7 ms·

Interesting hypothesis. Primordial black holes -- if they exist -- have a tendency to be very small, and very small black holes evaporate extremely quickly. The

Interesting hypothesis. Primordial black holes -- if they exist -- have a tendency to be very small, and very small black holes evaporate extremely quickly. The smallest ones couldn't have survived to the present day. An Earth-mass black hole is big enough to have survived until now, but I was under the impression PBHs that big were posited to be very rare.

subroutine 7y ago
They are looking at several objects between approx. 1-20 Earth-masses (presumably they were bigger during primordial creation; this is their size, now, after 13 billion years of evaporation).
- DuskStar 7y ago
  The bigger the black hole, the slower it evaporates. By the time you get up into planetary sizes they basically don't change mass on the time scale of stars. (From this wonderful calculator [0], a black hole with a mass of 6.0x10^24 kg - about that of the earth - would have a lifetime of ~5.75x10^50 years) So if there's a 1-20 Earth-mass black hole out there, it hasn't evaporated from anything appreciably larger to get to that point. 0: http://xaonon.dyndns.org/hawking/ http://xaonon.dyndns.org/hawking/
  - raducu 7y ago
    There go my hopes humanity could use this to convert mass to energy. How long would we have to wait for it to evaporate before we could use it for energy production?
- ajross 7y ago
  Earth-mass black holes have surface temperatures well under that of the cosmic background radiation and will not have evaporated, they'd have been literally accreting mass just from the background. You need to get down to the billion-ish tons range to find black holes that can have appreciably evaporated over the life of the universe.
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BurningFrog 7y ago
If they're in a solar system, they could keep devouring rocks and mini-planets to keep existing, no?
- dreamcompiler 7y ago
  Yes. They'd mostly be sustained by the solar wind.
- saagarjha 7y ago
  I’m sure that would give off huge amounts of gamma radiation.
  - sudhirj 7y ago
    Only when they cleared their orbits, around the beginning of the solar system. Doubt they'd see much impact now.
    - saagarjha 7y ago
      Earth is hit by a couple dozen tons of space material a day.
    - sliken 7y ago
      Can't reply directly to the child. The earth is quite larger than a black hole with an earth mass. Also the earth has a huge atmosphere to help capture things. A black hole with the same mass as the earth would be almost impossible to hit in comparison (9mm). No atmosphere to slow things down, very hard to hit, and you have to come within 1.5 * the radius before you can't escape without thrust. So sure, some dust would be capture, but nothing anywhere close to what the earth captures a day.
      - jamiek88 7y ago
        If you click on the time of the child comment it’ll open that thread separately and you can reply there. Also how you save comments. Someone told me the other day thought I’d pay it forward!
sliken 7y ago
Even a moon mass black hole would survive. The temperature would be about 2.7 kelvin, same as the CMB is now. So any blackhole created at or after the big bang will have stayed the same or gained mass. The threshold I could find for surviving since the big bang is around 10^11 kg which is quite a bit smaller than the moon. 10^11 kg is quite a bit compared to human scale, more than the mass of the Three Gorges Dam, but quite small on the astronomical scale (10^11 times smaller than the moon). So while an earth mass black hole is small (9mm), it will last quite long indeed. The evaporation time is proportional to the mass^3.
- rolleiflex 7y ago
  Something about your comment triggered a primal fear in me of tiny, immortal 9mm bullets of non-existence fired randomly in space at all directions until I remembered an earth-mass black hole, while about a centime wide, it's still has a full gravity field the size of Earth's. If one's coming our way, we probably can't do much, but we'll see it coming.
  - sliken 7y ago
    Seems unlikely. It emits close to nothing, it's temperature is less than 2.7 kelvin, and we'd have to be spectacularly lucky to see a micro lensing event with such a small mass. Hell we barely notice when a several km comet enters the solar system and I've read about fairly large asteroids getting closer to the earth than the moon before being noticed. We are going to notice a 9mm black hole? Maybe if it punctured Jupiter first.
    - xaedes 7y ago
      the fast moving mass of earth will distort orbits via gravity. that will be noticed
      - sliken 7y ago
        If you removed the earth, except for the moon the solar system would change very little. Sure if the earth came between Jupiter and it's moons there might be some chaos, but the likelyhood of that is minimal. Even if it did happen we would notice, but the earth massed black hole would be very unlikely to make it into the inner solar system at that point. The earth is a minor gravitational force in the solar system, most likely the effects would take significant time to notice... well after the nearest approach. Small gravitational tugs take many samples and significant time to notice. For example the outer solar system still doesn't add up... and we have no idea why.
verytrivial 7y ago
https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati... -- "Hawking radiation calculator" which will tell you how long a black hole will survive based upon e.g. mass. Very fun to play with.

subroutine 7y ago

They are looking at several objects between approx. 1-20 Earth-masses (presumably they were bigger during primordial creation; this is their size, now, after 13 billion years of evaporation).

