# How do we know so many details about Big Bang, but we do not know if Universe is finite/infinite?

I understand that this question may be too vague, but I am confused about the following:

On one hand we know a lot of "details" about Big Bang, like really precise details, evolution of universe, age of universe...

On the other hand we do not know size of Universe, or even if it is finite or not.

This seems quite weird to me, since even if we may never interact with Universe that is not observable those parts still interact with parts of our Observable Universe.

Some examples:

• I think we know very little from the Big Bang. – peterh - Reinstate Monica Oct 6 '19 at 17:08
• +1 This is a great question! I added a few examples to illustrate the wide range of (model-based) certainties but I think a good answer will cover that better. "a little" and "a lot" can of course be simultaneously valid since they are subjective relative comparisons, and what the phrase "we know" means can be addressed as well. – uhoh Oct 7 '19 at 0:13
• Determining whether the universe is finite or not is very hard, as discussed here: astronomy.stackexchange.com/q/32456/16685 – PM 2Ring Oct 7 '19 at 10:03
• – GeroldBroser reinstates Monica Oct 16 '19 at 5:23

By observing the observable universe we can gather a lot of data about the constituents of the universe now and in previous epochs, right back to the cosmic microwave background. This data places quite narrow constraints on viable models of the Big Bang. This is why cosmologists are confident about the details of the Big Bang, certainly from electroweak symmetry breaking onwards.

However, it is much more difficult to make an observation of the observable universe that could distinguish between a very, very large but finite universe and an infinite universe. That is why this is still an open question.