New geometric and field theoretic aspects of a radiation dominated
  universe
  The homogeneous and isotropic radiation dominated universe, following the
inflationary stage, is expressed as a spherically symmetric and inhomogeneous
spacetime upon a power law type conformal transformation of the null
(cosmological) coordinates. This new spacetime metric has many interesting
properties. While the static observers, at a fixed position in this new
spacetime, do not see any horizon, some non-static observers encounter a
horizon due to their motion which is analogous to the situation of Rindler
observers in Minkowski spacetime. The symmetry of the new metric offers a
unitarily inequivalent quantization of the massless scalar field and provides a
new example of particle creation. We calculate the particle content of the
cosmological vacuum state with respect to the static observer in this new
spacetime who, with respect to cosmological time, is freely falling in
asymptotic past and future but accelerated in between.
