- #1

Tertius

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- 10

- TL;DR Summary
- It seems the universe is not continuously symmetric with time because of a few factors, entropy, expansion, etc. But, is there a possibility of defining a sensible discrete temporal asymmetry for the universe?

Our current model (FLRW) is clear that the universe has a continuous temporal asymmetry. This is seen as the expansion factor grows with time, and thermodynamically with entropy.

A continuous transformation in the current model ##t \rightarrow t + dt## is not the same as ##t \rightarrow t - dt ##.

Because of general covariance, however, there is not an absolute notion of time. This mean we can basically shift any coordinate system some amount to avoid a discrete symmetry. This is the edge of my knowledge on this...

Is it possible to define some discrete temporal symmetry relating to the coordinate system on some manifold? Or does this ultimately amount to combining QFT with GR, so we can have a meaningful CPT symmetry for the universe?

Also: found this paper while researching the topic. Does this seem legit or is there a problem with the way the authors use discrete symmetries with the EFE? (Equation 1 shows the basis of what they are doing, with ##dx^\mu \rightarrow -dx^\mu##. Eventually, by extension from electrodynamics, they get to eq. 7, which is supposedly a CPT symmetrized EFE)

https://arxiv.org/pdf/1103.4937.pdf

A continuous transformation in the current model ##t \rightarrow t + dt## is not the same as ##t \rightarrow t - dt ##.

Because of general covariance, however, there is not an absolute notion of time. This mean we can basically shift any coordinate system some amount to avoid a discrete symmetry. This is the edge of my knowledge on this...

Is it possible to define some discrete temporal symmetry relating to the coordinate system on some manifold? Or does this ultimately amount to combining QFT with GR, so we can have a meaningful CPT symmetry for the universe?

Also: found this paper while researching the topic. Does this seem legit or is there a problem with the way the authors use discrete symmetries with the EFE? (Equation 1 shows the basis of what they are doing, with ##dx^\mu \rightarrow -dx^\mu##. Eventually, by extension from electrodynamics, they get to eq. 7, which is supposedly a CPT symmetrized EFE)

https://arxiv.org/pdf/1103.4937.pdf