Is the inverse-square law valid for all cosmological distances?

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Discussion Overview

The discussion centers on the validity of the inverse-square law at cosmological distances, particularly in relation to different Friedmann models (k=-1, 0, +1) and its implications for the observed acceleration of the universe's expansion. Participants explore theoretical frameworks, potential modifications to gravity, and the impact of these concepts on cosmological observations.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants argue that the inverse-square law is only valid at all distances in the k=0 Friedmann model, suggesting that its validity is contingent on the flatness of space.
  • Questions are raised about whether the current consensus model's value for k=0 assumes the validity of the inverse-square law, which could imply a circular argument.
  • There is speculation that modifications to gravity beyond General Relativity could potentially explain cosmic acceleration, although no compelling models have been presented that fit observations.
  • Some participants assert that the cosmological constant model (ΛCDM) adequately fits current cosmological data, while others express skepticism about its acceptance and propose alternative explanations for acceleration.
  • Concerns are voiced regarding the heavy reliance on the inverse-square law in cosmological methodologies, with some participants questioning its foundational assumptions and empirical verification at cosmological distances.
  • Participants discuss the possibility that the apparent acceleration in the universe's expansion could stem from incorrect assumptions about the inverse-square law's applicability to standard-candle distances.
  • There is mention of the potential for future weak lensing surveys to distinguish between modified gravity theories and dark energy models.
  • Some participants note that deviations from General Relativity have been explored but have not successfully explained observational discrepancies.

Areas of Agreement / Disagreement

Participants express a range of views on the validity of the inverse-square law and its implications for cosmology. While some agree on the adequacy of the ΛCDM model, others challenge its assumptions and propose alternative theories. The discussion remains unresolved with multiple competing perspectives on the role of the inverse-square law in cosmological observations.

Contextual Notes

Participants highlight the complexity of assumptions embedded in cosmological equations and methodologies, particularly regarding the inverse-square law. There are unresolved questions about empirical verification of the law at cosmological distances and the implications of different values for k in the Friedmann equations.

  • #31
Chalnoth said:
Your statement here has nothing to do with my question.

I can see that this discussion is not going to work out. :-)
 
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  • #32
Chalnoth said:
Why do you think that Newtonian Doppler shift is currently assumed?
Because distant Hubble objects are assumed to be stationary relative to their "local space" and so they are assumed to be not subject to relativistic time dilation which is a function of peculiar velocity relative to local space. Therefore the classic Doppler shift formula which does not include time dilation is used. In a flat-space-no-gravity model a receding object with a high relativistic velocity would have to be time dilated and so the relativistic Doppler equation would have to be used.

What model you use depends on what you measure, but what you measure depends on what model you assume.
 
  • #33
yuiop said:
Because distant Hubble objects are assumed to be stationary relative to their "local space" and so they are assumed to be not subject to relativistic time dilation which is a function of peculiar velocity relative to local space. Therefore the classic Doppler shift formula which does not include time dilation is used. In a flat-space-no-gravity model a receding object with a high relativistic velocity would have to be time dilated and so the relativistic Doppler equation would have to be used.
The peculiar motion of galaxies is generally pretty small compared to relativistic velocities, typically less than 1000 km/sec or so. It can't get much greater because any object that moves rapidly with respect to the expansion quickly catches up to the expansion. The only reason why some things move that fast at all is because they're in the vicinity of some nearby massive object that they're falling towards or in orbit around.
 
  • #34
Chalnoth said:
The peculiar motion of galaxies is generally pretty small compared to relativistic velocities, typically less than 1000 km/sec or so. It can't get much greater because any object that moves rapidly with respect to the expansion quickly catches up to the expansion.
I understand that, but in the flat-space-no-gravity model there is no peculiar motion. Peculiar motion only belongs to the Hubble flow idea where Hubble objects are receding at the same velocity as the "local space". The simplistic Special Relativistic interpretation does not consider a vacuum or space itself to be a substance with a measurable velocity, which has etheristic implications.
 
  • #35
yuiop said:
I understand that, but in the flat-space-no-gravity model there is no peculiar motion.
Okay, but that model doesn't describe our universe. So why consider it?
 

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