All Questions
58
questions with no upvoted or accepted answers
5
votes
0
answers
82
views
Bargmann–Wigner equations in NP formalism
Bargmann-Wigner equations describe free particles of arbitrary spin $j$, namely
$$(-\gamma^{\mu}\partial_{\mu}+m)_{\alpha_r \alpha_{r’}}\Psi_{\alpha_1,..,\alpha_{r’},...,\alpha_{2j}}=0$$
where we have ...
- 341
4
votes
1
answer
94
views
Cosmological constant phase transition?
I recently watched at a talk by Cumrum Vafa in which he stated that the cosmological constant allows us to define a time-scale $T_\Lambda=1/\sqrt{E_\Lambda}$. The time scale of this time is about 10¹¹ ...
- 6,611
3
votes
0
answers
126
views
Is Brans-Dicke theory really ruled out by solar system tests?
Brans-Dicke theory with small values of parameter $w$ are said to be ruled out by solar system general relativity tests like the Shapiro time delay test and the deflection of starlight by the sun.
But ...
- 5,925
3
votes
1
answer
115
views
Understanding calculation in Feynman's Lecture on Theory of Gravitation about age of universe and gravitational constant
I am reading Feynman's Lectures. In the chapter entitled "The Theory of Gravitation", there is the following part
If we take, in some natural units, the repulsion of two electrons
(nature’s ...
- 311
3
votes
0
answers
128
views
Metric vs coframe energy-momentum tensor in metric-affine gravity
Conventions
Latin indices represent components in the anholonomic frame and greek ones are for coordinate components.
I will call $R_{\mu \nu} := R_{\mu \rho \nu}{}^{\rho}$ (Ricci tensor) and $\bar{R}...
- 567
3
votes
0
answers
109
views
When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?
It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
- 131
3
votes
1
answer
243
views
Is it possible to directly test whether or not the vacuum gravitates?
According to GR, all sources of stress-energy (e.g. everything on the $T_{\mu\nu}$ side of the EFE) should gravitate (e.g. affect the curvature/$G_{\mu\nu}$ side of the EFE). We observe the expansion ...
- 1,677
3
votes
1
answer
120
views
Energy conservation in the Sachs-Wolfe effect
When energetic photons climb out of a potential well (eg Sachs-Wolfe effect) they lose energy. According to conservation of energy, where does this energy go? Into the gravitational potential making ...
- 928
2
votes
0
answers
90
views
Gravitational halos made of neutrinos...?
I have been recently interested in how halos made of standard model particles could be formed and behave.
After asking some questions in this site, I was told about how neutrinos could form such halos....
- 2,462
2
votes
0
answers
42
views
Did galaxies spin differently 4, 5, 6 billion years ago?
Today I read that the Matter-dominated era ended 4 billion years ago, and now we're in the Dark Energy dominated era.
Obviously, light from galaxies far enough away would be from earlier in the ...
- 121
2
votes
1
answer
71
views
Can empty space 'press' galaxies?
If gravity slows the effects of time, then empty space will see greater expansion than space inhabited by massive objects. So the space within a galaxy will be expanding more slowly than the space ...
2
votes
1
answer
136
views
Understanding the expansion of the universe
The universe is not only thought to be expanding but the rate at which it is expanding is said to be accelerating. Which of these models (if any) is a correct way of looking at this?
Model A:
Let’s ...
- 1,945
2
votes
0
answers
27
views
Is there an effect analogous to the slowing down of massive particles in Newtonian cosmology?
We know that massive particles slow down by $a^{-1}$ in the Robertson-Walker metric , where $a$ is the scaling factor.
If the particle velocity is non-relativistic, can we understand this slowing ...
2
votes
0
answers
158
views
How could the universe's expansion "remove the mean gravitational potential"?
I'm citing W. C. Saslaw's The Distribution of the Galaxies: Gravitational Clustering in Cosmology, chapter 25, where he adresses (what seems to be a Newtonian approximation of) the thermodynamic ...
- 2,463
2
votes
1
answer
102
views
Hubble bubble and the shell theorem?
According to the Hubble bubble theory a local void could explain deviations of the Hubble constant for measurements in close vicinity to out local group compared to the global Hubble constant and ...
