Can the Universe Transition from Finite to Infinite?

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Discussion Overview

The discussion revolves around the nature of the universe's curvature and whether it can transition between finite and infinite states. Participants explore cosmological models, the implications of curvature on the universe's shape, and the terminology used to describe these concepts.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions how a finite universe could become infinite, suggesting a contradiction in cosmological models regarding curvature.
  • Another participant asserts that physical entities that are finite cannot become infinite and vice versa, indicating that the universe must have started as either finite or infinite.
  • Some participants clarify that there are models with fixed curvature signs and that the curvature does not change from one spatial slice to another.
  • There is mention of a family of models that include both positive and negative curvature, with examples drawn from conic sections.
  • Participants discuss the confusion surrounding the terms used to describe curvature changes, noting that the curvature referenced in some contexts relates to the universe's acceleration or deceleration rather than spatial curvature.
  • Multiple shapes of negative curvature are acknowledged, with terms like "saddle" and "trumpet" being used descriptively, though one participant notes the limitations of language in conveying complex geometries.

Areas of Agreement / Disagreement

Participants express differing views on the nature of curvature and its implications for the universe's finitude and infinitude. There is no consensus on whether the universe can transition between these states, and the discussion remains unresolved regarding the specifics of curvature models.

Contextual Notes

Some statements depend on interpretations of curvature and the definitions used in cosmological models. The discussion includes unresolved questions about the nature of curvature changes and the terminology used to describe them.

Helios
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I want to get this simplest cosmology question asked once and for all because I don't get it. We have "cosmologicial models" which have the curvature of the universe going from positive to negative ( or maybe visa-versa ) at some stage ( or not? ). Now elsewhere we hear that a negatively curved universe is infinite and a positive one is finite. Now any student is going to ask "How can finitude become infinite?" ( or visa-versa? ) . Is what I've said flawed? Is there a contradiction? I am the amateur so the fault is surely mine. This must be a page one cosmology question with an elementary answer so can someone please explain? Someone can throw in some names of different universe shapes if they feel like it. Thanks.

edit--send to cosmology pls
 
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In response to one part of your question, physical things that are finite cannot become infinite and things that are infinite cannot become finite. The universe either started out infinite or it started out finite. No one knows, but however it started out, that's how it is now. If it is finite, there is a very strong consensus that it is unbounded because a universe that is finite and bounded would have a center and an edge and our universe has neither.
 
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No one is ever going to see "bounds" regardless of consensus. I did not mention bounds.
 
Helios said:
We have "cosmologicial models" which have the curvature of the universe going from positive to negative ( or maybe visa-versa ) at some stage

Which models are you talking about? I'm not aware of any models where the sign of the spatial curvature of slices of constant time changes.
 
Helios said:
We have "cosmologicial models" which have the curvature of the universe going from positive to negative ( or maybe visa-versa ) at some stage ( or not? ).
This is not correct the way you are thinking of it. A better way to say this is that there are a family of models where some have positive and some have negative curvature.

This is related to the fact that conic sections are a family of figures which include ellipses (positive curvature, finite) and hyperbolas (negative curvature, infinite).
 
DaleSpam said:
A better way to say this is that there are a family of models where some have positive and some have negative curvature.

And in each model in the family, the sign of the curvature is fixed; it doesn't change from one spatial slice to another. (I know you know this, but I want to make sure the OP understands it.)
 
Thanks. It must be acceleration vrs deceleration that has the sign change. This
https://www.physicsforums.com/threads/accelerating-expansion-and-mediocrity-priniciple.850180/
confused me because I read
"when curvature is crossing from negative to positive"
and I misconstrued the meaning. I take it that flat or negative curvature is the most popular belief. Now are there multiple negative shapes or one, because I've heard "saddle" and also "trumpet or horn" as descriptive of negative curvature?
 
Helios said:
I read
"when curvature is crossing from negative to positive"

The "curvature" referred to here is not spatial curvature; it refers to the curvature of the line on the graph describing our actual universe's deceleration/acceleration over time.

Helios said:
I take it that flat or negative curvature is the most popular belief.

Our best current model is that the universe is spatially flat.

Helios said:
are there multiple negative shapes or one

Multiple.
 
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Helios said:
Now are there multiple negative shapes or one, because I've heard "saddle" and also "trumpet or horn" as descriptive of negative curvature?
There aren't really good words in English for the shapes of 4D pseudo Riemannian manifolds. I am not sure that any word will convey the geometry correctly, that is why we use math.

Saddle refers to the spatial shape and horn refers to the time evolution, so both are legitimate descriptions.
 
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