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| Main Authors: | , , , |
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| Format: | Preprint |
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2026
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| Online Access: | https://arxiv.org/abs/2605.13016 |
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| _version_ | 1866909039345008640 |
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| author | Yura, Kaito Saga, Shohei Yokoyama, Shuichiro Ichiki, Kiyotomo |
| author_facet | Yura, Kaito Saga, Shohei Yokoyama, Shuichiro Ichiki, Kiyotomo |
| contents | We revisit regular primordial vector modes sustained by the anisotropic stress of free-streaming neutrinos. We consider two classes of neutrino-sector initial conditions, the neutrino velocity isocurvature mode ($ν\mathrm{VI}$) and the neutrino octupole mode ($ν\mathrm{OCT}$). We update their observational constraints using current cosmological data, and examine the impact of including the BICEP/Keck 2018 $B$-mode polarization data. From an MCMC analysis, we obtain the 95\% C.L. upper bounds on the vector-to-scalar ratio as $r_\mathrm{v}<1.55\times10^{-4}$ and $r_\mathrm{v}<1.04\times10^{-2}$ for the $ν\mathrm{VI}$ and $ν\mathrm{OCT}$ modes at the vector pivot scale $k_{0} = 0.01\,{\rm Mpc}^{-1}$, respectively. We then study two consequences of these bounds. First, we estimate the magnetic fields inevitably generated in the pre-recombination plasma associated with the vector modes. We find that the magnetic-field amplitude at recombination with a coherent length of $1~{\rm Mpc}$ is bounded by $B\sim\mathcal{O}(10^{-23})\,{\rm G}$ and $B\sim\mathcal{O}(10^{-21})\,{\rm G}$ for the $ν\mathrm{VI}$ and $ν\mathrm{OCT}$ modes, respetively, which is too small to provide the seed of magnetic fields observed today. Second, assuming the helical vector mode, we compute the induced CMB $EB$ spectrum. We show that even a fully helical primordial vector mode cannot reproduce the currently observed $EB$ signal while remaining consistent with parity-even CMB constraints. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2605_13016 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Revisiting constraints on primordial vector modes and implications for sourced magnetic fields and observed $EB$ power spectrum Yura, Kaito Saga, Shohei Yokoyama, Shuichiro Ichiki, Kiyotomo Cosmology and Nongalactic Astrophysics We revisit regular primordial vector modes sustained by the anisotropic stress of free-streaming neutrinos. We consider two classes of neutrino-sector initial conditions, the neutrino velocity isocurvature mode ($ν\mathrm{VI}$) and the neutrino octupole mode ($ν\mathrm{OCT}$). We update their observational constraints using current cosmological data, and examine the impact of including the BICEP/Keck 2018 $B$-mode polarization data. From an MCMC analysis, we obtain the 95\% C.L. upper bounds on the vector-to-scalar ratio as $r_\mathrm{v}<1.55\times10^{-4}$ and $r_\mathrm{v}<1.04\times10^{-2}$ for the $ν\mathrm{VI}$ and $ν\mathrm{OCT}$ modes at the vector pivot scale $k_{0} = 0.01\,{\rm Mpc}^{-1}$, respectively. We then study two consequences of these bounds. First, we estimate the magnetic fields inevitably generated in the pre-recombination plasma associated with the vector modes. We find that the magnetic-field amplitude at recombination with a coherent length of $1~{\rm Mpc}$ is bounded by $B\sim\mathcal{O}(10^{-23})\,{\rm G}$ and $B\sim\mathcal{O}(10^{-21})\,{\rm G}$ for the $ν\mathrm{VI}$ and $ν\mathrm{OCT}$ modes, respetively, which is too small to provide the seed of magnetic fields observed today. Second, assuming the helical vector mode, we compute the induced CMB $EB$ spectrum. We show that even a fully helical primordial vector mode cannot reproduce the currently observed $EB$ signal while remaining consistent with parity-even CMB constraints. |
| title | Revisiting constraints on primordial vector modes and implications for sourced magnetic fields and observed $EB$ power spectrum |
| topic | Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2605.13016 |