# Dark radiation: 21cm signals and laboratory tests

## TL;DR

This paper explores the possibility that dark radiation from decaying dark matter could produce detectable signals in 21cm cosmology and laboratory experiments, offering new avenues for testing dark matter models.

## Contribution

It introduces a novel scenario where non-thermal neutrinos and dark photons from dark matter decay can be tested via 21cm signals and laboratory experiments.

## Key findings

- Non-thermal neutrinos can leave detectable imprints in weakly interacting particle searches.
- Dark photons can exceed CMB photon density without current bounds, enabling new detection prospects.
- Conversion of dark photons into photons can be probed through cosmological 21cm observations.

## Abstract

It is entirely possible that our Universe is filled with dark radiation, such as SM neutrinos or new physics states, that are sourced by the decay of dark matter with cosmologically long lifetime. If non-thermal neutrinos produced such way carry sufficient energy, they can leave a detectable imprint in experiments designed to search for very weakly interacting particles: dark matter and underground neutrino experiments. Conversely, a very soft non-thermal population of dark photons sourced this way is allowed to exceed the number density of CMB photons by many orders of magnitude without being in conflict with current bounds. Equipped with a small probability of conversion into ordinary photons, the scenario becomes testable through the cosmological 21cm signal.