All Questions
142
questions
9
votes
4
answers
659
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 ...
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
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
-4
votes
1
answer
78
views
The background of the universe [closed]
Mass(-energy-momentum) curves space(-time).
However - what is space-time?
Is it (the background) necessarily flat(without something in it) ?
If there were an empty universe - is there still spacetime ...
- 63
0
votes
1
answer
72
views
Is it impossible to measure a gravitational field between points in time?
I will try and be as concise and clear as possible, but I'm still trying to understand my own question.
Timelike observers can move freely through space, which allows them to set up experiments such ...
- 57
2
votes
1
answer
135
views
How do you relate $\Omega_{k}$, the curvature term in the FLRW metric, to the radius of curvature?
I have assumed, for reasons a bit too detailed to go into here, that if $\Omega_{k}$, the curvature term in the FLRW metric, is equal to 1, then the radius of curvature is equal to 13.8 billion light ...
- 371
0
votes
0
answers
73
views
Does spacetime move? With respect to what?
Can spacetime itself rotate along a body, like a black hole? Would it move like a wave?
- 495
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 ...
- 2,472
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, ...
- 2,472
0
votes
0
answers
87
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 ...
- 2,472
1
vote
1
answer
133
views
Does dark energy cause the creation of spacetime?
The universe is expanding, in theory because of 'dark energy'. Does this mean that this dark energy is causing an increase in the amount of spacetime? I.e., does dark energy cause the creation of ...
- 1,011
0
votes
1
answer
38
views
Does the dating of the formation of the world take into account the effect of space-time curvature? [duplicate]
For example, when we calculated that the age of the universe is about 13.8 billion years, did we take into account the difference between the flow of time back at the early universe, where mass was ...
- 1
1
vote
1
answer
315
views
The dimensions of the universe and the scale factor
How to get the universe’s volume from the universe’s scale factor? Taking into account that the scale factor is dimensionless and the volume has dimensions of $cm^3$ or $m^3$ ?
In this paper for ...
- 395
0
votes
3
answers
502
views
Where in the universe does time pass fastest relative to time on earth?
It is my understanding that time essentially moves slower in higher gravitational fields relative to time on earth. Conversely, in lower gravitational fields, time passes faster relative to earth. Is ...
3
votes
1
answer
202
views
Penrose conformal diagram of Morris-Thorne wormhole
Consider the classical Morris-Thorne wormhole solution:
$$\tag{1}
ds^2 = dt^2 - dr^2 - (r^2 + a^2) \,d\Omega^2,
$$
where $a$ is a positive constant, $r > 0$ for one asymptoticaly flat spacetime, ...
- 7,592