Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics
Abstract
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 Neff. 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 Neff of several CMB-S4 survey configurations considered during its extensive design phase. The conceptual design reaches σ(Neff) < 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 Neff 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.
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BibTeX
@misc{trendafilova2026sensitivity,
title = {{Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics}},
author = {Cynthia Trendafilova and Srinivasan Raghunathan and Benjamin Wallisch and Joel Meyers and Kevork N Abazajian and Edoardo Altamura and Carlo Baccigalupi and Kimberly K Boddy and Thejs Brinckmann and Yuji Chinone and Gabriele Coppi and Francis-Yan Cyr-Racine and Jacques Delabrouille and Katherine Freese and Helena García Escudero and Martina Gerbino and Shamik Ghosh and Vera Gluscevic and Daniel Green and Daniel Grin and Kevin M Huffenberger and Mudit Jain and Lloyd Knox and Anto Lonappan and Marilena Loverde and Philip Lubin and Gabriele Montefalcone and Valentine Novosad and Marco Raveri and Christian L Reichardt and Sayan Saha and Murali M Saravanan and Emmanuel Schaan and Sara M Simon and Julien Tang and Scott Watson},
howpublished = {HAL},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.07453},
eprint = {2608.07453},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05734513v1},
}
RIS
TY - UNPB
TI - Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics
AU - Cynthia Trendafilova
AU - Srinivasan Raghunathan
AU - Benjamin Wallisch
AU - Joel Meyers
AU - Kevork N Abazajian
AU - Edoardo Altamura
AU - Carlo Baccigalupi
AU - Kimberly K Boddy
AU - Thejs Brinckmann
AU - Yuji Chinone
AU - Gabriele Coppi
AU - Francis-Yan Cyr-Racine
AU - Jacques Delabrouille
AU - Katherine Freese
AU - Helena García Escudero
AU - Martina Gerbino
AU - Shamik Ghosh
AU - Vera Gluscevic
AU - Daniel Green
AU - Daniel Grin
AU - Kevin M Huffenberger
AU - Mudit Jain
AU - Lloyd Knox
AU - Anto Lonappan
AU - Marilena Loverde
AU - Philip Lubin
AU - Gabriele Montefalcone
AU - Valentine Novosad
AU - Marco Raveri
AU - Christian L Reichardt
AU - Sayan Saha
AU - Murali M Saravanan
AU - Emmanuel Schaan
AU - Sara M Simon
AU - Julien Tang
AU - Scott Watson
PY - 2026
DA - 2026/08/07
PB - HAL
DO - 10.48550/arxiv.2608.07453
UR - https://hal.science/hal-05734513v1
AB - 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_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_eff of several CMB-S4 survey configurations considered during its extensive design phase. The conceptual design reaches σ(N_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_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.
ER -