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Questions tagged [event-horizon]

An event horizon is a type of boundary such that any information past this boundary is inaccessible to the observer it is defined for. Common examples are the Schwarzschild radius of a black hole (which is defined commonly for all observers outside this radius) and the cosmological event horizon (which is defined as a radius from an observer)

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Necessity of Singularity in General Relativity

The Schwarzschild solution is the standard example used to describe a black hole, its important points being the event horizon and the central singularity. But this solution is derived by assuming an ...
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End points of event horizon

I am reading The Nature of Space and Time by S. W. Hawking. In the last paragraph on page 16 he said that: event horizon may have past end points but don't have any future end points I understand ...
Talha Ahmed's user avatar
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Can you calculate the radius of a hypothetical singular surface inside a black hole from observing changes to its linear momentum?

Say there is a ball of unknown radius surrounded by a bubble. The ball represents a hypothetical singular surface inside a black hole and the bubble represents the event horizon. If you threw marbles ...
user414142's user avatar
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Can part of space be causally disconnected from the rest of the universe by being surrounded by black holes? [duplicate]

Is it possible for black hole event horizons to overlap and form a spherical wall around an island of space (that's not inside a black hole) while still being causally disconnected from the rest of ...
user3624007's user avatar
2 votes
3 answers
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Can a light signal from Earth reach a galaxy outside the Hubble Horizon?

Is this video on the FLRW metric (timestamp 29:00 minutes) mistaken in its claim that a light signal from Earth cannot catch up with a galaxy outside the Hubble horizon, due to the horizon receding at ...
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Black Hole Formation -- How Can an Event Horizon be Observed to Grow? [duplicate]

This is a question about black hole formation. To be clear, I’m not suggesting that black holes don’t form. It’s that I’m having trouble with the accepted explanation so there’s a flaw in my logic ...
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Multipole moment in the power spectra of the CMB

Why is the multipole moment l in the power spectra of the CMB corresponds to the radius (instead of diameter) of the sound wave horizon of the baryon-photon fluid? It seems to me that it should be the ...
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How to Understand Negative Energy in the Ergoregion?

I am trying to understand the Penrose process and having trouble explaining negative energy in the ergoregion. How I interpret it is: Energy is the dot product between the four momentum of the object ...
Gene's user avatar
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When do we talk about spaghettification or pancakification in black holes?

So I've been doing some research for a while now, and yesterday came across the video of PBS space time talking about what happens to quantum information in a black hole. In the thought experiment ...
Anais-Ellie Gucek's user avatar
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Can wormhole inside a black hole become an escape?

I did not major in Physics so not sure if this is a proper question; but according to some Google search there do exist papers discussing wormhole inside black hole like this, which I am not able to ...
Luke Lee's user avatar
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When it comes to getting closer to the Schwarzschild radius, how is discrete a limit?

From Keeton (2014) in Principles of Astrophysics: Using Gravity and Stellar Physics to Explore the Cosmos, Gravitational time dilation near a large, slowly rotating, nearly spherical body, such as the ...
olivierlambert's user avatar
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Why does the timelike killing vector become spacelike inside the ergoregion?

Why does the timelike killing vector become spacelike inside the ergoregion? Some textbooks make this claim and move on to explain negative energy, but I could not find any proof for this claim. I can'...
Gene's user avatar
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Realistic black holes

If I understand the answers provided in this Link Why singularity in a black hole, and not just "very dense"? Then the singularity at $r=0$ may just be a mathematical artifact, and may not ...
Precious Adegbite's user avatar
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Can a Kerr black hole become super-extremal?

Let's assume there is a large Kerr black hole, which is almost extremal and would become extremal with the addition of a small amount of mass $M$ with spin $J$ to make the final $J=M$. What if this ...
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What is the proof that the Schwarzschild metric is not static inside the horizon?

