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LIGO detects most massive black hole merger to date

(www.caltech.edu)
360 points Eduard | 8 comments | 14 Jul 25 20:06 UTC | HN request time: 1.096s | source | bottom
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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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1. gjm11 ◴[14 Jul 25 20:53 UTC] No.44565141[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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2. hnuser123456 ◴[14 Jul 25 21:03 UTC] No.44565260[source]▶
Up until the universe was around a few billion years old, its Schwarzchild radius would have been larger than even the co-moving (not just observable) universe's radius, but the initial momentum from the big bang was high enough to prevent collapse.
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3. jMyles ◴[14 Jul 25 21:36 UTC] No.44565640[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?

4. AnimalMuppet ◴[14 Jul 25 22:16 UTC] No.44565946[source]▶
That sounds suspiciously like "they were inside a region with enough mass to form an event horizon, but they escaped because they had enough momentum", which in turn sounds like "we can escape from inside an event horizon if we just move fast enough". Can you explain how that's not what you're saying?
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5. hnuser123456 ◴[14 Jul 25 22:29 UTC] No.44566046{3}[source]▶
I wish I had a straightforward answer to that. I'm sure the answer is some combination of cosmic inflation and dark energy, but by all means it appears the early universe either narrowly escaped, or simply is a black hole, that singularities are a flawed concept, that nothing is escaping the universe, and we are all stuck moving forward in time, and that the infinite future is the singularity.
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6. AnimalMuppet ◴[15 Jul 25 00:23 UTC] No.44566697{4}[source]▶
I don't have an answer either. But in my amateur opinion, of the available options, I lean toward "is a black hole". If all the mass we can see adds up to a black hole the size of what we can seen, then if you add all the stuff outside the light cone, it should add up to enough mass to make a black hole radius that includes the distance out to there.

But that leaves us with black holes forming inside a black hole, which I have absolutely no idea what to do with.

7. JumpCrisscross ◴[15 Jul 25 00:34 UTC] No.44566749[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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8. 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.)