A side from ordinary hydrogen, only four other nuclear species (H2,He3,He4 and Li7 are left over from the Big Bang. Of these, deuterium (2H) is the one whose obsei^ved abundance provides the most accurate determination of the crucial cosmological parameter ρb, 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 H1, the theory of Big Bang nucleosynthesis specifies ρb. (See the figure below.)

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