DuskStar 7y ago
The bigger the black hole, the slower it evaporates. By the time you get up into planetary sizes they basically don't change mass on the time scale of stars. (From this wonderful calculator [0], a black hole with a mass of 6.0x10^24 kg - about that of the earth - would have a lifetime of ~5.75x10^50 years) So if there's a 1-20 Earth-mass black hole out there, it hasn't evaporated from anything appreciably larger to get to that point. 0: http://xaonon.dyndns.org/hawking/ http://xaonon.dyndns.org/hawking/
- raducu 7y ago
  There go my hopes humanity could use this to convert mass to energy. How long would we have to wait for it to evaporate before we could use it for energy production?
ajross 7y ago
Earth-mass black holes have surface temperatures well under that of the cosmic background radiation and will not have evaporated, they'd have been literally accreting mass just from the background. You need to get down to the billion-ish tons range to find black holes that can have appreciably evaporated over the life of the universe.
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BurningFrog 7y ago

If they're in a solar system, they could keep devouring rocks and mini-planets to keep existing, no?

dreamcompiler 7y ago
Yes. They'd mostly be sustained by the solar wind.
saagarjha 7y ago
I’m sure that would give off huge amounts of gamma radiation.
- sudhirj 7y ago
  Only when they cleared their orbits, around the beginning of the solar system. Doubt they'd see much impact now.
  - saagarjha 7y ago
    Earth is hit by a couple dozen tons of space material a day.
  - sliken 7y ago
    Can't reply directly to the child. The earth is quite larger than a black hole with an earth mass. Also the earth has a huge atmosphere to help capture things. A black hole with the same mass as the earth would be almost impossible to hit in comparison (9mm). No atmosphere to slow things down, very hard to hit, and you have to come within 1.5 * the radius before you can't escape without thrust. So sure, some dust would be capture, but nothing anywhere close to what the earth captures a day.
    - jamiek88 7y ago
      If you click on the time of the child comment it’ll open that thread separately and you can reply there. Also how you save comments. Someone told me the other day thought I’d pay it forward!

sliken 7y ago

Even a moon mass black hole would survive. The temperature would be about 2.7 kelvin, same as the CMB is now. So any blackhole created at or after the big bang will have stayed the same or gained mass. The threshold I could find for surviving since the big bang is around 10^11 kg which is quite a bit smaller than the moon. 10^11 kg is quite a bit compared to human scale, more than the mass of the Three Gorges Dam, but quite small on the astronomical scale (10^11 times smaller than the moon). So while an earth mass black hole is small (9mm), it will last quite long indeed. The evaporation time is proportional to the mass^3.

rolleiflex 7y ago
Something about your comment triggered a primal fear in me of tiny, immortal 9mm bullets of non-existence fired randomly in space at all directions until I remembered an earth-mass black hole, while about a centime wide, it's still has a full gravity field the size of Earth's. If one's coming our way, we probably can't do much, but we'll see it coming.
- sliken 7y ago
  Seems unlikely. It emits close to nothing, it's temperature is less than 2.7 kelvin, and we'd have to be spectacularly lucky to see a micro lensing event with such a small mass. Hell we barely notice when a several km comet enters the solar system and I've read about fairly large asteroids getting closer to the earth than the moon before being noticed. We are going to notice a 9mm black hole? Maybe if it punctured Jupiter first.
  - xaedes 7y ago
    the fast moving mass of earth will distort orbits via gravity. that will be noticed
    - sliken 7y ago
      If you removed the earth, except for the moon the solar system would change very little. Sure if the earth came between Jupiter and it's moons there might be some chaos, but the likelyhood of that is minimal. Even if it did happen we would notice, but the earth massed black hole would be very unlikely to make it into the inner solar system at that point. The earth is a minor gravitational force in the solar system, most likely the effects would take significant time to notice... well after the nearest approach. Small gravitational tugs take many samples and significant time to notice. For example the outer solar system still doesn't add up... and we have no idea why.

verytrivial 7y ago

https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati... -- "Hawking radiation calculator" which will tell you how long a black hole will survive based upon e.g. mass. Very fun to play with.