Are Gravitational Wave Detections Becoming Normal? | Hacker News Reader

Are Gravitational Wave Detections Becoming Normal?

nautil.us

67 pointslxm23 comments

(I'm a Ph.D. student working on LIGO)

I think the author is missing the point a bit. LIGO is an observatory, and as it gets more sensitive, we certainly hope to see GW detections become normal!

It is already interesting and surprising that the universe contains so many intermediate mass black holes. This was unexpected, and our continued observation of them means that they are unlikely to be a statistical fluke.

Others have mentioned that LIGO also searches for other source types, like neutron star mergers, continuous GWs, and core-collapse supernovae. Assuming our predictions are right, these will be exciting to see, but eventually some of those observations will become routine.

However, you still gain a lot of information from routine observations! Routine observations let us start understanding the statistics of phenomena, which lets us figure out things like how common black holes are and when they evolve in our universe's history. It doesn't sound as sexy as a new discovery, but it's the kind of stuff you use to brainstorm and test new cosmological theories. You need it in order to do real science!

I'll also add that, as a new type of observatory, LIGO and Virgo are like an entirely new sense. It's like we can hear the universe all of a sudden, and our hearing is improving with time. What is really exciting to me is that we will most likely see things that nobody has predicted yet.

>"It is already interesting and surprising that the universe contains so many intermediate mass black holes. This was unexpected, and our continued observation of them means that they are unlikely to be a statistical fluke."

Interesting. Can you put a number on how unexpected this was? What is the probability "the universe contains so many intermediate mass black holes" independent of the LIGO data?

Not a professional, but AIUI the issue was the the models were just incomplete. These black holes are too big to be stellar collapse remnants, so they must have formed in the early universe. We don't have a good model for that yet that produces even stuff like the CBR spectrum or galaxy mass distribution. Nonetheless one of the things that no one was looking for was these medium black holes, so the fact that it's been observed is "surprising" in the sense of good news: it further constrains the big bang models.

This paper went out after the first detection in O1 (observing run 1) [1]. It discusses event rates inferred from the original event (GW150914) as well as previous predictions. CalTech also keeps a running list of detection papers, free for the public to read [2].

[1] https://arxiv.org/abs/1602.03842v3

[2] https://www.ligo.caltech.edu/page/detection-companion-papers

There’s two main formation pathways for black holes: ongoing stellar collapse and galaxy collapse in the early universe. The former makes stellar-mass black holes. The latter makes enormous million-stellar mass black holes or larger. That’s 5-6 orders of magnitude difference.

What’s interesting is that we can measure/infer the mass of a black hole from these gravitational waves and we are finding a lot of ~1000x stellar mass black holes. Too big to be stellar collapse, too small to be galaxy centers. Where do these come from?

The number for how many there should be is very close to zero. Presumably you’d only get one from a stellar black hole gobbling up enough mass to get that big, but 10 doublings is too much to have happened except by freak accident, and these are regularly occurring.

The current best theory is that they formed in the early universe before or during galaxy formation by some process that we don’t yet understand.

Where did you get this 1000x stellar mass figure from? According to https://en.wikipedia.org/wiki/List_of_gravitational_wave_obs... so far the observed merger components were between 7.5 and 35.4 solar masses.

The sample of stars in the night sky is biased by luminosity and longevity. I'd be curious to see a distribution of stellar masses that compensates for these two biases. On such a chart, extrapolating the trend in the high-mass range just before the error bars become too large would tell us whether 30-solar-mass stellar remnants are expected or unexpected by observation data.

Those are solar mass.

https://en.m.wikipedia.org/wiki/Intermediate-mass_black_hole

I mean, where did you get this 1000x figure as what "we're finding a lot of"?

There's examples in the article I linked to. (FWIW finding any at all, let alone the half dozen or so found in the last few years is "a lot" given the minuscule priors involved.)

I'm sorry I still don't see it. Your linked article says at the very top "There is as yet no unambiguous detection of an IMBH". Concerning gravitational waves, reference [7] says "We show that space based detectors such as the Laser Interferometer Space Antenna are likely to detect several of these sources". Reference [8] says "gravitational-wave observations could be used to accurately measure masses of black holes in merging binaries and probe the existence of intermediate-mass black holes". So no discovery yet, just theoretical work on what the signal would look like.

For LIGO, mergers are just the beginning. LIGO isn't just a merger detector, but an entirely new window of astronomy. There's also teams looking for stochastic background signals from the big bang (similar to the CMB), or a constant hum from spinning neutron stars. And not to mention, if the sensitivity continues to improve, the moment of merging from BH-BH collisions could lead to new breakthroughs in GR.

Also at some point we'll be able to hear neutron stars colliding and a black hole being born for the first time

I think the author may be making an incorrect assumption, that the recently published black-hole/black-hole merger was the "knock your sox off" detection leaked in August.

LIGO scientists have a pretty clear understanding of what a BH-BH merger sounds like in comparison to a neutron star merger. With a clear signal they would be unlikely to confuse one for the other. And the published BH-BH signature is very clear.

The sky location for the published BH-BH merger is in the constellation Eridanus. The rumored NS-NS merger was in galaxy NGC4993 in the constellation Hydra.

The published BS-BS merger was detected on August 14. The rumor leaks started August 18, hypothesizing a relationship to gamma ray burst GRB 170817A detected on August 17.

I hold out hope the NS-NS rumor is true, that paper is just taking a bit longer to prepare.

How would a NS merger compare with a BH merger?

If the rumours are true, we're about to find out!

> I hold out hope the NS-NS rumor is true, that paper is just taking a bit longer to prepare.

Mark October 16th on your calendar.

"For now, the faint sigh of spacetime rippling from the collision of two black holes is still momentous. But revelation turns inevitably to registration. One day, detecting phenomenal cosmic events through gravitational waves will not make the news."

Ok? Gravitational waves move the earth by about the width of one atom, so detecting them has always been more about verifying theories rather than anything else super exciting. What does the author expect?

Experimenters don't do their work just to verify theories. (Source: I'm a theorist, but used to do experiment.) They built these 'telescopes' because they want to observe the world via a new medium. They're going to be doing cutting edge astronomy with them for decades.

Dude, we still do cutting edge astronomy with optical telescopes (1608) and spectrographs (1876)! I'm pretty sure the awesomeness horizon of the GW observatories is a lot longer than a few decades. ;)

Fair point, although I meant these specific instruments, not the more general class of instruments.

The measured strain corresponds to a length distortion MUCH smaller than the width of one atom. An atom is about 1 angstrom in linear size, while a proton is 1/10000 that. The measured distortion corresponds to a distance smaller than that of a proton.

LIGO is really the 21st century telescope. Instead of receiving electromagnetic waves / photons, it receives spacetime waves / (gravitons???). As we have been building larger and larger telescopes, we would certainly build larger and larger LIGO's. Put them in space, put them on the moon, and put them on Mars. Galileo would be proud.

Physicists are already working on the -in-space version:

https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...

It got approved this summer for a planned launch date in 2034!