[MUSIC PLAYING] NARRATOR: Thanks to
CuriosityStream for supporting PBS Digital Studios. It's been conjectured that
the center of the Milky Way contains not one, but a
vast swarm of black holes, and now, we've
actually seen them. [MUSIC PLAYING] The core of our galaxy
is a wild place. The stars are so densely
packed that the night sky would be 500 times
brighter than our own. A supermassive black hole-- four million times
the mass of our sun-- lurks in the center. It flings nearby stars into
extreme slingshot orbits. It consumes anything that
gets too close, burping out blasts of x-rays. We know these things
because we see them, from our comfortable vantage
28,000 light years out in the galactic disk. But there's one particularly
terrible feature of the core that has only
been hypothesized until now. The central few light
years of the Milky Way is thought to contain
a vast swarm of smaller black holes that have reigned
in from the surrounding galaxy. In this episode of
Space Time Journal Club, we're going to delve into the
recent nature paper, Hailey, et al 2018, titled, "A
Density Cusp of Quiesce X-ray Binaries in the Central
Parsec of the Galaxy." In it, these astrophysicists
find powerful evidence that our own Milky Way core
is packed with hundreds, maybe thousands, of black holes. I'll get to how they found
these black holes in a minute, but first, I want
to ask, why did so many astrophysicists
already believe there must be a swarm of black
holes in the galactic core? Well, the simple answer is
straightforward-- dense things sink. Colder and hence
denser water or air, sinks to the bottom of the
ocean or the atmosphere. Dense elements, like
iron, sink to the centers of forming planetary bodies in a
process called differentiation. And the densest, stellar
objects, like black holes, sink to the centers of
galaxies or star clusters. We think black
holes must gradually sink to the center
of the Milky Way, although, the exact process
is a wee bit more complicated. Let me explain. Black holes form when the most
massive stars end their lives in spectacular
supernova explosions. After blowing off
their outer layers, if the remaining stellar
core is massive enough, it'll collapse
into a black hole. We've discussed this whole
process in an earlier episode. We expect the so-called
stellar-mass black holes to weigh in at between
five and 15 solid amasses, although, the recent
gravitational wave signals detected
by LIGO, suggest they may be even more massive. Even after blowing off most
of their mass in a supernova, these black holes are still
heavier than most stars. This means they migrate towards
the center of the Milky Way, in a process called
dynamical friction. It works like this. As a black hole
orbits the galaxy, it tugs on its
neighboring stars. Those stars are accelerated
towards the black hole and can gather behind it
in a gravitational wake. That over-density
behind the black hole, pulls the black hole
backwards reducing its speed. The black hole can also
slingshot stars outwards, losing momentum in
that process, too. The key is that the more
massive object-- usually, the black hole-- tends to donate its momentum
to the less massive object. The ultimate result
is that the black hole slows down and no longer
has the velocity it needs to maintain its circular orbit. Gradually, it falls towards
the galactic center. Now, this process takes
a really long time for a stellar-mass black hole. Over a few billion years, we
only expect the black holes from the central
several light years to have made much
progress inwards. However, there's
another process that can really drive a huge
number of black holes inwards. Our galaxy is surrounded
by these things called globular clusters. They're like ancient,
extremely dense mini-galaxies, containing millions of stars. Some nearly as old as
the universe itself. They exist in a swarm
surrounding the Milky Way, but sometimes, they're
captured by the Milky Way and dragged to its center
by this dynamical friction process. Because globular clusters
are much more massive than a single black hole,
they reach the galactic center a lot will quickly. Over the life of
the Milky Way, they have piled up in
the galactic core, forming a giant
nucleus star cluster. Those globular
clusters must have been full of ancient
black holes, which would be carried to the core
with their parent cluster. Those black holes would
then, sink even further to the center of the galaxy. Prior to this new result,
it had been calculated that this process should
lead to tens of thousands of black holes in
the central few light years of the Milky Way's core. So how did Hailey and team
spot these black holes? They're supposed
to be black, no? Well, that's true. Black holes are
effectively invisible, but things can be different if a
black hole and a companion star are in a binary orbit
around each other. If the companion
star gets too close, its outer regions can fall into
the gravitational influence of the black hole. Gas is siphoned off the star
into a whirlpool, an accretion disk around the black hole. That gas heats up to
crazy temperatures. To us, it looks like a
range of heat glows-- thermal radiation at
different temperatures, with the hottest glowing with
extremely energetic X-rays. These X-ray binaries are
seen throughout the galaxy. By the way, X-ray
binaries can also result from a neutron star
rather than a black hole cannibalizing its companion. But today, we're
