# Questions tagged [space-expansion]

Space expansion is a cosmological phenomenon wherein the proper distance between two spatial points for a given inertial reference frame increases from one moment of time to another. That is, space itself expands; the added distance is not due to relative motion of points or objects.

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### The color of deep space background of an arbitrary universe

While writing my notes on cosmology in general relativity and the Olber's paradox, I was wondering about the color of the deep background of space. Our universe is mostly black because light didn't ...
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### How do you explain cosmological red shifting in terms of gravitons?

We know that the photons from the big bang are continually being red shifted and losing more and more energy. In terms of the graviton view, how would you explain that? Where is the energy going? Are ...
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### Robertson-Walker metric and cosmic homogeneity

The Robertson-Walker metric is of the form $$\tag{1} ds^2 = dt^2 - a(t)^2 \Big(\frac{dr^2}{1 - kr^2} + r^2 d\theta^2 + r^2 \sin^2\theta \, d\phi^2 \Big).$$ My question is related to the $a^2(t)$ ...
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### Alternative values for the constant in the cosmological equation of state? (Mislaid list)

A couple of days ago I came across - but now cannot locate - an online list of about eight or nine alternative theoretical values for the constant in the cosmological equation of state (W = 1, -1/3, 0,...
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### Could the horizon problem be explained by a finite universe instead of inflation?

Suppose the universe is finite (either closed or open with a non-trivial topology), any point could be within the horizon of another.
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### What is this metric's scale factor?

While answering this question about a hypothetical 3-sphere universe $S^3$ expanding with a constant acceleration $\phi$ from a zero initial speed $$r=\dfrac{\phi}{2}t^2$$ I started from a generic ...
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### What happens when cosmic background microwaves enter a bound system?

Gamma radiation after spending eons in expanding space have now “stretched “ into microwaves. What happens when these waves enter a bound system like a galaxy or galaxy cluster. Expanding space ...
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### Is space expansion causing increasing distances between galaxies?

I read/heard that the expansion of the universe would lead to any observer not being able to see other galaxies as time goes on, due to their light not reaching them anymore. But there's something I ...
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### How was it possible for the big bang to occur?

I think the person who asked this Phys.SE question meant a different thing than any of the people who wrote an answer thought so I'm asking what I'm guessing the author probably really meant. If there ...
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### Big Rip: will cosmic event horizon's Unruh radiation balance accelerating expansion?

The way I've seen the expansion of the universe described, it seems to work a bit like an inside-out black hole: an event horizon beyond which there is no causal connection, light from beyond it can't ...
The first Friedmann equations which describes the expansion and content of the universe is as follows:  H^2 = (\frac{\dot{a}}{a})^2 = \frac{8\pi G}{3} \rho - \frac{kc^2}{a^2} + \frac{\Lambda c^2}{3}...