Unanswered Questions
1,856 questions with no upvoted or accepted answers
12
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0
answers
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Variation of the Einstein-Hilbert action in $D$ dimensions without the Gibbons-Hawking-York (GHY) term
Consider the standard Einstein-Hilbert action in $D \ne 2$ dimensions spacetimes :
\begin{equation}
S_{EH} = \frac{1}{2 \kappa} \int_{\Omega} R \; \sqrt{- g} \; d^D x,
\end{equation}
where $\Omega$ is ...
- 207k
11
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0
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445
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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
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664
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What are Galileons good for?
Lately I've seen many papers (for example "The galileon as a local modification of gravity"; 292 total hits on the arXiv) on types of field theories known as Galileons, and I'm wondering ...
- 22.1k
11
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365
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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
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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
2
answers
940
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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
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0
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295
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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
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500
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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
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0
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341
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Gauss-bonnet gravity constraints from string theory
Recently there have been advances in observational constraints of gravity theories that contain scalars coupled to the gauss-bonnet topological term:
http://arxiv.org/abs/0704.0175
http://arxiv.org/...
- 207k
10
votes
0
answers
259
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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
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3
answers
332
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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
100
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Different features of Gravity and Yang-Mills
I am reading a famous paper by S.Hawking - "Quantum gravity and path integrals" https://doi.org/10.1103/PhysRevD.18.1747.
On the third page left column there is a statement, after the ...
- 207k
9
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0
answers
262
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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
9
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1
answer
213
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Free higher spin fields and gravity
There are soft theorems that suggest that any massless boson with spin higher than 2 should be a free field theory and cannot have interactions. Does this mean that one cannot embed such fields into a ...
- 676
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283
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Does an evaporating black hole violate conservation of angular momentum?
Angular momentum is supposed to be conserved, but when a rotating black hole evaporates the Hawking radiation comes out in straight lines. Doesn't this violate conservation of angular momentum?
Does ...
- 358k