Infinite universe but finite beginning?

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Discussion Overview

The discussion revolves around the concept of the universe's size and structure in relation to the Big Bang theory, particularly addressing the idea of whether the universe is finite or infinite. Participants explore theoretical implications, visualizations, and the nature of the observable universe.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the Big Bang theory suggests the universe began as a very dense state, but there is uncertainty about whether it was finite or infinite at that time.
  • Others argue that if the universe was infinite at the beginning, it remains infinite now, while if it was finite, it remains finite, highlighting the current unknowns regarding the universe's overall size.
  • One participant notes that discussions often refer to the "observable universe," which is finite due to the finite speed of light and the universe's finite age.
  • Another participant mentions that visualizations of the universe as a "ball" are simplifications meant to aid understanding, emphasizing the concept of infinite density rather than a physical object.
  • A later reply introduces the idea of the universe potentially being finite but having a topology that allows for returning to the starting point, similar to traveling around the Earth.
  • One participant elaborates on the implications of traveling in a straight line in a finite universe, suggesting that it raises questions about the relationship between volume and size, and how infinite volumes could fit within finite sizes.
  • Another participant suggests a thought experiment involving infinite light speed to simplify geometric descriptions of the universe's structure and the concept of concentric shells representing different states of the universe.
  • There is a mention of the complexity of visualizing these concepts, with one participant comparing it to a video camera recording a TV screen, illustrating the challenges of understanding the relationship between observation and the universe's structure.

Areas of Agreement / Disagreement

Participants express various viewpoints on whether the universe is finite or infinite, with no consensus reached. The discussion includes multiple competing ideas and uncertainties regarding the nature of the universe's size and structure.

Contextual Notes

Participants acknowledge limitations in current understanding, including the dependence on definitions of "universe" and "observable universe," as well as unresolved mathematical and conceptual challenges in visualizing the universe's structure.

  • #31
so I read through the cosmological principle and I am having some trouble understanding where you guys pulled out that the universe is infinite. the only place that I think I might see it is in the part that says that when you look at the universe on a large enough scale, parts of it begin to look homogenous. (It is complex so I am just trying to understand what I read)

(I apologize for putting opinion into the conversation earlier, it will not happen again)
 
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  • #32
The present idea suggests we live in a special time...

First there is nothing
Then the Big Bang
Then acceleration of expansion (short fast inflation)
Then deceleration of expansion (and formation of structure)
Later, acceleration of expansion (long (infinite?) slow, dark energy)
Eventually, this acceleration of expansion has the universe approaching total vacuum (back to nothing)
 
  • #33
duhuhu said:
so I read through the cosmological principle and I am having some trouble understanding where you guys pulled out that the universe is infinite. the only place that I think I might see it is in the part that says that when you look at the universe on a large enough scale, parts of it begin to look homogenous. (It is complex so I am just trying to understand what I read)

(I apologize for putting opinion into the conversation earlier, it will not happen again)

The universe may be infinite or it may be finite. No one knows. I don't recall anyone stating categorically that it IS infinite and if they did they are offering unsupported personal speculation.
 
  • #34
phinds said:
The universe may be infinite or it may be finite. No one knows. I don't recall anyone stating categorically that it IS infinite

No, but we can say that our current best-fit model of the universe has it being spatially infinite. The "error bars" around the best-fit model are large enough, however, to include the possibility that it is finite.
 
  • #35
duhuhu said:
so I read through the cosmological principle and I am having some trouble understanding where you guys pulled out that the universe is infinite. the only place that I think I might see it is in the part that says that when you look at the universe on a large enough scale, parts of it begin to look homogenous. (It is complex so I am just trying to understand what I read)

(I apologize for putting opinion into the conversation earlier, it will not happen again)

Our universe 'according' to some solutions/math in Lambda-CDM model(somehow fits with homogenous-U) which is flat naturally has an "infinity" part to it. This is due to metric limitation in the friedman universe where ds2=−dt2+a2(t)[dr21−kr2+r2dΩ2] [assuming of course] 'if' k→0 the maximum possible distance (S) smax→∞ goes to infinity.. 'If' k>0 we have finite. And such special case of nontrivial topology torus (flat but finite).

Ok now. We have observations/data, but funny enough it is dynamic for obvious reasons. We have very small curve that show finite side of the argument and largely flat-infinite. Statistically by comparison we can't tell whether it is infinite or finite. ^^
 
  • #36
bahamagreen said:
The present idea suggests we live in a special time...

First there is nothing
Then the Big Bang
Then acceleration of expansion (short fast inflation)
Then deceleration of expansion (and formation of structure)
Later, acceleration of expansion (long (infinite?) slow, dark energy)
Eventually, this acceleration of expansion has the universe approaching total vacuum (back to nothing)

... It becomes mentally complicated when you use 'start and so on'/'nothing'. Try replacing 'nothing' to 'Everything' and the progression to 'phases/transitions'.
 
  • #37
What do you mean?

If there is anything, then everything cannot include nothing.

But if there is nothing, that includes everything... :)
 
  • #38
PeterDonis said:
No, but we can say that our current best-fit model of the universe has it being spatially infinite. The "error bars" around the best-fit model are large enough, however, to include the possibility that it is finite.

Agreed. I was more making the point that it is not known for a fact either way.
 

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