## Abstract

We present new Big Bang Nucleosynthesis (BBN) limits on the cosmic expansion rate or relativistic energy density, quantified via the number N_{ν} of equivalent neutrino species. We use the latest light element observations, neutron mean lifetime, and update our evaluation for the nuclear rates d + d ⟶ ^{3}He + n and d + d ⟶ ^{3}H+ p. Combining this result with the independent constraints from the cosmic microwave background (CMB) yields tight limits on new physics that perturbs N_{ν} and η prior to cosmic nucleosynthesis: a joint BBN+CMB analysis gives N_{ν} = 2.898 ± 0.141, resulting in N_{ν} < 3.180 at 2σ. We apply these limits to a wide variety of new physics scenarios including right-handed neutrinos, dark radiation, and a stochastic gravitational wave background. The strength of the independent BBN and CMB constraints now opens a new window: we can search for limits on potential changes in N_{ν} and/or the baryon-to-photon ratio η between the two epochs. The present data place strong constraints on the allowed changes in N_{ν} between BBN and CMB decoupling; for example, we find -0.708 < N_{ν}^{CMB} - N_{ν}^{BBN} < 0.328 in the case where η and the primordial helium mass fraction Y_{p} are unchanged between the two epochs; we also give limits on the allowed variations in η or in (η, N_{ν} ) jointly. We discuss scenarios in which such changes could occur, and show that BBN+CMB results combine to place important constraints on some early dark energy models to explain the H_{0} tension. Looking to the future, we forecast the tightened precision for N_{ν} arising from both CMB Stage 4 measurements as well as improvements in astronomical ^{4}He measurements. We find that CMB-S4 combined with present BBN and light element observation precision can give σ(N_{ν} ) ≃ 0.03. Such future precision would reveal the expected effect of neutrino heating (N_{eff} -3 = 0.044) of the CMB during BBN, and would be near the level to reveal any particle species ever in thermal equilibrium with the standard model. Improved Y_{p} measurements can push this precision even further.

Original language | English (US) |
---|---|

Article number | 046 |

Journal | Journal of Cosmology and Astroparticle Physics |

Volume | 2022 |

Issue number | 10 |

DOIs | |

State | Published - Oct 1 2022 |

## Keywords

- big bang nucleosynthesis
- cosmological neutrinos
- cosmology of theories beyond the SM
- particle physics - cosmology connection

## ASJC Scopus subject areas

- Astronomy and Astrophysics