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According to the equivalence principal, the long wavelength perturbations must not have any dynamical effect on the short scale physics up to ${cal O} (k_L^2/k_s^2)$. Their effect can be always absorbed to a coordinate transformation locally. So any physical effect of such a perturbation appears only on scales larger than the scale of the perturbation. The bispectrum in the squeezed limit of the curvature perturbation in single-field slow-roll inflation is a good example, where the long wavelength effect is encoded in the spectral index through Maldacenas consistency relation. This implies that one should be able to derive the bispectrum in the squeezed limit without resorting to the in-in formalism in which one computes perturbative corrections field-theoretically. In this short paper, we show that the $delta N$ formalism as it is, or more generically the separate universe approach, when applied carefully can indeed lead to the correct result for the bispectrum in the squeezed limit. Hence despite the common belief that the $delta N$ formalism is incapable of recovering the consistency relation within itself, it is in fact self-contained and consistent.
We develop a theory of nonlinear cosmological perturbations on superhorizon scales for a multi-component scalar field with a general kinetic term and a general form of the potential in the context of inflationary cosmology. We employ the ADM formalis
We propose a test of single-scalar inflation based on using the well-measured scalar power spectrum to reconstruct the tensor power spectrum, up to a single integration constant. Our test is a sort of integrated version of the single-scalar consisten
We revisit the question of how to calculate correlations of the curvature perturbation, $zeta$, using the $delta N$ formalism when one cannot employ a truncated Taylor expansion of $N$. This problem arises when one uses lattice simulations to probe t
We study tachyon inflation within the large-$N$ formalism, which takes a prescription for the small Hubble flow slow--roll parameter $epsilon_1$ as a function of the large number of $e$-folds $N$. This leads to a classification of models through thei
If observations confirm BICEP2s claim of a tensor-scalar ratio $rapprox 0.2$ on CMB scales, then the inflationary consistency relation $n_{t}=-r/8$ predicts a small negative value for the tensor spectral index $n_t$. We show that future CMB polarizat