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Would such "tiny" black holes be stable? I'm thinking Hawking radiation, or indeed the opposite, growing without bounds. If they are, are they considered as po

Would such "tiny" black holes be stable? I'm thinking Hawking radiation, or indeed the opposite, growing without bounds. If they are, are they considered as possible "particles" for dark matter? I guess that's what MACHOs are? And won't interact electromagnetically, only really by gravity.

dsr_ 2y ago
Both of these questions are considered in the article.
Tuna-Fish 2y ago
Hawking radiation gives you a lower bound for the size of a primordial black hole that still exists, at 10^15 g. Primordial black holes largely cannot grow, because they would be traveling very fast, likelihood of interaction with ordinary matter is very small, and there is no mechanism that would slow them down. So if there are some in our galaxy, they would just be zipping about at intergalactic speeds until the heat death of the universe, or until they hit the event horizon of a larger black hole and get absorbed. The present-day universe is just too sparse for them to form or accrete material.
- weitendorf 2y ago
  Not necessarily. That is only the case for black holes with no charge and angular momentum. Given high enough charge and angular momentum for a given mass, black holes become "extremal" and their hawking radiation/temperature approaches zero. It's argued these are unstable or maybe can't even exist for various reasons, but it's a pretty active area of physics AFAICT https://www.quantamagazine.org/mathematicians-prove-hawking-wrong-about-extremal-black-holes-20240821/ https://www.quantamagazine.org/mathematicians-prove-hawking-... That said, if these were primordial I'd expect their charges to not all be 0, so they'd interact with normal matter.
weitendorf 2y ago
Small black holes can be stable if they have a charge or high angular momentum. These are called extremal [0] black holes - the charge and/or angular momentum counteract the hawking radiation. Some physicists have argued that electrons (stable, very tiny) could technically be described as super-extremal black holes [1] [0] https://en.wikipedia.org/wiki/Extremal_black_hole https://en.wikipedia.org/wiki/Extremal_black_hole [1] https://en.wikipedia.org/wiki/Black_hole_electron https://en.wikipedia.org/wiki/Black_hole_electron

dsr_ 2y ago

Both of these questions are considered in the article.

Tuna-Fish 2y ago

Hawking radiation gives you a lower bound for the size of a primordial black hole that still exists, at 10^15 g. Primordial black holes largely cannot grow, because they would be traveling very fast, likelihood of interaction with ordinary matter is very small, and there is no mechanism that would slow them down. So if there are some in our galaxy, they would just be zipping about at intergalactic speeds until the heat death of the universe, or until they hit the event horizon of a larger black hole and get absorbed. The present-day universe is just too sparse for them to form or accrete material.

weitendorf 2y ago
Not necessarily. That is only the case for black holes with no charge and angular momentum. Given high enough charge and angular momentum for a given mass, black holes become "extremal" and their hawking radiation/temperature approaches zero. It's argued these are unstable or maybe can't even exist for various reasons, but it's a pretty active area of physics AFAICT https://www.quantamagazine.org/mathematicians-prove-hawking-wrong-about-extremal-black-holes-20240821/ https://www.quantamagazine.org/mathematicians-prove-hawking-... That said, if these were primordial I'd expect their charges to not all be 0, so they'd interact with normal matter.

weitendorf 2y ago

Small black holes can be stable if they have a charge or high angular momentum. These are called extremal [0] black holes - the charge and/or angular momentum counteract the hawking radiation. Some physicists have argued that electrons (stable, very tiny) could technically be described as super-extremal black holes [1] [0] https://en.wikipedia.org/wiki/Extremal_black_hole https://en.wikipedia.org/wiki/Extremal_black_hole [1] https://en.wikipedia.org/wiki/Black_hole_electron https://en.wikipedia.org/wiki/Black_hole_electron