Observing abundances
To test whether Big Bang nucleosynthesis (BBN) is the correct description of the early universe, we need to measure how much of those elements
are present in samples we can observe today. Astronomers determine relative abundances of elements in a distant object by observing the
amount of emission or absorption of light at certain wavelengths corresponding to atomic spectral lines. To do so requires finding both a location
where the gas composition has not changed much since BBN and where the physical conditions are favourable for forming observable spectral
lines. This latter condition, combined with the different atomic physics associated with each element, means that no two primordial abundances
can be accurately measured using the same object. Helium, for example, is measured by looking at light from small and irregular "blue compact
galaxies", while the amount of lithium is inferred from very old objects in our galaxy called "Spite-plateau" stars, which formed from nearly
primordial gas. The amount of deuterium, on the other hand, is determined by looking at how light from distant quasars is absorbed by a diffuse
cloud in our line of sight.