Acceleration proportional to what?

Click For Summary

Discussion Overview

The discussion revolves around the relationship between acceleration and distance in the context of cosmic expansion, particularly in relation to Hubble's law and various models of dark energy. Participants explore whether acceleration is proportional to distance from Earth or if different models apply, considering both theoretical and observational aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the acceleration of space with distant galaxies is proportional to their distance from Earth or if a different model applies.
  • Another participant explains that acceleration is time-dependent, noting that it was negative in the early universe and is currently positive, suggesting a shift in the interpretation of cosmic expansion through the scale factor rather than a constant Hubble parameter.
  • A participant introduces the Quintessence model of dark energy, proposing that if the acceleration varies for galaxies at different distances, it supports this model, which predicts varying dark energy density over time and space.
  • Another participant argues that even the cosmological constant model implies a time-dependent acceleration due to changes in matter and radiation density, suggesting that deviations from this prediction might indicate a need for a time-dependent cosmological constant.

Areas of Agreement / Disagreement

Participants express differing views on the nature of acceleration in cosmic expansion, with some supporting the Quintessence model and others favoring the cosmological constant model. The discussion remains unresolved regarding which model accurately describes the observed phenomena.

Contextual Notes

Participants note that the understanding of acceleration is influenced by the scale factor and that precise observations are necessary to distinguish between competing models of dark energy.

us40
Messages
29
Reaction score
0
acceleration proportional to what??

Hello,

We know that according to Hubble's law V=HD that velocity of distant galaxies are proportional to distance from Earth but what about acceleration?? Is acceleration of space with distant galaxies proportional to distance from Earth and one another or different model works for it??
 
Space news on Phys.org
Acceleration of space leads to deviations from Hubble's law.
The acceleration is a function of time. In the early universe, it was negative (so expansion slowed), now it is positive (so expansion is accelerating). Instead of the Hubble "constant" (which is not constant on cosmological timescales), it is better to interpret the evolution of the universe in terms of the scale factor. Without acceleration, the scale factor would increase with a constant derivative.

If you know the scale factor at the point where some light was emitted and the scale factor now (defined to be 1), you can calculate the redshift.
Example graph for the scale factor with various scenarios (red=accelerating) from NASA.
You can also calculate how bright those objects should appear to us, if you can estimate their true brightness. By comparing those observations with the model, you can measure the accelerated expansion. And this has been done with many measurements (from the supernova cosmology project)
 
But what about Quintessence model of dark energy?? If we measure acceleration of galaxies and if it is different for different galaxies which are currently at different distance from Earth than quintessence model is correct because it predict different dark energy density over place and over time otherwise cosmological constant model is true.
 
Even the cosmological constant model leads to a time-dependence of the acceleration, as the decreasing matter/radiation density leads to a acceleration of the acceleration (3rd derivative of the scale factor).
If the time-dependence deviates from this prediction, then a time-dependent cosmological constant might be an explanation. But that will need much more precise observations.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
4K
  • · Replies 23 ·
Replies
23
Views
4K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 15 ·
Replies
15
Views
4K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 28 ·
Replies
28
Views
3K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K