All Questions
71
questions
0
votes
1
answer
60
views
Is a closed universe symmetric?
Say our universe is closed, at some point in the future it will reach a maximum and then begin to contract, will it return to say the point we are now in exactly the reverse manner? For example, as ...
- 59
4
votes
1
answer
171
views
How are the symmetries of $g_{\mu\nu}$ reflected in those of $T_{\mu\nu}$?
For a homogeneous and the isotropic universe, the spacetime metric $ds^2$ is given by the FRW form in comoving coordinates: $$ds^2=dt^2-a^2(t)\Big[\frac{dr^2}{1-kr^2}+r^2(d\theta^2+\sin^2\theta d\phi^...
- 26.8k
2
votes
2
answers
143
views
With respect to what we are saying space is homogeneous or space-time is isotropic? [duplicate]
I don't really understand what we are talking about when we say space is homogeneous. What we are measuring? My notion is: it should depend on the entity and with respect to that entity one can decide ...
- 53
0
votes
2
answers
846
views
Einstein field equations are PDEs or ODEs?
This past semester, I just took an introductory course on G.R., which translates to a lot of differential geometry and then concluding with Schwarzschild's solution. We really didn't do any cosmology. ...
- 303
1
vote
1
answer
159
views
Why doesn't the universe look symmetrical?
If the universe was a dot lets say a point, and that dot expanded equally from all sides, then shouldn't the universe look more symmetric, maybe indentical, from that dot all around?
7
votes
3
answers
424
views
Wouldn't we have an additional conservation law in a spherical Universe?
According this answer, the recent WMAP experiment has only shown that if our Universe has a spherical geometry, then it should have at least a $3\cdot 10^{11}$ light year big radius.
Now consider the ...
- 8,247
0
votes
0
answers
36
views
How can the universe be homogeneous and isotropic while it has started with a big bang? [duplicate]
We assume the universe has been shaped by a explosion "Big Bang" but from what I understand explosions make the environment "not" homogeneous and "not" isotropic.
If that's right how can we assume the ...
- 474
3
votes
1
answer
192
views
Canonical form of structure constants and mutually orthogonal triad on the orbits of Bianchi cosmologies
In Class. Quant. Grav. 28 (2011) 185007: "Linearization of homogeneous, nearly-isotropic cosmological models" at the start of section 2.3 the authors (Andrew Pontzen and Anthony Challinor) claim that ...
- 1,563
2
votes
2
answers
399
views
Could dark energy just be another characteristic of the gravitational force?
Recent observations of the accelerating expansion of the universe have been quantified and for the time being given a name as to the cause: Dark Energy. And from what I've read from other, similar ...
- 11.7k
5
votes
3
answers
161
views
Why is there a matter-dark matter asymmetry?
It is said generally that nature is symmetric. For example if light behaves as both a particle and a wave, then matter must also do so, which turns out to be true. But we find that the Universe ...
- 51
0
votes
1
answer
229
views
Is the cosmological time grosso modo isochrone?
Is the cosmological time grosso modo isochrone? by analogy with space isotropy. Or else do we have possibly great differences by analogy with great voids in the space.
We know that it's not strictly ...
user46925
3
votes
2
answers
179
views
During the "Dark Ages" of the Universe's evolution, how lumpy (anisotropic) and dynamic was the mass distribution?
In the dark ages between recombination (~0.4 Myr post-BB) and reionization (~300 Myr post-BB) of atoms, there was not any condensed-phase matter (except maybe some form of dark matter), nor radiation ...
- 318
5
votes
2
answers
111
views
Is there a proof that space expanding produces observers at all points that see what we see?
I know that galaxies are moving away from us, and so can see that it's intuitive that if space was expanding, then the astronomical observations from Earth would be the same as at all other points in ...
- 799
3
votes
0
answers
347
views
How is translational symmetry related to Fourier decomposition?
The book (The Cosmic Microwave Background By Ruth Durrer) about cosmological perturbations says that because of translational symmetry of the background at a constant time, we can decompose our ...
- 431
7
votes
3
answers
3k
views
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