# Prospects and Limitations for Constraining Light Relics with Primordial Abundance Measurements

## TL;DR

This paper evaluates how primordial light element abundance measurements can constrain light relics, highlighting the potential and limitations compared to upcoming cosmic microwave background surveys.

## Contribution

It quantifies the observational uncertainties needed in primordial abundances to match future CMB constraints and identifies key nuclear reactions for improved measurements.

## Key findings

- Improved helium-4 measurements can enhance light relic constraints.
- Deuterium measurements are less competitive than CMB data.
- Nuclear reaction rate uncertainties impact the effectiveness of abundance constraints.

## Abstract

The light relic density affects the thermal and expansion history of the early Universe leaving a number of observable imprints. We focus on the primordial abundances of light elements produced during the process of Big Bang nucleosynthesis which are influenced by the light relic density. Primordial abundances can be used to infer the density of light relics and thereby serve as a probe of physics beyond the Standard Model. We calculate the observational uncertainty on primordial light element abundances and associated quantities that would be required in order for these measurements to achieve sensitivity to the light relic density comparable to that anticipated from upcoming cosmic microwave background surveys. We identify the nuclear reaction rates that need to be better measured to maximize the utility of future observations. We show that improved measurements of the primordial helium-4 abundance can improve constraints on light relics, while more precise measurements of the primordial deuterium abundance are unlikely to be competitive with cosmic microwave background measurements of the light relic density.