Unequal time correlators and the Zeldovich approximation
  The modeling of cosmological observables is based on the statistics of the
matter density, velocity and gravitational fields in the Universe as a function
of time. Typically, calculations are restricted to equal time correlations,
where any given fields are evaluated at the same redshift. For some
applications, it is necessary to make accurate predictions of unequal time
correlators, where the fields considered are evaluated at different redshifts.
In this work, we show that the Zeldovich approximation provides an accurate
(<10%) analytical prescription to model unequal time correlators, which we
validate against numerical N-body simulations. The Zeldovich approximation
introduces a scale-dependent exponential suppression of unequal time
correlators, which depends on cosmology and the redshifts of the fields
considered. Comparing the Zeldovich case to previous approximations, we show
that it can yield accurate predictions for wavenumbers that extend well into
the nonlinear regime. However, we also show that correlations over such scales
are typically suppressed by the geometry of the lightcone, and thus should
normally be negligible for cosmology with galaxy surveys. We discuss potential
exceptions, such as intrinsic galaxy alignments, where unequal time correlators
could play a role in the modeling of the observables.
