Density of the Universe
How will the universe end? Which force will win: gravity or expansion? Hubble constant is a measure of the current kinetic energy of the universe. The strength of the gravitational pull depends on the density of matter in the universe.
Critical Density
The greater the density, the greater the overall strength of gravity the higher the likelihood that gravity will eventually halt the expansion. Gravity can win if the current density of the universe exceeds 1029 g/cm3. This density is called the critical density.
The visible matter contributes only 1011 K, nuclear reactions converted protons into neutrons and back, keeping their numbers nearly equal. Between 1010 and 1011 K, neutron – proton reactions favor protons, because neutrons are heavier than protons.
Synthesis of Helium
Energy is required to convert protons to neutrons. At T < 1010 K, only neutrons can be changed into protons. However, fusion continued to operate and protons and neutrons combined into deuteriu Then deuterium fused into helium. During the early era of nucleosynthesis, helium nuclei were being destroyed by gamma-rays. At ~1 minute, gamma-rays were gone and the
Summary
It is easy to predict conditions in the early universe, it is harder to predict the behavior of matter and energy. Our current understanding of physics allows us to reconstruct the conditions in the universe back to 1010 sec. Observations support some predictions of the Big Bang theory.