The cosmic optical background excess, dark matter, and line-intensity mapping

Abstract

Recent studies using New Horizons LORRI images have returned the most precise measurement of the cosmic optical background to date, yielding a flux that exceeds that expected from deep galaxy counts by roughly a factor of two. We investigate whether this excess, detected at ∼ 4σ significance, is due to dark matter that decays to a monoenergetic photon with a rest-frame energy in the range 0.5 − 10 eV. We derive the spectral energy distribution from such decays and the contribution to the flux measured by LORRI. The parameter space that explains the measured excess with decays to photons with energies E 4 eV is unconstrained to date. If the excess arises from dark-matter decay to a photon line, there will be a significant signal in forthcoming line-intensity mapping measurements. Moreover, the ultraviolet instrument aboard New Horizons (which will have better sensitivity and probe a different range of the spectrum) and future studies of very high-energy γ-ray attenuation will also test this hypothesis and expand the search for dark matter to a wider range of frequencies.

Anisotropic cosmic optical background bound for decaying dark matter in light of the LORRI anomaly

“…where we have taken the speed of light to be unity. The Hubble parameter at the redshift z is given by H 4 where Ω Λ , Ω m and Ω r denote the density parameter of the dark energy, total matter and radiation, respectively. The photon spectrum at the 2 body decay of χ has a delta-function shape 4…”

“…This is about ∼ 4σ level above the expected flux from the Hubble Space Telescope (HST) galaxy counts. It would be exciting if this is a hint of the DM [3,4].…”