In Lecture Notes on General Relativity, Sean M. Carroll shows that the Schwarzschild metric is not only stationary but also static (Chapter 7, page 169, Eq. 7.20 and following interpretation). On the ...
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Removing the cosmic horizon in the de Sitter metric

The metric for de Sitter spacetime in static coordinates is $$ds^2 = \left(1-\frac{r^2}{\ell^2}\right)dt^2 - \frac {1}{1 - \frac{r^2}{\ell^2}}dr^2 - r^2\,d\Omega_2^2.$$ It is evident that there exists ...
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Birkhoff's theorem and Schwarzschild vacuum solution [duplicate]

Birkhoff's theorem states that any spherically symmetric solution of the vacuum field equations must be static and asymptotically flat, but the well-known Schwarzschild solution satisfies these ...
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Interior Solution for Black Hole in Particular

This paper seems to suggest that the interior metric for a black hole in particular (a.k.a not a different kind of spherically symmetric non-rotating body) is just the exterior Schwarzschild metric ...
user345249's user avatar
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How are objects inside a black hole affected by the gravity of objects outside the black hole?

There are many Q&As about whether something inside a black hole can escape the event horizon if another massive object gets close enough to pull it out. I realize the answer (I think universally ...
Peter Moore's user avatar
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Is there a formula relating the rate/speed a black hole event horizon will grow, to the density of the medium surrounding it?

I am thinking about the average density of the space around the black hole, not the density immediately adjacent to the event horizon which might be different. It will probably be best to model the ...
John Hobson's user avatar
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Event horizon is a null surface - what about the angular coordinates?

From the Schwarzschild metric $$ds^2=(1-2m/r)dt^2-(1-2m/r)^{-1}dr^2-r^2(dθ^2+\sin^2⁡θ dϕ^2)$$ on the surface $r=2m$ (setting $dr=0$) we have $$ds^2=-r^2(dθ^2+\sin^2⁡θ dϕ^2).$$ This looks spacelike ($...
Khun Chang's user avatar
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How do we know that a black hole radius is not significantly contracted for a stationary outside observer?

It is my understanding that just as special relativity contracts length with velocity general relativity contracts length with gravity. Would this mean the radius of a BH is smaller than it would ...
Joe's user avatar
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What happens to the ring singularities when two Kerr black holes merge?

Imagine two Kerr black holes with ring singularities oriented in different axes (e.g. one horizontal and the other one vertical). If they merge, what will happen to these singularities? Will they form ...
Flamethrower's user avatar
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5 answers
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Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?

Once you start studying black holes, one of the first things you'll probably hear is that from an outsider's perspective objects falling into the black hole take an infinite time to do so due to time ...
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6 answers
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Singularity of a black hole: point or solid sphere? [duplicate]

A black hole is defined by its event horizon. The event horizon has a Schwarzschild radius of, $$r_s=\dfrac{2GM}{c^2}$$ Technically, this means that any body of mass, $M$, with a radius smaller than ...
Hritik RC's user avatar
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1 answer
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Time required to reach Black Hole's Event Horizon from outsider perspective?

Let's imagine a pair of particles that is entangled. One (call it $P_1$) is released and then falls to a black hole from a distant $x_0$, (for example $x_0=5r_s$) and velocity $v_0(=1/2c)$, while the ...
Nhat Nguyen's user avatar
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0 answers
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Effects of quantum jitters at the event horizon?

General relativity would seem to have a black hole’s event horizon be perfectly smooth on even the smallest scales. Theorists studying black hole event horizons are aware of the possibility of quantum ...
user86742's user avatar
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1 answer
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Do black holes evaporate prior to crossing their event horizon? [duplicate]

Bob falls into a black hole, watched by Alice who is far away. Alice sees Bob asymptotically approach the event horizon while his clock asymptotically approaches 12:00 noon as his image redshifts (...
Matt's user avatar
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Electrons keeping dynamical quantum fluctuations?

I was thinking about this paper (https://arxiv.org/abs/1405.0298) where the authors argue that there wouldn't be dynamical quantum fluctuations in a De Sitter space as fluctuations would be static ...
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What are regular event horizons?

I am studying uniqueness theorems of Black Holes and I often see the word "regular event horizon" instead only event horizon. Many textbooks and literature do not define this term. Please ...
Talha Ahmed's user avatar
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2 answers
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Does someone falling into a spinning black hole see the end of the universe?

It is well known that if you fall into the Schwarzschild black hole you cannot see the entirety of the outside spacetime since there are photons which cannot catch up with you before you reach the ...
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1 answer
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Is it possible that the mass of a black hole is located at the event horizon?

