Minkowski Diagram: Expanding Universe

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Discussion Overview

The discussion revolves around the representation of Minkowski diagrams in the context of an expanding universe, specifically addressing whether such diagrams can accurately depict non-flat spacetime models without acceleration or black holes.

Discussion Character

  • Debate/contested, Technical explanation

Main Points Raised

  • One participant questions how to draw a Minkowski space diagram with expanding space, suggesting a specific approach.
  • Another participant asserts that expanding space models, such as FRW spacetimes, are not flat and therefore cannot be represented as Minkowski spacetime diagrams.
  • A third participant references the Milne model as a potential example related to the discussion of Minkowski diagrams in an expanding universe.
  • Another response emphasizes that while it is reasonable to attempt to visualize a (1+1) dimensional slice of spacetime, it cannot be considered a Minkowski diagram unless it truly represents Minkowski spacetime. They also note the existence of multiple representations in curved spacetime.

Areas of Agreement / Disagreement

Participants express disagreement regarding the validity of drawing Minkowski diagrams for expanding space. There is no consensus on whether such diagrams can accurately represent the discussed models.

Contextual Notes

Participants highlight the distinction between flat and curved spacetime, indicating that assumptions about the nature of the spacetime being represented are critical to the discussion.

sqljunkey
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Is this how you would draw the Minkowski space diagram with space expanding, (without acceleration or black holes) ?
upload_2019-1-2_17-59-7.png
 

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sqljunkey said:
the Minkowski space diagram with space expanding

There is no such thing. The "expanding space" spacetime models (the FRW spacetimes) are not flat, so they're not Minkowski spacetime. So you can't draw a Minkowski spacetime diagram of them at all.
 
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It's reasonable to want to draw a map of a (1+1) dimensional slice of spacetime. However this isn't a Minkowski diagram unless it's Minkowski spacetime. There's also more than one way to do it in curved spacetime, much as there are multiple flat representations of the surface of the Earth. For example, see figure 1 in Davis and Lineweaver https://arxiv.org/abs/astro-ph/0310808
 

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