Early cosmic inflation and the expansion of the universe.

[Herbascious J]

My question is regarding the early inflationary phase of the Big Bang. As I understand it, inflation is what gave rise to the expansion energy of the universe. Meaning, inflation gave the 'push' so to speak that set the everything moving apart. This makes sense because obviously the universe has a tremendous amount of energy behind it's expansion. However, I am confused about a certain point...

If inflation is the mechanism behind the momentum of the expansion of the universe, then shouldn't the universe be expanding WAY more quickly. As I understand it, inflation was extremely rapid. So, when inflation 'shut off' what slowed down the expansion to what we observe with the Hubble flow? I realize without inflation the universe will stop doubling exponentially, but it seems something had to 'put on the brakes'. What am I missing? Thanks.

 

[mfb]

Be careful with the scale of expansion. Inflation was very quick, but only relative to the size of the universe at that time. Afterwards, gravity slowed it a bit, and some billion years ago the expansion accelerated again (but that effect is small compared to the other scales). I'm not sure how fast exactly expansion was, but it does not have to be much quicker than today.

 

[George Jones]

The Hubble parameter gives the relative expansion rate of the universe, i.e., the rate of expansion divided by size. Assuming the scale of the universe has increased by a factor of 10^30 since inflation ended, I calculate that Hubble parameter was about 10^58 times larger at the end of inflation than it is now.

 

[Herbascious J]

Oh, I see, that is the source of my misunderstanding then. I had assumed that the universe had achieved a fairly comparable size during inflation. But it makes sense that it shut off at a much smaller scale (many orders of magnitude) this of course allows the relative velocities of different regions to all spread out, and appear slower locally, as we see today. Thanks so much.

 

[Chalnoth]

The Hubble parameter gives the relative expansion rate of the universe, i.e., the rate of expansion divided by size. Assuming the scale of the universe has increased by a factor of 10^30 since inflation ended, I calculate that Hubble parameter was about 10^58 times larger at the end of inflation than it is now.

Hmm, I think your estimate is a bit off, because that would fit the entire observable universe within a single Planck length at the end of inflation.

 

[mfb]

Hmm, I think your estimate is a bit off, because that would fit the entire observable universe within a single Planck length at the end of inflation.

I get 400µm. 40 billion light years / 10^30

Give or take a factor of 2 for radius vs. diameter, definition of size and so on, but that is far away from the Planck scale.

 

[Chalnoth]

I get 400µm. 40 billion light years / 10^30

Give or take a factor of 2 for radius vs. diameter, definition of size and so on, but that is far away from the Planck scale.

Ugh, sorry. My mistake. I was taking the change in expansion rate rather than the change in scale factor.

That sounds much more reasonable.