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Comment by stefco_ | Hacker News Reader

I'm a grad student with LIGO.

Yeah, the rate of intermediate mass black hole mergers was totally unexpected and has caused a bunch of active discussion on formation mechanisms.

In short, intermediate mass black holes are too big to be generated from star collapses, which means they (likely) come from smaller BH mergers. But because these things take so long (GWs radiate energy very slowly until the bodies are very close and fast), we didn't previously expect there to be large populations of them. It has been surprising to see that there are enough of them to sustain a pretty decent IMBH merger rate.

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How likely is it that there are feeding mechanisms for black holes that make them this large?

I guess the main problem is that you'd need friction to slow down matter that's attracted by the black hole, so it doesn't just orbit it...

I don't keep up with the latest material here, but there is some stuff I've picked up.

For one thing, it's probably not ordinary matter falling into the black holes. When ordinary matter falls into a black hole, it usually forms a bright, glowing accretion disk. We would see that with an ordinary telescope, especially given just how much matter would need to be accreted. (We're talking about dozens of suns worth of mass for each of these intermediate mass black holes!)

You can also have three-body interactions. A pair of black holes in a circular orbit at a decent distance will merge on a timescale of billions of years. But if a third black hole comes in and disrupts the orbit, it can knock those two black holes closer together, at which point the GW power gets much higher and the merge can finish on a reasonable timescale.

For these reasons, some papers I've seen suggest that there might be large collections of black holes that are dense enough for three-body interactions to be relatively common.

Is anyone in the community skeptical of what you're actually observing though? If this rate was so unexpected, isn't there a possibility that what's being observed is not what you think it is?

Could they be reflected waves reaching 'the end of the pond' and coming back and so you see a lot more of them than that there are actual events?

(Probably a very silly question)

pray tell what reflects gravity

If the universe isn't infinite, isn't the idea that things sort of "wrap around"?

No the idea is that the universe is expanding. It's not infinite but it's expanding faster than C and therefore it's impossible to reach the edge.

Yeah but another idea is that it is isotropic, which is borne out by observation. This leaves a few obvious possiblities:

1. It's infinite and homogeneous 2. It's finite but periodic (like the Earth; go straight long enough and end up where you were) 3. It's slowly varying, so that within our Hubble volume (the volume within the horizon receding from us at the speed of light, beyond which we can't see) everything looks isotropic; in other words, it varies, but slowly

So what you're saying is true from our position in space, but it should also (if points 1 and 2 are true) be true everywhere in space, and the question of the large scale structure of the universe remains valid.

Will we ever hear neutron stars colliding with other large masses any time soon?

LIGO also searches for BH/NS mergers and for NS/NS mergers. None have been announced yet, but they are definitely something we look for.

BH/NS and NS/NS mergers are exciting because they involve conventional matter and are likely to give off neutrinos and light as well as GWs. This means that EM followup is more likely to be worthwhile.

The smaller mass of neutron stars means they are more common, and the NS event-rate is expected to be much higher than the BH event rate. But BH events are louder due to the extra mass, so we can search in a much much larger volume of space, which so far has been favorable for BBH (binary black hole) merger detections.

Another interesting thing about BNS mergers is that, because they get closer/faster before merging, their signal should climb to higher frequencies and spends much, much longer in our sensitive band than BBH mergers (this is also true for lighter BBH mergers vs. heavier ones). This is really cool because it means that BNS mergers can be fit much more tightly to model waveforms, allowing for more precise parameter estimation (among other things) given the same peak signal-to-noise ratio.

NS-NS collision, everything doesn't disappear into an event horizon, wouldn't they deform and maybe some mass gets ejected, so would the impact event sound more complex/irregular?

It would be cool if we could hear a NS-NS collision of sufficient masses such that we could hear a BH being born for the first time.

Will we reasonably ever hear a supermassive BH collision? I would assume their loudness to rarity ratio is just too low.

How cool would be to be bale to see a video of that merger? One can only dream..

These gorgeous, physically-correct simulations are about as close as you'll get for now: https://www.black-holes.org/explore/movies

One problem is that supermassive BHs merge at very low frequencies; they merge before their orbits get fast.

LIGO's sensitivity range is in about the same range of frequencies as the human ear. NSNS mergers reach our sensitivity sweet spot for a few seconds; intermediate mass BHBH mergers get into the low end of our sweet spot. Supermassive black holes merge at very low frequencies outside of our sensitive band.

LISA is a planned space-based observatory that would be sensitive in those bands: https://lisa.nasa.gov/

Yes, NNS and BNS collisions are very messy. NNS are also very likely to form a BH.

> Will we reasonably ever hear a supermassive BH collision?

Hopefully! They'll certianly be looked for. There are some extremely massive SMBHs out there (e.g. Blazar QSO B0218+357), and it's reasonable to think that at least some of them are the result of mergers of SMBHs rather than, say, a rain of smaller BHs (and dust, gas and stars) onto an already big BH (which may also happen).

https://www.youtube.com/watch?v=vw2sLcyV7Vc

https://www.youtube.com/watch?v=ow9JCXy1QdY

http://newscenter.lbl.gov/2017/08/02/new-space-simulations-n...

But we won't detect them with LIGO. We will need a lower-frequency detector, like the space-based LISA design, to do that.

It's just like how we have optical telescopes for visible light and radio telescopes for light at different frequencies. Different interferometer designs will pick up different phenomena based on what their sensitive band is.

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