MATT O'DOWD: This episode is supported by the Great Courses
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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.
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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?
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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?
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