A black hole is a volume from which photons, or any matter, can not 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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What is the mass density distribution of a Schwarzchild black hole?

According to Schwarzschild metric the density element $$ \frac{\mathrm dm}{\mathrm dV} $$ becomes infinite on the event horizon if there is mass $$ \mathrm dm > 0 $$very near at the outside of ...
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The difference between sub-Eddington and super-Eddington accretion disk models

I am studying accretion disks around active galactic nuclei (AGN). I have seen two sets of models, sub-Eddington and super-Eddington accretion disks. What do these terms refer to in this case? I ...
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Ricci scalar for Black Hole

What is the value of Ricci scalar at $r=0$ inside Black Hole? Since $R_{\mu \nu}=0$ is vacuum solution and valid outside event Horizon of black hole where there is no mass energy density. But inside ...
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Universe as a black hole - how did everything escape? [duplicate]

While listening to physics lectures, they glossed over things escaping from black holes insinuating it doesn't happen other than radiation... When talking about the period between Planck Epoch & ...
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271 views

Coordinate Singularity in Metric

Suppose I have some metric $$ds^2=g(t)dt^2+\frac{1}{r}dr^2$$ which has a singularity at $r=0$. However, if I make the coordinate transformation $u=\frac{1}{r}$, then I get: $$ds^2=g(t)dt^2+r^3 du^...
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How do we know singularities/black holes actually bend time?

Recently I read up on some articles about singularities / black holes, how they form and how they affect their surroundings. Just to clarify, I am no physicist and I found those articles on the ...
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How much mass can colliding black holes lose as gravitational waves?

Concerning the recent detection of gravitational waves produced by colliding black holes, it has been reported that a significant percentage of the combined mass was lost in the resulting production ...
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How long can a particle survive inside of the horizon of a black hole?

By examining the causal structure of a Schwarzschild black hole, one can see that a particle in region II is unable to escape to $r=\infty$. Such diagrams do not show, however, how long a particle ...
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Chasing someone who has fallen into a black hole

Assume that my friend and I decided to explore a black hole. I parked the spaceship in a circular orbit safely away from the horizon. He puts on his spacesuit with a jet pack and carefully travels ...
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272 views

What are the virtual particles generated during the Hawking radiation?

What are the virtual particle pairs generated during the Hawking radiation? If a photon is emitted by Hawking radiation, what is its negative energy partner which fell into the black hole? Does it ...
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Is the universe actually formed according to the big bang theory? [on hold]

Big-Bang pretty much explain the origin of universe, but still many people don't believe it. Isn't it possible that our universe come out of a white hole which have been pulled out from a black hole ...
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136 views

Does time and time dilation exist in a black hole?

Since being near stronger gravitational fields causes time dilation, can this be generalized to the region beyond the event horizon? i.e , is it meaningful to say that someone outside the black hole ...
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Killing Horizon for Kerr Black Hole

I have some confusion about Killing Horizon of BH. Since a Killing Horizon (KH) is a null hyper-surface at which killing vector $k^{\mu}$ is null; $k^{\mu}k_{\mu}=0.$ For time translation symmetry $...
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Hawking radiation for closely orbiting black holes

Suppose we have two black holes of radius $R_b$ orbiting at a distance $R_r$. I believe semi-classical approximations describe correctly the case where $R_r$ is much larger than the average black body ...
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QPO and super-orbital periods

Are super-orbital periods from pulsars/neutron stars the same thing that the Quasi-Periodic Oscillations that apply not only to pulsar but also to more general compact objects like black holes?
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25 views

Killing Horizon for a space-time metric

How many Killing Horizon (KH) are possible for a particular S.T. metric. Because there is a surface gravity associated to each KH, which is constant over KH surface. Do a Black Hole have many surface ...
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109 views

How to get null tetrad by metric?

How to get null tetrads ${l^a,n^a,m^a,\overline{m}^a}$ for this metric? This on is from Ryder's book (Introduction to general relativity) page 268 $g^{\mu\nu}=\begin{pmatrix} 0 & \frac{1}{c} &...
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35 views

If a neutron decays inside event horizon of a negatively charged black hole,can the electron from decay escape? [closed]

Suppose a neutron is decaying inside event horizon of a negatively charged black hole and an electron is produced, is that electron possible to escape from black hole due to charge repulsion with the ...
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51 views

Can light change the shape of a black hole?

If light orbits a black hole without ever getting closer or further from it, and the black hole pulls on the light from all directions nearly simultaneously, does the black hole extend at its equator? ...
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54 views

Black Hole: mass density or energy density? [closed]

Recently, I read a quora post in which the OP asked the following question: Can a black hole be formed from accelerating a body and increasing its relativistic mass to the level of a Schwarzschild ...
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Cygnus X-1, HDE 226868 radial velocity and mass function

Where could I find the data of the Radial Velocity for the visible companion (an O class star) for the binary system of Cygnus X-1? Also for this system, we have the mass function $$ f(M)=\dfrac{PK_C^...
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255 views

Coupling between galaxy spin and central black hole spin

What is the relationship between the spin of a galaxy and the spin of its corresponding black hole? Associated questions: Do they always have the same axis of rotation? Do they always spin in the ...
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124 views

What happens to objects sucked into a black hole after the black hole evaporates away?

