Abstract
Light dark photons are subject to various plasma effects, such as Debye screening and resonant oscillations, which can lead to a more complex cosmological evolution than is experienced by conventional cold dark matter candidates. Maintaining a consistent history of dark photon dark matter requires ensuring that the superthermal abundance present in the early Universe (i) does not deviate significantly after the formation of the cosmic microwave background (CMB), and (ii) does not excessively leak into the Standard Model plasma after big band nucleosynthesis (BBN). We point out that the role of nonresonant absorption, which has previously been neglected in cosmological studies of this dark matter candidate, produces strong constraints on dark photon dark matter with mass as low as ${10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$. Furthermore, we show that resonant conversion of dark photons after recombination can produce excessive heating of the intergalactic medium (IGM) which is capable of prematurely reionizing hydrogen and helium, leaving a distinct imprint on both the Ly-$\ensuremath{\alpha}$ forest and the integrated optical depth of the CMB. Our constraints surpass existing cosmological bounds by more than 5 orders of magnitude across a wide range of dark photon masses.
Highlights
As many once-favored models of particle dark matter become increasingly constrained, candidates other than those resulting from weak-scale thermal freeze-out have been the subject of growing focus and development
We show that resonant conversion of dark photons after recombination can produce excessive heating of the intergalactic medium (IGM) which is capable of prematurely reionizing hydrogen and helium, leaving a distinct imprint on both the Ly-α forest and the integrated optical depth of the cosmic microwave background (CMB)
We provide a review of the signatures imprinted on the CMB from energy transfers between the dark and visible sectors in the Appendix A, and focus below only the formalism adopted for computing the current limits and projected sensitivity
Summary
As many once-favored models of particle dark matter become increasingly constrained (see e.g., [1,2,3,4,5]), candidates other than those resulting from weak-scale thermal freeze-out have been the subject of growing focus and development. WITTE absorption of dark photons, subsequently heating baryonic matter; if this heating is sufficiently large, it may destroy the thermal equilibrium of the Milky Way’s interstellar medium [29], that of ultrafaint dwarf galaxies such as Leo T [30], or cold gas clouds in the Galactic Center [31] This idea has been used to project the sensitivity that could be obtained from future 21 cm experiments which observe absorption spectra during the cosmic dark ages [32]. We identify (and describe in a unified manner) the resonant and nonresonant contributions to both of these classes of observables We find that these simple and robust requirements lead to extremely stringent constraints for light photon dark matter, covering dark photon masses all the way down to ∼10−22 eV.
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