Have you ever looked up at the sky and wondered: Where does it all end? Does the universe have an edge? If yes, what lies beyond? And if there is something, then isn’t it wrong to call it the “edge” of the universe since there is something beyond it? As much as we would want to have it answered, there is no answer to this question except maybe in mathematical terms; I will warn you that it is not an easy job to understand it. And when you do, you will definitely be more confused than you are right now but I bet you will understand the question in hand much better.
Imagine you’re standing on a hill; now toss a piece of rock in the air and watch its trajectory. It should look something like this:
Nothing unexpected happens, you know the gravity of the Earth will pull it back to the ground and that is what you will see.
Now throw it a little harder (much harder). It will follow a similar path but it will fall farther away. It is easy to conclude that the harder you throw something, the farther away it will fall.
Remember the fact that the Earth is not flat? It is actually round, so if you throw this piece of rock farther enough, the Earth will start to curve below it. In fact, if you throw it hard enough, the Earth will just curve below it and it will not find a place to land, like in the image below. You have just put that piece of rock in orbit around the Earth. This is how we put satellites in the sky.
Now take another rock and throw it towards the sky really hard; it will never come down and will escape the Earth’s gravitational field entirely, like in the image shown below. This speed at which the piece of rock will escape the Earth’s orbit is the Earth’s escape velocity. In other words, if you can’t travel at or more than this speed, then your fate is sealed and you can never leave Earth. The escape velocity for Earth is 11,200 meters per second.
Everything that has mass has an escape velocity.
A blackhole is an object whose escape velocity is larger than the speed of light. So even light can’t escape it. Hence the name — blackholes.
Something that is lying on Earth’s surface is attracted towards the center of the Earth and the magnitude of this attraction depends on the Earth’s radius. In other words, if the Earth was more dense, i.e. if it had the same mass but a smaller size, it would pull on you harder.
If you continue making the Earth smaller and smaller, keeping the mass unchanged, at one point the gravitational pull of the Earth will resemble a blackhole. Or, its escape velocity will exceed the speed of light.
Karl Schwarzchild was a German physicist who hypothesized the existence of blackholes after analyzing Einstein’s theory of relativity.
Anything with mass has a Schwarzchild radius, i.e. if we shrink a massive object it will eventually become a blackhole.
The Schwarzchild radius of Earth is about 8.7 mm. This means that if you shrink Earth down to this size, it would become a blackhole.
Since everything that has mass has a Schwarzchild radius and we know that the universe has mass, that means the universe should have a Schwarzschild radius too. Therefore, if we squeeze down the entire universe to its Schwarzschild radius, then it should become a blackhole.
It turns out that the size of the universe is equal to its Schwarzschild radius, i.e. the size of the universe is the size that universe would have if the universe was a blackhole! Well then it means that the universe is a blackhole. A blackhole with an entire existence inside it. A blackhole with blackholes inside it which might have universe of their own inside them which would contain their own blackholes then and so on indefinitely.
It means the size of the universe is the not only the size it would have if it were a blackhole but also that its size is equal to the biggest size it could have possibly had, since the age of the universe is the longest possible stretch of time and the speed of light is the fastest speed possible at which an object can travel.
Now go wash your face and read this again.