  We study the renormalized energy-momentum tensor of cosmological scalar
fluctuations during the slow-rollover regime for power-law inflation and find
that it is characterized by a negative energy density at the leading order,
with the same time behaviour as the background energy. The average expansion
rate appears decreased by the back-reaction of the effective energy of
cosmological fluctuations, but this value is comparable with the energy of
background only if inflation starts at a Planckian energy. We also find that,
for this particular model, the first and second order inflaton fluctuations are
decoupled and satisfy the same equation of motion. To conclude, the fourth
order adiabatic expansion for the inflaton scalar field is evaluated for a
general potential V(\phi).
