Steven Weinberg reviews and extends the "in-in" formalism to calculate higher-order Gaussian and non-Gaussian correlations in cosmology. Previous calculations used tree graphs, but loop graphs are also considered. The contributions of loop graphs depend on the behavior of the inflaton potential near horizon exit for some theories. A one-loop calculation is presented. The paper discusses the late-time behavior of cosmological correlations, showing that for certain theories, these correlations depend only on the inflaton field's behavior near horizon exit. The "in-in" formalism is used to evaluate time-ordered products of field operators. The paper proves a theorem about the late-time behavior of cosmological correlations, showing that they depend only on the inflaton field's behavior near horizon exit if certain conditions are met. The theorem is applied to various theories, including those with additional scalar fields. The paper also discusses the challenges of integrating over internal wave numbers and the importance of renormalization in removing ultraviolet divergences. A sample one-loop calculation of a cosmological correlation is presented, showing how the results depend on the inflaton field's behavior near horizon exit. The paper concludes that the contributions of loop graphs depend on the inflaton field's behavior near horizon exit, and that this allows for confident calculations of loop contributions.Steven Weinberg reviews and extends the "in-in" formalism to calculate higher-order Gaussian and non-Gaussian correlations in cosmology. Previous calculations used tree graphs, but loop graphs are also considered. The contributions of loop graphs depend on the behavior of the inflaton potential near horizon exit for some theories. A one-loop calculation is presented. The paper discusses the late-time behavior of cosmological correlations, showing that for certain theories, these correlations depend only on the inflaton field's behavior near horizon exit. The "in-in" formalism is used to evaluate time-ordered products of field operators. The paper proves a theorem about the late-time behavior of cosmological correlations, showing that they depend only on the inflaton field's behavior near horizon exit if certain conditions are met. The theorem is applied to various theories, including those with additional scalar fields. The paper also discusses the challenges of integrating over internal wave numbers and the importance of renormalization in removing ultraviolet divergences. A sample one-loop calculation of a cosmological correlation is presented, showing how the results depend on the inflaton field's behavior near horizon exit. The paper concludes that the contributions of loop graphs depend on the inflaton field's behavior near horizon exit, and that this allows for confident calculations of loop contributions.