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| Format: | Preprint |
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2026
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| Accès en ligne: | https://arxiv.org/abs/2604.00062 |
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| _version_ | 1866917375811518464 |
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| author | Kar, Poulastya Koranga, Bipin Singh |
| author_facet | Kar, Poulastya Koranga, Bipin Singh |
| contents | Sterile neutrinos arise naturally in extensions of the Standard Model and can affect cosmological evolution even with subdominant abundance. Their impact is often described by a constant shift in the effective number of relativistic species, Delta Neff, assuming a radiation-like equation of state. However, for finite mass sterile neutrinos with incomplete thermalization, the equation of state evolves with time. In this work, we develop an analytic framework treating sterile neutrinos as a dynamical cosmological fluid with a time-dependent equation of state. Starting from the Boltzmann equation in an expanding Friedmann-Lemaitre-Robertson-Walker background, we show that suppressed active-sterile oscillations lead to a reduced Fermi-Dirac distribution characterized by a thermalization parameter less than unity. We compute the resulting energy density and pressure and incorporate them into the Friedmann equations. We identify distinct regimes, including a relativistic phase, a transition phase, and a matter-like behavior. For GeV scale sterile neutrinos, their contribution at matter-radiation equality is effectively matter-like, shifting the equality epoch in proportion to their energy fraction. Observational constraints indicate that this fraction remains small. This framework connects microscopic production physics to cosmological expansion and goes beyond the standard Delta Neff description. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2604_00062 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Sterile Neutrinos as a Dynamical Cosmological Fluid: Implications for the Expansion History and Matter-Radiation Equality Kar, Poulastya Koranga, Bipin Singh High Energy Physics - Phenomenology Sterile neutrinos arise naturally in extensions of the Standard Model and can affect cosmological evolution even with subdominant abundance. Their impact is often described by a constant shift in the effective number of relativistic species, Delta Neff, assuming a radiation-like equation of state. However, for finite mass sterile neutrinos with incomplete thermalization, the equation of state evolves with time. In this work, we develop an analytic framework treating sterile neutrinos as a dynamical cosmological fluid with a time-dependent equation of state. Starting from the Boltzmann equation in an expanding Friedmann-Lemaitre-Robertson-Walker background, we show that suppressed active-sterile oscillations lead to a reduced Fermi-Dirac distribution characterized by a thermalization parameter less than unity. We compute the resulting energy density and pressure and incorporate them into the Friedmann equations. We identify distinct regimes, including a relativistic phase, a transition phase, and a matter-like behavior. For GeV scale sterile neutrinos, their contribution at matter-radiation equality is effectively matter-like, shifting the equality epoch in proportion to their energy fraction. Observational constraints indicate that this fraction remains small. This framework connects microscopic production physics to cosmological expansion and goes beyond the standard Delta Neff description. |
| title | Sterile Neutrinos as a Dynamical Cosmological Fluid: Implications for the Expansion History and Matter-Radiation Equality |
| topic | High Energy Physics - Phenomenology |
| url | https://arxiv.org/abs/2604.00062 |