You can make a black hole with only matter, but Hawking radiation involves the emission of both matter and antimatter. If a star made out of matter (including baryons) collapses into a black hole, and then the black hole evaporates, will Baryon number conservation be violated? Can this process be played in reverse to turn equal parts matter and antimatter into mostly matter?
24576
The  says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge.  For me it is puzzling why local quantities are not included, i.e. quantum numbers different from electrical charge. Lack of such parameters means breaking of the conservations laws (for a black hole made of baryons, Hawking radiation then is 50% baryonic and 50% anti-baryonic).  The question is:   If lack of baryonic number as a black hole parameter is a well established relation?   OR   It is (or may be) only an artifact of lack of unification between QFT and GR?
12757
A star's collapse can be halted by the  due to the Pauli exclusion principle. In extreme relativistic conditions, a star will continue to collapse regardless of the degeneracy pressure to form a black hole. Does this violate the Pauli exclusion principle? If so, are theorists ok with that? And if it doesn't violate the Pauli exclusion principle, why not?
122988
I'm making several assumptions, not sure if any are correct:   there is a black hole at the center of a galaxy the black hole is eating the galaxy   Eventually the galaxy will be gone, right?  Has this been observed? Do we know what happens afterwards?  Posting here since astronomy got merged into physics
84917
I am wondering whether a black hole can absorb photons whose wavelengths are greater than the horizon size of the black hole. I have seen different contradicting opinions on the answer. For instance , the accepted answer claims that the answer is no, without giving any justification whatsoever. While on , I think all the answers are pointing that yes, a black hole should be able to absorb photons whose wavelengths are greater than the size of the horizon of the black hole.  If the answer is yes, then black holes are perfect blackbodies, right? Or is it more complicated, because, like some answer on physicsforums claim, a black hole might have only a small probability to absorb photons whose wavelengths are greater than the horizon size (though no mathematical justification is given), thus even though it might be possible for a BH to absorb such photons, it might still not be a blackbody.  Edit: Here's what I've been told by a QFT specialist:      I think your question is not answerable. "A single photon" requires   knowing how to quantize the EM field near a black hole. That is an   open problem. And also, "single" particles don't objectively exist in   accelerated frames. See the Unruh effect.   I am not satisfied with the current two answers. They do not deal with the above comment at all. Furthermore they seem to contradict each other and they aren't crystal clear.
104246
Black holes do not have any property as baryonic or non-baryonic matter, as is stated by the no-hair theorem. Regarding conventional theories, in different models of cosmology, like Lambda-CDM, are black holes regarded as baryonic matter, and does the black hole matter significantly affect the baryonic density?
91404
We know from big bang nucleosynthesis that baryonic matter accounts for about 5% of the universe's total mass-energy density. What is the current best estimate of how much of this is in the form of stars? I'm guessing that this would be known only very roughly. It seems like you would just have to survey large volumes of space for stars and larger structures made out of stars. Although galaxies may pretty efficiently cycle their hydrogen and helium through stars, I would assume that there is a lot of hydrogen out there in the spaces between superclusters that has never had a chance to form a star and never will.  related:
102824
What happens to all of the electrons and protons in the material of a neutron star?  Could there ever be an electron star or a proton star?
114435
I like to know when a dying star collapsed into a black hole, is there anything inside or on the event horizon that is interacting with higgs field?
73600
I know what anti-matter is and how when it collides with matter both are annihilated. However, what about anti-photons? Are there such things as anti-photons?  I initially thought the idea preposterous. However I am curious because, if anti-photons don't exist, then anti-matter could theoretically transfer its energy to normal matter - through the mechanism of light. Is it right?
22611
The photograph of the black hole, at the centre of galaxy M87, is now a part of human history. We are told that the red-colour is caused by hot gas, matter &amp; radiation around the "edge", the Event Horizon. Given the power of the gravitational forces emanating from black hole, why aren't these three things drawn into the abyss? If light is bent/ drawn into the BH, how does this red image arise? The logic, here, would be that a body, on the Event Horizon, is immune from the gravity--which sounds pretty fatuous?
105732
From the perspective of a matter-based observer does antimatter:   Increase in entropy (and therefore decrease in entropy in its own time)   OR   Decrease in entropy (and therefore increase in entropy in its own time)   Option 2 would seem to explain why we don't see much anti-matter (it all went into energy.)  Which is correct? Does antimatter increase or decrease in entropy over time?
15082
I've heard that, from the perspective of an external observer, something falling into a black hole will eventually look "frozen": light waves will move to the infrared and further into lower frequencies.  But what about that unfortunate individual falling into such a black hole? What will he/she see when looking back at our universe?   For instance, I wonder, will it be the ultimate end of the Universe? Can someone who just crossed events horizon of the black hole still receive messages from our side?
2284
In  (ignoring Hawking radiation), why is a  black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a  black hole black?
44670
This is really bugging me. When you look up some educational text about stars life, this is what you find out:   Gravity creates the temperature and pressure to start fusion reactions. The fusion proceeds to heavier and heavier cores ending with iron, which remains in the centre of the star. One moment, all light cores are depleted and the gravity wins over the power of the fusion reactions, now absent. The core of the star collapses into high density object, which may vary depending on the star mass. And the top layers of the star explode.   And I just cannot find clear explanation why. According to what I imagine, the top layers of the star should just fall into the collapsing core.  Is that because of the 3rd Newtons rule?  Or do the stars have some need to end with a cool boom?
59098
Classically, black holes can merge, becoming a single black hole with an horizon area greater than the sum of both merged components.  Is it thermodynamically / statistically possible to split a black hole in multiple black holes? If the sum of the areas of the product black holes would exceed the area of the original black hole, it seems to be a statistically favorable transition by the fact alone that would be a state with larger entropy than the initial state
32425
So, once something is inside a black hole's event horizon, it can only move towards the center. This is fine for a point-object. But 3D solid objects rely on molecular forces to stay in one piece. These forces act in all directions inside the solid. But there could not be any interaction between atoms at different distances from the singularity, right?  So, what happens to a solid (let's assume a crystalline lattice, for simplicity) hypothetically placed inside the event horizon? Does it get cleaved in ultra-thin concentric layers, centered on the singularity?
98947
A number of times I have encountered in text-books and articles that neutrinos might contribute only a small fraction to dark matter. The reason has to do with the fact that if all of the dark matter consisted of neutrinos, then small-scale structures in the Universe could not have formed yet, because, as they say, neutrinos "wash out" small fluctuations. However, none of these texts provided a reference to any specific sources explaining in detail what is meant by "washing out". After all, neutrinos are notorious in their weak interaction with baryonic matter, so if there is a small-scale fluctuation of baryons, then how background neutrinos can prevent it from growing further if they practically do not interact with baryons? I guess the question boils down to calculating cross-sections of interactions at specific temperatures. I would appreciate comments and references to sources addressing this particular issue.