The Physics Colloquium: Brian Fields (University of Illinois Urbana-Champaign) "Big-Bang Nucleosynthesis: the Rise of the Elements in the Early Universe"

Abstract:

We will give an overview of the status of big-bang nucleosynthesis (BBN), which describes the production of the lightest nuclei--deuterium, 3He, 4He, and 7Li--during the first three minutes of cosmic time.   This is one of the few arenas in nature where all four fundamental forces play essential roles.  Standard BBN assumes that the simplest and most robust physics is at play, combining the Standard Model of particle physics, nuclear cross-section measurements, and the neutron lifetime; this leaves only a single free parameter--the cosmic density of baryons, i.e., ordinary matter.  We will emphasize the transformative influence of cosmic microwave background (CMB) experiments, which pin down the cosmic baryon density to exquisite precision.  This allows BBN to make tight predictions for the primordial light element abundances, which can be compared with observations:  for deuterium the agreement is spectacular, and for helium the agreement is excellent.  This CMB/BBN concordance marks a great success of the hot big bang, and BBN and the CMB together now sharply probe cosmology, neutrino physics, and dark matter physics at times around 1 second.  But this success is tempered by lithium observations that are significantly discrepant with BBN+CMB predictions;  we will summarize possible resolutions to this "lithium problem."    Turning to 4He, we will present new observations that have achieved a leap in precision.  We will discuss the implications of these observations for cosmology and physics beyond the Standard Model.   We conclude with an outlook on future CMB, astronomical, and laboratory measurements that can better probe new physics in the early Universe, and help shed light on the solution to the lithium problem.