Black Holes: The Weirdest Things Around#

Imagine this: you might be inside a black hole, that’s inside another black hole, which is inside another black hole. It’s possible that everything in existence is just black holes going all the way down.

Turns out, black holes are way, way stranger than most people think. They don’t just bend the rules; they actually break the universe much more than usually explained. They can destroy time and space! And get this – they might even create infinite universes as they do it.

But before we dive into all that, let’s try building a black hole ourselves, just out of air.

How to Make a Black Hole (and Surprises About Density)#

Anything can become a black hole if you squeeze it down enough to a critical limit.

• To turn Earth into a black hole, you’d need to squeeze it down to the size of a coin.
• For the Sun, you’d need to squeeze it down to the size of a small city.

The usual way people explain black holes is that a lot of mass gets concentrated into a really tiny space, becoming super dense, and then it collapses into a black hole.

But here’s a surprising twist! You actually don’t need any ultra dense stuff to make them, especially the big ones. We’re skipping some complicated math here, but the main thing you need to know is this: The larger black holes get, the less dense they are.

So, really large black holes are actually kind of thin, density-wise. Let’s look at some examples:

• A sun-mass black hole is only about 6 km wide. Its density is roughly like one Himalayan range per cubic meter. That’s pretty dense!
• The supermassive black hole at the center of our Milky Way galaxy has a mass of 4 million suns. It’s about 24 million kilometers in diameter. But its density is much lower: only about 6 blue whales per cubic meter. Less dense than a mountain range!
• Now look at the ultramassive black hole IRAS 20100−4156. It has a mass of 3.8 billion suns and is as wide as a solar system. Because it’s so gigantic, its density is incredibly low – it’s only as dense as air!

This leads to a wild idea: at least theoretically, you could take a gigantic balloon and fill it with an unbelievable amount of air – undecillions of tons. The moment this balloon of air gets to the size of a solar system, something amazing happens: an event horizon suddenly forms, and it turns into a supermassive black hole. No squeezing or violence required!

A Universe-Sized Black Hole?#

Okay, let’s think even bigger. What if we tried to make a black hole the size of the entire universe?

The part of the universe we can see from Earth, the observable universe, is a giant sphere with a radius of 45 billion light-years. It’s filled with a staggering amount of stuff: hundreds of billions of galaxies, tons of gas, and lots of other things. If you add up all the mass, it comes out to about a million billion billion suns.

That sounds like a lot, right? But spread out over such a huge volume, the universe on average is not very dense. If we took all the galaxies, stars, gas, and energy and spread them out evenly inside the observable universe’s volume, the average density would be about 5 hydrogen atoms per cubic meter. You can think of this as a super thin “cosmic air” that makes up the universe.

So, what happens if we take a balloon as big as the observable universe and fill it with this “cosmic air”? Well, it turns out that all the mass in the observable universe is more than enough to create a black hole. In fact, it’s enough to make a black hole 10 times larger than the observable universe itself!

This seems to point to a crazy conclusion: we should be living deep inside a truly gigantic, cosmic-sized black hole!

But there’s a snag, a “catch”. We know that our universe is expanding. And an expanding universe is definitely 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 this simple way we just imagined it.

Except… the universe might have a wild, mind-bending trick up its sleeve. To figure out how, let’s do the only logical thing: jump into a black hole and die! (Just kidding, it’s a thought experiment!).

What Happens Inside a Black Hole#

We usually picture black holes as simple spheres with a singularity right at the center – a point where all the mass is so concentrated that our normal math just gives up. But honestly, that’s a bit of a lie. They are SO much weirder than that.

From the outside, a black hole looks like a normal, black sphere. But once you cross the point of no return, the inside is where things get really strange. Black holes warp the fabric of the universe so much that at the event horizon, space and time actually switch their roles.

Think about a normal sphere: space inside is limited (finite), but time keeps going forever. Inside a black hole, it’s flipped: space goes on forever (infinite), but time is limited (finite).

So, once you’re inside, you find yourself in what looks like an infinite universe with no center. The geometry is incredibly complicated, so we’re simplifying a lot, but basically, you could walk forever in one direction, or walk in another direction and incredibly, end up right back where you started.

But remember, time is finite inside. It’s running out! After a while, you’d start to notice that space itself is changing. In one direction, space is being stretched, while in all other directions, space is shrinking. It’s like the whole universe around you is being squeezed, kind of turning into a collapsing spaghetti.

Sooner or later, this entire black hole universe collapses into itself. Every single part of space is turning into a singularity. This means the singularity isn’t a place at the center, or in any specific direction. It’s an event in the future for anything that falls inside. (We even made a whole video about this if you want to learn more!).

