MATT O'DOWD: This episode is
supported by the Great Courses Plus. In the very first instant
after the Big Bang, the density of
matter was so great everywhere that vast numbers
of black holes may have formed. These primordial black
holes may still be with us. [MUSIC PLAYING] There's no longer any question
that black holes exist. LIGO's recent observation
of gravitational waves from merging black holes
is a stunning confirmation of this fact. Of course, we already
thought they must exist. As long as a volume of space
contains a high enough density of mass or energy,
general relativity tells us that a
black hole will form. In the modern universe,
there is only one natural way to get such insane densities. That's in the core of the most
massive stars when they die. The process is awesome, and we
look at it in a previous video. But that's the modern universe. Once upon a time,
the entire universe had the density of
a stellar corpse. In fact, soon
after the Big Bang, the density of the
universe was vastly higher. So why didn't all the
matter in the universe become black holes then? Well, actually,
some of it may have formed what we call
primordial black holes, and they may still
be around today. Let's back up a bit. In order to make a black
hole, extremely high density isn't enough. You need a density differential. Otherwise, there's no
preferred direction for all that
gravitational attraction. Also, the gravitational
pull needs to be strong enough to overcome
the expansion of the universe. Now, matter in
the early universe was pretty smoothly spread
out, and the universe was expanding fast. That means most of it avoided
collapsing into black holes. And that's a very good
thing, by the way. However, it wasn't
perfectly smooth. There were lumps. The oldest light we can see is
the cosmic microwave background radiation. It reveals tiny differences
in the density of matter from one point in
space to the next. The universe was
very slightly lumpy at the moment the
CMB was created, about 400,000 years
after the Big Bang. These density fluctuations
were enough to kick-start the formation of
galaxies, but certainly not enough to immediately
collapse into black holes. Yet if we rewind time,
those fluctuations must have been much stronger. It's thought that these
fluctuations originally formed when the entire
observable universe was smaller than a single atom. Back then, quantum fluctuations
caused a sort of static fuzz across the minuscule cosmos. There are several different
stories for the initial size and growth of
these fluctuations, and cosmic inflation
certainly plays a role. But it's well within the
possibility of many models that some of these fluctuations
were, at some point in the early expansion,
intense enough to resist the local
expansion of the universe and form a black hole. Some highly speculative
Big Bang physics also predicts
primordial black holes. For example, the collapse
of cosmic string moves and the collision
of bubble universes? Awesome. Now, these models can predict
a huge range of possible masses for primordial
black holes-- PBHs, as we like to call
them in the biz. PBHs could have been
formed at a few grams to tens of thousands of
times the mass of the sun, depending on which
formation model you go with. Or they might not exist at all. That's a big possibility. If they do exist, then there's
probably a particular mass range that most
of them formed at. Discovering PBHs and
learning their masses would tell us a huge
amount about the earliest moments of our universe. We need to hunt for
these black holes or their influence in
the modern universe. First of all, we aren't going
to find primordial black holes less than around a
billion tons, or the mass of a small asteroid. They would have all evaporated
away due to Hawking radiation. I'll get back to that. Black holes larger than
this should still be around, but they'd be very difficult to
spot, being so black and all. If PBHs are rare, then it
may be impossible to confirm or disprove their
existence entirely. However, there is
a question that we can answer with some certainty. Could primordial black
holes be dark matter? This is a slightly
terrifying possibility that 80% of the
mass in the universe is in the form of countless,
swarming black holes. That's a lot of
primordial black holes, and so we expect them to leave
their mark on the universe in different ways. For one thing, if these little
knots of warped space time are everywhere, then
they should produce obvious gravitational lensing. We'd expect them
to frequently pass in front of other space stuff. Depending on PBH mass, this
would cause a twinkling effect-- microlensing. In stars in our galaxy,
in distant quasars, even in gamma ray bursts. Well we just don't see enough of
this twinkling, which rules out a lot of possible masses. There's also the fact
that swarms of black holes would mess up
their surroundings. As the heavier ones
buzz around the galaxy, they should pull apart
loosely bound binary systems and have an effect on the
structure of star clusters. The smallest should
fall into neutron stars, causing them to either
explode or become black holes themselves. But we see loosely
bound binaries, and normal star clusters,
and plenty of neutron stars. These arguments
let us rule out all but a very narrow
set of mass ranges for primordial black holes as
an explanation for dark matter. The options we're
left with are either lots of PBHs with masses similar
to a large asteroid like Ceres, so around 10 to the
power of 21 kilograms, or a much smaller number
of really big PBHs around 20 to 100
times the Sun's mass. Now, this last
possibility is sketchy. Some scientists think
that the voracious feeding of lots of really big
primordial black holes would have left their mark
on the cosmic microwave background. However, others argue that
the recent LIGO detection of the merging of two
approximately 30-solar-mass black holes is evidence
in favor of this idea. With new observations from both
