How new physics affects primordial neutrinos decoupling: Direct simulation Monte Carlo approach

Abstract

Cosmological observations from big bang nucleosynthesis and the cosmic microwave background (CMB) offer crucial insights into the Early Universe, enabling us to trace its evolution back to lifetimes as short as 0.01 s. Upcoming CMB spectrum measurements will achieve unprecedented precision, allowing for more accurate extraction of information about the primordial neutrinos. This provides an opportunity to test whether their properties align with the predictions of the standard cosmological model or indicate the presence of new physics that influenced the evolution of the MeV-temperature plasma. A key component in understanding how new physics may have affected primordial neutrinos is solving the neutrino Boltzmann equation. In this paper, we address this question by developing a novel approach—neutrino direct simulation Monte Carlo (DSMC). We discuss it in depth, highlighting its model independence, transparency, and computational efficiency—features that current state-of-the-art methods lack. Then, we introduce a proof-of-concept implementation of the neutrino DSMC and apply it to several toy scenarios, showcasing key aspects of the primordial plasma’s evolution in the presence of new physics. Published by the American Physical Society 2025