- #1
spaghetti3451
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Is the Hubble constant decreasing over cosmological timescales?
Variation of Hubble constant in cosmological time
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Discussion Overview
The discussion revolves around the behavior of the Hubble constant over cosmological timescales, particularly its variation during different epochs of the universe, including the radiation-dominated and inflationary epochs. Participants explore theoretical implications and relationships involving the Hubble parameter.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- Some participants question whether the Hubble constant is decreasing over cosmological timescales.
- One participant explains that the Hubble constant tends to infinity in the early universe due to the relationship between recessional velocities and distances, drawing an analogy to the function f(x)=1/x as x approaches 0.
- There is a discussion about the validity of the relation $$H = 1.67 \sqrt{g_{*}} \frac{T^{2}}{M_{P}}$$ during the radiation-dominated epoch and whether it applies during the inflationary epoch, with some asserting it does not.
- Participants inquire about the correct relation for the Hubble parameter during the inflationary epoch, suggesting it depends on the equation of state of the inflation-driving component.
- One participant offers a resource link that may provide further information on the topic.
Areas of Agreement / Disagreement
Participants express differing views on the behavior of the Hubble constant and its relationships during various epochs, indicating that multiple competing views remain and the discussion is unresolved.
Contextual Notes
Some claims depend on specific definitions and assumptions regarding the equations of state and the nature of inflation, which are not fully resolved in the discussion.
- #2
Bandersnatch
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Yes:
- #3
spaghetti3451
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Why did the Hubble constant tend to infinity in the early universe?
- #4
Bandersnatch
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Mostly for the same reason that f(x)=1/x goes to infinity as x goes to 0. Remember that H is the proportionality factor between recessional velocities and distances: H=V/D. As the distances between receding objects decrease to 0, H goes to infinity.
An additional effect comes from V being larger in the past, but that's secondary.
An additional effect comes from V being larger in the past, but that's secondary.
- #5
spaghetti3451
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The relation
$$H = 1.67 \sqrt{g_{*}} \frac{T^{2}}{M_{P}}$$
is valid during the radiation-dominated epoch. Is it valid during the inflationary epoch?
$$H = 1.67 \sqrt{g_{*}} \frac{T^{2}}{M_{P}}$$
is valid during the radiation-dominated epoch. Is it valid during the inflationary epoch?
- #6
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No.spaghetti3451 said:
- #7
spaghetti3451
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What would be the correct relation for the Hubble parameter in the inflationary epoch?
- #8
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That would depend on the equation of state for whatever is driving inflation. If what drives inflation behaves as a cosmological constant, then the Hubble parameter would be constant during inflation.
- #9
spaghetti3451
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Would you share a review article which discusses this in more detail?
- #10
Bandersnatch
Science Advisor
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Would this be of any help?
https://ned.ipac.caltech.edu/level5/Watson/Watson5_3.html
https://ned.ipac.caltech.edu/level5/Watson/Watson5_3.html
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