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### Is it accurate to say that nothing can travel faster than $c$ in a GR context, where more space can be created? [duplicate]

Years ago, my brother and I had an argument where I was trying to convince him that nothing could travel faster than the speed of light. I was pursuing this in the context of Special Relativity. My ...
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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 ...
9k views

### Why is the observable universe so big?

The observable universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?
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### No FTL information implies no FTL travel?

The general consensus in the scientific community is that it is impossible to transmit information faster than light. There is also speculation that it might be possible to open wormholes or travel ...
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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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### Interpretation of cosmological redshift

I was trying to understand why we cannot explain the observed redshift of distant galaxies using special relativity and I came upon this article by Davis and Lineweaver. Unfortunately when I arrive ...
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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 ...
178 views

### 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 ...
207 views

### 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 ...
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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### 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 ...
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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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### 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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### Definition of speed of photon coming from /galaxy moving away faster then the speed of light?

This is not the duplicate, because my question is not answered there, my question is about the definition of the speed of the photon and the galaxy itself. I understand that only photons reach us ...
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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 ...