arXiv subjects

Murali M. Saravanan

Publications and source records attributed to Murali M. Saravanan.

3 recordsLinked to original sources

arXiv2026

Big Bang For Your Helium Buck

We study the implications of a recent measurement of the primordial helium fraction from the Large Binocular Telescope for cosmological inference from the cosmic microwave background. We show that LBT establishes the robustness of cosmological parameters to theoretical assumptions about big bang nucleosynthesis: its empirical calibration of the helium fraction enables constraints on cosmology and inflation that are agnostic to BBN but as precise as those that instead enforce standard BBN predictions. Future CMB surveys require at most a marginal improvement in precision over LBT to maximize their BBN-agnostic constraining power when the radiation density is free (and no more than a factor of two improvement across all cases we consider). We then apply the LBT measurement to a number of scenarios that feature new physics in BBN and the CMB. First, we constrain nonstandard radiation sectors (interacting and free-streaming) and search for evolution of the radiation abundance between nucleosynthesis and recombination. We then test models that alleviate the Hubble tension and the tension between CMB and baryon acoustic oscillation data, including self-interacting light relics and varying fundamental constants; LBT precludes most but not all of the models we consider via their effect on BBN. Finally, we use the LBT measurement as an indirect but independent test of the neutron lifetime anomaly, inferring values that are consistent with "bottle" experiments and $2.4 σ$ below "beam" experiments.

astro-ph.CO↗

arXiv2026

Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics

Neutrinos and other light relics leave characteristic imprints in the cosmic microwave background anisotropies, making their observation a sensitive probe of the particle content and thermal history of the early universe. The energy density in these relativistic species is parameterized by their effective number $N_\mathrm{eff}$. Measuring this parameter at the percent level, which is a long-standing science goal of CMB-S4 and other experiments, would test a wide range of well-motivated physics within and beyond the Standard Model of particle physics. In this paper, we present Fisher-matrix forecasts of the projected sensitivity to $N_\mathrm{eff}$ of several CMB-S4 survey configurations considered during its extensive design phase. The conceptual design reaches $σ(N_\mathrm{eff}) < 0.03$ over its seven-year observing period, while the revised configuration achieves the same precision over a longer timescale. We complement these results with a cosmic-variance-limited survey over the same multipole range to quantify the room for improvement accessible with additional instrumental, observational, and theoretical efforts. Finally, we discuss the broad implications of precise $N_\mathrm{eff}$ measurements for the radiation sector, big bang nucleosynthesis, light thermal relics, and other early-universe physics. The forecasts presented in this work are performed with the publicly released DRAFT (Dark Radiation Anisotropy Flowdown Team) tool. It provides an end-to-end pipeline from simulated foreground maps and component separation to delensing and projected sensitivities for any cosmological parameter, and it can be directly applied to other cosmic microwave background survey designs.

astro-ph.CO↗

arXiv2025

Abundance and properties of dark radiation from the cosmic microwave background

We study the cosmological signatures of new light relics that are collisionless like standard neutrinos or are strongly interacting. We provide a simple and succinct rephrasing of their physical effects in the cosmic microwave background, as well as the resulting parameter degeneracies with other cosmological parameters, in terms of the total radiation abundance and the fraction thereof that freely streams. In these more general terms, interacting and noninteracting light relics are differentiated by their respective decrease and increase of the free-streaming fraction, and, moreover, the scale-dependent interplay thereof with a common, correlated reduction of the fraction of matter in baryons. We then derive updated constraints on various dark-radiation scenarios with the latest cosmological observations, employing this language to identify the physical origin of the impact of each dataset. The "PR4" reanalyses of Planck CMB data prefer a larger primordial helium yield and therefore also slightly more radiation than the 2018 analysis; we investigate the differences between the two releases that drives these shifts. Smaller free-streaming fractions are disfavored by the excess lensing of the CMB measured in lensing reconstruction data from Planck and the Atacama Cosmology Telescope. On the other hand, baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument drive marginal detections of new, strongly interacting light relics due to that data's preference for lower matter fractions. Finally, we forecast measurements from the CMB-S4 experiment.

astro-ph.CO↗