Inflation

So we think the Universe began with a Big Bang, but there are things that we don't expect based on observations of the Universe.

For example, if we look 10 billion light-years away (the distance light travels in 10 billion years), we see a particular galaxy. Nothing strange about that. If we look 10 billion light-years in the opposite direction, we see another galaxy, that strangely is very similar to the first galaxy. Wait a minute. These galaxies are each 10 billion light-years away from us, but 20 billion light-years from each other. How are they so similar? We know the Universe is only 14 billion years old, so there is no way these galaxies could not have interacted with each other to make them seem the same. This is true for any object farther apart then the age of the Universe. How can this be? This is referred to the Horizon Problem, in that two regions of space that have no business interacting with each other because they are so far apart that light from one has not had enough time since the beginning of the Universe to reach the other. It tells us that the Universe is isotropic, but it should not be so if the expansion of the universe is constant.

 

There is another problem, as well. If we look at the overall Universe, it is relatively uniform, or homogeneous. Just by going back in time at the current rate of the expansion of the Universe, this should not be so. There is no reason that the Universe should be uniform. In fact, it is so uniform, that we see the actual density of the Universe is very close to the critical density (the density at which the Universe will evolve as a flat universe - we'll get to this later). This is called the is very close to the Flatness Problem. Why is the Universe so uniform?

 

Cosmic Microwave Background Radiation from WMAP

Image Credit:

NASA / WMAP Science Team

The answer is inflation. What happened during inflation was that the Universe at about 10e-36 sec ABB rapidly accelerated its expansion. At about 10e-33 or 10e-32 sec ABB, inflation stopped. The best way to think of this is before inflation, the size of the Universe was smaller than the age of the Universe. Everything could interact with everything else that existed. After inflation, the age of the Universe became smaller than the size of the Universe. Now, distant objects cannot interact (i.e. cannot receive radiation from each other unless they are physically closer to each other than the age of the Universe).

 

Unfortunately, we do not know why inflation occurred (remember, we can't see anything before recombination), but we can make observations to see that inflation most likely occurred.