Unanswered Questions
2,235 questions with no upvoted or accepted answers
13
votes
0
answers
381
views
Are there any experimental bounds on the ratio of neutrinos to antineutrinos in the universe?
In the Standard Model, both baryon number and lepton number are conserved quantities (excluding the theoretical possibility of sphaleron processes which are exceeding rare, at least at non-"near in ...
CommunityBot
- 1
12
votes
0
answers
573
views
Time diffeomorphisms breaking in inflation
I am currently working on the topic of inflation.
It seems that at the stage of inflation, the universe can be described as a de Sitter space. In such a space, all spacetime diffeomorphisms are ...
CommunityBot
- 1
11
votes
0
answers
445
views
View of the sky from inside a black hole
Consider an observer located at radius $r_o$ from a Schwarzschild black hole of radius $r_s$. The observer may be inside the event horizon ($r_o < r_s$).
Suppose the observer receives a light ray ...
- 1,398
11
votes
0
answers
430
views
Equation of motion for cyclic model of the universe
I recently started to study the cyclic universe. I came across this article [1]. My question is about the action used for describing the cyclic model:
$$S = \int d^{4}x\sqrt{-g}(\frac{1}{16\pi G}R-\...
- 1,064
11
votes
0
answers
365
views
Significance for LQG of Sen's result on entropy of black holes?
Sen 2013 says,
...we apply Euclidean gravity to compute logarithmic corrections to the entropy of various non-extremal black holes in different dimensions [...] For Schwarzschild black holes in ...
CommunityBot
- 1
11
votes
1
answer
457
views
Aren't black holes required to exist forever in our frame of reference instead of evaporating?
I know that for an observer far away, nothing ever crosses a black hole horizon (due to time dilation), while in the frame of reference of a falling observer the horizon is nothing special on its way ...
- 2,083
10
votes
1
answer
3k
views
Lagrangian for Perfect fluid Stress-Energy tensor
The wiki article on the Einstein-Hilbert action for General Relativity says that the stress-energy tensor $T_{\mu\nu}$ is related to the Lagrangian of matter, $\mathcal{L}_M$, by
$$T_{\mu\nu}=-2\frac{\...
CommunityBot
- 1
10
votes
2
answers
940
views
Conservation of Komar mass
The definition of Komar mass in GR is associated with one asymptotically flat end. However, a hypersurface may contain more than one end, such as the spacelike Einstein-Rosen bridge in Kruskal ...
CommunityBot
- 1
10
votes
0
answers
295
views
What really are exotic supersymmetric black holes?
I have just read (in the black holes chapter 14 on p244 of this book Ref.1) that in string theory, when one adds an (electric?) charge $Q$ to a static black hole, one can arrive at an exotic ...
- 756
10
votes
0
answers
500
views
Can a nearly-extremal black hole be stable against Schwinger vacuum breakdown?
I was doing some basic algebra to estimate the range of possible masses $M$ and electric charge $Q$ for a nearly extremal Reissner-Noström black hole. I want to see if the logic is correct
the ...
- 14.5k
10
votes
0
answers
259
views
Can we have consistent histories inside a black hole?
A consistent history is a POVM set of observables corresponding to a time-ordered product of projection operators. For gauge theories, not any old operator will do, only gauge-invariant observables. ...
- 3,745
9
votes
3
answers
332
views
Quantum pressure and chemical potential for a Schwarzschild black hole?
Just as Hawking showed that even Schwarzschild black holes have a temperature, shouldn't they also have a pressure and chemical potential? Are there any analytical formulae of those as well as
$$ T_{...
CommunityBot
- 1
9
votes
0
answers
203
views
Is it a coincidence that Big Bang nucleosynthesis lasted about one free neutron half-life?
The free neutron half-life is about 10 minutes. Big Bang nucleosynthesis, which mostly involves protons and neutrons interacting, lasted about 20 minutes.
Question: Is it a coincidence that these ...
- 173
9
votes
0
answers
300
views
The color of deep space background of an arbitrary universe
While writing my notes on cosmology in general relativity and the Olber's paradox, I was wondering about the color of the deep background of space. Our universe is mostly black because light didn't ...
- 7,592
9
votes
0
answers
262
views
Does Hawking radiation need an apparent horizon and when does it switch on during stellar collapse?
I've read that Hawking radiation is implicitly linked with the existence of an apparent horizon (1). This seems a slightly less onerous than linking Hawking radiation with a genuine bona fide event ...
- 207k