Infinite Universe: Can It Ever End?

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Discussion Overview

The discussion revolves around the concept of whether an infinite universe can ever end, exploring both spatial and temporal interpretations of "end." Participants engage in theoretical considerations related to general relativity and cosmological models.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that an infinite universe cannot end in space, as it would extend indefinitely.
  • Others argue that, according to general relativity (GR), an infinite universe must continue to expand forever and cannot come to an end in time.
  • One participant questions the assertion that an infinite universe cannot end in time, suggesting that a universe with a negative cosmological constant and open curvature could potentially recollapse.
  • Another participant acknowledges the possibility of a flat universe recollapsing under similar conditions but notes that such models have not been proposed to describe our actual universe.
  • There is a discussion about the implications of quantum gravity and whether something occurs after a recollapse, indicating uncertainty about the nature of "end" in this context.

Areas of Agreement / Disagreement

Participants express differing views on the possibility of an infinite universe ending, with some supporting the idea that it cannot end in space or time, while others propose alternative models that could allow for recollapse. The discussion remains unresolved with multiple competing views.

Contextual Notes

Limitations include the dependence on specific cosmological models and assumptions about the nature of the universe, as well as unresolved questions regarding the implications of quantum gravity.

Einstein's Cat
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Could an infinite Universe ever end?
 
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No. By definition.
 
Einstein's Cat said:
Could an infinite Universe ever end?

If by "end" you mean "end in space", then obviously not, as DaveC426913 pointed out.

If by "end" you mean "end in time", i.e., "come to an end", then according to GR, no; any spatially infinite universe must keep expanding forever.
 
PeterDonis said:
If by "end" you mean "end in time", i.e., "come to an end", then according to GR, no; any spatially infinite universe must keep expanding forever.

I didn't know that. Care to elucidate?
 
Hornbein said:
Care to elucidate?

There are no solutions in the FRW family of spacetimes that are spatially infinite and recollapse; all of the spatially infinite solutions expand forever. The only solutions that recollapse are spatially finite (closed universes--but not even all of those recollapse, some, with positive cosmological constant, expand forever).
 
PeterDonis said:
If by "end" you mean "end in space", then obviously not, as DaveC426913 pointed out.

If by "end" you mean "end in time", i.e., "come to an end", then according to GR, no; any spatially infinite universe must keep expanding forever.
I'm not sure that's true. You could have universe with a negative cosmological constant and open curvature that recollapses. However, it might not "end" in the sense that quantum gravity might demonstrate that something happens after the recollapse.
 
Chalnoth said:
You could have universe with a negative cosmological constant and open curvature that recollapses.

Hm, yes, you're right. (I think a flat universe could also recollapse with a negative cosmological constant.) AFIAK, nobody has proposed that such a model might describe our actual universe, so it would not be relevant in a practical sense, but as a matter of theory, yes, such a model could be spatially infinite and recollapse.

Chalnoth said:
it might not "end" in the sense that quantum gravity might demonstrate that something happens after the recollapse.

Yes, agreed; this would be true of closed models that recollapse as well.
 

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