Light Speed & Expanding Galaxies: A Cosmological Mystery

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Discussion Overview

The discussion centers around the apparent contradiction of observing galaxies that are receding from us at speeds exceeding that of light due to the expansion of the universe. Participants explore the implications of this phenomenon on visibility and the nature of cosmic expansion.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question how we can see galaxies that are receding faster than the speed of light, suggesting that they should disappear from view.
  • Others clarify that galaxies are not expanding themselves but are moving away due to the expansion of the universe, likening it to sound waves from a receding object.
  • A participant notes that certain galaxies may become unobservable if they are receding faster than light, while others may still be visible as their light travels toward us.
  • It is mentioned that the expansion of space occurs between galactic clusters, which does not affect the internal dynamics of those clusters where gravity prevails.
  • One participant states that receding galaxies will redshift into obscurity rather than disappearing suddenly.
  • Another participant asserts that, according to current models, galaxies will eventually fade away due to their increasing distance.

Areas of Agreement / Disagreement

Participants express differing views on the implications of galaxies receding faster than light, with some agreeing on the mechanics of visibility and redshift, while others raise questions about the nature of cosmic expansion and its effects on observation. The discussion remains unresolved regarding the long-term visibility of these galaxies.

Contextual Notes

Participants acknowledge that the discussion involves complex concepts related to cosmological expansion, redshift, and the gravitational effects within galactic clusters, which may not be fully resolved in the current exchange.

binbots
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If galaxies aer expanding fast than the speed of light how are we able to see them. At the moment this expansion surpassed the speed of light shouldn't these galaxies disappear or remain unchanged visually?
 
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Galaxies are not expanding.

Do you mean moving away from us due to the expansion of the universe? You can see galaxies that appear to be moving away from us faster than the speed of light for much the same reason you can hear an object moving away from you faster than the speed of sound. The speed of light is not dependent on the speed of the moving galaxy.

There is a limit, though, based on the universe expanding so fast the distance is increasing faster than the light moves.
 
binbots said:
If galaxies aer expanding fast than the speed of light how are we able to see them. At the moment this expansion surpassed the speed of light shouldn't these galaxies disappear or remain unchanged visually?

Certain galaxies might be presently unobservable because they are receding from us at faster than light speed due to cosmological expansion. But the light of others might still be streaming on its way toward us and so we see them as they were prior to reaching that superluminal velocity. Keep in mind that the galaxies themselves cannot exceed the 186,000 speed limit. It's the space between galactic clusters that is expanding at that velocity and beyond and thus creating insurmountable distances between us and them and making their present light forever undetectable for us.

Also, we are talking about areas of space where gravity doesn't cancel out cosmological expansion.
Galactic clusters containing from as few dozen to millions of galaxies are not internally affected by that expansion. In fact, within these clusters, galaxies collide and merge. Some. like our Magellanic clouds which orbit our Milky Way are satellites of other galaxies. Andromeda, the largest member of our own galactic cluster is presently approaching us and seems destined to collide with our our galaxy in the far future. The the distancing is between super clusters of galaxies where gravity cannot predominate.

BTW
By unobservable I mean that the light being emitted by them now will never reach us.
 
Last edited:
Receeding galaxies merely redshift into obscurity, they will never suddenly 'disapperar' from view.
 
so they will eventually fade away?
 
Yes, according to current models.
 

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