Posts Tagged ‘black hole evaporation’

Black Hole Evaporation versus CMBR

May 5, 2018

Black holes evaporate. At least, that’s what most physicists tell us.

What I stumbled into recently was a conjecture that they can’t evaporate beyond a certain point if the input is greater or equal to their output. This was mentioned on Quora by some physicist as a response to a related question. I thought it was interesting enough to mention it here.

Really large black holes are colder than the cosmic microwave background radiation (CMBR), which is about 2.73 degrees. The radiation going into a black hole is actually greater than the radiation leaving the black hole. The only way a black hole could radiate is if it’s very small and already radiating hotter than the CMBR (plus whatever particles fall into it, adding to its mass).

The limit where the size/mass of the black hole is equal to CMBR input is about 1% Earth mass, about 4×10^22 kilograms, based on Susskind’s formula and Hawking’s formula. This would create a black hole smaller than a millimeter. But black holes can’t even form without at least three or more Solar masses to begin with.

So, any black hole larger than a millimeter is going to keep growing. Presumably, primordial black holes smaller than 10^11 kg, created during the Big Bang, would have evaporated by now. This leaves a range of possible primordial black holes from 10^11 to 10^22 kg as possible existing evaporators, since they would be hotter than the background radiation.


Primordial black holes would form because of high density and radiation. It would be crazy to think that their mass wouldn’t quickly grow far beyond Earth mass when surrounded by a buffet of dense gas and radiation. Just the nature of the formative process suggests that they will never radiate faster than mass/energy is added to them from their environment, and will always grow larger in size.

I really WANT them to exist, however. My next SF story kind of depends on it.