Abstract
Light new vector bosons can be produced gravitationally through quantum fluctuations during inflation; if these particles are feebly coupled and cosmologically metastable, they can account for the observed dark matter abundance. However, in minimal anomaly-free $U(1)$ extensions to the Standard Model, these vectors generically decay to neutrinos if at least one neutrino mass eigenstate is sufficiently light. If these decays occur between neutrino decoupling and cosmic microwave background (CMB) freeze-out, the resulting radiation energy density can contribute to $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}$ at levels that can ameliorate the Hubble tension and be discovered with future CMB and relic neutrino detection experiments. Since the additional neutrinos are produced from vector decays after Big Bang Nucleosynthesis (BBN), this scenario predicts $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}>0$ at recombination, but $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}=0$ during BBN. Furthermore, due to a fortuitous cancellation, the contribution to $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}$ is approximately mass independent.
Highlights
Cosmological inflation elegantly accounts for the observed flatness, isotropy, and homogeneity of the Universe
In this paper we have studied the fate of massive vector particles produced gravitationally from inflationary fluctuations
If these vectors only interact with the Standard Model (SM) via kinetic mixing, for m < 2me, the only allowed decay is V → 3γ which is sharply suppressed, so V is generically metastable can serve as a dark matter candidate [15]
Summary
Cosmological inflation elegantly accounts for the observed flatness, isotropy, and homogeneity of the Universe. Couple feebly to neutrinos and that at least one neutrino mass eigenstate is sufficiently light to allow V → νν decays If such decays occur after neutrino decoupling, but before CMB photon decoupling, there is an irreducible contribution to ΔNeff that is potentially observable with future CMB-S4 experiments [20] and a modified relic neutrino spectrum observable at PTOLEMY [21,22]. Such a contribution of ΔNeff can alleviate the discrepancy between early and late time measurements of the Hubble constant (for recent reviews see [23,24])
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