How big is the universe?

Answer

We cannot deduce this exactly from the observations.

What we are quite convinced of is that we know how old the universe is, which is 13.73 billion years. As far as we can see, there is something to see, so there is universe.

If the universe had been what it is today from the beginning, we could see as far as light travels in 13.73 years, which is 13.73 light-years. But we know that the universe is growing. As a result, something whose light was sent to us, say, 10 billion years ago, is now more than 10 billion light-years away from us. So how far the furthest we see really is depends on how the expansion went. From our knowledge of it, we deduce that the farthest reaches of the universe that we can see are about 40 billion light-years away from us. So that is a lower limit of the size of the universe.

We think today that the universe expanded enormously very quickly just after its formation. The part of the universe that we can see could therefore be only a small part of the whole, but it is difficult to determine directly how small it is.