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### At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=\dfrac{v}{c}$ won't apply)?
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### Does our Galaxy move at the speed of light? [closed]

I am not a physicist, but I have a few questions that bother me for a while: As we do not know if the Universe is infinite or not I assume we do not know our whereabouts in it; We know,though, there ...
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### What should I consider as an observer to measure the speed of cosmic objects?

I mean for example if earth is the observer, then there might be entire galaxies travelling faster than the speed of light relative to earth. So according to Einstein relativity this shouldn't be ...
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### Two particles at different points on a curved manifold

Stupid question, but why two particles at different points on a curved manifold do not have any well-defined notion of relative velocity? For instance n cosmology the light from distant galaxies is ...
204 views

### How can an affine geodesic exist on a curved manifold?

I'm reading Inverno's "Introducing Einstein's Relativity". I think I understand how the author derives the affine geodesic equation. He considers the tangent vector to any curve as the vector field. ...
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### Does length contraction apply to the far galaxies moving due to expansion?

From Wikipedia: Universe in an expanding sphere. The galaxies farthest away are moving fastest and hence experience length contraction and so become smaller to an observer in the centre. Does ...
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### How can light be stretched (redshifted) if the speed of light is constant?

If light travels at the same speed how can it be stretched to provide red shift that is used in measuring the expansion of the universe?
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### Can light in a medium travel faster than speed of light $c$ in vacuum?

Refractive index of medium $m$, $n_m$, is given by $$n_m = \sqrt{\mu_r\epsilon_r}$$where $\mu_r$ and $\epsilon_r$ are permittivity and permeability of the medium respectively. And, velocity of light ...
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### Does general relativity say (or assume) that there is a universe-wide present all observers and objects live in?

I understand that in general relativity all observers agree on what it is they see (looking at the same object or event) when they account for the effects on their observations of the gravitational ...
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### Inertial mass increase of the farthest receding galaxies

If the farthest galaxies that we can see are receding at near the speed of light, shouldn't this increase their inertial mass as well relative to our galaxy?
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### Superluminal speed at cosmic horizon [duplicate]

The observable universe is limited by a cosmic horizon. Galaxies beyond the horizon move away from us faster than light, so we cannot see them. Imagine a spaceship in the vicinity of our horizon. If ...
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### Regarding expansion of the Universe and effect on the visible Universe

Does the faster than light expansion of the Universe have the following consequence: The extent of our visible universe increasing by 1 light-day every day, but due to expansion the actual contents of ...
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### How can string theory produce a globally Lorentz non-invariant theory like GR?

Locally, General Relativity is Lorentz invariant. Globally, it is not. e.g. Galaxy's sufficiently far apart are travelling more than the speed of light relative to each other. Although no particles in ...