Due to gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the horizon, never actually reaching it. If it were to somehow enter the event horizon, ...
DexterJ Arnold's user avatar
1 vote
2 answers
72 views

Hawking-like radiation will probably be difficult to detect (even if it can be detected). Why is that?

I was having a conversation in another physics forum about horizons (like the event horizon of a black hole, or a cosmological horizon) emitting Hawking radiation and I mentioned that if the universe ...
vengaq's user avatar
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Is the Hilbert space of a Black Hole factorizable?

I am doing research on black hole information and have a question: Is the black hole's Hilbert space factorizable? In other words, can we write the Hilbert space of the black hole as a multiplication ...
TheFyziker's user avatar
1 vote
1 answer
91 views

Could I peel a black hole or at least sneak a peek beyond the event horizon with compact matter?

If you created an annular ring of compact matter, delicately balancing it and continuously adding matter to the ring in a stable orbit around a dormant black hole, could you peer beyond the ...
Travis R's user avatar
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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 ...
zahra's user avatar
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1 vote
2 answers
115 views

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 ...
Árpád Szendrei's user avatar
3 votes
2 answers
109 views

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 ...
Javier Ramos's user avatar
2 votes
0 answers
71 views

On vacuum energy in a de Sitter universe?

I have a couple of questions about de Sitter cosmological horizons. Initially I made a single post containing the two questions, but after some suggestions, I asked them in two separate posts. This ...
vengaq's user avatar
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2 votes
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On entropy in a de Sitter horizon in a universe undergoing accelerated expansion?

I have a question about de Sitter cosmological horizons: In this video by Leonard Susskind (https://youtu.be/n7eW-xPEvoQ?t=2324) he explains the concept of the Poincaré recurrence using particles and ...
vengaq's user avatar
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3 votes
1 answer
734 views

How many null directions are there?

The metric signature of spacetime is usually given as ($3,1$), but spaces can also be ($3,n,1$). Null surfaces include photons and event horizons, which exist, so is $n$ actually $ > 1$ in the ...
Miss Understands's user avatar
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1 answer
121 views

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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Range that the Schwarzschild metric is valid

The Schwarzschild metric is the metric calculated from the field equation outside of the black hole. This condition of region (outside of the matter) was the reason why we could use $T_{\mu\nu}=0$. ...
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1 answer
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Why do we see a black hole shadow despite the fact that there is hot gas all around it and not all radiation is absorbed by the horizon?

If the accretion disk surrounds the black hole (like that seen in M87 by the Event Horizon Telescope) but is too far away from the black hole to have its light captured, why do we still see a black ...
Jumales's user avatar
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Why can Newtonian gravity provide the correct value of the Schwarzschild radius? [duplicate]

By using Newtonian gravity, we can equate the kinetic and potential energy of a test mass in order to obtain the escape velocity of an object from a large mass $M$: $$\frac{1}{2} m v^2 = \frac{GMm}{r}$...
e-diamond's user avatar
1 vote
1 answer
81 views

Don't all objects that collapse have an apparent event horizon and so Hawking radiates?

So say there is an object that is in the form of gas and dust and a core that weighs 10 earths is in the center and there is a sphere of gas around it that weighs 50 Earths, so the final mass is only ...
Roghan Arun's user avatar
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1 vote
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In the reference frame of an observer an arbitrarily large distance away, can event horizons form in finite time? [closed]

There are similar questions answered already but the answers disagree. I understand that simultaneity/time is local in GR and that in a given region of space, an event horizon and singularity can form ...
K340's user avatar
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1 answer
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What happens to entangled particles at a Black Hole? [duplicate]

I am probably going to ask this wrong, so please don't flame me. I am not a scientist or student.... If an entangled particle enters an event horizon but has not reached the singularity, can we still ...
Rick's user avatar
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What is the difference between the cosmological and the black hole horizons in thermodynamics?

I want to know the different thermal behaviors between cosmological and black hole horizons, such as temperature, entropy and so on.
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Are black holes XXXL stars?

Am I right to think that black holes are stars that are so dense and heavy that radiation and matter cannot leave from it's surface?
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