
Are Gravitational Wave Detections Becoming Normal? - prostoalex
http://nautil.us//blog/are-gravitational-wave-detections-becoming-normal?utm_source=Nautilus&utm_campaign=a94cfa3e65-EMAIL_CAMPAIGN_2017_09_28&utm_medium=email&utm_term=0_dc96ec7a9d-a94cfa3e65-60565561
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stefco_
(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.

~~~
nonbel
>"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?

~~~
kobeya
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.

~~~
stromgo
Where did you get this 1000x stellar mass figure from? According to
[https://en.wikipedia.org/wiki/List_of_gravitational_wave_obs...](https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations)
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.

~~~
kobeya
Those are solar mass.

[https://en.m.wikipedia.org/wiki/Intermediate-
mass_black_hole](https://en.m.wikipedia.org/wiki/Intermediate-mass_black_hole)

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

~~~
kobeya
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.)

~~~
stromgo
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.

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scottmsul
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.

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

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dzdt
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.

~~~
raverbashing
How would a NS merger compare with a BH merger?

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

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hacker_9
_" 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?

~~~
auntienomen
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.

~~~
xenophonf
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. ;)

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

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jxy
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.

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

[https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...](https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna)

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

See also this feasibility study:

[https://en.wikipedia.org/wiki/LISA_Pathfinder](https://en.wikipedia.org/wiki/LISA_Pathfinder)

