Resilience of long modes in cosmological observables
  By a careful implementation of gauge transformations involving
long-wavelength modes, we show that a variety of effects involving squeezed
bispectrum configurations, for which one Fourier mode is much shorter than the
other two, cannot be gauged away, except for the unphysical exactly
infinite-wavelength ($k=0$) limit. Our result applies, in particular, to the
Maldacena consistency relation for single-field inflation, yielding a local
non-Gaussianity strength $f_{\rm NL}^{\rm local} = - (5/12)(n_S-1)$ (with $n_S$
the primordial spectral index of scalar perturbations), and to the $f_{\rm
NL}^{\rm GR} = -5/3$ term, appearing in the dark matter bispectrum and in the
halo bias, as a consequence of the general relativistic non-linear evolution of
matter perturbations. Such effects are therefore physical and observable in
principle by future high-sensitivity experiments.
