A side from ordinary hydrogen, only four other nuclear species ( and are left over from the Big Bang. Of these, deuterium is the one whose obsei^ved abundance provides the most accurate determination of the crucial cosmological parameter the present‐day average mass density of ordinary baryonic matter (made up of protons and neutrons) in the universe. In principle, if one knows the primordial abundance of any one of these light nuclei, relative to the theory of Big Bang nucleosynthesis specifies (See the figure below.)
New Measurements of Ancient Deuterium Boost the Baryon Density of the Universe
Bertram Schwarzschild; New Measurements of Ancient Deuterium Boost the Baryon Density of the Universe. Physics Today 1 August 1996; 49 (8): 17–18. https://doi.org/10.1063/1.881505
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