The Baryon Density Through The (Cosmological) Ages
  The light element abundances probe the baryon density of the universe within a few minutes of the Big Bang. Of these relics from the earliest universe, deuterium is the baryometer of choice. By comparing its primordial abundance inferred from observations with that predicted by Big Bang Nucleosynthesis (BBN), the early universe baryon density is derived. This is then compared to independent estimates of the baryon density several hundred thousand years after the Big Bang and at present, more than 10 billion years later. The excellent agreement among these values represents an impressive confirmation of the standard model of cosmology. 
Gary Steigman (The Ohio State University)