Big Bang nucleosynthesis and physics beyond the Standard Model

TL;DR

Big Bang nucleosynthesis provides a stringent, parameter-sensitive probe of physics beyond the Standard Model by linking light-element abundances to the early-universe expansion history and particle content. The work synthesizes relativistic cosmology, BBN theory, and observational data to constrain relics (massless and massive) and their interactions, and applies these constraints to neutrinos, SUSY, GUTs, and dark-matter scenarios. It emphasizes the importance of accurate primordial abundance determinations and discusses how BBN bounds complement collider experiments in shaping viable new physics models. The findings reinforce that a consistent, data-driven extension of the SM must respect the tight BBN constraints on $N_ u$, $\\eta$, and the lifetimes/decay channels of exotic particles, with broad implications for inflation, baryogenesis, and dark matter.

Abstract

The Hubble expansion of galaxies, the $2.73\dK$ blackbody radiation background and the cosmic abundances of the light elements argue for a hot, dense origin of the universe --- the standard Big Bang cosmology --- and enable its evolution to be traced back fairly reliably to the nucleosynthesis era when the temperature was of $\Or(1)$ MeV corresponding to an expansion age of $\Or(1)$ sec. All particles, known and hypothetical, would have been created at higher temperatures in the early universe and analyses of their possible effects on the abundances of the synthesized elements enable many interesting constraints to be obtained on particle properties. These arguments have usefully complemented laboratory experiments in guiding attempts to extend physics beyond the Standard $SU(3)_{\c}{\otimes}SU(2)_Ł{\otimes}U(1)_{Y}$ Model, incorporating ideas such as supersymmetry, compositeness and unification. We first present a pedagogical account of relativistic cosmology and primordial nucleosynthesis, discussing both theoretical and observational aspects, and then proceed to examine such constraints in detail, in particular those pertaining to new massless particles and massive unstable particles. Finally, in a section aimed at particle physicists, we illustrate applications of such constraints to models of new physics.