- #1
jeremyfiennes
- 323
- 17
- TL;DR Summary
- The two seem contractory. How are they reconciled?
The GR predictions for the universe's size are those of fig.a. Whereas the Hubble expansion is exponential, fig.b. How are the two reconciled?
Universe Expansion: GR vs Hubble Reconciled
In summary, the conversation discusses the predictions for the size of the universe based on General Relativity (GR) and the Hubble expansion. The GR predictions are shown in figure a, while the Hubble expansion is shown in figure b. The two are not reconciled and can represent different possible futures of the universe, depending on the composition of matter and dark energy. The conversation also mentions a third graph, which is about Doppler shift and not relevant to the discussion.
- #2
PeterDonis
Mentor
- 45,656
- 22,670
Where are you getting that figure from?jeremyfiennes said:
What "Hubble expansion" are you talking about? Where are you getting that figure from?jeremyfiennes said:
- #3
jeremyfiennes
- 323
- 17
I don't know. My memory doesn't go back that far. But it us implicit in this one which I found in wikipedia:
"Possible velocity vs. redshift functions; patterned after Davis & Lineweave, CC BY-SA 3.0." The 'linear' curve is the Hubble law case.
"Possible velocity vs. redshift functions; patterned after Davis & Lineweave, CC BY-SA 3.0." The 'linear' curve is the Hubble law case.
- #4
Bandersnatch
Science Advisor
- 3,485
- 3,000
All four curves in the OP are allowed under GR. Without knowing the context, we won't be able to answer why they were labelled like that.
The first graph shows the evolution of the scale factor with time in universes with matter+radiation (although the latter doesn't really show in such simplified representation) depending on the ratio of density vs critical density - these are labelled 'flat' and 'closed'. For the one labelled 'open' to curve upwards like that, the composition must include dark energy. If it were approaching linear expansion instead of exponential, all three would represent the possible futures of the universe as could be encountered in textbooks pre-1990s, before dark energy was seriously considered. As is, the 'open' one is a bit out of place in its company, and mislabelled, but permissible under GR nonetheless.
The graph labelled 'b) Hubble' shows the time evolution of the scale factor in universes where the only component present is dark energy. It's the only case where expansion is exponential. It is also equivalent to the Hubble parameter never changing, so it's what you get from naively assuming that the Hubble constant in the Hubble law is constant in time.
All kinds of scale factor evolutions are permissible under GR - exponential, linear, approaching steady state, contracting. Which one is appropriate depends on what's in the universe.
The graph in post #3 shows a completely different thing. That one's about how Doppler shift works.
The first graph shows the evolution of the scale factor with time in universes with matter+radiation (although the latter doesn't really show in such simplified representation) depending on the ratio of density vs critical density - these are labelled 'flat' and 'closed'. For the one labelled 'open' to curve upwards like that, the composition must include dark energy. If it were approaching linear expansion instead of exponential, all three would represent the possible futures of the universe as could be encountered in textbooks pre-1990s, before dark energy was seriously considered. As is, the 'open' one is a bit out of place in its company, and mislabelled, but permissible under GR nonetheless.
The graph labelled 'b) Hubble' shows the time evolution of the scale factor in universes where the only component present is dark energy. It's the only case where expansion is exponential. It is also equivalent to the Hubble parameter never changing, so it's what you get from naively assuming that the Hubble constant in the Hubble law is constant in time.
All kinds of scale factor evolutions are permissible under GR - exponential, linear, approaching steady state, contracting. Which one is appropriate depends on what's in the universe.
The graph in post #3 shows a completely different thing. That one's about how Doppler shift works.
- #5
jeremyfiennes
- 323
- 17
Thanks.
- #6
PeterDonis
Mentor
- 45,656
- 22,670
Need I point out that this means they are not valid references?jeremyfiennes said:
Since we have no valid basis for discussion, this thread is closed.
1. What is the difference between GR and Hubble's Law?
General Relativity (GR) is a theory that describes how gravity works in the universe, while Hubble's Law is an observation that states the further away a galaxy is, the faster it appears to be moving away from us.
2. How does GR explain the expansion of the universe?
According to GR, the expansion of the universe is caused by the curvature of space-time due to the presence of matter and energy. This curvature causes objects to move away from each other, leading to the expansion of the universe.
3. How does Hubble's Law fit into the concept of universe expansion?
Hubble's Law provides observational evidence for the expansion of the universe. It shows that galaxies are moving away from each other at a rate proportional to their distance, which supports the idea of an expanding universe.
4. Can GR and Hubble's Law be reconciled?
Yes, they can. While GR and Hubble's Law may seem to contradict each other, they are actually complementary. GR provides the theoretical framework for understanding the expansion of the universe, while Hubble's Law provides observational evidence for this expansion.
5. How does the reconciliation of GR and Hubble's Law impact our understanding of the universe?
The reconciliation of these two theories has greatly enhanced our understanding of the universe. It has allowed us to better explain the expansion of the universe and has led to the development of the Big Bang theory, which is currently the most widely accepted explanation for the origin and evolution of the universe.
Similar threads
-
Special and General Relativity
-
Special and General Relativity
-
Advanced Physics Homework Help