Use the Hubble expansion relation (9.1.1), the temperature scaling relation (9.1.3), and the energy density relation before the electron-positron annihilation (9.3.6b) to show that the temperature as a function of time during the first second of the universe was \(T(t) \approx 10^{10} \mathrm{~K} \sqrt{\frac{0.99 \mathrm{~s}}{t}}\).