  The regularized vacuum fluctuation related to a conformally coupled massless
scalar field defined on a space-time with dynamical horizon is computed with
respect a radially moving observer in a generic flat Friedmann-Robertson-Walker
space-time. Two simple measurement prescriptions are given in order to remove
the ambiguity associated with the short distance singularity of the correlation
function. In some cases, it turns out that one is dealing with a quantum
thermometer, recovering a proposal due to Buchholzet al. in order to determine
local temperature in the framework of quantum field theory. In general, by
arranging the detector so that it does not register for inertial motion in flat
space, the regularized quantum fluctuation may be used as a probe of space-time
geometry and, in particular, may provide informations on the Hubble parameter.
As an aside, it is not possible in general to fully decouple the effect of the
detector motion from the universe expansion, a fact that could be interpreted
as a kind of Machian effect which can be traced back to the worldwide nature of
the vacuum.
