Questions tagged [black-holes]

A black hole is a region of spacetime from which nothing can escape. More formally, the future light cone of any observer within the black hole is completely contained in the black hole, and the black hole region is not within the past light cone of any observer that goes to spatial infinity in an infinite amount of time.

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Angular momentum, black holes and up-tunneling events in the vacuum

To give some context on the matter, I found these interesting articles (https://arxiv.org/abs/2007.11428 & https://arxiv.org/abs/2003.04927) where the authors analyzed, among other things, the ...
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Crossing the Event Horizon of a Supermassive Black Hole [closed]

It seems like this would be instantly fatal, even if the gravity and density was gentle. The bottom line is an event horizon is an event horizon, and it blocks the flow of information. As you pass ...
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Are we inside a black hole? universe is expanding but we are moving towards singularity, like inside a black hole where the space around us infinite? [closed]

Like we are attracted towards the great attractor even the the space is expanding, like when we are in side a black hole we strech so much that the surroundings expand, as if we are moving towards the ...
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Angular velocity of stationary observer

I am studying Kerr metric in Boyer Lindquist coordinates and having trouble in understanding the components of angular velocity of stationary observer with constant r and $\theta$ motion w.r.t ...
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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 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 ...
28 votes
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Wouldn't Miller's planet be fried by blueshifted radiation?

In Interstellar, wouldn't Miller's planet be fried by blueshifted radiation? The 61,000x time dilation multiplier would make even cosmic background radiation photons into extreme UV photons. I was ...
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1 answer
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Significance of simultaneous transformation of $t$ and $\phi$ in kerr metric

The Kerr metric in Boyer-Lindquist coordinate is invariant under simultaneous transformations $t \rightarrow -t$ and $\phi \rightarrow -\phi$ but not invariant if we apply one transformation only. ...
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Perimeter of Kerr's event horizon

We can compute the "proper circumference" of the Schwarzschild event horizon integrating its line element along its perimeter at a fixed $t$. This would be the minimum length of a rope that ...
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What happens to the angular momentum of two merging black holes?

Let's suppose that two black holes of roughly equal mass in a binary system forming from say a large mass stellar binary system are in orbits around their center of mass. Further suppose that we are ...
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Is it possible to have a stable black hole that does not evaporate?

Black holes would presumably evaporate in the long future via Hawking radiation. However is this inevitable? Or are there any mechanisms that would compensate the lost mass due to Hawking radiation ...
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Could primordial black holes have been supermassive?

Usually when primordial black holes are mentioned, they are imagined to be tiny. Could they have been supermassive? This would explain the existence of quasars in early universe.
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Expressing Schwarszchild coordinates with Cartesian coordinates

Im sure weve all seen the Kruskal-Szekeres diagram before (see below) Im currently trying to draw this myself for a project, but Im struggling to understand Schwarszchild Coordinates. I tried reading ...
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When does a collapsing star become a black hole?

It’s said that from an outside observer’s reference frame, a black hole (as well as the event horizon) takes forever to form because of runaway time dilation, which causes falling matters to slow down ...
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Why is it difficult for 2 black holes to merge?

Apparently as part of the merger of galaxies, sometimes the central supermassive black holes of colliding galaxies get really close, however once they come to a distance of about <1 parsec, it ...
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Magnitude of Black hole entropy

I was looking with a friend of mine through time travel related stuff, and for some reason we ended up at the Hawking equation for black hole entropy which is: $$S= \frac{\pi Akc^3}{2hG}$$ where: A:...
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Is it correct to claim that Hawking-Page phase transition is related to the breaking point at the Page-time?

In the evaporation process of a Black Hole in an AdS space we have a Hawking-Page phase transition. We know that such a phase transition can be exhibited by a singular behavior. At the other hand, ...
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What is the locus of the velocity vectors of a spaceship moving in some distance from a black hole?

Someone with a background in mathematics and limited knowledge of physics asks the following question: Starting from rest, a spaceship is momentarily pushed with a specific amount of kinetic energy in ...
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Could any new structures be formed after the heat death of the universe?

When the universe would reach a maximal state of entropy, heat death would presumably be reached and no structures would be left after the last black hole would evaporate. However, is this really true?...
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Could a black hole be a Bose-Einstein condensate of gravitons?

Gravitons are bosons. As such they should obey the Bose-Einstein statistics at low temperature. And this means they should form a Bose-Einstein condensate at temperature close to absolute zero. ...
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Are light rays always bent in any gravitational field, or just in a strong one like a black hole?

This is a follow-up question after this one: Yes, the curvature is enough to bend lasers. In fact, there is an interesting feature in Schwarzschild spacetime that might answer your question well. ...
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Schwarzschild metric in Isotropic coordinates

As one wants to jump to Isotropic coordinates in order to write the Schwarzschild metric in terms of them, one does this coordinate transformation: $$r=r'\left(1+\frac{M}{2r'}\right)^2$$ So we start ...
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The final parsec "problem"

Many and perhaps all galaxies seem to contain supermassive black holes of about $10^7 M_\odot$ at their centres. Determining their origins is of great astrophysical interest. In what I understand to ...
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What are the odds of a rogue planet that enters into a galaxy reaching the black hole at the center of the galaxy?

