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LIGO detects most massive black hole merger to date

(www.caltech.edu)
360 points Eduard | 2 comments | 14 Jul 25 20:06 UTC | HN request time: 0s | source
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perdomon ◴[14 Jul 25 20:19 UTC] No.44564794[source]▶
What happens when black holes collide? Does one black hole “consume” the other? Do they become a larger black hole? Does it get more dense or just larger?
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hnuser123456 ◴[14 Jul 25 20:31 UTC] No.44564906[source]▶
They become a larger black hole, mostly conserving mass, minus a few percent to gravitational waves. However, their mass is proportional to their radius, not volume, so it gets LESS dense. If you laid out a bunch of black holes in a line, just barely not touching, and let them merge, suddenly, the whole sphere of space enclosing the line becomes black hole. It also turns out that a black hole with the mass of the universe would have a volume about the size of the universe.
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JumpCrisscross ◴[14 Jul 25 20:44 UTC] No.44565021[source]▶
> turns out that a black hole with the mass of the universe would have a volume about the size of the universe

Mass and energy.

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gjm11 ◴[14 Jul 25 20:53 UTC] No.44565141{3}[source]▶
Is that intended to be a correction? (I don't think the original statement needs correcting, other than by replacing "universe" with "observable universe" in both places.)
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1. JumpCrisscross ◴[15 Jul 25 00:34 UTC] No.44566749{4}[source]▶
Mass alone doesn’t do it. You need energy, namely the CMB, to push the observable universe close to its Schwarzschild limits.
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2. gjm11 ◴[15 Jul 25 19:43 UTC] No.44575062[source]▶
Ohh, I see, you mean "mass" should have been "mass and energy" rather than e.g. that (mass,volume) should have been replaced by (mass,energy) or something.

I confess I just ... take it for granted in this kind of context that "mass" or "energy" or "mass+energy" all mean the same thing. Someone who wants to refer just to the total amount of matter will say something like "the total mass of the matter in the universe".

It's commonplace for physicists to write just "mass" when talking about this sort of thing. E.g.,

P T Landsberg, "Mass scales and the cosmological coincidences", Annalen der Physik, https://onlinelibrary.wiley.com/doi/10.1002/andp.19844960203:

"Theories involving the parameters h, c, G, H (in a usual notation) are considered. A huge ratio of 10^120 of the mass of the universe (m_u) to the smallest determinable mass m_0 in the period since the big bang occurs in such theories."

(Not cherry-picked; I went to the Wikipedia article on "Black hole cosmology", noted that it just says "mass" rather than "mass-energy" or whatever, and followed the link in the attached footnote. Also, so far as I know, not crankery; Landsberg was an eminent physicist.)