The biggest problem astronomers have with trying to understand the universe is that we’re limited by what we can see. This means, of course, that to know about something, light from that something has to have had time to get to us. As a cosmic speed limit goes, the speed of light is pretty high, but considering the distances involved in space, that’s still a pretty strict limit. Since we know the universe began 13.7 billion years ago, the most distant thing visible to us has been sending light our direction for 13.7 billion years.
The section of the universe that we can actually receive information from is known as the “observable universe”, and that’s all we have to work with in deciphering how the universe works. This is not a catastrophic problem, because anywhere we look in our section of the universe, physics seems to work in the same way, so we assume that physics should work the same way everywhere, even the parts we can’t observe. The down side of this, of course, is that we have no idea how much universe we’re missing. If it’s beyond this cosmic horizon imposed by the length of time since the Big Bang, the information just hasn’t had time to get here.
For all intents and purposes, the universe may well be infinite. Sometimes saying that the space in our universe is infinite makes the mathematics of describing the universe a little easier to manage.
We probably won’t ever be able to determine the size of the entire universe, since we don’t have an infinite amount of time to observe. But we can predict an awful lot about how the universe behaves, how it began, and how it will unfold in the future, just based on the portion of the universe we can see.
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