Gazal Geshnizjani
A measure for local cosmological expansion rate
We calculate the back reaction of cosmological perturbations on
a general relativistic variable which measures the local expansion
rate of the Universe. Specifically, we consider a cosmological
model in which matter is described by a single field.
We analyze back reaction both in a matter dominated Universe and in
a phase of scalar field-driven chaotic inflation. In both cases,
we find that the leading infrared terms contributing to the
back reaction vanish when the local expansion rate is measured
at a fixed value of the matter field which is used as a clock,
whereas they do not appear to vanish if the expansion rate is
evaluated at a fixed value of the background time. We discuss
possible implications for more realistic models with a more complicated matter
sector.
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