Cosmic Optical Background Excess, Dark Matter, and Line-Intensity Mapping.

Area of Science:

• Cosmology
• Particle Physics
• Astrophysics

Background:

• Recent observations from New Horizons's Long Range Reconnaissance Imager (LORRI) provide the most precise measurement of the cosmic optical background (COB).
• The measured COB flux is approximately double the expected value from deep galaxy counts, a discrepancy noted at a significance of ~4σ.

Purpose of the Study:

• To investigate the hypothesis that the observed COB excess originates from the decay of axionlike dark matter particles into monoenergetic photons.
• To determine the parameter space (masses and effective axion-photon couplings) for axionlike particles that could explain the LORRI observations.

Main Methods:

• Calculation of the spectral energy distribution (SED) for photons emitted from axionlike dark matter decay.
• Assessment of the contribution of these decay photons to the flux measured by LORRI.
• Analysis of the parameter space for axionlike particles, assuming they constitute all dark matter.

Main Results:

• The study identifies a parameter space for axionlike particles, with masses between 8-20 eV and effective axion-photon couplings of 3-6×10⁻¹¹ GeV⁻¹, that can account for the observed COB excess.
• This parameter space remains unconstrained by previous observations.

Conclusions:

• The observed cosmic optical background excess is consistent with a signal from decaying axionlike dark matter.
• Future line-intensity mapping measurements are expected to provide a significant signal, enabling the discrimination of this dark matter decay hypothesis from alternative explanations.