# Can galaxies recede from us at velocities greater than c?

## Main Question or Discussion Point

Can galaxies recede from us at velocities greater than c?
If yes, don't they violate the law of special relativity?

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## Answers and Replies

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Mentz114
Gold Member
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

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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 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.

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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?
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.