If we know approximately how long it’s been since the Big Bang, and we know approximately how fast the universe expands/has been expanding, why can’t we get a good estimate on how big the non-observable universe is? Or more specifically, why can’t we figure out the radius on how far matter has spread out since the Big Bang?
In: Planetary Science
Scientists actually don’t know how fast was the universe expanding. There is a possibility the rate of expansion changed for some unknown reason. (This might explain some discrepancies in cosmologists measurements. But maybe not.)
Even assuming the rate was constant, you need to know also the initial size. For example if you know how big the whole universe was when things were closer together (let’s say when universe was half the size), you could just multiply that by 2 and get current size. Problem is the universe might actually have been infinite. That would mean now it’s 2 times infinite, so… it’s infinite.
Maybe somehow an infinite extremely dense universe came into existence which then expands and cools down but it’s still infinite. When you look at current expanding universe and simulate it backwards, you’ll get to conclusion that at some time everything had to be really close together. But no-one lived back then so no-one could check what happened for sure. We can only use current understanding of physics to piece together clues, and these clues need to be observed by telescope first, so they need to be within observable universe.
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