The singularity isn’t somewhere you can travel to – it’s something that happens in time. Once it happens, you and everything else that fell into the black hole will be mercilessly crushed into an infinitely small region with infinite gravity and infinite energy. Time, space, none of it makes sense anymore; they stop existing in any way we’d recognize.

The Big Bounce: A Universe Reborn?#

Is that the end? Well, maybe not. This collapse of the black hole universe into a singularity looks eerily similar to one of the ideas for the end of our own universe: The Big Crunch. In this scenario, long after the Big Bang expansion, the whole universe eventually stops expanding and collapses back into a singularity.

But, if there’s a Big Crunch, there might also be a Big Bounce. Imagine a rubber ball you’ve squeezed too much; it doesn’t just stay squashed, it suddenly rebounds and expands again. Could space do the same? It’s possible that after collapsing, space might expand again, giving birth to a new universe!

And here’s the funny part about this scenario: nothing changes at all outside the black hole. To anyone watching from the outside, it’s still just a black sphere of nothingness. And yet, on the inside, a whole new universe has been born!

So, maybe our universe was born like this. Maybe we are all actually living inside a black hole.

Infinite Black Hole Universes and Cosmic Selection#

If our universe can create black holes that create new universes, which then create more black holes that create even more new universes… this process of cosmic self-reproduction would be subject to a kind of natural selection.

Think about it: a Big Bang is a chaotic and messy event. It’s likely that the new “daughter” universes born this way wouldn’t always be exactly identical to their “mums”. Maybe the physics would be slightly different, with some fundamental values being a bit higher or lower.

This could lead to variety:

• Some universes might have conditions perfect for creating loads of stars, planets, and black holes.
• Others might end up as a uniform cosmic soup where no stars, planets, or black holes can form.

If all new universes are born inside black holes, then over vast cosmic timescales, any universe line that doesn’t create tons of black holes would simply die out. They wouldn’t produce any “daughters” to continue their line. The universes with the specific physics and conditions needed to create loads of black holes would become the most common. They would spawn the most daughter universes.

It’s like survival of the fittest, but instead of organisms, it’s universes competing. Our observable universe alone has already created at least 10^17 black holes so far. That’s a lot.

So, maybe the reason our universe has the physics and laws it does is because it was born after a very long process of cosmological selection. This process would have favored universes that were exceptionally good at producing tons of black holes.

And this would have a rather lovely side effect. If universes are optimized to create as many new black hole universes as possible, they are also optimized to create loads of galaxies and stars. And that, almost by accident, creates the conditions for life to emerge.

So, the universes that are the best at creating new universes might also be the best at creating life. If this idea is true, who knows how many bazillions of black hole universes might be out there? Each with stars and planets, potentially home to others like us.

Is Our Universe Like This? And The Power of Curiosity#

So, is our universe actually like this? The honest truth is, we don’t know. These ideas are based on real science and work out on paper according to our current understanding, but they are still speculative and currently not testable.

Also, these ideas of cyclic universes or universes birthing other universes don’t really answer the biggest questions, like why the universe exists in the first place or why it is the way it is. Instead of giving final answers, these are really just new questions in disguise. So, keep that in mind before getting too carried away.

But isn’t it just absolutely wonderful and heartwarming that we’re living in a universe where ideas as huge and mind-bending as this are even thinkable? There might be so much life out there. And if new universes are constantly being created, maybe life itself will go on forever.

We’ve heard whispers of a chosen one. A special birb who possesses the power to shine a light into the vast darkness of the universe, uncovering the great mysteries of the world. We are all born with this power. But only a few are able to truly master it.

We use this very same power here at kurzgesagt to dig up the latest science and come up with the most exciting videos for you. This special power is called: curiosity.

And the special birb… could be you!

Our Guide to Curiosity is now available on our shop. It’s designed as an adventure that will help you to think like a kurzgesagt birb and unlock your full curiosity potential. Join us across 160 thrilling, interactive pages specifically created to awaken your curiosity and change your perspective on the world forever.

But be prepared! We will 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 around you.

But don’t worry, the birbs will be right there with you every step of the way. And hey, you can even bring a friend along!

These 18 curiosity quests were developed right here at the kurzgesagt headquarters. After hundreds and hundreds of hours of research and a whole lot of trial and error, we believe we’ve created the ultimate curiosity adventure. It was honestly the most fun we’ve ever had making a product – and we are just so incredibly excited to finally share this experience with you.

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