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LIGO detects most massive black hole merger to date

(www.caltech.edu)
360 points Eduard | 1 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. jMyles ◴[14 Jul 25 21:36 UTC] No.44565640{4}[source]▶
Is that true though?

Can't we generalize to say that we observe that black holes have a similar density (which is to say, proportion of mass to volume) any sample of the observable universe sufficiently large as to be roughly uniform?

In other words, doesn't this observation scale both down (to parts of the universe) and up (beyond the cosmological horizon, presuming that the rough uniformity in density persists), at least for any universe measured in euclidian terms?

It's very possible that I'm wrong here, and I'd love to be corrected.

...I also think we have to acknowledge that "similarly" is doing a fair bit of work here, as we're not accounting for rate of expansion - is that correct?