# How are the temperature, density, and size of the Universe calculated during the first few minutes after the Big Bang?

I'd like to know the assumptions cosmologist use when they give a timeline of temperature, density, and size of the universe from $$T=0$$ to $$T=$$ a few minutes later, as Steven Weinberg did in his book, "The First Three Minutes". Specifically, do they use an estimate of the total mass of the Universe, the rate at which the Universe is expanding, the shape of the Universe, or, are some or all of these unnecessary? If they are needed, what are their values?

• The total mass of the universe is undefined. See physics.stackexchange.com/questions/2838/… . Cosmological models do not attempt to describe t=0, which is a singularity. – Ben Crowell Apr 2 '18 at 1:05
• Everything is changing rapidly, as described the differential equations of GR applied to the Robertson-Walker metric. All three Friedmann models look much the same at the big bang, so shape seems unimportant. Thermodynamics relate pressure to density and composition, with hidden assumptions about approximate equilibrium. Key parameters include the ratio of entropy to baryon number, and the number of leptons. Having too many neutrinos or antineutrinos could throw off the abundance of heavy elements. – Bert Barrois Apr 2 '18 at 11:55
• And don't forget the power spectrum of density fluctuations that will ultimately lead to galaxy formation and clustering. – Bert Barrois Apr 2 '18 at 11:55