Ok, I tweeted something strange and need to clarify. Black holes have a Hawking temperature, & as far as we know, all BHs in the universe are *way* colder than the cosmic microwave background CMB: between 9 and 18 orders of magnitude colder. The amount of radiation they absorb 1/ https://twitter.com/duetosymmetry/status/1302800702310354944

from the CMB is way more than the amount they emit due to Hawking radiation. What I wrote before was that they are "being heated" by the CMB. What I wrote was wrong!!! Ordinary things, when they absorb more energy than they emit, get hotter. Not black holes! 2/

Black holes have what is called a negative heat capacity. That means when you add energy, the temperature goes *down*. So, when black holes absorb radiation from the cosmic microwave background, their mass goes up. & the Hawking temperature is inversely proportional to mass, 3/

So when BHs absorb radiation from the cosmic microwave background, they actually get colder! The cosmic microwave background is *cooling* black holes by giving them energy! 4/4