CDI Perturbations
CDI stands for "Cosmic Damping" which is not an exact match; however, the idea of pre-inflationary axion models generating disturbances or vibrations in space-time can be compared to a dense forest where vibrations cause echoes.
Technical definition
Pre-inflationary axion dark matter isocurvature perturbations are hypothetical fluctuations in the universe's density that could have been generated by axions before inflation.