Why do the observed amounts of hydrogen, helium, and deuterium support the big-bang model?

The big-bang model is strongly supported by the observed cosmic abundances of light elements, which were synthesized shortly after the universe began.1

Primordial Nucleosynthesis

When the universe was only a few minutes old and temperatures had dropped to approximately ${10}^9$ K, conditions allowed for the formation of deuterium through the capture of neutrons by protons. Once deuterium formed, a rapid chain of nuclear reactions assembled most available neutrons and deuterium into helium nuclei.2

Calculations based on the big-bang model—accounting for neutron decay and the expansion of the universe—predict a primordial mass fraction of approximately 25 percent helium and 75 percent hydrogen. These values align closely with astronomical observations, a result that steady-state theory could not explain, as stars alone cannot account for such high helium concentrations.21

The Role of Deuterium

The observed abundance of deuterium serves as a sensitive probe of the density of ordinary matter in the early universe. Because deuterium is easily consumed in stars, the small residual amount observed today allows cosmologists to calculate the density of ordinary matter present during primordial nucleosynthesis. This calculation confirms that ordinary matter accounts for only a fraction of the total mass in the universe.3