Suppose an object falls into a black hole that's so massive that it wouldn't get torn apart at the event horizon. What happens to it after the black hole evaporates away? According to the theory ...
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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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539 views

Doesn't the Schwarzschild metric combined with Hawking radiation imply that nothing ever gets past the event horizon of a black hole?

According to the General Theory of Relativity, the coordinate time distance per spacetime distance traveled by a particle freely falling into a black hole gets closer and closer to $0$ as the particle ...
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Zero gravity at center of astronomical bodies [closed]

All theories agree that the net gravity at center of all astronomical bodies is zero. Are black holes exception to this? Is gravity zero at the center:) of singularity? Is anything like center of ...
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Why can't a nucleus become a black hole?

A nucleus is very small and very dense. Its density is approximately $2.3 \times 10^{17}~\mathrm{kg/m^3}.$ So why can't a nucleus itself become a black hole?
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Removing temperature from black hole

Suppose I take the metric of a three dimensional AdS-Schwarzschild black brane (Note not actually a black hole!): $ds^2 = \frac{L^2 dr^2}{r^2 W} + \frac{r^2}{L^2} \left[ - W dt^2 + dz^2 + dx^2 \right]...
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640 views

Black hole no-hair theorems vs. entropy and surface area

I was revisiting some old popular science books a while ago and two statements struck me as incompatible. No-hair theorems: a black hole is fully-described by just a few numbers (mass, spin etc) ...
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How dense is the material in the core of a black hole? [duplicate]

If I would like to find out the answer of this question, I get two different one: The material is collapsed into a tiny dot. This density is called planck density, and it is so dense, all of the ...
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52 views

What's new about Hawking's theories in relation to 't Hooft's?

What's new about Hawking's theories in relation to 't Hooft's about resolving the black hole information paradox? Thanks "The information is stored in a super translation of the horizon that the ...
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Black Hole mass

Where is the material that has past the event horizon of a black hole stored/kept? Does it become pure energy once it passes the event horizon? Does the singularity at the center of the black hole ...
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Does a planet/star itself lose mass when it 'emits' gravitational waves?

Orbiting planets and stars can create gravitational waves, as seen with the LIGO. But are they also losing mass-energy. Beside that, the kinetic energy associated with their orbital angular momentum ...
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What does it look like for a ball falling to the event horizon observed by distant static observer?

Here is the picture used in susskind&Lindesay's book ''An Introduction to Black Holes, Information and String Theory Revolution'' I understand very well that the ball will be contracted at the ...
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1answer
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Computation of photon trajectory near a black hole [closed]

I've been given the task to represent the trajectory of a photon near a Schwarzschild Black hole. I've read the physics behind it, and I've derived the equation of photon orbit near a schwarzschild ...
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Can a black hole fall into another black hole's innermost stable orbit?

Can a black hole, in principle, fall into another black hole's innermost stable circular orbit?
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Is the Hawking radiation of a charged black hole thermal?

Suppose you have a Schwarzschild black hole of mass $M$ and angular parameter $a = 0$ (no rotation). Question: is it possible to throw a charge $Q$ at a faster rate than it will be re-radiated? Will ...
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Capacitors in space [closed]

I'm new here and I have a question for all of you. What happens to a capacitor in open space? There are different kind of them, which one of these is better to use there? And... We know that if we ...
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Lower bound in the size of (naturally formed) black holes?

I suppose most of you will agree with me that Black Holes (BH) are among the coolest and most mysterious objects in our Universe. Theoretically predicted a century ago by the Einstein's theory of GR, ...
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Does fusion continue to happen as a star collapses into a black hole (beyond our view)?

I know that we can't actually see a black hole form, because of time dilation, and we would see a frozen image just before the final collapse past the event horizon, slowly redshifting to nothing... ...
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Computing the path of photons near a black hole

For a simulation, I want to compute the path that light follows near a black hole. Non-relativistically, a massive point particle in a central newtonian gravitational field follows either an ellipse, ...
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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 ...
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Fast moving black hole with low rest mass [duplicate]

Sorry for probably very stupid questions but.. If the massive body with small mass (one of the twins, for example) accelerates until the speed close to the speed of light, the mass of this body as ...
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What subatomic particle is a blackhole composed of?

In a black hole, what is the "state" in which the mass that is present is in? Are they there as tightly packed neutron or soup of quark or are particles broken down into 10D strings and coiled as ...
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Position / velocity / acceleration vs time graphs when falling towards a black hole

In Newtonian physics, as far as I understand it, for a small object falling towards a massive object, the graphs of position, speed and acceleration vs time look like this: (If the massive object ...
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How does the mass of black holes become so much larger than an average star [duplicate]

How does the mass of black holes become so much larger than the mass of an average star? Since black holes are formed from the collapse of large stars, how come the mass of black holes is of ...
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Formation of supermassive black holes

Scientists have found very bright source of light which they call quasar and the are found to be supermassive black holes. So these black holes are so massive that they cannot be formed by a supernova....