Big Bang Nucleosynthesis as a Probe of New Physics

TL;DR

This paper reviews how Big Bang Nucleosynthesis acts as a sensitive probe of new physics by examining how additional relativistic content, time-varying fundamental constants, non-thermal energy injection from decays/annihilations, and catalysis by charged relics can alter light-element abundances. It analyzes the resulting changes to the primordial yields of ${}^4\mathrm{He}$, D, ${}^3\mathrm{He}$, ${}^7\mathrm{Li}$, ${}^6\mathrm{Li}$, and heavier isotopes, and outlines the observational and theoretical constraints that limit such scenarios. The review emphasizes the lithium problem as a key tension and discusses potential resolutions ranging from astrophysical depletion to new particle physics mechanisms (e.g., WIMP energy injection and catalysis by charged or strongly interacting relics). It also highlights how forthcoming CMB measurements and collider results can complement BBN bounds in constraining beyond-Standard-Model physics during the early universe.

Abstract

Big bang nucleosynthesis (BBN), an epoch of primordial nuclear transformations in the expanding Universe, has left an observable imprint in the abundances of light elements. Precision observations of such abundances, combined with high-accuracy predictions, provide a nontrivial test of the hot big bang and probe non-standard cosmological and particle physics scenarios. We give an overview of BBN sensitivity to different classes of new physics: new particle or field degrees of freedom, time-varying couplings, decaying or annihilating massive particles leading to non-thermal processes, and catalysis of BBN by charged relics.

Figures (9)

• Figure 1: Time and temperature evolution of all standard big bang nucleosynthesis (SBBN)-relevant nuclear abundances. The vertical arrow indicates the moment at $T_9 \simeq 0.85$ at which most of the helium nuclei are synthesized. The gray vertical bands indicate main BBN stages. From left to right: neutrino decoupling, electron-positron annihilation and $n/p$ freeze-out, D bottleneck, and freeze-out of all nuclear reactions. Protons (H) and neutrons (N) are given relative to $n_b$ whereas $Y_p$ denotes the ${}^4\mathrm{He}$ mass fraction.
• Figure 2: Change of nuclear abundances relative to their SBBN values as a function of the "dark radiation" component $\rho_{dr}/\rho_{\mathrm{SM}}= \Delta\rho_{\mathrm{SM}}/\rho_{\mathrm{SM}}$. The vertical band shows the allowed amount of dark radiation that keeps $Y_p$ in the $0.24\div 0.26$ window.
• Figure 3: Contours of $Y_p$, D/H and ${}^7\mathrm{Li}$/H are plotted in the parameter space of variable neutron-proton mass difference $\Delta m_{np}$ and deuteron binding energy $E_d$, normalized to their current values. The 5-10% downward change in $E_b$ can significantly reduce ${}^7\mathrm{Li}$ abundance.
• Figure 4: Consequences of late decays of a heavy 1 $\mathrm{TeV}$ mass particle $X$ that releases half of its rest mass in the form of electromagnetic energy. The threshold of ${}^4\mathrm{He}$ disintegration is clearly visible below 1 keV. Primary abundance flows are indicated by solid arrows whereas the dashed arrow indicates the secondary transformation of $A=3$ nuclei into ${}^6\mathrm{Li}$. The model is excluded by the overproduction of D, ${}^3\mathrm{He}$, and ${}^6\mathrm{Li}$.
• Figure 5: Effects on element abundances in response to an elevated (thermal) neutron content, sourced by a decaying species, at early times. Although neutrons are mostly incorporated into D, they also transfer ${}^7\mathrm{Be}$ nuclei into the ${}^7\mathrm{Li}$ reservoir from which they are more susceptible to proton burning. This mechanism suppresses the overall outcome of the $A=7$ elements.