# 21 cm cosmology and spin temperature reduction via spin-dependent dark matter interactions

## TL;DR

This paper proposes a novel mechanism where spin-dependent dark matter interactions directly lower the hydrogen spin temperature during cosmic dawn, but finds current models are constrained by stellar cooling bounds.

## Contribution

It introduces a new dark matter interaction model that could explain the 21 cm absorption feature by reducing the spin temperature without affecting gas temperature.

## Key findings

- Significant spin temperature reduction is excluded by stellar cooling constraints.
- A simple asymmetric fermionic dark matter model with a pseudo-vector mediator is incompatible with observations.
- Alternative dark sector models might still achieve the same effect under different constraints.

## Abstract

The EDGES low-band experiment has measured an absorption feature in the cosmic microwave background radiation (CMB), corresponding to the 21 cm hyperfine transition of hydrogen at redshift $z \simeq 17$, before the era of cosmic reionization. The amplitude of this absorption is connected to the ratio of singlet and triplet hyperfine states in the hydrogen gas, which can be parametrized by a spin temperature. The EDGES result suggests that the spin temperature is lower than the expected temperatures of both the CMB and the hydrogen gas. A variety of mechanisms have been proposed in order to explain this signal, for example by lowering the kinetic temperature of the hydrogen gas via dark matter interactions. We introduce an alternative mechanism, by which a sub-GeV dark matter particle with spin-dependent coupling to nucleons or electrons can cause hyperfine transitions and lower the spin temperature directly, with negligible reduction of the kinetic temperature of the hydrogen gas. We consider a model with an asymmetric dark matter fermion and a light pseudo-vector mediator. Significant reduction of the spin temperature by this simple model is excluded, most strongly by coupling constant bounds coming from stellar cooling. Perhaps an alternative dark sector model, subject to different sets of constraints, can lower the spin temperature by the same mechanism.