Questions tagged [space-expansion]
The expansion of the universe is a phenomenon wherein, at scales much larger than galaxies, the distance between objects grows over time. This phenomenon is often described as "expansion of space", although there is no difference between space expanding and objects moving apart.
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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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Trying to reproduce curves with angle of CMB anisotropies as a function of distance and curvature parameter
I am looking for a way to get, by a simple numerical computation, the 3 curves on the following figure:
For this, I don't know what considering as abcissa (comoving distance ?, i.e
$$D_{comoving} = ...
user87745
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How do you explain cosmological red shifting in terms of gravitons?
We know that the photons from the big bang are continually being red shifted and losing more and more energy. In terms of the graviton view, how would you explain that? Where is the energy going?
Are ...
user84158
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How fast is a true vacuum bubble going to expand?
If a true vacuum bubble will occur in our universe, how fast is it going to expand?
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At what expansion rate (H) would virtual particles be ripped apart into real particles, and what might be the density (temperature) of such an event?
Is it even possible for a virtual particle-antiparticle pair to be torn apart by cosmological expansion? Virtual particle-antiparticle pairs are ripped apart near black hole event horizons, creating ...
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What kind of matter's energy density scales as the inverse of the scale factor
We know that radiation energy density scales as $a^{-4}$ with EoS parameter ($w=\frac{1}{3}$), matter as $a^{-3}$ with ($w=0$), curvature as $a^{-2}$ with ($w=-\frac{1}{3}$).
Then which kind of matter ...
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Quantum tunneling of dark energy from black holes
There was an article posted recently suggesting there is a very deep connection between dark energy and black holes.
https://phys.org/news/2019-09-black-holes-dark-energy.html
I was thinking this ...
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Robertson-Walker metric and cosmic homogeneity
The Robertson-Walker metric is of the form
$$\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).$$
My question is related to the $a^2(t)$ ...
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Hypothetically, could the interior of a black hole look exactly like the universe that surrounds us?
I do understand that we can't experimentally verify anything we imagine about the interior of a black hole.
If we were to apply what we know about the physics of the observable universe and assume ...
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Cosmological perturbations and energy in an expanding universe?
I was reading an interesting book from cosmomogist Viatcheslav Mukhanov Physical Foundations of Cosmology and I had a specific question about it:
It is usually said that energy conservation is ...
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Cosmic strings increasing internal energy as the Universe expands?
I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales.
At some part (point 2.4) he cites several article, including one by Rees and ...
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Confused about size of the universe in the past
From Wikipedia, I got that the photons of the cosmic microwave background radiation originated when the spherical volume of space which will become the observable universe was 42 million light-years ...
user338734
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Black hole vs Dark energy
This is a follow-up question to this answer.
The original question was whether there is a maximum size limit to a black hole in our universe. The answer given was that if the event horizon coincided ...
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Hubble's law for approaching, rather than receding, galaxies
Hubble's law states that, the recessional velocity $v_r$ of a distant object, relative to an observer, is
$$
v_r = H_0D + v_{pec}
$$
where $v_{pec}$ is the peculiar velocity, $H_0$ is the Hubble ...
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On the definition of comoving coordinates
In chapter 8 Carroll's Introduction to General Relativity: Spacetime and Geometry, he defines a set of coordinates to be 'comoving' if the metric is free of cross terms $dt\,du^i$ and the coefficient ...
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