How the Universe Works

Cosmologists are battling over the universe's greatest enigma, black holes.

We've never seen them.

It's near impossible to study them.

And their existence challenges everything we think we know about space.

The black hole represents the absolute limits of what we understand about nature.

The truth is we have almost no idea what these things are and how they work.

Black holes are at the very heart of cosmology.

Yet some scientists question if they're even real.

Black holes present lots of paradoxes.

And the simplest way to resolve the paradoxes would be if black holes didn't exist at all.

Solving the mysteries of black holes will push our understanding of physics to the edge of reality.

But it's the only way to discover if black holes really exist.

Black holes are the monsters of the Universe.

Terrifying cosmic giants that devour all they encounter.

But black holes petrify scientists for very different reasons.

They challenge our theories to the breaking point.

This is at the forefront of theoretical physics.

When it comes to the detailed nature of black holes, it would not surprise me if we got it all wrong.

The science of black holes is so challenging that some scientists question whether they exist at all.

Yet, despite their fearsome reputation, we've never actually seen one.

Black holes are everywhere.

They're all over the Universe.

They're all throughout our galaxy.

But that doesn't mean that they're easy to find.

They're black.

And space is black.

And black on black is kind of hard to see in a picture of space.

This is paradoxical.

Because scientists believe black holes are born in the aftermath of the brightest explosions in the Universe.

Rising from the corpses of detonated stars, many times larger than our sun.

Stars exist because of an epic battle.

It's a battle between the heat they generate in their cores, trying to expand

them, and their own mass causing gravity, trying to contract them.

But over time, it winds up losing that battle.

Stars are powered by nuclear fusion in their cores.

But their fuel source doesn't last forever.

Eventually, when a star's power output drops, it can no longer produce enough

energy to overcome the force of its own gravity.

The result?

The core collapses, flashes out a tremendous amount of energy in a last

gasp, blows out the outer layers, and a supernova explosion.

And if the star has enough mass to begin with, what's left in the end is a black hole.

A star that burns for 10 million years collapses to form a black hole in a period of seconds.

As it collapses, the outer region of the star hits the core, triggering a huge explosion.

A supernova.

We see the bang, but not the remnant.

A dead core with the enormous mass of the star crushed down into an infinitesimal tiny area.

From this minuscule, high-mass core, a black hole is born.

The flow of gravity is so strong that nothing can escape, not even light.

But how can scientists claim that black holes exist?

If we can't even see them?

You could say that about the existence of the atom.

We knew they had existed for decades, centuries, before we had actually seen one in some sort of imaging device.

And so it's the same sort of thing with black holes.

Just because you can't see it doesn't mean it's not there.

Not seeing black holes but knowing they're there is a possibility.

Just like we know that wind is there even though we can't see air.

Air is invisible.

Yet when the wind blows, its effects can be measured.

It's the same with black holes.

You just need to know what to look for.

While they emit no light themselves, black holes are tremendous sources of x-rays.

And that's because as things get close to a black hole, they're accelerated by the gravity.

And they can heat up to millions of degrees.

Million degree gas gives you lots of x-rays.

To find and measure these telltale x-rays, scientists turned to the New Star Space Telescope.

In 2017, it spotted a burst of x-rays in a cluster called 47 Tucana, at the edge of the Milky Way.

When scientists analyzed the data, they realized they were looking at two objects orbiting each other, very closely.

All we see is that there's a star being ripped apart, and gas is spiraling down to a very dense, very dark object.

So something weird is going on.

As one of the objects accretes matter off the other, it causes it to emit x-rays.

And those x-rays can be used then to trace out the orbits and therefore extract the mass.

When scientists calculated the size and mass of the two objects, they found that

the first is the fading corpse of a sun-like star, and that the second object is tiny.

Yet this tiny object has the mass of a giant.

Could this be an elusive black hole?

What we're talking about here is an object that is very massive, very small,

very dense, with intense gravity.

But it turns out there are lots of different ways to create an object like that.

There is another type of ultra-dense object out there in the Universe called a neutron star.

Neutron stars form in the same way we think black holes form, when stars die and explode.

But then, neutron stars collapse down into a tiny ball of matter.

The gravitational attraction of a neutron star is enormous, pulling in gas,

dust, and asteroids.

But light can still escape.

Black holes and neutron stars are kind of cousins.

But in the case of a neutron star, it didn't have quite enough mass to collapse out of control.

So you can sort of think of it as just barely hanging back from collapsing into a black hole.

It's easy to see the difference between a neutron star and a black hole,

because a neutron star has a surface you can see light coming off of, and if

something falls at it, it crashes into the surface, whereas black holes have no surface.

The surface of a neutron star is a place you don't want to be.

The enormous mass of the tiny body means its gravity.

Its gravity is huge, and it would crush a human in an instant.

But the mass of a neutron star is still smaller than that of a black hole.

