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Sure black holes are awesome, but how about black holes being
captured by the screaming vortex of a quasar, where
they merge and grow like some monstrous version of a solar system. This insane hypothesis
is getting closer to reality, at least according to the papers in today’s Space Time Journal club.
In September 2015 the laser interferometer gravitational wave observatory - LIGO - detected
its first gravitational wave from the merger of two black holes. That was stunning enough,
but the real promise lay ahead. Every time we learn to observe the universe in a new
way we discover new things. When we figured out how to see in radio waves quasars and
supernova remnants lit up the sky, When we learned to see in neutrinos the core of the
Sun became visible to us. But now that our vision extends to gravitational waves, what
else might we discover?
The black hole mergers themselves were not so surprising. Einstein’s general relativity
predicted gravitational waves and astrophysics predicted black hole mergers. When two very
massive stars are in binary orbit with each other, they may end their lives to leave a
pair of binary black holes. And in very dense environments like the cores of galaxies, lone
black holes may find each other and form a binary pair. However the pair forms, it can’t
last forever. As they circle each other, black holes whip the fabric of space into expanding
ripples - gravitational waves - which saps orbital energy from the system. The black
holes spiral closer and closer together. In the last instant they coalesce into a single
black hole, and the powerful gravitational waves produced in the last fraction of a second
are what LIGO detects - sometimes from over a billion light years away.
We expected to see black hole mergers, but there was some striking surprises. For one
thing, many of the merging black holes were too massive to have been formed by the collapse
of stellar cores. That is if our understanding of stellar evolution is half as good as we
think it is. This led astrophysicists to think of new ways to produce black hole mergers.
Here’s the most awesome possibility: what if black hole mergers actually occur in orbit
around supermassive black holes, embedded deep in the whirlpools of searing gas that
surround some of these monsters? Well today on Space Time Journal Club we’ll be looking
at a pair of 2019 papers that talk about this possibility. We have Yang Yang Imre Bartos et al., which predicts
the properties of black holes that merge this way, and Barry McKernan Saavik Ford et al.
which proposes a way for us to actually test this hypothesis.
The argument goes like this: we know that the center of almost every galaxy contains
a supermassive black hole of millions to billions of times the mass of the Sun. Recently we’ve
also learned that the galactic center likely also contains a swarm of perhaps tens of thousands
of stellar-mass black holes. These are the remnants of dead stars, typically a few to
a few tens times the mass of the Sun. They rained down on the galactic center over billions
of years as massive stars formed and died in the surrounding galactic core. This has
been a theoretical prediction for some time, but we’ve recently found evidence of the
Milky Way’s black hole swarm - and yeah, we covered that in a previous episode.
OK, so, a black hole swarm surrounding a supermassive black hole sounds like a recipe for black
hole collisions. Actually not so much - black holes are so compact that they never collide
outright - they need to merge by first forming a binary pair and then falling together. Now
binary black hole pairs surely do exist in the dense galactic center, but they may have
trouble merging in such a dense environment. Regular glancing encounters with other stars
or black holes can tear binary pairs apart before they can spiral together.
But there’s a way to massively accelerate the mergers of these black holes: all you
need is a little quasar. For the most part the supermassive black holes at galactic centers
are, well, black. But occasionally gas from the surrounding galaxy will find its way into
the galactic center and form an incandescent vortex - an accretion disk - as it plummets
into the insane gravitational field of the central monster. In the case of the largest,
most well-fed black holes this results in the quasar phenomenon, and their accretion
disks glow bright enough to be visible from the other side of the universe. More generally,
these feeding supermassive black holes are called active galactic nuclei.
This is how supermassive black holes can grow to such enormous sizes, but what does the
presence of an accretion disk mean for the swarm of stellar mass black holes? The orbits
of those black holes are mostly random, so the swarm forms a spheroid a few light years
across. The accretion disk is quite a bit smaller than the full swarm, but there should
still be plenty of black holes orbiting in the region. These will punch right through
the accretion disk twice every orbit. On each pass a streamer of gas is dragged out of the
disk, tugged by the black hole’s gravitational field. Momentum is transferred from black
hole to gas, slowing the black hole down a bit and causing its orbit to decay - much
like how a satellite’s orbit will decay if it’s too close to Earth’s atmosphere.
Eventually these disk-crossing black holes should be swept into the accretion disk. There
they gorge on the gas of the disk and grow in mass much, much faster than they could
in almost anywhere else in the galaxy. So now we have one way for these black holes
to get big. But in order to be detected by LIGO, they also need to merge with each other.
There are two ways this can happen: If a binary black hole pair gets captured by the disk,
the surrounding gas saps their orbital energy much more quickly than by gravitational radiation
alone. This means they can spiral together before being ripped apart by a glancing blow
with another object.
And accretion disks also allow lone black holes to find each other. This is really cool,
because the process is similar to how planets form. When a massive object is embedded in
a rotating disk, it will exert a gravitational tug on the surrounding particles. Depending
on the local properties of the disk, that can cause the object to either gain or lose
angular momentum. If it gains angular momentum the size of its orbit increases, so it moves
further out in the disk - or “migrates outward”. If it loses angular moment it migrates inwards.
But the local properties of a disk will change with the distance from the center. In some
regions you get inward migration and in some regions outward migration. The boundaries
between these regions are called migration traps - no migration occurs there. So an embedded
object will wander inwards or outwards until they find one of these traps, and they’ll
remain stuck there for some time.
