Expert: Courtney Seligman - 2/9/2011

Question
Courtney,
My understanding is that the inflation theory related to the big bang was devised primarily to deal with the horizon problem of the universe being too alike throughout.  I understand that some explanation was wanted because scientists could not believe such an event would not be chaotic, as are all "explosions" in our experience.
My question is:  Is there any credible science that assumes exactly that; that from whatever was there emerged a universe with exactly the evenness and distribution that we see in the WMAP?
It's a much simpler explanation, which I know science generally  prefers.
And why not?  Just because we intuit that it was chaotic does not require it to have been so.  In fact, if it started with a point of pure energy, then there could have been zero differentiation at the very start.  
I wonder if what we should actually be asking, is why was there any differentiation at all, even the slight bit required for physical existence.
Thanks,
Paul

Answer
First, a summary of how the Inflation theory was developed:

The original theory of the Cosmic Fireball was that the Universe started off several tens or hundreds of thousands of light years across. But if that were the case, we would expect some areas to be a little denser or less dense, and a little hotter or cooler, which would lead to noticeable differences in the Cosmic Microwave Background, which we do not observe. Hence the idea that perhaps the Cosmic Fireball started off as a point of zero size, so there was no room for any difference in characteristics, save for quantum fluctuations (which ARE observed in the CMB, in about the way they should be, if the Universe did start off at zero size).

However, if the Universe started at zero size and expanded to tens or hundreds of hundreds or thousands of light years in size at the speed of light (the normal maximum speed for such things), then it would have been very hot and very dense for so long that there would have been a lot of nuclear reactions during that time, which would have made the composition of the early Universe noticeably different from what we observe.

So how can we explain the uniform CMB, which seems to require a starting point of zero size, and the lack of "wrong" nuclear reactions, which would occur if the early Universe expanded at the speed of light? We invent a theory -- Inflation -- which supposes that you can go from zero size to tens, hundreds, or even billions of light years across in essentially no time at all. How that could happen is beyond currently known laws of physics, but the idea is that during that very short period of time, the laws of physics were different from those we are used to.

So although there are some questions about what caused the Inflation, and how things worked during its extremely short interval, there isn't much doubt that the theory described above is the correct one. Anything else just won't explain the observations.