There's an upper limit to how massive they can be, just under three times the mass of the sun.

They simply can't get any bigger, because then they would collapse to form a black hole.

So if we see an object out there that's not emitting light, and has more than

three times the mass of the sun, it can't be a neutron star.

The tiny object discovered by NuSTAR does have enormous mass.

But size and mass alone are not enough to prove it's a black hole.

Cosmologists need more evidence.

They can't see black holes, but is there another way to find them?

What if they could hear them?

Almost all astronomy is based on black holes.

On the electromagnetic spectrum.

Looking at visible light, X-ray light, radio light.

Gravitational waves are something else entirely.

And they're generated from some of the most exotic processes in our Universe.

So they give us a direct look at these processes that we simply can't see.

Literally can't see it, but we can hear that through gravitational waves.

Gravitational waves give us this wonderful window into the universe.

Think of two massive cars colliding.

Boom, when they do, they radiate sound.

And then we can tell whether or not that collision occurred and maybe even how far away it was.

It's like that when black holes collide.

So by listening for a black hole collision, could scientists conclusively prove they exist?

Black holes are gravitational giants of the Universe.

But we've only found circumstantial evidence that they exist.

For definitive confirmation, cosmologists are listening for proof in the hidden world of gravitational waves.

There are gravitational waves going through this room all the time.

Every time I move my hands like I just did, I create gravitational waves.

The problem is, gravity is so weak that you don't detect those gravitational waves.

In order to detect those disturbances in space and time, you have to have

cataclysmic events involving massive objects.

Black holes are some of the densest objects in the Universe, so we should be

able to hear and measure the waves created when they collide.

LIGO, THE LASER INTERFEROMETRY GRAVITATIONAL WAVE OBSERVATORY,

LISTENS FOR WAVES THAT CAN COME FROM OVER A BILLION LIGHT YEARS AWAY.

IN 2017, LIGO HEARD AN ENORMOUS CRASH.

Two very massive objects collided at near the speed of light in one of the most

energetic events that we have ever witnessed in the history of humankind.

Two ultra heavy, ultra dense objects whirled around each other, hurling powerful gravitational waves through space.

The closer they fell towards each other, the more gravitational energy they threw out.

Finally, they collided in one of the most violent events in the Universe.

The smash sent out immense gravitational waves that rippled across intergalactic space.

Until, eventually, LIGO detected them.

Listening to a gravitational wave is like listening to a musical instrument.

If it's making certain tones or certain vibrations, you can figure out the size of

the musical instrument, the type of the musical instrument, who's playing the musical instrument.

The thing that's really amazing about the LIGO detection is it allowed us to measure

the mass of these objects and how quickly they coalesce together.

So we actually have an idea how dense they must have been.

And with modern physics, we say, well, it has to be a black hole.

But the question is, have we missed something?

The information gathered by LIGO is groundbreaking.

But some scientists think that the gravitational waves could have come not

from black holes, but from something even more mysterious.

It's possible that what we identify as black holes in our universe are really another object like Gravastar.

Possible.

There's a capital P on that, possible.

A Gravastar is what scientists call an exotic compact object.

This bizarre theoretical body has exactly the same mass and gravitational pull as a

black hole, but it's made of exotic matter.

A gravistar would be impossible to see with the naked eye, and because it forms

differently to a black hole, it has a strange, incredibly dense surface.

In the formation of what we think of as a black hole, the catastrophic gravitational

collapse of a dense object, maybe it doesn't go all the way down to become an infinitely dense point.

Instead, maybe there's some interaction that prevents the formation of the black hole.

And instead, you have a tight little dense ball, which is what we call a gravistar.

So the LIGO data could be the signature of two black holes colliding, but it also

could be the signature of two gravistars colliding.

Right now, we can't tell them apart.

So for now, gravitational waves have led to a dead end in the hunt for black holes.

We can't be completely sure that we're hearing them, and we already know we can't see them.

But what about the mayhem they leave behind?

Even though you can't see the black hole itself, it's going to leave behind a trail of destruction.

And that is something you can see.

So, could analyzing such devastation lead to solving the mystery of black holes?

This is.

The Hydra A galaxy cluster, 840 million light years from Earth, it's a region of

space filled with galaxies and dense intergalactic gas, but a dark and

massively destructive force is at work here, one that's blasting holes in the gas that are bigger than the Milky Way.

This is the Hydra A galaxy cluster, 840 million light years from Earth, it's a region of space in the Milky Way.

Could a black hole be responsible?

It's almost as if an intergalactic bomb has exploded to blow these cavities out.

Some of these cavities are tens or even hundreds of thousands of light years across.

And to create a cavity that large requires an immense source of energy and a powerful engine that's driving it.

To discover what's creating the cavity.

Scientists combined images taken at different wavelengths, they revealed something remarkable.

The cavities are being carved out by enormous jets emanating from a galaxy in the center of the cluster.