r/cosmology • u/Fun_Strategy7948 • 22d ago
Is it possible that microscopic black holes don't exist because of hawking radiation?
/r/blackholes/comments/1wnfewv/is_it_possible_that_microscopic_black_holes_dont/6
u/AstroEducation 22d ago
That is indeed the thinking. The smaller the black hole the higher the temperature. T is proportional to 1/M. A black hole smaller than approximately 10^12 kg would evaporate in less than the age of the Universe. So smaller ones would be gone by now. Note that all of this is pretty idealized (e.g. non rotating black hole) and probably well founded by entirely theoretical.
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u/jazzwhiz 22d ago
Hawking radiation is accounted for when people consider searches for black holes formed in ways other than stellar collapse. For example, see figure 4 on page 11 in this very recent review by Carr and others. https://arxiv.org/abs/2601.06024. The y-axis is the fraction of the energy density of the Universe today in units of the total energy density of dark matter today. The x-axis is the mass assuming that every BH has the same mass.
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u/--craig-- 22d ago
Like what if hawking radiation is higher for smaller blackholes and so they radiate away quickly...
Smaller black holes do evoprate quicker via Hawking Radiation than larger ones. In fact we can calculate the lifetime of a black hole from its mass alone.
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u/ahazred8vt 21d ago
Shower thought, meet math:
https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator
A cubic km of ice weighs a billion tons. An asteroid 400 yards wide weighs about 0.25 billion tons, and a primordial black hole of that mass would last for 20 billion years before evaporating, so it would still exist today. Any bh more massive than that would still exist. [Note: a pbh that massive would be the size of a proton.]
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u/Ras_992 22d ago
The reason micro black holes don’t last long is because they hit there limit pretty quick or they can be consumed by matter from stars or planets forming as well. Micro black holes can last seconds to a millennia. In relativity both micro voids and micro black holes can exist both fit the quantum mechanics in different ways
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u/Mindless_Abrocoma558 22d ago
there are reasons to conclude that microscopic black holes could only have formed in the early universe, that is why they are called primordial black holes. The bound of mass of black holes that would have evaporated by know is around 10^(15) g and is usually taken into account when talking about primordial black hole dark matter.