It seems "the first three minutes" are well established... back to the first second? 10 seconds? What about the "hadron epoch" going back to the first microsecond? Or before that? I know there are plenty of theories like inflation and GUT that go very far back, but it seems that there is little evidence for those or at least to choose between versions. By "observational evidence" I don't just mean looking through telescopes, but also amounts of helium and lithium, which is what brings us back to at least the first three minutes.
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1$\begingroup$ 380,000 years for CMB. Nothing before that. $\endgroup$– safesphereCommented Jul 20, 2018 at 8:19
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$\begingroup$ Although you can't (electromagnetically) see before the CMB you can deduce things by what the CMB looks like -- if it's very uniform it tells you things about how things must have been before that. That's not direct observational evidence though. $\endgroup$– user107153Commented Jul 20, 2018 at 8:23
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1$\begingroup$ Related: physics.stackexchange.com/q/11136/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Jul 20, 2018 at 9:11
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We can observe matter, light and gravitational waves. The oldest optical observation is that of the cosmological microwave background. According to cosmology it dates back to 379000 years after the big bang. The oldest matter we observe is pristine intergalactic hydrogen helium mixture. This matter never participated in star formation and dates back to a few minutes after the big bang, when the big bang nucleosynthesis occurred. Gravitational waves have only just been discovered so who knows what is out there!
