Measuring up to data
The Big Bang nucleosynthesis model allows the total baryon density of
the universe to be calculated for each of the light elements: deuterium,
helium-3, helium-4 and lithium-7 (green lines). By measuring these
abundances today (red boxes), astronomers can therefore get a
handle on events that took place when the universe was just a few
minutes old. Since the 1990s the helium-4 abundances as measured
by independent groups do not agree (top), probably indicating that
uncertainties in these measurements have been underestimated
(dotted line). The deuterium abundances measured in eight different
locations also disagree to some extent (middle), while measurements
of primordial helium-3 abundances are too difficult to constrain the
baryon density. The measured lithium abundance implies a baryon
density that is in conflict with the independent determination obtained
from the WMAP data (vertical band), though the difference could arise
from some unknown astrophysical process.