All Questions
525
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
9
votes
4
answers
651
views
Understanding expansion of the Universe as things flying apart
Say that we have a Universe uniformly filled just with matter (let's not bring dark energy into this). And say that we fill it with very light particles (so that the gravitational interaction between ...
0
votes
0
answers
47
views
Inflation in background free models of the universe
There are many authors who are attempting to construct a model of physics that doesn't rely on the objective existence of spacetime. This is part of the work in quantum gravity. This leads to things ...
- 1,430
1
vote
1
answer
71
views
Age of universe vs Hubble time in Milne universe
Consider an empty universe where energy density $\varepsilon = 0$, thus the Friedmann Equation can be reduced into:
$\dot a^2= -\frac{kc^2}{R_O^2}$
$k$ is the curvature of space, $R_0$ is the radius ...
- 411
4
votes
3
answers
199
views
Change of variables from FRW metric to Newtonian gauge
My question arises from a physics paper, where they state that if we take the FRW metric as follows, where $t_c$ and $\vec{x}$ are the FRW comoving coordinates:
$$ds^2=-dt_c^2+a^2(t_c)d\vec{x}_c^2$$
...
- 293
7
votes
3
answers
5k
views
If we consider the spacetime of the universe to be four-dimensional, does the Big Bang lie in its center?
Apologies for the (hopefully now somewhat less) clickbait-y title. Now, of course, I know that the Big Bang did not happen at any point connected to a single point in our current $3$-dimensional ...
- 1,897
2
votes
1
answer
123
views
Could the universe have a form of a $T^3$-torus?
Cosmological measurements suggest that we live in a flat universe. However, what might be less clear is its topology. So could the flat universe have the form of a $T^3$-torus, i.e. the torus whose ...
- 9,778
1
vote
1
answer
80
views
Do we still not know whether the universe had a beginning? [duplicate]
The Question:
According to Prof. Brian Cox in the first 30 seconds of this YouTube video from 4 years ago, we do not know whether the universe had a beginning. Is it still the case that we do not ...
- 105
-4
votes
2
answers
141
views
What model tells us there is nothing outside the universe?
Is there an existing model or theory that shows there is nothing outside of the universe that interacts with anything inside the universe? Or to put it in other words, is there a model or theory ...
- 4,783
1
vote
1
answer
80
views
Cosmic web shape
Does anybody know why the cosmic web is shaped like a web? I feel like it would be more likely that it is more like a galaxy with a supermassive black hole in the center, if that were true, what would ...
2
votes
1
answer
100
views
Reducing Tensor-rank by fixing an argument
Assume for example that you are given a (2,0) tensor $T^{\mu\nu}$ and you want to create
a vector, i.e., a (1,0) tensor out of it. Is it possible to just fix an index of
$T^{\mu\nu}$ while keeping the ...
- 56
0
votes
3
answers
126
views
Does science have evidence that the simulation hypothesis is true? [closed]
It is often written that the simulation hypothesis cannot be proven or disproved.
There is also a lot of talk about the fact that the simulation hypothesis is not science.
But the people also write ...
- 109
0
votes
0
answers
100
views
Movement of galaxies vs expansion of space
I can readily accept the theory that the universe is expanding as a mathematical model to explain the fact that all galaxies are moving away from each other, but I have difficulty understanding ...
- 1
1
vote
1
answer
95
views
What happens if we let time expand in the FLRW metric?
If we multiplied the time differential (dt) by a scale factor that depends on time in the FLRW metric, what would this imply on cosmology? In particular, what are its implications on the cosmological ...
- 468
0
votes
2
answers
119
views
Time in the Standard Model of Cosmology
Beyond a formal preference for background independence, what is stopping us from setting cosmological time as a de facto universal timeline, analogous to newtonian absolute time? General relativity ...
- 1