Calculating universe age vs. size of universe

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Discussion Overview

The discussion revolves around the age of the universe and how it relates to its size, particularly in hypothetical scenarios where the universe is either similar to or significantly larger than the observable universe. Participants explore the implications of these scenarios on the universe's age and the Hubble constant.

Discussion Character

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

Main Points Raised

  • One participant questions how the universe's age might differ if the universe is "almost exactly" the size of the observable universe versus "approximately" 100 times its size.
  • Another participant asserts that the two scenarios do not affect the universe's age from the established 13.73 billion years.
  • A claim is made that the Hubble constant would not be altered by the size of the universe due to Birkhoff's theorem, which suggests that matter outside the observable universe does not influence the gravitational dynamics within it.
  • A participant expresses concern about the assumptions underlying the determination of the universe's age, prompting a request for clarification on these assumptions.
  • Assumptions listed include the accuracy of General Relativity, the homogeneity of the observable universe, and its isotropy, with a note that these assumptions are frequently scrutinized but currently lack unambiguous evidence against them.
  • Recent observations are mentioned that suggest a potential large source of gravitational attraction outside the observable universe, which may influence the motion of galactic clusters.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement. While some assert that the universe's age remains unchanged regardless of size, others raise questions about the assumptions involved in calculating that age and the implications of new observations.

Contextual Notes

Limitations include the dependence on the assumptions of General Relativity, homogeneity, and isotropy, which are subject to ongoing debate and scrutiny. The discussion also touches on the potential impact of external gravitational influences, which remains unresolved.

BrianConlee
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Hello,

I was pondering the age of the universe and reading about how it was calculated, but then I began questioning something.

Two scenarios, BIG IF. (I know, it's a big IF. I'm sure I'll get a lot of comments about how it's not possible, or improbable. I get it. but...) IF:

-We determine the exact size of the universe in both of these scenarios-

1. The universe is "almost exactly" the size of the observable universe. (yes I know that's qualitative not quantitative, but you know what I mean)

2. The universe is "approximately" 100 times the size of the observable universe. (see scenario 1 comment)


Ok, so this begs the question: How do these two scenarios differ in the universe's age FROM 13.73 billion years?

Would this alter the Hubble constant possibly?

Thanks.
 
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Ok, so this begs the question: How do these two scenarios differ in the universe's age FROM 13.73 billion years?
They don't.
Would this alter the Hubble constant possibly?
No. There's a theorem (Birkhoff) that sperical shells of matter (the universe outside the observable part) have no gravitational influence whatsoever on what's inside.
So the evolution is the same whether the universe is as big as a cherry stone or infinite.
 
Thanks for the response. It was really bothering me how they could determine the age without knowing the at least approximate true size.

Can I ask while we're on the topic, what assumptions are being made in order to determine the age?
 
There has been some recent observations which suggest that there may be a large source of gravitational attraction outside the observable universe. It appears that a large group of galactic clusters are traveling in some particular direction, over and above the expected expansion.
 
Can I ask while we're on the topic, what assumptions are being made in order to determine the age?
The assumptions are:
- General Relativity is an accurate enough description of gravity, space and time
- The (observable) universe is homogeneous
- The universe as we see it is isotropic

The last assumption is more or less experimental fact (the alleged dark flow would be a perturbation, if it exists).
The second assumption has a reasonable basis in observations, and is quite natural given the observed isotropy.
The first assumption seems also safe, given the succes of GR in all tests done so far.

All of these assumptions are being questioned all the time. However, as of now, there is no unambiguous evidence against them, and there have been no really viable alternatives proposed.

With the assumption, you're constrained to the Friedman models of the universe. The current LCDM model is the one best fitting the observations.
 

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