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> Prior to this new finding, all the black holes that have been identified have also had a companion star—they are discovered due to their impact on light emitt
by OsrsNeedsf2P 2y ago
> Prior to this new finding, all the black holes that have been identified have also had a companion star—they are discovered due to their impact on light emitted by their companion star. Without such a companion star, it would be very difficult to see a black hole.
It seems like we think there's many more of these black holes, but we just can't see them
- adastra22 2y agoLone stars are actually the exception, so not radically more as you might think. But there are also binary black holes.
- btilly 2y agoThis only covers stellar black holes. (Note that this black hole is believed to be a stellar black hole.) Those statistics could change quickly if you add to it a currently unknown number of primordial black holes that arose around the Big Bang. If those primordial black holes are mostly on their own, and are both numerous and small, they make a potential candidate for dark matter. They could also be potentially small enough to be evaporating in our current era. This has been suggested as a potential source of a very high energy neutrino that was found in February. See https://www.livescience.com/space/black-holes/evidence-for-stephen-hawkings-unproven-black-hole-theory-may-have-just-been-found-at-the-bottom-of-the-sea https://www.livescience.com/space/black-holes/evidence-for-s.... (Note that this is just a single observation. We are a very long way from being able to obtain strong experimental evidence for such speculative theories.)
- adrianN 2y agoIs black hole evaporation experimentally confirmed in any way?
- btilly 2y agoThe only thing that I've seen cited as potential evidence either way is that one neutrino, with an energy too high for other mechanisms that we have thought of. But it is just a single neutrino. And it may be produced by a mechanism we haven't yet thought of. The black holes that we know about are large. Large black holes are supposed to emit so little radiation that we'd never be able to detect it.
- ithkuil 2y agoIf the hawking temperature is higher than the CMB, would there be any net evaporation at all? A BH needs to be truly tiny for it to be hotter than the CMB
- deleted 2y ago[deleted]
- once_inc 2y agoLarge black holes do have significant accretion disks that do impact their visibility and can and will spew forth vast amounts of radiation. Anything caught in their event horizon gets pulled in and is never seen again. Anything near it bumps into other things in orbit around it, which lights it up like a candle. Note that there is also a 'weight class' of black holes that are significantly large that their event horizon would be visible to the naked eye, but which don't have an accretion disk. We currently don't know enough to make scientific estimates on how many of each weight class exists.
- btilly 2y agoThat is light coming from the accretion disk, and things falling it. The light does not come from the black hole itself.
- adastra22 2y agoYes and no. The mechanism for evaporation is a straightforward implication of matter/antimatter pair production in the vicinity of the event horizon. This general phenomenon (minus the event horizon) is observed every day in particle accelerators. But the signal would be too small to detect from any black hole we do know about.
- MichaelZuo 2y agoI thought there were too many constraints to make PBHs a significant contributor?
- btilly 2y agoMy understanding is that we can rule out having too many really small ones (we'd see them evaporating) and we can rule out too many stellar sized ones (gravitational lensing), but we haven't ruled out black holes in size ranges of a moon to a planet. But I don't track this field. So there could well be research that I don't know about which puts bigger constraints on it.
- cryptonector 2y agoPBHs can't be too small, since they'd have evaporated by now, but if they're large enough to last several times longer than our current universe age then perhaps there could be lots and we not be able to see them.
- GolDDranks 2y agoMy understanding is that they can't be _ultramicroscopic_, but they can be quite small. According to the Wikipedia: > A primordial black hole with an initial mass of around 10^12 kg would be completing its evaporation today And according to this: https://www.omnicalculator.com/physics/schwarzschild-radius https://www.omnicalculator.com/physics/schwarzschild-radius Mass: 10e12 kg → Schwarzschild radius of 1.5e-14 m which is smaller than a hydrogen atom (5.3e−11 m). I wonder what would be the mass that'd keep a black hole in thermal equilibrium with the current background radiation... (Edit. whoops, minus sign in a wrong place made the calculations haywire, fixed now.)
- casperc 2y agoHow does a black hole this small stay together. I assumed it was gravity pushing against the atomic forces. Is it a case of once you black, you never go back?
- GolDDranks 2y agoNobody really knows for black holes of these sizes, because we don't have a theory of quantum gravity, but I think that answer according to general relativity would that the curvature is steep enough that escaping would require going faster than c, the universal speed limit, so it doesn't happen.
- clarionbell 2y agoExactly. Once your object reaches the right radius it's done. It will 'evaporate' eventually. But it won't revert back. An interesting result is that you could, and this is very much beyond our engineering, manufacture black holes like this.
- o11c 2y agoThat depends on how you're counting. About 2/3 of star systems have only one star. About 2/3 of stars exist in a system with multiple stars. However, the higher the mass, the more likely it is to be in a multiple-star system. For stars with a mass high enough to form a black hole ... hm, Wikipedia says "at least 80%" but makes it unclear which statistic is being measured.
- cryptonector 2y ago> But there are also binary black holes. A binary black hole system will keep any other companions quite far from the binary, and perhaps not even allow companions at any distance. What is the density of lone black holes and black hole binaries that we could have in -say- our galaxy before we would notice them frequently in the same way as in TFA? Well, presumably LIGO would sense them, or could if they knew what to look for.
- adastra22 2y agoLIGO does sense them, or at least the infrequent merges.
- cryptonector 2y agoYes, the mergers. I'm just wondering about binaries that are millions of years from merging.
- chasil 2y agoA type-1a supernova peer would produce this effect, leaving only the black hole (or the oversize star that would become it). I don't know any other types where the star is completely destroyed.
- qoez 2y agoFun to realize there's barely any way for us to detect if there's one heading for us right this very moment.
- scoofy 2y agoThankfully the speed of light is slow enough to where that's not a huge concern... that is, unless the Nemesis hypotheses is actually a black hole and not a brown dwarf or planet. https://en.wikipedia.org/wiki/Nemesis_(hypothetical_star) https://en.wikipedia.org/wiki/Nemesis_(hypothetical_star)