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."
