Question

What determined the temperature, density, and ratio of hydrogen to helium of the universe soon after the Big Bang?

Answer 1

In 1929 the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts. Redshift occurs when a light source moves away from its observer: the light's apparent wavelength is stretched via the Doppler effect towards the red part of the spectrum. Hubble’s observation implied that distant galaxies were moving away from us, as the furthest galaxies had the fastest apparent velocities. If galaxies are moving away from us, reasoned Hubble, then at some time in the past, they must have been clustered close together.