# On gravitational wave-Cherenkov radiation from photons when passing through diffused dark matters

## TL;DR

This paper investigates the theoretical possibility of gravitational wave-Cherenkov radiation (GWCR) caused by photons passing through diffuse dark matter, concluding that the energy loss is negligible over cosmic timescales, thus justifying its neglect in astrophysics.

## Contribution

First calculation of photon energy loss via GWCR in diffuse dark matter, demonstrating the effect is negligible over the universe's age.

## Key findings

- Photon energy loss time exceeds the Hubble time
- GWCR effect is negligible in astrophysical contexts
- Supports ignoring GWCR in cosmological models

## Abstract

Analogy to Cherenkov radiation, when a particle moves faster than the propagation velocity of gravitational wave in matter ($v>c_{\rm{g}}$), we expect gravitational wave-Cherenkov radiation (GWCR). In the situation that a photon travels across diffuse dark matters, the GWCR condition is always satisfied, photon will thence loss its energy all the path. This effect is long been ignored in the practice of astrophysics and cosmology, without justification with serious calculation. We study this effect for the first time, and shows that this energy loss time of the photon is far longer than the Hubble time, therefore justify the practice of ignoring this effect in astrophysics context.