Questions tagged [black-holes]

A black hole is a volume from which matter cannot escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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Physical reality of inner event horizon and inner ergosurface in a rotating black hole in D. Wiltshire et al. “The Kerr spacetime”

In chapter 1/The Kerr spacetime-a brief introduction by Matt Visser of D. Wiltshire, M. Visser, S.M. Scott "The Kerr spacetime - Rotating black holes in general relativity" the author presents a ...
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Tensor product of universe Hilbert space and black hole Hilbert space equals?

I'm a newbie struggling to parse concepts important in black hole, holographic principle and related issues. My query comes from Thomas Thiemann’s Modern Canonical Quantum Gen. Relativity, which ...
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Redshift z=40 and gravitational waves

Why z=40 is the critical value of the redshift (more or less) to decide find out primordial gravitational waves around it? I have read that is related to the following issue: black hole mergers ...
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Celestial Body Approaching Speed of Light, “Observed” as a Black Ellipsoid?

I have an interesting question, one which I think may not work haha, but please bear with me. Let's work with our sun, with mass of about $2 \times10^{30}$ kg. According to the following formula, ...
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Dimensional reduction from even to odd dimensions and Chern-Simons terms

I'll be taking some Lagrangians from this paper to try and keep personal typos out of the discussion I have been looking at the dimensional reduction of Einstein-Maxwell-Dilaton theories and I am ...
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Positive frequency modes and lightlike coordinates

My question is about defining positive frequency modes of radiation in a generic (static) spacetime. I mean, take a black hole and Eddington-Finkelstein null coordinates; $u$ and $v$ are Killing ...
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Circumference of a circle in Schwarzschild metric in different frames

Suppose that we want to measure the circumference of a circle of radius $R+h$ around a spherical star of radius $R$. The metric there is the Schwarzschild metric, $$ds^2=-\big(1-\frac{r_S}{r}\big)dt^...
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Temperature of Kerr black hole and conical singularity

For spherical static black holes, for example, the metric may take the form \begin{equation} ds^2=-f(r)dt^2+\frac{dr^2}{f(r)}+r^2d\Omega^2_d \end{equation} One can use conical singularity method to ...
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Can rotating black hole have toroidal event horizon with penetrating relativistic jet(s)?

It is conjectured that a rotating black hole has at its center a ring-shaped singularity. Thus, at the center of the ring-shaped singularity the gravitational field must be zero (similar to ...
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Can gravitational waves be absorbed near a quasar?

As the Quasar Wikipedia article explains, a quasar is powered by gas falling into a supermassive black hole at the center of a galaxy. The Luminous infrared galaxy article says that, in extreme cases, ...
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For a charged black hole, does the charge appear to lie on the event horizon?

In considering a charged (Reissner–Nordström) black hole, I was trying to figure out if the charge acts as though it is at the singularity or rather behaves as though it is a surface charge density on ...
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889 views

What does a white hole look like?

If I were looking at a black hole with an accretion disk, I presume it would appear something like this: I.e., the black hole is as black as black can be, and the disk on the far side of the hole is ...
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On a Doppler-shift-induced radiative dissipation of angular momentum for the accretion disk of a black hole

A recent question explored the possibility that the accretion disk of a black hole could wind down and lose its angular momentum through radiative process, and Rob Jeffries' excellent answer there ...
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How massive is the most massive known structure — the Great Attractor — and how much mass will it get?

So I asked a question about dark flow in which I inquired whether we were moving towards the "great attractor" and asking if there was visualizations of what that meant. While I received no answer, ...
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Hawking radiation of fermions?

In most of literature, Hawking radiation is referred to a black-body radiation, with a temperature $ T =1/8\pi kM $ for Sch black holes. However, I reckon it is perfectly possible for black holes to ...
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How entropy connect to “death” of a black hole?

I realized that everything in the universe can only increase in size and not decrease. Because the entropy is growing, but in the other hand, according to hawking a black hole which not "devour" ...
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Do black holes comply with the principle of unitary evolution?