I am wondering if anybody has ever calculated the odds of a rogue planet, which has been traveling through interstellar space and then enters into a galaxy, being able to travel all the way to the ...
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Types of singularities

I am confused about the types of singularities. According to my limited knowledge there are two types of singularity. One is space like singularity ( a curvature singularity enclosed within a null ...
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The inside of a black hole

I am looking for a confirmation (or correction) of my thinking about the nature of the black holes. As I am not a physicist and only a physics enthusiast, my understanding is probably very simplified, ...
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Magnetic dipole near black hole

Usually it is said that black holes cannot have electric or magnetic dipole, only electric charge and angular momentum are allowed quantities besides mass So, it would seem that black holes behave as ...
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Do black holes have a size?

I'm wondering if one can say that a black hole is an object "made of matter" that has a size (as a size, I'm not talking about the size of the event horizon). I would like to know if one can ...
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Scattering approach for massless scalar Hawking radiation and Bogoliubov coefficients in Schwarzschild metric

Reflection coefficient from the scattering approach in tortoise coordinates, looks exactly like relationship between modulus squared of Bogoliubov coefficients. However I'm not able to figure out a ...
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Where does the energy density of black holes fit in a figure with the different energy densities of the universe like matter and radiation? [closed]

Based on the black hole mass, giving that lower mass black holes have higher mass densities than higher mass black holes. Energy Densities `The density is dependent only upon the mass of the black ...
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Is this an actual photo of frame dragging?

Is this new image (below) of polarized light surrounding Sagittarius A, showing actual frame dragging being captured by the magnetic field? The image is from this article If not, how would a photo ...
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Is it possible to refind entry trajectory after leaving a rotating black hole?

I'm asking about the case where an infalling object travels a path right through a rotating black hole, intact. I want to provide a simple parallel for the purposes of question clarity: A horse can ...
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Is the curvature so extreme at the event horizon, that you could see curved laser beams?

I have read this: Because the spacetime curvature at the horizon is so great that there is no light-like world line the extends beyond the horizon. Why does time stop in black holes? If the ...
3 votes
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Is the QFT Hamiltonian on an eternal Schwarzschild black hole background unbounded below?

Consider the $t=0$ Cauchy slice of the maximally extended Schwarzschild black hole. Let the parts of the slice to the left and right of the bifurcation surface have Hilbert spaces $\mathcal{H}_L $ and ...
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Lagrangian for Kerr-Newman black holes

I am trying to write down the action that is extremized by Kerr-Newman solutions in General Relativity. Specifically, I am interested in parametrizing the Lagrangian by the mass $M$, angular momentum $...
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Deriving a path-integral expression for a thermal density matrix with position-dependent temperature

I've been fiddling with deriving a path-integral expression for a thermal partition function with a position-dependent temperature but I'm not sure how to get started on this. Concretely, I'm trying ...
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Gravitons (real) passing through a black hole

This is not a duplicate. I do not ask about the gravitational field of the black hole, or why it extends farther out then the event horizon. I do understand that the gravitational field is described ...
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Hawking Radiation paradox

How does Hawking radiation help in information conservation? Doesn't it create a paradox, as the particle-antiparticle pair created at the event horizon has nothing to do with the matter that has ...
6 votes
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How to find that there is a conical singularity in the BTZ black hole?

Considering a non-rotating and non-charged 2+1 dimensional black hole, known as the BTZ black hole which obtained by adding a negative cosmological constant $\Lambda=-\frac{1}{l^2},l\ne0$ to the ...
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Big Bang Escape Velocity

When our entire section of the universe was in a single hot dark dense state, right before our big bang, what was the escape velocity?
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The center of the Schwarzschild black hole

My question is why, when expressing the Schwarzschild solution in isotropic coordinate system, the coordinate system is considered valid only outside the event horizon. For simplicity, we assume that ...
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Calculating dynamical friction of a black hole passing through solid matter

Following up this question I am trying to calculate the dynamical friction of a black hole passing through solid matter $$\frac{d\mathbf{v}_M}{dt} = -16 \pi^2 (\ln \Lambda) G^2 m (M+m) \frac{1}{v_M^3}...
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Charge separation equilibrium equation for non-rotating blackhole at Eddington limit with spherically symmetric accretion?

What is the formula for equilibrium charge on a non-rotating black hole with spherically symmetric hydrogen-1 inflow at the Eddington limit of luminosity based on photon scattering acceleration of ...
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Black hole information paradox

I read that it is generally believed that information is preserved in black hole evaporation, and people's views only diverge when it comes to how information is preserved. Is this true?
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Reaching a turning point in photon trajectory

Given the geodesic equations for a photon in a Schwarzchild or Kerr metric (provided by a near BH for example), the radial equation has usually two possible signs: \begin{equation} \dfrac{dr}{d\tau}= ...
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Would a black hole be slowed down as it passed through a large solid object?

Say I get careless and drop a black hole. It falls through the Earth, passes through the core, and comes out on the other side. Idealised as point masses, the black hole would reach an equal height on ...
10 votes
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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 ...
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What happens if $ a^2 > M^2 $ in Kerr metric?

(Boyer-Lindquist coordinates and $ c = G =1 $ taken) As I know, line element in Kerr metric $ d s^2 = - \left( 1 - \frac{2Mr}{\rho^2} \right) d t^2 - \frac{4 M a r \sin^2 \theta}{\rho^2} d \phi d t + \...
6 votes
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Are worldlines towards the origin with the Schwarzschild metric finite in time and length?

A recent spacetime video about Kerr's objection to the existence of singularities has made want to clear up something about geodesics towards the origin in the Schwarzschild solution. It is said that ...
7 votes
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Curing spacetime singularities by a higher-curvature gravitational Lagrangian

It is well known that the self-energy of a point charge is diverging in the classical (Maxwell) electrodynamics. In 1930's, Born and Infeld introduced their nonlinear electrodynamics (with a ...

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