It's deductions from the Standard Model of particle physics, not accelerator data, that tell of what we think we know about conditions in the early Universe. I am not a particle physicist, so I am not qualified to judge how valid these deductions are, but when the community of particle physicists tells us that they are confident that theory has enabled them to extrapolate physical phenomena back to 10^-43 second, I'm inclined to accept their statements at least tentatively as well-founded. These statements are not made lightly: They are not published in reputable journals until other physicists up to date on the latest findings endorse them through the peer review process. I cited only one place where the 10^-43 second figure for the beginning of the GUT era can be found. I've seen it in a number of different publications.