In infant planetary systems, a disk of gas and dust surrounds the newly-formed star - a
protoplanetary disk. Lumps of coagulated ice and dust migrate to these traps, where they
can find each other and build into planets. In the case of accretion disks, the “planets”
are black holes - captured single black holes end up in the same migration trap, eventually
finding each other and forming binary pairs, which then quickly merge. This mechanism helps lone black
holes find each other, so it should massively boost the number mergers. And boosting the
mergers also helps black holes to grow in mass more quickly. Multiple black holes can
end up in the same migration trap and merge one at a time, ultimately reaching enormous
sizes. This is one of the calculations of Yang and collaborators: they figure that black
holes merging in this way should have much higher masses than via the “traditional”
empty-space mergers - with 50-solar-mass mergers being relatively common. Another paper by
Jillian Bellovary and co. predicts that behemoth “intermediate mass” black holes can form,
with 1000s of times the mass of the Sun.
So all of this sounds exciting and fun - and it may explain why so many surprisingly massive
black hole mergers are observed. And if we spot more and more high-mass mergers that
will be further evidence in favour of the hypothesis. Yang and co. also predict a particular
distribution of black hole spins - again to be tested with more LIGO observations.
But what about a more direct test? That’s where the paper by McKernan and collaborators comes
in. If black holes merge in empty space then the event should invisible - it should emit
no electromagnetic radiation. But it’s different in an accretion disk. A few things happen
right after merger that could lead to a bright burst of light to accompany the gravitational waves.
These captured black hole binaries will be surrounded by their own mini-vortices of gas.
When they finally merge, they release a burst of gravitational radiation so powerful that
it can carry away up to several percent of the original mass of the two black holes.
Gas that was orbiting the binary suddenly finds itself moving too quickly for the reduced
gravitational field of the final black hole. It creates an expanding expanding shock-front
that then collides with the gas of the surrounding accretion disk. Some of the shocked gas will
then fall back in, resulting in a burst of accretion into the new, merged black hole.
Finally, the release of gravitational waves delivers a kick to the final black hole - a
bit like the recoil of a gun. This drives the black hole and its surrounding gas through
the accretion disk, causing more shocks as gas is rammed together. All of this violent
motion produces a flash of ultraviolet radiation - and those flashes may be visible to telescopes
on Earth right after the gravitational waves arrive.
Now it’s going to be a challenge spotting these. The combined resolution of
the two LIGO and the VIRGO observatories locates a gravitational wave source to a pretty large
blob on the sky, which will typically contain hundreds of active galactic nuclei and many
thousands of regular galaxies. Any of those could be the source of the black hole merger.
But since LIGO started operation we now have advanced follow-up systems in place. As soon
as a candidate wave is detected, multiple telescopes scan that region of the sky to
search for electromagnetic signatures. If the merger was inside an accretion disk then we
might see a temporary increase in the light from one of the active galaxies — a fading
flash that is brightest at ultraviolet wavelengths.
The researchers are currently scouring the follow-up observations of past black hole
mergers for just such a signature, and will be keeping a close eye on future mergers.
With LIGO now detected a black hole merger event every week or so, there’s a good chance
this will be spotted — assuming accretion disk mergers are really happening, and that
all the assumptions of the model hold up.
Anyway, like I said: gravitational wave astronomy will reveal many cosmic mysteries and strange
phenomena. Now we have the amazing possibility of black holes merging and growing to enormous
size while trapped within the blazing vortices around even vaster supermassive black holes.
Awesome, sure, but what else would you expect from this supremely badass and frankly, totally
metal Space Time.
So we're a little behind in comment responses due to the holidays - in fact, due to this
one - coming to you from the hot Australian summer. We're going to catch up on comments
over the next couple of episodes, but today we're doing comments from our episode on cosmological
natural selection - Lee Smolin's idea that maybe new universes are born inside black holes.
A number of you made a similar observation/question: Shouldn't a universe that's born inside a black hole
be limited in mass by the amount of stuff that falls into that black hole? Well, the answer
is ... not necessarily. There's at least one mechanism in which you can make an arbitrarily gigantic
universe from very little matter and that is inflation, in which the rapid exponential expansion
of space can create a ridiculously large universe that multiplies the density of a high-energy
quantum field powering inflation. Check out our episodes on cosmic inflation to expand
on that answer.
A couple of you point out that the idea of black holes birthing universes still doesn't
explain where the first something-from-nothing actually came from. That's true, but I don't think
it' fair to cite this as a weakness of the idea. The hypothesis is trying to explain
the apparent fine tuning of the fundamental constants, not the origin of everything. Ideas
like cosmological natural selection and eternal inflation are helpful because they reduce the
amount of work required by that initial creationary event - it only needs to produce a spacetime
capable of exponential growth - after that the fundamental constants sort of take care of
themselves.
Xzayler asks what happens when a black hole is absorbed by another black hole? Do the
universes collide? Do the constants average out? The answer to this is ... no one has
any idea. The cosmological natural selection hypothesis doesn't really define the ongoing
connection between the initial black hole and the universe it spawns. That new universe
is birthed in the formation of the black hole singularity. If it grows by some inflation-like
expansion into an entirely new spacetime then it may not care about the later evolution
of its parent black hole - perhaps unaffected by whether it grows or merges or evaporates.
Really kids these days, no respect at all. On that note New Message feels a bit better learning that even whole
universes can be a disappointment to their parents.
Many people also commented that they'd thought of the whole black holes creating new universes
thing independently to Lee Smolin. I'm making a list, guys. If it turns out the whole cosmological natural selection
thing is true, I'll send that list to the Swedish Academy of Sciences. I can see it
now" "This year's nobel prize in physics is split between aclaimed theoretical physicist
Lee Smolin, xxfishytomatoxx, and 57 others from the internet who totally thought
of it years ago dude."