What is the temperature inside a black hole?  It may be very cold, or very hot. Or just the surrounding temperature.  Or maybe neither, because the concept of temperature makes no sense inside somehow?
120692
Since it cannot radiate light and therefore neither heat, if any reactions which generate heat were to occur inside a Black Hole, that would seem to imply an infinitely high ever increasing temperature,  or perhaps none whatsoever, that the intense radiation from the accretion discs and plasma jets is a necessary transformation of mass into a form which can cross the event horizon... I have never seen any reference to the temperature of a singularity, or if such a thing could have one, is there any theory of what it might be?
1703
I was wondering if it is possible for a single photon to form a black hole if it has a small enough wavelength.If so, what would this wavelength be? I came across this question because I am reading about loop quantum gravity, and have heard that it is 'untestable'. I know that the loops making up spacetime in loop quantum gravity are thought to have a size in the order of $10^{-35}$m, and I was wondering whether making a photon with a wavelength small enough to investigate this would form a black hole.  I am aware of the two posts here, but neither seem to really answer my question because they refer to several photons:  and .  Thank you.
6012
What's the entropy of the universe today? How does one go about calculating this? I've heard the statement that black holes account for the bulk of the entropy in the universe today, but don't know why this would be true or the relationship between black holes and entropy.
78728
Recently I read an interesting article about . I was puzzled because I thought before that  has definite meaning in thermodynamics: it tells about how fast atoms jiggle. Now if temperature can be negative, this means temperature has wider meaning...  I wonder if there is any deep physical meaning of temperature, not mathematical one.
84731
Why does there have to be a singularity in a black hole, and not just a very dense lump of matter of finite size? If there's any such thing as granularity of space, couldn't the "singularity" be just the smallest possible size?
11701
My understanding is that light can not escape from within a black hole (within the event horizon).  I've also heard that information cannot propagate faster than the speed of light.  It would seem to me that the gravitational attraction caused by a black hole carries information about the amount of mass within the black hole.  So, how does this information escape?  Looking at it from a particle point of view: do the gravitons (should they exist) travel faster than the photons?
9760
Black holes have very high gravitational force that tends to crush everything. So as we know atoms in a molecule have inter-atomic spacing between them and further electrons also revolve at a certain distance to the nucleus and inside the nucleus there is also some empty space. So does the strong force of a black hole crush them all the way, leaving no space in between them? And, if that happens, then it should give rise to a completely new species of atoms and molecules, right?
38164
I know that there's no evidence at this point for "white holes" however would it even be mathematically possible for a black hole to be connected to a white hole (total opposite so everything would be expelled, after some really extreme physical conditions)? Maybe with a wormhole connecting them? If this was even possible (if the black hole or connection could actually be created and be stable enough), would that matter be expelled into a different universe, etc? Maybe even a different region of spacetime? Just curious, as it would be a cool idea.
35600
Observations:   I have read that for a free-falling observer within the event horizon of a black hole that all lines of sight will end at the singularity which is black. I also look up and see that the sky is mostly black. I also know that by measuring the red-shifts of the galaxies that they are all accelelerating towards that blackness. (rather than accelerating away from a big-bang event, which makes less intuitive sense) The red-shifts are greatest for those galaxies closest to the singularity (i.e. furthest away from us).   I therefore conclude from these consistent observations that all the matter in at least the finite observable region of the universe in which we are located, is accelerating due to gravity towards the singularity of a super-massive black hole.  In the spirit of good science I will happy accept being wrong, but I need to understand why I am wrong.  Predictions from the theory:  The light from the universe outside the event horizon of the black hole should be visible as a small disc or point light source in the direction facing away from the singularity. Of course, if this happens to be inconveniently located on the far side of the milky way's core from us, then we will be unable to observe it.  Further Speculations:  It may be that in regions of the universe far ouside the influence of this black hole (e.g. >50 billon light years away) that the sky is white hot due to Olber's paradox.  The black hole may have been formed from a large quantity of anti-matter, which might explain the local perponderance of ordinary matter.  The microwave background radiation may be a result of Hawking radiation at the event horizon (which we are viewing fromn the inside), or, depending upon the non-euclidian geometry, we may be observing the result of matter being destroyed at the singularity.
29252
The  (or conjecture), suggests that black holes can be entirely described by their mass, angular momentum and charge. All other details of the BH formation are lost.  Is there a simple way of understanding why you would be able to tell the difference between a BH made from protons and one made from electrons, but not between one made from matter rather than anti-matter?  i.e. What is special about charge, and why aren't lepton number, baryon number, strangeness etc, also black hole properties?
72880
I’m studying a PhD in Theoretical Physics (quantum field theory, to be specific), but I would like to learn more about black holes (and related areas. Does anyone have any suggestions for textbooks/available notes on these subjects for people without much background in them? Obviously I’m very comfortable with mathematics and physics in general so I’m not looking for “pop science” material.
4222
Reading about  I understood that black holes lose energy over time - which is logical in some way (otherwise they would be there forever and heat death would never technically happen)  But - how exactly does it "evaporate"? What happens when it no longer contains enough mass within it's  ? Does it explode somehow? Transforms into "regular matter"? Simply vanishes over time? Or?
49054
Imagine a sphere of black holes surrounding a piece of space. Will this piece be separated from the rest of normal spacetime (at least for some time, till these black holes finally attracted themselves).  So, seen from the outside, we have a black hole, but with a non-singular interior.  Is this possible?
57634
Firstly, I understand that we have no observational evidence for 'wormholes'. They are theorised solutions to general relativity equations.  That said, if macroscopic wormholes do exist---how do they work? I've been thinking about this for the past couple of days, and I have some questions.   What does the mouth of a wormhole look like? I've seen ; is that more or less accurate? If I place one end of a wormhole in a high magnetic field, and the other end on my desk, will all my ferrous objects fly into the wormhole? If so, with what force? What does the magnetic field look like at each end of the wormhole? If the distance between the mouths of the wormhole was comparable to the force that the magnets exert, would the magnet 'pull' on itself and draw itself into the wormhole?   Do any of these questions even make sense?
116959
I have heard both that  is the smallest length that there is in the universe (whatever this means) and that it is the smallest thing that can be observed because if we wanted to observe something smaller, it  would require so much energy that would create a black hole (or our physics break down). So what is it, if there is a difference at all.
21255
Yes, I am aware of observations of "massive objects", for example near the center of the galaxy. But is there any evidence that the object in question is a black hole, with all its commonly attributed properties? Or are black holes just an unconfirmed artifact of general relativity, i.e. something that is an abstract mathematical concept and not a real object?  EDIT: That is not an answer to my question. I did not ask about means of detecting them, I am aware of those means. I'm interested if there's anything suggesting that the properties commonly attributed to black holes, other than large mass, have anything to do with reality, or are completely abstract and deduced from math.
66497
According to NASA      a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area   According to the documentary  about Black Holes     Gravity here is so strong even light cannot escape   Which leads me to believe that Black Holes basically are masses so compact but great which gives them very strong gravitational influence (Newton's Gravitational Law) that light is not allowed to go out of, hence a "Black" hole.  If my above understanding of Black Holes is correct, then why is those Spiral Galaxies pictured using Hubble Telescope is showing a big shiny ball instead of a "Black" hole ? What is the obvious clue that I am missing here ?  Here is the image of Andromeda Galaxy image taken using Hubble.