Gauge bosons associated to new gauge symmetries under which the standard
model particles are not charged are predicted in many extensions of the
standard model of particles and interactions. We show that under very general
conditions, the average energy-momentum tensor of these rapidly oscillating
vector fields is isotropic for any locally inertial observer. This result has a
fundamental importance in order to consider coherent vector fields as a viable
alternative to support models of dark matter, dark energy or inflation.Comment: 4 pages, 1 figure. Contributed to the 9th Patras Workshop on Axions,
WIMPs and WISPs, Mainz, June 24-28, 201