Claus Kiefer, "Quantum gravity", 3rd ed., page 220/221, says in chapter 7 "Quantization of black holes": "A theory of quantum gravity should give a definite answer to the question of whether ...
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Kruskal-Szekeres spacetime

How does one determine the Kruskal Szekeres regions? I know the Schwarzschild solution in Kruskal-Szekeres coordinates is given by $$(ds)^2 = \frac{32(GM)^3}{r}\exp(-r/2GM)(-dT^2 + dX^2) + r^2d\Omega^...
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Does large angular momentum cause swirly bridges when two black holes merge?

Attached is an image that shows the bridge formed between two adjacent black holes. It shows three example mergers. Two non-rotating black holes are shown in the first example merger. In the case of ...
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Is Poplawski's “Cosmology with torsion” a “variable speed of light” cosmology?

In his 2010 paper "Cosmology with torsion", Poplawski mentions that, in the hypothesized formation of a new temporally-local universe within a black hole, its "expansion is not visible for observers ...
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Does Black hole tears and compresses the nucleus of atoms?

Part 1 Stephen Hawking describes the flight of a fictional astronaut who, passing within a black hole's event horizon, is "stretched like spaghetti" by the gravitational gradient (difference in ...
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Continuation to Euclidean BTZ Black hole

BTZ black hole in Lorentzian signature is given by $$ ds^2= -fdt^2+f^{-1}dr^2+r^2(d\phi + N^{\phi} dt)^2 $$ $$ f=-M+r^2+\frac{J^2}{4r^2},~~~~~ N^{\phi}=-\frac{J}{2r^2} $$ $f$ can be wriitten as $$ f=\...
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Dirac equation in higher dimensional spacetimes

I would like to understand the Dirac equation in higher dimensional curved spacetimes. I have seen there are some crucial points like the spin connection and the introduction of the Newman-Penrose ...
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What is the mass and density distribution of a Schwarzchild black hole in the viewpoint of distant observer?

According to Schwarzschild metric: $$ c^2~\mathrm d\tau^2 = \left(1-\frac{r_s}{r}\right)c^2 ~\mathrm dt^2 - \left(1-\frac{r_s}{r}\right)^{-1}~\mathrm dr^2 - r^2\left(\mathrm d\theta^2 + \sin^2\theta~...
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black hole potential well

Let d be the distance (in some coordinate system) from a particle to the event horizon of a black hole. For a particle starting at distance d>0 above the event horizon to rise away to a very large (...
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How to find tetrads corresponding to a given metric?

In General Relativity, sometimes doing calculations with a set of orthonormal tetrads $h^{a}_{\mu}$ where $h^{a}_{\mu}$$h^{b}_{\nu}$$\eta_{ab}=g_{\mu\nu}$ for some metric $g_{\mu\nu}$ is easier ...
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Are the quasinormal modes scalar quantities?

I am studying the so-called quasinormal modes (QNMs) in the context of the AdS/CFT correspondence and I got stuck. For instance, if I choose a weird patch of coordinates for the, say, AdS5-...
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what is the metric of N-sheeted $AdS_3$?

Suppose the AdS$_3$ metric is given by $$ ds^2 =d\rho^2+cosh^2\rho d\psi^2 +sinh^2 \rho d\phi^2 $$ what is the n-sheeted space of it? Can the n-sheeted BTZ be constructed from it by identifications ...
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How can the energy of a spinning black hole be converted into magnetic field?

I've read about the Blandford-Znajek process, and what I understood is that the black hole is treated as if it were a conducting sphere spinning rapidly with an accretion disk around it, and the ...
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Lense-Thirring Precession of short GRBs

Stone et al. 2014 have proposed that the jet resulting from the merger of a black hole (BH) and neutron star (NS) may precess due to Lense Thirring torques A large rotating mass e.g. the Earth, can ...
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Effective potential for Kerr incorrect?

I am self-learning GR. Background I have been following Christopher Hirata's lecture notes on Kerr geodesics. In Equation 38, the effective potential $V(r)$ is given by: $V(r)=(1-\epsilon)^2r^4-2Mr^...
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Choice of the coordinate system in a spherically symmetric solution

For a static spherically symmetric solution, the metric can be written as $$ds^2=-A(r)^2 dt^2+\frac{dr^2}{B(r)^2}+C(r)^2d\Omega^2$$ In some cases, we can write $C(r)=R$ and interpret then $C$ (or $R$)...
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How do gravitational waves and black holes interact?