interested in black holes. The brightest X-ray binaries
are aggressively gobbling up their companion stars, but
that ravenous phase probably doesn't last all that long. X-ray binaries likely spend most
of the time in a quieter phase, with the gas just trickling
slowly from the companion star. These quiescent
X-ray binaries should be seeing much more frequently
than the active ones. Frequently enough that
if the galactic core is full of black holes,
then it should also contain quiescent
X-ray binaries. Hailey and team used the
orbiting Chandra X-ray Observatory to hunt for
these, and surprise, surprise, they found them. They spotted 92 point-like
X-ray sources within one parsec, or around three light years,
of the galactic center. These were potential
X-ray binaries, but there are other
astrophysical critters that also shine bright in X-rays. One we expect to be common
in the galactic core are magnetic cataclysmic
variables, also called polars. Polars are a bit
like X-ray binaries, except instead of a black
hole or a neutron star, you have a white dwarf with
a powerful magnetic field. Those magnetic fields
act like a dam, allowing gas from the companion
star to build up and then, fall very suddenly
onto the white dwarf, producing a burst of X-rays. But these polars produce
a very different spectrum to X-ray binaries. Polars only glow at a single
extremely high temperature, while X-ray binaries glow at
both high and low energies, due to the large temperature
range of the accretion disk. That allow the researchers to
weed out the X-ray sources that had the wrong spectra. After weeding out polars and
other uninteresting sources, there remained 13 probable
quiescent X-ray binaries, which appeared to be the
type powered by black holes. Now, 13 doesn't sound like
a swarm, but remember, only a small fraction
of black holes are seen as X-ray binaries. The researchers
extrapolate that there would need to be
at least hundreds of stellar-mass black holes
in the central few light years in order to get these
13 X-ray binaries. Now, that's tens of thousands
of times the black hole density anywhere else in
the galaxy so yeah, it's a swarm of black holes. If the sun was near
the galactic core, the nearest black hole would be
inside the solar systems Oort cloud. Besides being very cool
and kind of freaky, this result is especially
important for the new field of gravitational wave astronomy. Now, we keep seeing these
gravitational wave signals from black hole merges, and
as I've discussed previously, they're kind of confusing. If black holes are
so densely packed in the centers of
galaxies, then we should probably know
that, if we want to understand the source of
these gravitational waves. So, next time you see the
Milky Way in the night sky, find the bright patch, just to
the edge of the constellation of Sagittarius. Consider what lies
beyond that dusty veil. Not just one
gigantic black hole, but also, a swarm of
hundreds, maybe thousands, of smaller black
holes, in what has to be the craziest
and most terrifying environment in nearby spacetime. Thank you to CuriosityStream for
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and use the promo code, space time, during
the sign-up process. Last week, we talked about
some of the incredible ways for detecting gravitational
waves beyond LIGO. You guys had a lot to say. Majestic potato asked,
whether a supernova can produce gravitational
waves detectable from Earth? Actually, yes, and LIGO itself
may be able to see them. The trick is that supernova
can't be spherically symmetric. Gravitational waves are produced
when the quadrupole moment of a mass distribution changes. In non-techno speak,
they're created in non-spherically or circularly
symmetric movements of mass. So if the explosion
of a supernova is concentrated, say,
more on one side, then LIGO could potentially
see the resulting gravitational waves. Juxtaposed stars asks
whether, theoretically, you could build an engine
to extract power from gravitational waves
via the sticky bead method? Sure, for the right
definition of theoretically. That is, the laws
of physics allow it, the laws of engineering
may beg to differ. You need a phenomenal
amount of matter spread over a vast region. There may be more efficient ways
to gather energy at that scale, like a good old
fashioned Dyson sphere. A couple of you asked whether
the gravitational waves interfere with each other? Or could even be used in
a two-slit experiment? Well, the answer to the
first is definitely, yes. Two gravitational
waves crossing paths will add together at any
one point in space and time. This is either constructively
producing a stronger stretching or contraction of
space or destructively, meaning their
effect cancels out. And the two-slit experiment-- well, in principle, yes. You'd need a material capable
of blocking gravitational waves. We now know that they can
lose energy to matter, but you would need
a lot of matter. I don't know, maybe a
cosmic scale wall of neutron stars with two gaps in it? And what would you see? Well, to answer that, I'd need
a theory of quantum gravity so let me get back to you. The rogue wolf notes, that
stellar gravitational wave detectors, like
pulsar timing arrays, are a bit like using the
rustling of leaves and grass to see the wind. I don't have anything
to add to that. Sir, you are a poet.
Man that guy is hard to watch.