Exponential expansion of inflation

Click For Summary

Discussion Overview

The discussion centers on the nature of the expansion of the inflationary universe, specifically why it is described as "roughly" exponential. Participants explore the implications of vacuum energy and the conditions under which exponential expansion occurs, touching on theoretical aspects of cosmology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about the terminology "roughly exponential" and seek specific references to support this claim.
  • It is noted that cosmology texts, such as Steven Weinberg's "Cosmology," mention the expansion as "more or less exponentially" due to the influence of slowly varying vacuum energy.
  • One participant argues that only a vacuum energy that is exactly constant would lead to an exactly exponential expansion, suggesting that the presence of other matter affects this characterization.
  • Another participant questions whether there is indeed other matter present during inflation, referencing the Standard Model fields being in their vacuum states and having zero vacuum expectation value for energy during inflation.
  • A later reply acknowledges a misunderstanding regarding the current near-exponential expansion, clarifying that the inflationary field varies slowly enough to allow for near-exponential expansion.

Areas of Agreement / Disagreement

Participants express differing views on the conditions that lead to exponential expansion, particularly regarding the role of vacuum energy and the presence of other matter. The discussion does not reach a consensus on these points.

Contextual Notes

Participants reference specific texts and quotes but do not provide a comprehensive resolution to the nuances of vacuum energy and its effects on expansion rates. The discussion reflects varying interpretations of the inflationary model.

Ranku
Messages
434
Reaction score
18
The expansion of the inflationary universe is said to be roughly exponential. Why is it called "roughly" exponential?
 
Space news on Phys.org
Ranku said:
The expansion of the inflationary universe is said to be roughly exponential.

In what source? Please give a specific reference.
 
It's mentioned in most cosmology texts, e.g. Cosmology by Steven Weinberg, in Inflation chapter.
 
Ranku said:
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?
 
"...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.
 
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.
 
PeterDonis said:
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.
 
Chalnoth said:
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.
 
PeterDonis said:
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.
 
  • #10
Thank you for clarifying the issue.
 

Similar threads

Undergrad Questions about the accelerating Hubble expansion
  • · Replies 36 ·
2
Replies
36
Views
6K
Undergrad Energy conservation in an expanding universe
  • · Replies 2 ·
Replies
2
Views
4K
Undergrad Exponential expansion and the Cosmological Constant
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Some questions about Cosmic Inflation
  • · Replies 4 ·
Replies
4
Views
5K
Graduate Gravitational Instability and accelerated explanation of Universe?
  • · Replies 19 ·
Replies
19
Views
3K
High School Does dark energy or cosmic inflation explain flatness?
  • · Replies 37 ·
2
Replies
37
Views
6K
Undergrad Developing Inflation Theory w/o Dark Energy: Alan Guth 1980
  • · Replies 1 ·
Replies
1
Views
2K
Graduate Early cosmic inflation and the expansion of the universe.
  • · Replies 6 ·
Replies
6
Views
3K
Undergrad What is the significance of slow-roll inflation in cosmology?
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad How long did inflation last in an eternal inflating universe?
  • · Replies 35 ·
2
Replies
35
Views
5K