the universe be almost exactly the same temperature everywhere? I know what you're thinking. "Right after the big bang, the universe was all basically a point, so shouldn't everything have gotten stirred up then?"
Image by Ilias Strachinis/Shutterstock
Oh, you're a clever one, but remember that not only was the universe small at the beginning it was also very young. There wasn't much time to get things smoothed out. As it happens, the youngness should be more important than the smallness. Without inflation, distant regions of space separated by more than 1 degree on the sky should be completely independent of one another. They shouldn't necessarily look anything alike. In fact, if randomness plays any sort of role in the universe (and according to quantum mechanics, it does), we'd be really surprised to find different parts of the sky — the very distant sky, not stars in our galaxy or nearby — looking anything alike.
Inflation to the rescue. If you allow even a tiny fraction of a second to stir the pot, then inflate the mixed cosmic goop up to ginormous scales, lo and behold! The universe will look more or less uniform to us today.
Credit: NASA WMAP project
It's not completely boring.
Of course, the universe isn't completely uniform. It would kind of suck if it were, since a completely smooth universe can't make things like stars and galaxies and us. Those things form when you start with a region that's just a wee bit more dense than average, and it attracts more stuff gravitationally and it grows and grows.
But where do the seeds come from? The picture above is kind of famous, and might give us some hints. It's a snapshot of the universe as it appeared about 380,000 years after the big bang — way, way after the period of inflation. The cosmic microwave background is literally the first images we can get of the universe, it shows the warm (red) and cool (blue) spots at that time. Of course, hot and cool are relative things. We're really only talking about something like a hundred thousandth of a degree or so.
Even so, why aren't things perfectly smooth?
Quantum mechanics. I've written a number of times about the fact that on microscopic scales pairs of particles constantly jump into and out of existence. In black holes, if you can irreparably split these pairs apart before they have a chance to recombine, you get to keep them.
The same was true in the early universe. On a microscopic scale, the density looked just like white noise, with little fluctuation occurring on ridiculously short timescales. Then whoosh! the pairs got split apart by the inflating universe, and couldn't ever recombine. What we now see as structure was quite literally the static of the first instant.
The universe is flat.
Since everything expanding underneath you, doing geometry on the large scale universe is extremely tough. I don't think this is a problem that plagues your daily lives, and I don't want to get into a whole thing about flat universes versus closed universes versus open ones (though I'm happy to do so another time, or for that matter, I talk about a fair amount in my book), but it might help to get the basic idea if you think about how things look on the earth.
The earth is, roughly, a sphere. This means, for example, that maps of the whole earth get distorted if you draw them on a flat piece of paper. On the other hand if the earth really were flat, we wouldn't have any such problems. You could fall off the edges, of course, but you wouldn't have any difficulty making maps, which for our purposes is vastly more important.
As far as we can tell, the universe is really, really close to flat, maybe exactly flat. Again, this simply means that geometry on scales of the entire observable universe work the way that Euclid would have liked. The problem is that if you start with a universe which is just a little bit curved, it quickly becomes very, very curved. In other words, the only way for the universe to be nearly flat now, is if it were insanely close to flat at the beginning of time (with something like 60 digits of precision). This seems unlikely to happen by chance.
On the other hand, if I take a balloon (clearly curved) and blow it up to ginormous scales, it very quickly looks flat to any ants who happen to be standing upon it. This is the same reason why the earth (round) looks flat in your everyday life. As long as the furthest horizons of the universe are much smaller than the size of the balloon, everything's flat enough for government work.
Some while ago, I talked about magnetic monopoles. One of the ways that they might form (if they exist at all) is that there could a mis-match between the electromagnetic field in nearby regions of space. The monopoles could essentially form near the edges. But if that's the case, they should be relatively uncommon. Inflation gets rid of this problem as well, since by blowing up the volume of space, you instantly obliterate the density of everything inside of it. In one quick sleight of hand, we decide that we shouldn't see any magnetic monopoles and indeed (so far) we haven't.
Image via sdecoret/Shutterstock
How does it work?
Having gone a bit long on why of inflation, I'm going to go a bit easier on you for how of inflation. There's not just a single inflationary model (Hell, even though most cosmologists buy into some form of inflation, we're generally not wedded to the idea), but all of them have certain things in common.
The trick, as always, is to invoke E=mc^2. Energies and masses are easily exchangeable for one another, so despite the common misconception, which means that energy is a source of gravity just like mass is. Back in the early days (as now) the universe is filled with fields, and those fields have energies. The inflaton (not a misspelling) field started at a relatively high energy, and dropped (ultimately) to a lower one. That's it, although as you might suppose the mathematical details are a bit hairier. Along the way, a lot of energy gets liberated, and that goes straight into gravity in fairly complicated ways, including an exponential expansion. By the time the field settles into into it's final, lower state, inflation is done.
This isn't as crazy as it sounds. The Higgs field (the proposed particle responsible for mass), behaves in a very similar way. And if this exponential expansion sounds familiar, it should. It's exactly the same sort of thing that happens with dark energy, the mysterious substance that's causing the expansion of the universe to accelerate even now. The difference, however, is that the energies in the inflationary expansion were something like 10^100 times higher.
Some of you will no doubt be deeply concerned about the problem that the universe expanded "faster than light." Take a deep breath. It'll be okay. For one thing, there's no such thing as the universe expanding faster than light. The units are completely different. Normal speeds (like the one for light) are in meters per second or miles per hour or something like that, but the expansion rate of the universe is measured in the amount of time it takes to double in size. See? Different units.
But I take your point, at early times (and even now, though only for very, very distantly separated galaxies), two points in space move away from one another at faster than light speeds. The key is realizing that since even light is dragged along with the expansion of the universe, you can't exploit the expansion to beat light in a fair race. The speed of light limit has always been about not overtaking lightbeams.
Inflation can get much weirder than faster-than-light expansion, however. Some regions of space might have had a slightly different inflaton field to start than others. This means that inflation might have lasted just a wee bit longer, and sprouted into universes with slightly different properties. But that wee bit means a huge difference since space was expanding so quickly. Some parts of those long-inflating regions might themselves continue to expand a little extra, and on an on. In fact, this ultimately gives us eternal inflation, a literally infinite multiverse. It might even give you hope for meeting, and potentially teaming up with, your evil twin.
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on twitter, facebook, twitter or our blog.) He is an Associate Professor of Physics at Drexel University. Feel free to send email to firstname.lastname@example.org with any questions about the universe.