All Questions
Tagged with cosmology special-relativity
127
questions
2
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0
answers
64
views
Proving that the CMBR remains thermal after a Lorentz boost
Suppose $S$ is a frame in which the CMBR is thermal and isotropic. I'm taking this to mean it follows the Planck Blackbody distribution $$n(\omega)=\frac{1}{\pi^2 c^3}\frac{\omega^2}{e^{\beta\hbar\...
- 248
2
votes
0
answers
48
views
Does expansion of space over time assume a particular space/time dichotomy?
Regarding the expansion of the Universe, Wikipedia states:
The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe ...
- 222
2
votes
3
answers
218
views
Does our past light-cone have some extension in three-dimensional space?
On spacetime diagrams cosmologists represent our past light-cone as a two-dimensional surface extending back in time, on which are situated all light-emitting events which we can observe today. This ...
- 928
3
votes
0
answers
39
views
Is it valid to add energy densities of *interacting* perfect fluids?
In several papers on interacting perfect fluids in cosmology, the authors assume that we still can add the energy densities and pressures of the individual fluids, as if there wasn't any interaction:
\...
- 7,592
16
votes
6
answers
2k
views
Question on Roger Penrose's argument on using particles as clocks
In Roger Penrose's book Cycles of Time under section 2.3 (space-time, null cones, metrics, conformal geometry), Penrose makes the following argument which states that only particles with mass can be ...
- 498
3
votes
0
answers
109
views
Can Lorentz symmetry be broken?
I am used to symmetry breaking in condensed matter, where we have a preferred reference frame (superconductivity, ferromagnets, etc). But what about Lorentz symmetry? What would such symmetry breaking ...
- 2,625
-1
votes
1
answer
446
views
What is meant by dark matter being non-relativistic and why is this?
What does it mean by dark matter being non-relativistic and why is it non-relativistic?
0
votes
1
answer
615
views
Four-Vector For Number Flux of Particles
Imagine a four-velocity $U^\mu(x)$ defined all over spacetime. Define the number-flux four-vector as
$$N^\mu = nU^\mu$$
where $n$ is the number density of a collection of particles in their rest frame....
- 201
1
vote
1
answer
67
views
Is the fact that we're moving with a certain speed with respect to the CMB special-relativity consistent?
As a sidenote to an exercise about the aberration of CMB at the dipole level, which scope was to find the peculiar velocity we have with respect to the cosmic background (assuming the doppler effect ...
- 577
1
vote
1
answer
155
views
Lower limit to the distance between two mouths of a wormhole?
This is more like a conceptual question. Wormholes are tunnels connecting two different parts of the same universe or connecting two parts in different universes. Taking the former one as an example; ...
- 3,464
1
vote
3
answers
59
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Indirectly breaking the speed of light through inflationary phase
How do you define velocity? Rate of change of "space" between two objects-and that will give relative velocity between them. Now in inflationary phase the universe expanded faster than light ...
- 483
2
votes
1
answer
947
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Comoving coordinates and frames in Cosmology
In FLRW metric, I am reading that all galaxies are given a comoving coordinate $\lbrace x^i, i=0,1, 2,3\rbrace$( Schutz) . Since GR can be done in any coordinate system, using comoving coordinates is ...
- 2,797
0
votes
2
answers
290
views
Flow of time in a block universe
On asking how there could be a flow of time in a block universe one user said
"In the block universe concept
there is no physical flow of time. The flow of time is a purely mental sensation that ...
user282467
1
vote
0
answers
52
views
What implications does special relativity have on the accelerating expansion of the universe? [duplicate]
Under special relativity we have the Lorentz factor:
$$
\gamma = \sqrt{\frac{1}{1-\frac{v^2}{c^2}}}
$$
Which essentially mathematically describes how the relative speeds between objects can never ...
- 261
1
vote
1
answer
480
views
Coordinate transformation to find the Minkowski metric
I have the following cosmology exercise:
i) calculate the evolution scale factor for an open empty universe and write down its spacetime metric in terms of coordinate $\chi$
\begin{equation}
d\chi = \...
- 155