Less isn't more: Cosmological bounds on the neutrino masses are robust to changes in the neutrino abundance
Abstract
We investigate how neutrino-mass constraints from cosmology depend on the assumed thermal history of the universe. Photon injection after neutrino decoupling would decrease the neutrino abundance inferred from the temperature of the cosmic microwave background (CMB), potentially loosening the upper limit on their masses. We first evaluate how the cosmological neutrino-mass bound is altered by the decay of massive particles into photons and dark radiation after Big Bang nucleosynthesis. To test the robustness of this constraint more generally, we also assess the impact of varying the temperature of the cosmic neutrino background without assuming a specific physical mechanism. We perform MCMC analyses of both frameworks with primary CMB observations from Planck, CMB lensing measurements from Planck and ACT, and baryon acoustic oscillation data from DESI. For the degenerate mass ordering, the 95% credible limit tightens from ∑ mν<0.0691 eV in a standard thermal history to ∑ mν<0.0652 eV if the massive particles decay only into photons, while the injection of dark radiation in addition to photons slightly relaxes this limit to ∑ mν<0.0710 eV. The same pattern holds for the normal and inverted orderings, and the decay scenario shifts the bound on the sum of the neutrino masses by at most $0.004$ eV for fixed mass splittings. Allowing model-agnostic changes in the neutrino-to-photon ratio yields a 95% credible limit of ∑ mν<0.0724 eV for the degenerate ordering, indicating that the stringency of our neutrino-mass bounds is not driven by constraints on the decay scenario. We find that the neutrino temperature and the sum of the neutrino masses are positively correlated, which implies that reducing the pre-recombination radiation density will only worsen the emerging tension between cosmological bounds on the neutrino masses and the measured mass splittings.
Related
- Cosmological constraints on extra radiation and early dark energy with lepton asymmetry and varying quark masses
- Cosmological preference for a positive neutrino mass at 2.7σ: A joint analysis of DESI DR2, DESY5, and DESY1 data
- Reanalyzing DESI DR1: IV. Percent-level cosmological constraints from combined probes and robust evidence for the normal neutrino mass hierarchy
- The host halo masses of AGNs and quasars at z ∼ 3 − 7 with TNG-Cluster, FLAMINGO and other cosmological galaxy simulations
- The Structural Abundance Crisis of Massive Galaxies in Current Cosmological Simulations
BibTeX
@misc{figueroa2026less,
title = {{Less isn't more: Cosmological bounds on the neutrino masses are robust to changes in the neutrino abundance}},
author = {Sebastian J. Figueroa and Alexander C. Sobotka and Adrienne L. Erickcek},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.15970},
eprint = {2609.15970},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.15970},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Less isn't more: Cosmological bounds on the neutrino masses are robust to changes in the neutrino abundance
AU - Sebastian J. Figueroa
AU - Alexander C. Sobotka
AU - Adrienne L. Erickcek
PY - 2026
DA - 2026/09/14
PB - arXiv
DO - 10.48550/arxiv.2609.15970
UR - https://arxiv.org/abs/2609.15970
AB - We investigate how neutrino-mass constraints from cosmology depend on the assumed thermal history of the universe. Photon injection after neutrino decoupling would decrease the neutrino abundance inferred from the temperature of the cosmic microwave background (CMB), potentially loosening the upper limit on their masses. We first evaluate how the cosmological neutrino-mass bound is altered by the decay of massive particles into photons and dark radiation after Big Bang nucleosynthesis. To test the robustness of this constraint more generally, we also assess the impact of varying the temperature of the cosmic neutrino background without assuming a specific physical mechanism. We perform MCMC analyses of both frameworks with primary CMB observations from Planck, CMB lensing measurements from Planck and ACT, and baryon acoustic oscillation data from DESI. For the degenerate mass ordering, the 95% credible limit tightens from ∑ m_ν<0.0691 eV in a standard thermal history to ∑ m_ν<0.0652 eV if the massive particles decay only into photons, while the injection of dark radiation in addition to photons slightly relaxes this limit to ∑ m_ν<0.0710 eV. The same pattern holds for the normal and inverted orderings, and the decay scenario shifts the bound on the sum of the neutrino masses by at most $0.004$ eV for fixed mass splittings. Allowing model-agnostic changes in the neutrino-to-photon ratio yields a 95% credible limit of ∑ m_ν<0.0724 eV for the degenerate ordering, indicating that the stringency of our neutrino-mass bounds is not driven by constraints on the decay scenario. We find that the neutrino temperature and the sum of the neutrino masses are positively correlated, which implies that reducing the pre-recombination radiation density will only worsen the emerging tension between cosmological bounds on the neutrino masses and the measured mass splittings.
ER -