« on: 21/08/2019 16:33:12 »
Estimated mass of SMBH at center of our galaxy: 4 million solar masses.
If black holes evaporate via Hawking radiation, a supermassive black hole with a mass of 1011 (100 billion) M☉ will evaporate in around 2×10^100 years . Would galaxies drift apart without their blackholes??
Total estimated mass of our galaxy 700 billion solar masses.
So the SMBH only makes up 0.0006% of the mass of the galaxy. It's loss would have an insignificant effect on the dynamics of the galaxy.
For comparison the Sun converts about the same percentage of its mass to light in just under 1,000,000 years. Ergo, A galaxy would loose mass via radiation loss at a faster rate than it would by SMBH evaporation, even if the SMBH were free to evaporate.
Secondly, For a BH to even lose net mass via evaporation, It would need to lose energy/mass at a faster rate than it takes in. Matter, starlight, and even the Cosmic background radiation all all sources which will feed the BH.
The basic rule is that the Hawking temp of the BH has to higher than the average ambient temp of its surroundings. Even if we remove all other sources, the CMBR still remains. Hawking Temp goes down with size, and beyond a certain size, a BH will be below the CMBR temp. This mass is approximately equal to that of our moon.
Ergo, before any SMBH at the center of any galaxy can even start to experience a net loss of mass via Hawking radiation, the universe will have to have expanded and cooled by a great deal, the galaxies would have had to used up their source of hydrogen for forming new stars, and remaining husks of old stars will have to have cooled significantly.( considering the fact that small red dwarfs can burn for trillions of years, and It can take a quadrillion years (or longer) for the white dwarf formed at the end of solar mass range stars to cool to black dwarf stage, you are going to be for a long wait. If the "Big Rip" hypothesis is correct, run away expansion of the universe would have torn galaxies, star systems and even stars themselves apart before then, so it would be a bit of a moot point.
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