Join us on an interactive adventure across 160 thrilling pages that will change your perspective on the world forever.

Available now on the Kurzgesagt shop.

You might be inside a black hole that's inside a black hole that's inside a black hole.

Everything in existence could be black holes all the way down.

It turns out black holes are much weirder than you think and they break the universe much more than is usually explained, destroying time and space.

And they may actually create infinite universes in the process.

But before we can get to that, let's first build a black hole out of air.

Everything can become a black hole if you squeeze it to a critical limit.

You'd need to squeeze Earth down to the size of a coin for it to turn into a black hole.

The Sun needs to be squeezed down to the size of a small city to become a black hole.

And if a lot of mass is concentrated in a really tiny space, you get something super dense.

This is usually how black holes are explained.

Stuff becomes super dense and collapses into a black hole.

But actually, you don't need any ultra-dense stuff to make them.

We're ignoring some maths here, but all you really need to know is one thing.

The larger black holes get, the less dense they are.

So, really large black holes are kind of thin.

A Sun-mass black hole is only about 6 kilometers wide and has a density of about 1 Himalayan range per cubic meter.

The supermassive black hole at the center of the Milky Way has a massive 4 million Suns, a diameter of 24 million kilometers, and a density of 6 blue whales per cubic meter.

The ultramassive black hole Iris 2100-4156 has a mass of 3.8 billion Suns and is as wide as the solar system.

But because it's so large, it is only as dense as air.

This means, at least in theory, that if you take a gigantic balloon and fill it with under zillions of tons of air, the moment it gets to the size of a solar system, an event horizon suddenly forms and it turns into a supermassive black hole without violence or squeezing.

So now, let's think big.

What do we need to make a black hole the size of the Universe?

A Universe-sized black hole.

The chunk of the Universe that we can see from Earth is a sphere with a radius of 45 billion light-years, filled with hundreds of billions of galaxies, lots of gas, and a bunch of other things.

If you add them up, it has the mass of about a million billion billion Suns.

Which sounds a lot, but on average, the Universe is not very dense.

If we break up all the galaxies, stars, gas, and energy, and spread them equally inside the volume of the Universe, we get an average density of about 5 hydrogen atoms per cubic meter.

You can imagine this as the sort of ultra-thin cosmic air that makes up the Universe.

What would happen if we take a balloon as big as the observable Universe and fill it with cosmic air?

Well, it turns out that all the mass in the observable Universe is more than enough to create a black hole.

Actually, it's enough to make a black hole ten times larger than the observable Universe.

But that can only mean one thing.

We should be living deep inside a truly gargantuan cosmic-sized black hole.

There's one catch, though.

We know that our Universe is expanding, and an expanding Universe is not what you'd expect to see if you were inside a black hole.

So our Universe can't be a black hole, at least not in the naive way we've just described.

Except there's a wild and mind-bending trick the Universe could play on us.

To find out how, let's jump into a black hole and die.

A whole Universe born inside a black hole.

We usually imagine black holes as spheres with a singularity at their center, a point where all their mass is concentrated so much that our maths breaks down.

But this is a lie. They are so much weirder.

From the outside, a black hole looks like a normal black sphere, but the inside is where things stop making sense.

Black holes warp the Universe so much that at the event horizon, space and time switch their roles.

Inside a normal sphere, space is finite, but time goes on forever.

But inside a black hole, it's the other way around.

Space goes on forever, but time is finite.

So once inside, you see an infinite Universe with no center.

The geometry is too complicated, so we're simplifying.

But basically, you could walk forever in one direction, or walk in another direction and arrive at the same place again.

But not only that, inside a black hole, time is finite, and it's now running out.

So after a while, you start to notice that space itself is slowly changing.

In one direction, space is being stretched, while in all other directions, space is shrinking.

The whole Universe is being squeezed, kind of turning into a collapsing spaghetti.

Sooner or later, the whole black hole Universe collapses into itself.

All of space, every single part of it, is turning into a singularity.

So the singularity of a black hole is not at its center or in any direction at all.

It's in the future of whatever falls inside.

We made a whole video about this if you want to learn more.

So the singularity is not a place where you can go, it's an event in time that happens.

