The paper by Steven Weinberg reviews and extends the "in-in" formalism to calculate higher-order Gaussian and non-Gaussian correlations in cosmology. The "in-in" formalism is used to evaluate tree and loop graphs, with a focus on whether the contributions of loop graphs depend only on the behavior of the inflaton potential near the time of horizon exit. Weinberg presents a sample one-loop calculation and discusses the challenges and potential insights that loop corrections can provide. The paper also introduces a class of theories with a single inflaton field and additional massless scalar fields, and proves a theorem about the late-time behavior of cosmological correlations. The theorem states that if ultraviolet divergences cancel in the integrals over internal wave numbers, then cosmological correlations depend only on the behavior of the inflaton field near the time of horizon exit. However, there are theories where this result does not apply, such as those with self-interactions for additional scalar fields.The paper by Steven Weinberg reviews and extends the "in-in" formalism to calculate higher-order Gaussian and non-Gaussian correlations in cosmology. The "in-in" formalism is used to evaluate tree and loop graphs, with a focus on whether the contributions of loop graphs depend only on the behavior of the inflaton potential near the time of horizon exit. Weinberg presents a sample one-loop calculation and discusses the challenges and potential insights that loop corrections can provide. The paper also introduces a class of theories with a single inflaton field and additional massless scalar fields, and proves a theorem about the late-time behavior of cosmological correlations. The theorem states that if ultraviolet divergences cancel in the integrals over internal wave numbers, then cosmological correlations depend only on the behavior of the inflaton field near the time of horizon exit. However, there are theories where this result does not apply, such as those with self-interactions for additional scalar fields.