All Questions
13
questions
20
votes
10
answers
4k
views
Ambiguity in applying Newton's shell theorem in an infinite homogeneous universe
Newton's shell theorem has two corollaries:
The gravitational attraction of a spherically symmetric body acts as if all its mass were concentrated at the center, and
The gravitational acceleration ...
- 10.6k
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
12
votes
2
answers
843
views
Do the standard cosmology models spontaneously break Lorentz symmetry?
In standard cosmology models (Friedmann equations which your favorite choice of DM and DE), there exists a frame in which the total momenta of any sufficiently large sphere, centered at any point in ...
- 1,869
4
votes
1
answer
180
views
How does spatial homogeneity make sense in general relativity?
Homogeneity of space implies that the laws of physics are form invariant under translations $(x, y, z)\rightarrow (x+a, y+b, z+c)$. This makes sense for Newtonian mechanics and special relativity.
...
- 8,640
1
vote
0
answers
43
views
Could inhomogeneties and/or topological defects break the fundamental symmetries of the universe?
I have heard that some types of inhomogeneties and topological defects (like cosmic strings) in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, ...
- 2,462
11
votes
3
answers
2k
views
What is the exact meaning of homogeneity in cosmology?
I understand that, in general, homogeneity is the physical attribute of being uniform in composition (" of the same form at every point"), but I'm slightly confused when it is used in cosmology as ...
- 3,063
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
5
votes
2
answers
3k
views
Why isn't our universe symmetric?
Why were random variations introduced in the spherically symmetric universe after Big Bang which made it non-symmetrical. Since the outcome of a coin toss depend on factors such as torque applied, air ...
- 297
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
2
votes
1
answer
164
views
Does the homogeneity of space imply that the expansion of the universe is uniform?
Obviously, homogeneity implies that the density is the same everywhere at any time. However, does this imply that the expansion is uniform? By uniformity, I mean that if I pick three galaxies to form ...
- 1,335
2
votes
2
answers
288
views
Topology in cosmology
Usually in cosmology, we make the hypothesis that the universe is isotropic.
Which conditions does this hypothesis impose on the topology of the universe? Does it fix completely the topology? Are all ...
- 1,335
1
vote
2
answers
601
views
Expanding universe and Time-translation invariance
My understanding is that there are 5 implicit assumptions when introducing the Lagrangian formulation.
Assumptions:
We appear to know that the principle of stationary action is true for the universe
...
- 143
0
votes
1
answer
118
views
Does the homogeneity and isotropy of space imply that the expansion of the universe is uniform?
I have asked this question. Now I wonder what could happen if I take a step further. If space is assumed to be BOTH homogeneous AND isotropic, can I prove that the expansion of the universe is uniform?...
- 1,335