The observable universe. Does it mean the edge of space where nothing else is? Is it where the universe is currently at in its expansion after the big bang? Or is it just a barrier that our telescopes are yet to look beyond, and there are just more galaxies past it?
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It’s the speed of light times the age of the universe.
The universe is 13.6 B years old. That’s how much time has elapsed since the Big Bang. The observable universe is, thus, 13.6B light years from Earth in all directions. Light from anything beyond that, even if it was emitted when the universe began, couldn’t reach Earth yet because light is limited to a speed of 1c. Because that light can’t reach us yet, we can’t see it.
It’s much like those radii they use when someone is kidnapped. If they were kidnapped an hour ago, the kidnapper could have only taken them as far as they can travel by car in an hour if they used a car. Someone kidnapped in Pittsburgh an hour ago could have reached Steubenville, OH because the kidnapper could drive from Pittsburgh to Steubenville in an hour. But the victim couldn’t be in Chicago because it isn’t possible to get from Pittsburgh to Chicago by car in an hour. If the kidnapper had an airplane, now they could be in Chicago, but they couldn’t be in Tokyo because an airplane won’t make it from Pittsburgh to Tokyo in an hour.
As time passes, the radius increases. If they made it to Steubenville in an hour, in another hour they could be as far as Cambridge. In another hour after that they could be in Columbus. In another hour they could be in Toledo.
In 400M years, the observable universe will have a radius of 14B light years.
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