All Questions
17
questions with no upvoted or accepted answers
4
votes
1
answer
209
views
Is gravitational particle production due to symmetry breaking?
A well-known fact about QFTs in curved spacetimes is that there is a phenomenon of particle production in expanding universes, these being described by the line element $$ds^2=-dt^2+b^2(t)d\vec x^2.$$
...
- 503
3
votes
0
answers
184
views
Antiparticles, CPT and leptogenesis
When people are being careful they'll tell you that antiparticles are the CPT conjugates of particles. You can't say that they are C conjugates or CP because these, while they do reverse the charge, ...
- 841
3
votes
0
answers
347
views
How is translational symmetry related to Fourier decomposition?
The book (The Cosmic Microwave Background By Ruth Durrer) about cosmological perturbations says that because of translational symmetry of the background at a constant time, we can decompose our ...
- 431
1
vote
0
answers
43
views
Could inhomogeneties and/or topological defects break the fundamental symmetries of the universe?
I have heard that some types of inhomogeneties and topological defects (like cosmic strings) in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, ...
- 2,462
1
vote
0
answers
101
views
(Wald's GR book) Isotropy implying constant curvature
Context
In Wald's GR book, p. 94 of chapter 5, he gives an argument for why isotropy and homogeneity implies constant curvature.
In summary, it goes like this: We can take the induced metric, $h_{ab}$,...
1
vote
0
answers
110
views
How to justify studies on fractal cosmology?
What are the arguments (theoretical and experimental) to support or to justify a fractal distribution of matter on all scales in cosmology?
I see only one justification:
On some small scales, matter ...
- 7,592
1
vote
0
answers
336
views
Spontaneous symmetry breaking by axions?
I am just reading at the beginnin of this nice article, that axions could be responsible for spontaneously breaking of a symmetry in the early universe.
Does anybody know which symmetry is alluded to ...
- 9,581
0
votes
1
answer
146
views
Do symmetries indicate that laws of physics are invariant?
Concerning symmetry in fundamental physics, it is usually said that symmetry indicates that laws of physics are invariant independently of something
For example, time translational symmetry indicates ...
- 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
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
177
views
Equation of motion for scalar field
I am trying to derive the equation of motion for a scalar field in flat and homogenous space time where the metric is $g_{\mu \nu}=diag(-1,a^2(t),a^2(t),a^2(t))$ and the Lagrangian is given by
$$\...
- 31
0
votes
0
answers
35
views
Role of isotropy argument in fixing the form of $T^\mu_\nu$ of cosmology (Padmanabhan's book)
The following lines from Gravitation Foundation and Frontiers by T. Padmanabhan (he uses Latin indices for spacetime and Greek indices for space, which is unusual)
The assumption of isotropy ...
- 11.8k
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