Our Universe consists of about 22% Dark Matter (DM). Such matter is the main stuff of galaxies. Also, Black Holes (BH) live in galaxies. We know that DM is subject to gravitational interaction. So, BH should absorb DM, and increase its event horizon. Comparing such a process with Hawking radiation, what process is more prevailing?
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4$\begingroup$ All but the smallest black holes (if there are any small ones) will grow by absorbing the cosmic microwave background radiation, among other things. $\endgroup$– Andrew SteaneCommented Oct 22, 2019 at 11:46
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$\begingroup$ Are you asking how much dark matter is absorbed versus how much is created through Hawking radiation? $\endgroup$– MichaelCommented Oct 22, 2019 at 18:33
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$\begingroup$ I'm asking about rather increasing/decreasing event horizon due to absorption/evaporation processes. But, it is interesting question, after absorption DM, BH can be release it as a Howking radiation. $\endgroup$– SergioCommented Oct 22, 2019 at 18:42
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1$\begingroup$ The larger a black hole, the less Hawking radiation it emits. Eventually, a black hole becomes so cold that even the gains from absorbed cosmic microwave background radiation are larger than the losses from Hawking radiation. That's the case for all black holes of stellar size, the CMB radiation is still several orders of magnitude too hot. Stellar black holes simply cannot evaporate with so much CMB radiation around. $\endgroup$– cmaster - reinstate monicaCommented Oct 22, 2019 at 22:19
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2 Answers
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It is called dark matter because the equations of motion of galaxies as seen with our instruments cannot be explained unless there is extra mass than derived from the luminous mass seen. There are also other observations that require this extra dark mass.
There are two main proposals for what this mass is composed of, MACHOS and WIMPS.
MACHOS :
A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter that emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs are not luminous, they are hard to detect. MACHOs include black holes or neutron stars as well as brown dwarfs and unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.
Depending on the geometry of the black hole under study, MACHOs would also be gravitationaly attracted as luminouw mass is, and will be eaten up by the black hole increasing its mass and horizon.
Weakly interacting massive particles (WIMPs) are hypothetical particles that are one of the proposed candidates for dark matter. There exists no clear definition of a WIMP, but broadly, a WIMP is a new elementary particle which interacts via gravity and any other force (or forces), potentially not part of the standard model itself, which is as weak as or weaker than the weak nuclear force, but also non-vanishing in its strength. Many WIMP candidates are expected to have been produced thermally in the early Universe, similarly to the particles of the standard model according to Big Bang cosmology, and usually will constitute cold dark matter.
These due to their weak interactions have the problems that BenCrowell is discussing, i.e. would have smaller probaility of ending into a black hole than MACHOs, although some of them would have the inevitable end. It will depend on a specific particle and specific calculation to get at those probabilities.
Hawking radiation energy loss goes inversely to the mass of the black hole, so astrophysically recorded black holes lose very little energy due to this radiation. Certainly if dark matter is MACHOs there is no comparison of absorbed matter to loss due to radiation. For WIMPs I would guess it as also true, because seen back holes are so large, but it needs a specific model to get numbers.
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Black holes are small, so they make small targets. Also, matter can't get to the event horizon easily because of conservation of angular momentum. It tends to swing past on a hyperbolic or elliptical orbit. A cloud of baryonic matter experiences friction, so some of it is still able to get in. Dark matter doesn't interact strongly with other matter, so this frictional mechanism is absent.
Hawking radiation is negligible for astrophysical black holes.
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3$\begingroup$ But can we be certain that dark matter doesn't interact with itself in some bizarre way to create something akin to friction? $\endgroup$– NiRVANACommented Oct 22, 2019 at 15:02
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3$\begingroup$ @Astik: Our knowledge of DM is not really as poor as you might be imagining. If there were some efficient dissipative mechanism of the kind you suggest, dark matter would have different dynamics, which we could detect based on its mass distribution. $\endgroup$– user4552Commented Oct 22, 2019 at 18:52
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$\begingroup$ You've correctly pointed out that the rates of dark matter absorption and Hawking radiation emission are negligible for astrophysical black holes, but you haven't answered the OP's question of which one is less negligible. $\endgroup$– tparkerCommented Oct 23, 2019 at 2:17
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$\begingroup$ @Astik "can we be certain that dark matter doesn't interact with itself in some bizarre way to create something akin to friction?" - This refers to "dark sector". There is no efidence of its existence: en.wikipedia.org/wiki/Hidden_sector $\endgroup$ Commented Oct 24, 2019 at 4:49