Undergrad Exponential expansion of inflation

[Ranku]

The expansion of the inflationary universe is said to be roughly exponential. Why is it called "roughly" exponential?

 

[PeterDonis]

The expansion of the inflationary universe is said to be roughly exponential.

In what source? Please give a specific reference.

 

[Ranku]

It's mentioned in most cosmology texts, e.g. Cosmology by Steven Weinberg, in Inflation chapter.

 

[PeterDonis]

It's mentioned in most cosmology texts, e.g. Cosmology by Steven Weinberg, in Inflation chapter.

Ok, so can you give a specific quote and page/section reference?

 

[Ranku]

"...there was an earlier period of inflation, when the energy density of the universe was dominated by a slowly varying vacuum energy, and a(t) grew more or less exponentially." Pg. 201.

 

[PeterDonis]

I would say the "more or less exponentially" is a consequence of the "slowly varying vacuum energy". As I understand it, only a vacuum energy that is exactly constant will lead to an expansion that is exactly exponential.

 

[Chalnoth]

I would say the "more or less exponentially" is a consequence of the "slowly varying vacuum energy". As I understand it, only a vacuum energy that is exactly constant will lead to an expansion that is exactly exponential.

It's more that only in a universe with nothing in it but vacuum energy will the expansion be exactly exponential. It's not quite exponential because there's other matter around.

 

[PeterDonis]

It's not quite exponential because there's other matter around.

Is there, though? As I understand the basic inflation model, the Standard Model fields are all in their vacuum states during inflation, and all of these fields have zero vacuum expectation value for energy; they only get reheated to highly non-vacuum, high temperature states at the end of inflation.

 

[Chalnoth]

Is there, though? As I understand the basic inflation model, the Standard Model fields are all in their vacuum states during inflation, and all of these fields have zero vacuum expectation value for energy; they only get reheated to highly non-vacuum, high temperature states at the end of inflation.

Argh, sorry, you're right. I was thinking of the current near-exponential expansion. Never mind.

Yes, expansion during inflation isn't quite exponential because the field that drives inflation isn't constant. The field just varies slowly enough that near-exponential expansion occurs.

 

[Ranku]

Thank you for clarifying the issue.