Ultralight dark matter or dark radiation cosmologically produced from infrared dressing

TL;DR

This paper investigates how infrared dressing of heavy particles by massless particles in a cosmological setting can produce ultra-light dark matter or dark radiation, revealing a universal decay process and entanglement growth over time.

Contribution

It introduces a non-perturbative dynamical resummation method to study particle evolution, showing a universal power-law decay and entanglement in the production of dark radiation.

Findings

Power-law decay of initial particle amplitude with anomalous dimension

Asymptotic state as an entangled mixture of heavy and massless particles

Energy-momentum tensor describes two covariantly conserved fluids

Abstract

Infrared dressing of bosonic or fermionic heavy particles by a cloud of massless particles to which they couple is studied as a possible production mechanism of ultra light dark matter or dark radiation in a radiation dominated cosmology. We implement an adiabatic expansion valid for wavelengths much smaller than the Hubble radius combined with a non-perturbative and manifestly unitary dynamical resummation method to study the time evolution of an initial single heavy particle state. We find a striking resemblance to the process of particle decay: the initial amplitude of the single particle decays in time, not exponentially but with a power law with anomalous dimension $\propto t^{-\Delta/2}$ featuring a crossover to $t^{-\Delta}$ as the heavy particle becomes non-relativistic in both bosonic and fermionic cases suggesting certain universality. At long time the asymptotic state is an entangled state of the heavy and massless particles. The entanglement entropy is shown to grow under time evolution describing the flow of information from the initial single particle to the final multiparticle state. The expectation value of the energy momentum tensor in the asymptotic state is described by two indpendent fluids each obeying covariant conservation, one of heavy particles and the other of relativistic (massless) particles (dark radiation). Both fluids share the same frozen distribution function and entropy as a consequence of entanglement.