2.2 Eternal Inflation

summary

Back to course

Could inflation still be part of the story?

The theory still has power to explain a lot and to make predictions, even if it doesn’t explain the horizon and flatness problems. Maybe there’s a deeper mystery about the initial conditions. Unfortunately, those things aren’t true either.
The reason for this is an effect known as eternal inflation.
Recall the ball on a hill analogy, with the hill representing the value of the inflaton field at every point in space.
However, when we include quantum mechanics in our analogy, we realize that the field won’t have the same value everywhere in space due to fluctuations. In some places it will be further down the hill than others—it will occupy some distribution.

Introducing quantum mechanics to the inflationary theories yields significant effects.

What is really happening is that different regions are rolling down this hill at different rates, occasionally being buffeted around by quantum fluctuations.
Some regions will reach the bottom of the hill before others, ending inflation in that region, cooling and expanding, forming matter and radiation, while other regions are still inflating, and will be hotter as they end inflation later. This ostensibly explains the hot spots and cold spots in the cosmic microwave background radiation—so we can’t avoid these fluctuations even if we wanted to, because they are part of quantum physics and the reality of nature, and are key to the prediction of the CMB.
But with these quantum fluctuations also comes something you don’t want. There exist rare fluctuations which are way behind the rest, farther up the hill. And although they may only be delayed by what appears to be a small amount, this small delay causes a tremendous result.
These procrastinating regions are still inflating while the regions that made it down the hill have stopped. And inflation is an incredible stretching mechanism, so within literally a matter of instants, what you perceived to be a rare unlikely region of space becomes exponentially most of space; and the region you thought was typical (the region that made it down the hill) is now a tiny little patch or bubble surrounded by the procrastinating regions of space that are continuing to inflate.
In other words, when you properly include quantum physics in your thinking, the entire picture of the inflationary universe turns inside out—the regions that end inflation are very atypical, and the regions that haven’t yet ended inflation (that you thought were rare) are actually occupying most of the volume.

Eternal inflation has huge ramifications for the inflationary picture.

The procrastinating regions inflating to occupy the majority of space occurs again and again and again. When you properly take into account the effects of quantum physics plus inflation, you get something quite dramatic—eternal inflation. Most of the universe at a given time is inflating, with patches of space that have ended inflation (perhaps containing matter and radiation), but they occupy an infinitesimal fraction of the entire universe. And this occurs eternally
You could qualify it: you might claim the parts of the universe which are inflating are not habitable, and the patches that end inflation can be habitable, and we live inside one of those patches. We might be satisfied with that logic if we could say that all such patches had the same physical and cosmic properties, but again, quantum physics does not allow this claim to be made.
Since the process lasts eternally, and since quantum physics allows all possibilities, every possibility will be explored given enough time (and eternity is enough time). Not only will they occur, they will occur an infinite number of times!
There will be bubbles that have regions which are flat and have simple cosmic microwave background radiation curves that agree with the data just like we see in our own universe. But for every one of those, there will be ones that are not flat (curved) and ones that don’t have microwave backgrounds that conform to the data we’ve seen.
Eternal inflation allows for every conceivable combination of physical and cosmic properties, exhibited an infinite number of times.
Whereas you were hoping to explain why the universe has the properties that it does, what happened instead was a universe was produced in which anything can happen. Alan Guth remarked: “Anything that can happen will happen—and it will happen an infinite number of times.