Black Holes from the Dawn of Time

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?