Following the first detection of gravitational waves yesterday (11 Feb. 2016) by LIGO, I have a couple of questions about how gravitational waves and black holes interact. Assume that there is a ...
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What is the global Virasoro symmetry generators in BTZ spacetime?

This is the case of AdS3, how about BTZ? The picture is from arxiv:1506.01353.
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What is the inertial mass of a black hole?

Or the inertial mass of any spherically symmetric object, can I calculated by measuring very accurately the spacetime distortion this object produces in its surroundings? With 'inertial mass' I mean ...
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Boundary condition for holographic Green's function

I understand that to obtain retarded Green's function using AdS/CFT one needs to put ingoing wave boundary condition at the horizon, when one is considering a finite temperature system. For system ...
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Near horizon limit of near-extreamal black brane

It is known that the near horizon limit of a ($d+1$) dimensional extremal charged black hole (BH) is $AdS_2\times S^{d-1}$. I was looking at this paper by Faulkner et al. They consider a ($d+1$) ...
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Black Hole - Between event horizon and singularity

Dear Physics Board Users What is between the singularity and the event horizon? If the gravitation gets bigger and bigger coming nearer to a black hole, is then the gravition inside even bigger that ...
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Rigorous Proof of General Relativity's Non-renormalizability?

The answer to this question and the comments on it implies that general relativity has not been rigorously shown to be non-renormalizable for all loop diagrams -- only shown for two loops. However, ...
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Help me calculate the Euclidean action of a gravitating system!

I recently read Gibbons and Hawking's paper Action integrals and partition functions in quantum gravity, Phys. Rev. D 15 (1977) 2752. I am interested in repeating their calculations. It is fairly ...
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Nonsingularity of the Euclidean black hole metric

How can one show that the nonsingularity of the Euclidean black hole metric is required for thermal equilibrium of the original black hole in Lorentzian signature? It is mentioned in Prof. P. K. ...
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Accretion disks on neutron star binaries

Why does hydrogen gas from accretion disks not constantly get sucked onto/into a neutron star or into a black hole? I understand that some gets sucked into the black hole and some may come down and ...
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Is there a map which shows all the black holes in our galaxy?

I am looking for a map which shows all black holes in the Milky Way. See this exoplanet map for what I mean. If it is not existing (yet). Is there at least an online catalogue which summarizes for ...
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What are the equations of motion that model near light speed orbits of a massive body about incredibly massive bodies?

In Kip Thorne's recently published book, The Science of Interstellar, he describes, by means of an illustration, the complex nature of a spacecraft orbiting a massive black hole with velocities ~ 0....
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Gravitational singularity

Is it possible that the gravitational singularity actually turns out to be a genuine singularity once we have a true theory of quantum gravity in place. There is a lot of talk about singularity but ...
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Electric and magnetic field in a black hole

I have many questions about this topic: Does the electric field of a charged black hole look like this? I mean how can it have an electric field if nothing can escape from a black hole, and what is ...
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What exactly is black hole complementarity and why is it necessary for solving black hole information paradox?

What exactly is black hole complementarity and why is it necessary for solving black hole information paradox? The first question is about the story of vacuum fluctuation causing Hawking radiations. ...
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Statistics of Black Holes

I am hoping to collect information about the sampling distribution of black hole sizes, densities, and their relative spatial distribution within galaxies and other formations. By sampling ...
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Entangled event horizons

Assuming it is possible in principle to entangle the degrees of freedom of the event horizons of two black holes, and that this is something that can be done, either after the black hole is formed, or ...
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Reissner-Nordström Black Holes

The Reissner-Nordström black holes are described by the metric, \begin{align} ds^2 = -\left(1-\frac{2M}{r}+\frac{Q^2}{r^2}\right)dt^2 + \frac{1}{1-\frac{2M}{r}+\frac{Q^2}{r^2}}+r^2d\Omega^2 \end{...