Does light from steady sources bear any observable imprint of the dispersive intergalactic medium?

摘要

There has recently been some interest in the prospect of detecting ionizedintergalactic baryons by examining the properties of incoherent light frombackground cosmological sources, namely quasars. Although the paper by\cite{lieu13} proposed a way forward, it was refuted by the later theoreticalwork of \cite{hir14} and observational study of \cite{hal16}. In this paper weinvestigated in detail the manner in which incoherent radiation passes througha dispersive medium both from the frameworks of classical and quantumelectrodynamics, which led us to conclude that the premise of \cite{lieu13}would only work if the pulses involved are genuinely classical ones involvingmany photons per pulse, but unfortunately each photon must not be treated as apulse that is susceptible to dispersive broadening. We are nevertheless able tochange the tone of the paper at this juncture, by pointing out that becausecurrent technology allows one to measure the phase of individual modes of radiowaves from a distant source, the most reliable way of obtaining irrefutableevidence of dispersion, namely via the detection of its unique signature of aquadratic spectral phase, may well be already accessible. We demonstrate howthis technique is only applied to measure the column density of the ionizedintergalactic medium.