All Questions
93
questions
2
votes
1
answer
310
views
Wigner's classification in curved space
Wigner classfied elementary particle as unitary irreducible representations (UIR) of the Poincaré group.
Suppose the spacetime is curved with symmetries $G$. Should the elementary particles in this ...
- 1,354
1
vote
0
answers
159
views
Abel deprojection formula in static and spherically symmetric spacetimes
Given a fluid spherically distributed with density $\rho(r)$ in 3-dimensions in flat-spacetime; the projected surface density $\sigma(R)$ (onto two dimensions) can be obtained by the well known ...
3
votes
1
answer
208
views
Full Bondi-Metzner-Sachs (BMS) or asymptotic group are the same and have equal interpretations?
I had red about supertranslations or even superrotations. But I just discovered there are also superboosts and superLorentz ( I suppose this is for superrotations and superboosts).
Is the full BMS ...
- 6,611
0
votes
1
answer
276
views
In GR, is 'Static' the same as 'Time-symmetric'?
I recently attended a talk where the person stated a uniqueness result for static vacuum spacetimes whereby he came to a conclusion about a type of spacetime (a 4-manifold) by studying 3-manifolds ...
- 1,410
0
votes
0
answers
66
views
Curvature and Symmetries of spacetime
Is there any relation between symmetries of spacetime and the curvature invariants? For example is spherical symmetric spacetimes, necessarily have positive curvature? Could we define any spherical ...
- 159
1
vote
1
answer
163
views
How do we know that the actual universe has no Killing vector fields?
This article states the following:
The infinitude of conserved energies constructed via Noether’s theorem
suffers a startling reversal as soon as Special Relativity is
superseded by General ...
- 519
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
2
votes
2
answers
266
views
Einstein equations in the spherically symmetric, static case
This question is not about the solutions but much rather about the equations we write in GR for a spherically symmetric, static vacuum 4D spacetime.
The Einstein equations are
$$G_{\mu\nu}=0\;\;\;\...
- 1,874
1
vote
2
answers
837
views
Difference continous - discrete symmetry
I am trying to understand the difference between the two types of symmetries.Wiki
Wikipedia says that
Translation in time : $t \rightarrow t + a$
is a $\textbf{continuous}$ symmetry, for any real $...
- 149
0
votes
1
answer
161
views
Is universe symmetric about a point?
We have a good amount of discussion and theories on the formation of universe.
I want to ask is universe symmetric about a point?
I think that the answer should depend upon the uniformity of ...
- 3,377
1
vote
2
answers
736
views
Non-static spherical symmetry spacetime
The Schwarzschild solution is a static spherically symmetric metric. But I wanted to know that how would the space-time interval look in a Non-Static case. I tried to work it out and got
$$ds²= Bdt² -...
- 1,490
3
votes
1
answer
196
views
Redefinition of spacetime coordinates for Noether's Theorem
In the derivation of Noether's theorem some authors consider not only redefinitions of the fields
\begin{equation}
\phi(x) \rightarrow \phi'(x) = \phi(x) +\delta\phi(x)
\end{equation}
but also ...
- 364
1
vote
1
answer
108
views
Correlation between velocity and homogeneity of spacetime and isotropy of space
Considering only inertial frames of reference and constant velocities, does the fact that any velocity, with the exception for the speed of light in a vacuum, can be transformed, via an accurate ...
- 145
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