All Questions
18
questions
1
vote
2
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133
views
Is the FRW metric, based on spatial homogeneity and isotropy, rotationally and translationally invariant? If so, how?
The spatial part of the Minkowski metric, written in the Cartesian coordinates, $$d\vec{ x}^2=dx^2+dy^2+dz^2,$$ is invariant under spatial translations: $\vec{x}\to \vec{x}+\vec{a}$, where $\vec{a}$ ...
- 11.8k
2
votes
1
answer
100
views
Topological phase transitions for the whole universe...?
Physicist Grigory Volovik has put forward some ideas about the universe undergoing a topological phase transition (especially in the early stages of the universe). He published a book called "The ...
- 2,462
0
votes
0
answers
66
views
No symmetries in the universe at the Big Bang...?
I apologize in advance if this is a stupid question but...
According to some scenarios about the beginning of the universe (namely cosmological inflation), in layman terms, everything was born out of ...
- 2,462
0
votes
0
answers
44
views
Effects of anisotropy and non-homogeneity in the universe's symmetries...?
I was reading Philip W Anderson's essay "More is Different" (https://www.tkm.kit.edu/downloads/TKM1_2011_more_is_different_PWA.pdf) and at some point he links the isotropy and homogeneity of ...
- 2,462
0
votes
0
answers
62
views
Spacetimes where symmetries vary from place to place?
Are there spacetimes or metrics where symmetries (like Poincaré, Lorentz, diffeomorphism, translational... invariances) are only local and the symmetries of one local neighbourhood are not, a priori, ...
- 2,462
1
vote
1
answer
81
views
What if the universe was not uniform...?
In this popular science article, they say that if our universe resulted to be non-uniform (that is highly anisotropic and inhomogeneous) then the fundamental laws of physics could change from place to ...
- 2,462
0
votes
0
answers
86
views
Spacetimes, metrics and symmetries in the theory of relativity?
I was discussing this paper with a couple of physicists colleagues of mine (https://arxiv.org/abs/2011.12970)
In the paper, the authors describe "spacetimes without symmetries". When I ...
- 2,462
0
votes
1
answer
105
views
If the universe is infinite, would it be homogenous?
I know, that we can't really know the answer to that, but what is the current state of understanding?
We seem to assume that on very large scales, the universe is homogenous. As I understand it, this ...
- 866
-3
votes
1
answer
170
views
Must spacetime be homogeneous?
According to Einstein's equations of general relativity, space must be homogeneous. It can't have an edge or a centre. Is the same true of 4-dimensional spacetime – must it also be homogeneous?
- 371
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
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
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
0
votes
1
answer
161
views
Is universe symmetric about a point?
We have a good amount of discussion and theories on the formation of universe.
I want to ask is universe symmetric about a point?
I think that the answer should depend upon the uniformity of ...
- 3,377
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
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?