Once it happens, you and everything else that fell inside the black hole will be mercilessly crushed into an infinitely small region with infinite gravity and infinite energy.

Time, space, none of it matters anymore.

Both kind of stop existing in ways that we would recognize.

And then, is this the end?

Well, maybe not.

This collapse of the black hole Universe into a singularity looks like one of the scenarios for the end of our Universe, the Big Crunch, where long after the Big Bang, the whole Universe collapses into a singularity again.

But if there's a Big Crunch, there might be a Big Bounce, like a rubber ball that you've squeezed too much and that suddenly rebounds, space might expand again.

So a new Universe could be born inside a black hole.

The funny thing about this scenario is that nothing has changed in the slightest outside the black hole.

Watching from the outside, it's still a black sphere of nothingness.

And yet, on the inside, a new Universe has been born.

So maybe our Universe was born like this, and we are all actually inside the black hole.

But if our Universe can also create black holes, they might give birth to new Universes.

Is our black hole Universe also just part of a Universe further up that's also a black hole inside another Universe?

Is there an end to it?

Is there one original Universe?

Is the cosmos black holes inside black holes inside black holes?

Infinite Black Hole Universes

If the Universe creates black holes that create Universes, that then create new black holes that create new Universes, this cosmic self-reproduction would be subject to natural selection.

A Big Bang is a chaotic and messy event, so it's possible that the new daughter Universes would not always be fully identical to their mums.

Sometimes physics may be slightly different, with some fundamental values higher or lower.

So some Universes might be able to create loads of stars, planets and black holes.

Others might not, maybe creating a uniform cosmic soup where no stars, planets and black holes form.

But if all Universes are born inside black holes, in the long run all Universe lines that don't create loads of black holes would die out.

The Universes with the conditions for loads of black holes would become the most common and spawn the most daughter Universes.

But with Universes instead of organisms.

Our observable Universe alone has created at least 10 to the power of 17 black holes so far.

So maybe our Universe has the physics and laws it has because it was born after a long process of cosmological selection that favoured the production of tons of black holes.

And that would have a lovely side effect.

If Universes are optimised to create as many new black hole Universes as possible, they're optimised to create loads of galaxies and stars.

And thereby also, by accident, the conditions for life to emerge.

So Universes that are the best at creating new Universes are also the best at creating life.

If this scenario is true, who knows how many bazillions of black hole Universes might be out there.

All with stars and planets potentially home to others like us.

So, is our Universe like this?

The truth is we don't know.

While these ideas are based on real science and work on paper, they're speculative and not testable.

Also, cyclic Universes don't actually explain why the Universe exists in the first place, or why it is the way it is.

Instead of providing answers, these are really just new questions in disguise.

So keep that in mind before getting too excited.

But isn't it just wonderful and heartwarming that we're living in a Universe where ideas as big as this one are even thinkable?

There might be so much life.

And if new Universes are created constantly, maybe life will go on forever.

We've heard rumours of a chosen one.

A special verb who has the power to illuminate the vast darkness of the Universe, uncovering the great mysteries of the world.

Curiosity.

And the special verb could be you.

Our Guide to Curiosity is now available on our shop.

It's an adventure that will help you to think like a Kurzgesagt verb and unlock your curiosity potential.

Join us across 160 thrilling, interactive pages designed to awaken your curiosity, changing your perspective on the world forever.

But be prepared.

We'll send you on epic quests to find incredible creatures, challenge you to build unprecedented structures, ask you to cook up something surprising, and solve riddles to unveil hidden truths about the world.

But don't worry.

The verbs will be with you every step of the way.

And you can also bring a friend.

The 18 Curiosity Quests were developed right here at the Kurzgesagt headquarters.

After hundreds and hundreds of hours of research and lots of trial and error, we created the ultimate curiosity adventure.

It was the most fun we ever had making a product and we're so excited to finally share this experience with you.

So head over to the Kurzgesagt shop and find out if you are the chosen verb.

Every Kurzgesagt product you buy directly funds another moment we get to spend working on our videos.

And you get a special piece of Kurzgesagt in return.

Thanks for being a part of our story and making this channel possible.