Measurement of the dispersion of radiation from a steady cosmological
  source
  The `missing baryons' of the near universe are believed to be principally in
a partially ionized state. Although passing electromagnetic waves are dispersed
by the plasma, the effect has hitherto not been utilized as a means of
detection because it is generally believed that a successful observation
requires the background source to be highly variable, \ie~the class of sources
that could potentially deliver a verdict is limited. We argue in two stages
that this condition is not necessary. First, by modeling the fluctuations on
macroscopic scales as interference between wave packets we show that, in
accordance with the ideas advanced by Einstein in 1917, both the behavior of
photons as bosons (\ie~the intensity variance has contributions from Poisson
and phase noise) and the van-Cittert-Zernike theorem are a consequence of
wave-particle duality. Nevertheless, we then point out that in general the
variance on some macroscopic timescale $\tau$ consists of (a) a main
contributing term $\propto 1/\tau$, plus (b) a small negative term $\propto
1/\tau^2$ due to the finite size of the wave packets. If the radiation passes
through a dispersive medium, this size will be enlarged well beyond its vacuum
minimum value of $\Delta t \approx 1/\Delta\nu$, leading to a more negative (b)
term (while (a) remains unchanged) and hence a suppression of the variance
w.r.t. the vacuum scenario. The phenomenon, which is typically at the few parts
in 10$^5$ level, enables one to measure cosmological dispersion in principle.
Signal-to-noise estimates, along with systematic issues and how to overcome
them, will be presented.
