Question: If the universe is "infinite" how can you tell it is infinite? How large is this infinite? Would it be possible to ever reach the part of the universe that's the furthest away?

Matthew Malek answered on 17 Mar 2014:

Hi Chloe! That’s a really good question!

We have no good means to determine the ultimate size of the universe, whether it is finite or infinite. When discussing our knowledge of the universe at its largest scales, we tend to talk about the “observable universe”. That’s the part of the universe where light has had time to reach us.

Since the speed of light is finite, when you look at things that are further away in distance, you are simultaneously looking further back in time. The universe is about 13.7 (or 13.8) billion years old. Anything close enough to have light reach us within that time is in the “observable universe”. Anything further away is not.

Since we cannot make direct observations of the non-observable universe, we can only speculate about what might be there. And there is a LOT of speculation! Some people have even proposed some really mind-blowing ideas, like the concept of an infinite “multiverse” with pockets where the laws of physics could be different.

Unless we ever find a means to probe beyond the observable universe (wormholes??), I’m afraid that speculation will be all we can do about these areas. Without the ability to observe, those regions are outside the realm of science.