Why do we need inflation to explain the homogeneity of CMB?

petergreat

Can't we simply assume that the initial condition for the universe is perfectly spherically symmetric, and the problem is solved? In other words, can't we make the CMB homogeneous just by imposing homogeneous initial conditions? The fluctuations can be explained by quantum effects. Of course there are other things such as the flatness problem which seem to be quite naturally explained by inflation, but I never feel sure about the homogeneity problem. After all, we know very little about how the big bang happened, and we can't rule out any particular kind of initial conditions, such as a homogeneous one.

Chronos

Inflation nicely explains why the CMB almost perfectly homogenous in every direction. That is one of its more appealing features.

friend

Inflation in turn only begs the question as to why the expansion was uniform.

bapowell

Uniform expansion follows from the requirement that the field driving inflation be uniform in space across the initial inflationary patch. Why do you suggest that the expansion should not have been uniform?

bapowell

I think this is precisely the point. Sure, the initial conditions could have been perfectly homogeneous. There is nothing that forbids this in principle. But, as you say, the IC's could have been inhomogeneous as well. In fact, there are lots more ways of not being homogeneous than there are of being homogeneous. So, given a generic initial state, one would not expect it to be perfectly homogeneous. By just putting such a condition in 'by hand' is considered by many to be a serious fine tuning of initial conditions. Inflation is preferred in this case because, while one still needs a sufficiently homogeneous region of spacetime in order to get inflation started, the region across which we demand such homogeneity is smaller than today's currently observable universe.

friend

If you're going to suppose that fields can be uniform as the universe grows to astronomical dimensions, then we could just as easily suppose that the matter fields were uniform with a larger universe as well. I fail to see the difference.

bapowell

The difference is that the initially uniform inflationary patch can be made uniform through a causal mechanism (the scalar field is correlated across the inflationary spacetime). This is very different then imposing uniformity on the matter fields across cosmological distances without inflation. This is the horizon problem -- no causal mechanism could generate such homogeneity.

But that's not what I was asking. I was asking about your comment regarding why inflation begs the question that the expansion was uniform.