Does negative gravity play a role in the expansion of the universe?

Click For Summary

Discussion Overview

The discussion revolves around the concept of negative gravity and its potential role in the expansion of the universe, particularly in relation to the Friedmann equation and the implications of spacetime geometry. Participants explore the nature of space expansion versus space curvature, the concept of "repulsive gravity," and the relationship between the cosmological constant and inflationary theory.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion regarding the expanding universe and the Friedmann equation, referencing a quote about gravity's repulsive nature in the early universe.
  • Another participant clarifies that while spacetime is curved due to gravity, space can expand without necessarily being different from spacetime curvature.
  • There is a discussion on the concept of "negative gravity" or "repulsive gravity," with one participant arguing that it should not be viewed as a force acting on space, but rather as a geometric property resulting from a positive cosmological constant.
  • A follow-up question is raised about the relationship between the inflaton field in inflationary theory and the cosmological constant, with a participant suggesting that the inflaton field is similar but much stronger.
  • Another participant agrees that while the inflaton field behaves like a positive cosmological constant, it is not a true constant, as its energy density changes slowly during inflation.

Areas of Agreement / Disagreement

Participants express differing views on the terminology and conceptualization of negative gravity and its implications for space expansion. While there is some agreement on the nature of the inflaton field, the discussion remains unresolved regarding the interpretation of negative gravity and its role in the expansion of the universe.

Contextual Notes

Participants note the potential misunderstanding arising from the terminology used to describe negative gravity and its effects. The discussion also highlights the complexity of relating concepts like space expansion, spacetime curvature, and the behavior of the inflaton field.

michaeljmj
Messages
11
Reaction score
0
These questions shows my lack of understanding of the expanding universe specifically the Friedmann equation. Here is a quote from Brian Greene (The Fabric of the Cosmos, page 273): “The early universe provided an arena in which gravity exerted its repulsive side with a vengeance, driving every region of space away from every other with unrelenting ferocity.”
Relativity proves that gravity curves spacetime but isn’t space expanding different from space curvature? In other words, how does negative gravity exert a force on space?
If the universe stopped expanding, like in a “bounce” theory, would the expanse of space (containing the Higgs field and cosmological constant) contract as the density of mass and energy increased?
 
Space news on Phys.org
michaeljmj said:
Relativity proves that gravity curves spacetime

Yes, spacetime, not space.

michaeljmj said:
isn’t space expanding different from space curvature?

It can be--space can be expanding but still be flat--but it is not different from spacetime curvature--if space is expanding, spacetime is curved.

michaeljmj said:
how does negative gravity exert a force on space?

Space expanding due to "negative gravity" or "repulsive gravity" is not a "force on space". (This is one reason I don't like Greene's terminology here, since it invites that very misunderstanding.) It's just how the geometry of spacetime works out when you have a positive cosmological constant. There is no "force" since all the objects whose motion tells about the expansion of space are in free fall, feeling no force.
 
PeterDonis said:
Space expanding due to "negative gravity" or "repulsive gravity" is not a "force on space". (This is one reason I don't like Greene's terminology here, since it invites that very misunderstanding.) It's just how the geometry of spacetime works out when you have a positive cosmological constant. There is no "force" since all the objects whose motion tells about the expansion of space are in free fall, feeling no force.
Thanks for the specific answer. A follow-up question, in the inflationary theory, the inflaton field was similar to a positive cosmological constant only much stronger. Right?
 
michaeljmj said:
Thanks for the specific answer. A follow-up question, in the inflationary theory, the inflaton field was similar to a positive cosmological constant only much stronger. Right?
Yes. Of course it wasn't truly a constant, but certainly the energy density changed very slowly during the bulk of the inflationary expansion.
 

Similar threads

Undergrad Can gravitational waves gain energy in an expanding FRW spacetime?
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad Expansion of space & expansion of matter within space
  • · Replies 17 ·
Replies
17
Views
3K
Graduate Is the metric basis for space expanding as the universe expands?
  • · Replies 2 ·
Replies
2
Views
2K
High School Why doesn't cosmic inflation violate conservation of energy?
  • · Replies 3 ·
Replies
3
Views
4K
High School Does Negative Mass Impact Expansion in Vacuum Space?
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Why is the observed local region of space flat?
  • · Replies 25 ·
Replies
25
Views
3K
Undergrad Where Does Earth's Counter Gravity Go in an Expanding Universe?
  • · Replies 72 ·
3
Replies
72
Views
9K
High School Do we still live in a Big Bang Singularity?
  • · Replies 14 ·
Replies
14
Views
4K
Graduate Question: Accelerating Universe = Net Positive Energy?
  • · Replies 7 ·
Replies
7
Views
3K
Graduate The expansion of the universe and gravity: One question.
  • · Replies 6 ·
Replies
6
Views
4K