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I am curious to know if anyone has ever sat down and calculated what the diameter of a black hole would be, in kilometers, if it were to contain all the mass of all the black holes that are currently in the Universe.

I have recently read that it is estimated that there may be as many as 40 trillion black holes in the Universe and that their combined mass make up about 1% of the total mass of the Universe.

So, if there was just one gigantic, supermassive black hole in the Universe which contained all the total mass of all the black holes currently in the Universe, what would its diameter be?

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    $\begingroup$ Wikipedia has the mass of the observable universe and the formula for the radius of a black hole. You can calculate it and answer your own question! $\endgroup$
    – Ghoster
    Commented Feb 13, 2023 at 18:14
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    $\begingroup$ Related: If the observable universe were compressed into a super massive black hole, how big would it be? $\endgroup$
    – Ghoster
    Commented Feb 13, 2023 at 18:25
  • $\begingroup$ @Ghoster, thanks for those links. I will check them out. $\endgroup$
    – user57467
    Commented Feb 13, 2023 at 18:30
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    $\begingroup$ A Schwarzschild black hole of radius $R$ has mass $c^2R/(2G)$ and density $3c^2/(8\pi GR^2)=\rho_c(c/(RH))^2$, with $\rho_c$ the critical density. So a critical-density spherical universe of radius $c/H$ has the same mass-radius relation as a black hole. Given how much of the universe's mass is in black hole's, I imaginary the answer to your question is in the mega- or even gigaparsecs. $\endgroup$
    – J.G.
    Commented Feb 13, 2023 at 20:08
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    $\begingroup$ Unfortunately, we don't know how many black holes are in the universe because we can't see the whole universe. $\endgroup$
    – David White
    Commented Feb 14, 2023 at 1:04

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