Can we be sure about there was nothing before big bang?
The Big Bang is not an "initial singularity", though pop science presentations often misleadingly imply that it is. The Big Bang is the hot, dense state that the Universe was in at the earliest times we have reliable knowledge of. There was certainly something before that; our current best theory is that it was a period of inflation.
We cannot. Wish there were more questions about cosmology and cosmonogy that easy to answer.
Certainly we cannot be sure there was not : ^)
If you are really interested, I'd suggest you have a look at the December 2014 article by Yi-fu Cai and Edward Wilson-Ewing called "LambdaCDM Bounce Scenario"
they are very interested in ways that pre-bounce hypotheses can be TESTED by checking against observational data. that is really challenging.
they make the straightforward assumption that the contracting pre-bounce phase was like our expanding universe (the LambdaCDM model that everybody uses) same physics, same type of geometry and material. that is at least a simple place to start.
To get the article (which has a mix of technical parts with less technical introduction and conclusions) you just need to google
"LambdaCDM bounce" and get the "arXiv.org" free online PDF version.
there is a lot of current pre-"big bang" research available, but I would recommend this because comparatively straight forward.
I'll get the URL for you, although googling the title is also very quick.
See what you can get our of pages 12 and 13, the conclusion at the end which are entirely in words, no equations.
What would precede nothing?
How could we know? I happen to think , Like Thales and some other ancient Greek philosophers, that something cannot come from nothing, and something cannot turn into nothing. This is tantamount to saying something has always existed. How can no-thing exist? These are my opinions. But that is logic, which the universe seems to violate every so often just to mess with our minds.
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