Can galaxies recede from us at velocities greater than c?

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Discussion Overview

The discussion centers on whether galaxies can recede from us at velocities greater than the speed of light (c) and the implications of such a phenomenon on the laws of special relativity. It explores theoretical frameworks, cosmological models, and the nature of space expansion.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that galaxies can recede from us at velocities greater than c without violating special relativity, due to the expansion of space itself rather than the galaxies moving through space.
  • Others argue that in a cosmological model where distant objects are receding in free-fall, relativity is not compromised, as the receding matter is not being accelerated.
  • A participant mentions that there will be an event horizon at a certain distance from the observer, which is a consequence of this model.
  • It is noted that special relativity does not apply in non-flat spacetimes, and in flat spacetimes with a cosmological constant, the equations from special relativity may not hold.
  • One participant emphasizes that it is space itself that is expanding, leading to the perception of galaxies receding, rather than the galaxies moving through space at high speeds.

Areas of Agreement / Disagreement

Participants express multiple competing views on the topic, with no consensus reached regarding the implications of galaxies receding faster than c and the applicability of special relativity in such scenarios.

Contextual Notes

Limitations include the dependence on specific cosmological models and the assumptions regarding the nature of space expansion and the behavior of galaxies within those models.

aachenmann
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Can galaxies recede from us at velocities greater than c?
If yes, don't they violate the law of special relativity?
 
Last edited:
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If you adopt a cosmoloy where distant things are receeding from any observer, but doing so in free-fall, the answers are 'yes' and 'no'.

Because of the 'yes', there will be an event horizon at a certain distance from the observer.
Because the receeding matter is in free-fall, thus not being accelerated, relativity is not compromised. There is a GR cosmological model which predicts this.

Check it out here -

en.wikipedia.org/wiki/Friedmann-Lemaître-Robertson-Walker_metric
 
Last edited:
Mentz114 said:
If you adopt a cosmoloy where distant things are receeding from any observer, but doing so in free-fall, the answers are 'yes' and 'no'.

Because of the 'yes', there will be an event horizon at a certain distance from the observer.
because the receeding matter in free-fall, thus not being accelerated, relativity is not compromised. There is a GR cosmological model which in fact predicts this.
Special relativity does not apply since such spacetimes are not flat. Also in flat spacetimes with a cosmological constant the equations from special relativity no longer apply.
 
Last edited:
aachenmann said:
Can galaxies recede from us at velocities greater than c?
If yes, don't they violate the law of special relativity?

Galaxies themselves probably aren't receding from us, but it is space itself that is expanding, thus the galaxies get farther away from us.

So, yes, in theory a galaxy could "recede" from us at speeds greater than C, because it is space itself that is expanding, i.e. the galaxies are NOT actually moving through space at the speeds we perceive them to be.
 

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