regular telescopes and LIGO, we're rapidly closing all
of these mass windows. Before too long, we'll
either spot the signature of primordial black
holes at these masses, or discover that PBHs
are actually very rare, and that they're
certainly not dark matter. This latter is more
likely, but we'll see. Of course, primordial
black holes that have already evaporated
due to Hawking radiation definitely are not dark
matter, and that rules out any PBHs lighter than
about a billion tons. But that last stage of Hawking
evaporation is very fast. In fact, it's explosive. It's possible that certain types
of very short gamma ray bursts are these final flashes from
PBHs evaporating in our galaxy. Some highly speculative stuff,
but also some highly awesome possibilities. It wouldn't be right
to end a discussion on primordial black
holes without talking about what would happen if
one passed through the Solar System. Even a close encounter
with a black hole as massive as the Sun or higher
would be pretty catastrophic. If it passed anywhere
near the planetary system, the gravitational tug would
disrupt the planet's orbits. Even if it passed by the
outskirts of the Solar System, it could shake up the Oort cloud
and send a nice rain of comets to pepper the
inner Solar System. Of course, regular black
holes from supernovae can, and perhaps
have, done that. Having high mass
primordial black holes just makes it more likely. If PBHs are closer to the
mass of a large asteroid, then they're too low in mass
and probably moving too fast to do any gravitational damage. They'd just zip right through
the solar system unnoticed. It's a different matter
if one hit the Earth. Traveling at a couple hundred
kilometers per second, it'd punch straight
through the planet, but certainly leave
a narrow column a vaporized rock behind it. These sorts of hits
would be incredibly rare and may never happen. However, if
primordial black holes have approximately the minimum
possible mass to not have evaporated-- around
a billion tons-- these would be
much more abundant than asteroid mass PBHs. In fact, they may pass
through the planet frequently. A billion-ton black hole
has an event horizon around the size
of a proton, so it would pass through the planet
as though the Earth were made of air. However, through a
deposit something like a billion joules of Hawking
radiation on its way through. This should leave
detectable traces in crystalline material
in Earth's crust. In fact, perhaps
geologists will be the first to discover the
primordial black holes. If they're out there,
someone will figure it out. I mean, how long
can the universe expect to hide vast
numbers of holes punched in the fabric of spacetime? Thanks to The Great Courses Plus
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thegreatcourses.com/spacetime. We recently talked
about what life might look like in the ocean
of Jupiter's moon Europa. You guys had a lot to say. Parameth asks,
"How can we prevent cross contamination between
Earth organisms and Europa ones?" Well, this will take
some serious care. Any instrument
searching for life will have to be thoroughly
sterilized and protected from contamination before
reaching its destination. But perhaps we'll
find life that's different enough to
Earth life that there's no possibility that it was
from some contamination. Turcan Fred asks about what
sort of insane pressures you'd expect to find
at 100 kilometers deep in Europa's ocean. Well, Europa's surface gravity
is only about 13.5% that of Earth's. So even at 100 kilometers depth,
the pressure is only about 20% higher than at the deepest
point of the Earth's ocean. That's in the Mariana Trench,
and there's plenty of life down there, as James
Cameron was kind enough to go down and find out for us. A few of you wonder
why we're so fixated on water as a basis for life. For example, Saturn's moon Titan
has lakes of liquid methane, and perhaps life could
have formed in these. However, water has
some properties that might make it
uniquely awesome for life. Besides being an
excellent solvent, which is critical for moving
molecules around to interact with
each other, water is also a dipole, meaning it
has more positive charge on one side and more
negative on the other. This leads to all sorts
of useful behavior, like surface tension, capillary
action, and hydrogen bonding. All of these are important
to life as we know it. In addition, water has an
extremely high specific heat, meaning it takes a lot of energy
to change its temperature. This is important for the
stability of water-based life. Liquid methane is just not as
good on any of these points. Many Of you criticized my
pluralization of "octopus." I said "octopi." Some of you pointed out that
it should be "octopuses." First, English is
a fluid language, a mishmash of more languages
than any other modern tongue. And octopi is a
sufficiently common usage that's in essentially
all major dictionaries. So I used the Latin plural of
an etymologically Greek word. When the hordes of octopodes
rise from sunken R'lyeh to harken his coming, these
silly grammatical quibbles will seem kind of
frivolous, don't you think? [MUSIC PLAYING]
I didn't really understand the scale of black holes until he said a billion ton black hole would be the size of a proton.
God I hate the comments on these videos. I miss the days when the comments were mostly people asking relevant questions or at least trying to talk about it.
It makes me kinda sad that about half the top comments are all "but the universe is only 2000 years old" troll baiting. Or just low effort jokes. "If you look carefully into a Black Hole, what you'll see on the other side is Adรจle saying hello.๏ปฟ"
Dozens of comments saying "whoa this blew my mind" or "I'm so smart I understood all of it" "I'm high and this is cool". It's just all worthless stuff.
:(
I know it's inevitable on youtube, but damn. It disappoints me. At least they somehow manage to find worthwhile comments to answer at the end still.