All Questions
Tagged with cosmology metric-tensor
180
questions
25
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8k
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How does the Hubble parameter change with the age of the universe?
How does the Hubble parameter change with the age of the universe?
This question was posted recently, and I had almost finished writing an answer when the question was deleted. Since it's a shame to ...
- 358k
22
votes
3
answers
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What is meant when it is said that the universe is homogeneous and isotropic?
It is sometimes said that the universe is homogeneous and isotropic. What is meant by each of these descriptions? Are they mutually exclusive, or does one require the other? And what implications rise ...
- 3,439
22
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6
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Interpretation of a singular metric
I'm interested to find out if we can say anything useful about spacetime at the singularity in the FLRW metric that occurs at $t = 0$.
If I understand correctly, the FLRW spacetime is a combination ...
- 358k
13
votes
3
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Space is expanding so what is time doing? [duplicate]
Space is expanding and as we know space and time are intrinsically linked to be now known as spacetime.
What is happening to time during expansion? Is there more time, longer time or is the time part ...
- 2,421
12
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1
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What are the allowed topologies for a FRW metric?
Given a spacetime that has the maximal amount of spacelike translations and rotations, what are the possible topologies it may take? I am mostly wondering about the "time" topology since the spatial ...
- 16.5k
11
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1
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Gravity with more than one metric tensor
As weird as it sounds, yes, there are gravity theories with more than one metric tensor. This is called bimetric gravity.
My question to those who have encountered bimetric gravity before:
a) ...
user106422
10
votes
4
answers
3k
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Perturbative expansion of the metric and its inverse
As I understand it, in the context of cosmological perturbation theory, one expands the metric $g_{\mu\nu}$ around some background metric (in this case the Minkowski metric) such that $$g_{\mu\nu}=\...
- 3,207
9
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4
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Einstein tensor in Friedmann equations : where is the missing $c^2$?
I would like to demonstrate the several forms of the Friedmann equations WITH the $c^2$ factors. Everything is fine ... apart that I have a missing $c^2$ factor somewhere.
In all the following $\rho$ ...
- 1,109
8
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4
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738
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How can we distinguish between the metric expansion of space and the speed of light slowing over time?
How can we distinguish between the metric expansion of space and the speed of light slowing over time?
I understand that the universe is expanding. But it seems that our main measurement techniques ...
- 3,556
8
votes
3
answers
522
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Why are particles described with Poincaré symmetry even though space seems inhomogeneous?
Poincaré transformation consists of translation, rotation, and boosting. And by assuming the physical quantities are invariant and equations are covariant under the transformations, we build the ...
- 479
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2
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Non-stationary spacetime
What is an example for a spacetime that is non-stationary that is considered as a description of something in nature?
So far all the spacetimes I encounted have always been stationary (Schwartzschild,...
- 2,024
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3
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Why does the FLRW metric assume constant curvature?
So the FLRW metric takes the following form in reduced-circumference polar coordinates.
$$\mathrm ds^2 = -c^2 \mathrm dt^2 + a^2(t) \left(\frac{\mathrm dr^2}{1 - k\, r^2} + r^2 (\mathrm d\theta^2+\...
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7
votes
2
answers
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Is the de Sitter Universe static?
In the book Tolman R.C. Relativity, thermodynamics, and cosmology (3pr., Oxford, 1949) I read that the de Sitter Universe is static as well as the Einstein Universe.
But the de Sitter Universe is ...
- 2,635
7
votes
3
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Does isotropy imply homogeneity?
This question comes from exercise 27.1 in Gravitation by Misner, Thorne and Wheeler. They required the following:
Use elementary thought experiments to show that isotropy of the universe implies ...
- 3,622
6
votes
3
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277
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Two Robertson-Walker observers, at what time will a light signal be received?
Here is a question I have that is inspired by this question here.
The spacetime metric of a radiation-filled, spatially flat ($k = 0$) Robertson-Walker universe is given by$$ds^2 = - dT^2 + T[dx^2 + ...
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