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Amplitudes meet Cosmology: A (Scalar) Primer
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We review the most recent progress in our understanding of quantum mechanical observables in cosmology in the perturbative regime. It relies on an approach that considers them directly as functions of the data at the space-like boundary at future infinity prescinding from the explicit time evolution. It takes inspiration from the on-shell formulation of perturbative scattering amplitudes developed in the past 20 years: starting with the requirement of consistency with some fundamental principles such as causality, unitarity and locality, it provides different ways of phrasing and extracting predictions. In this review, we aim to provide a pedagogical treatment of the most recent insights about the analytic structure of the perturbative quantum mechanical observables in cosmology, its relation to fundamental principles as well as physical processes, and how such observables and their features emerge from novel well-defined mathematical objects with their own first principle definition. The review is divided in three parts: Part 0 discusses the definition of quantum mechanical observables in cosmology and some general principles; Part I reviews the boundary approach to the analysis and computation of the perturbative wavefunction of the universe; Part II provides an introduction to the combinatorial-geometrical description of cosmological processes in terms of cosmological polytopes.
Forward citations
Cited by 5 Pith papers
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All Tree-Level Massive Cosmological Correlators via Spectral Gluing
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Perturbations of Plane Waves and Quadratic Quasinormal Modes on the Lightring
Second-order gravitational perturbations on plane waves are solved with a GHP master equation and tensor harmonics, yielding quadratic quasinormal mode ratios and selection rules.
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Resummation of Cosmological Correlators and their UV-Regularization
All-order necklace cosmological correlators are resummable only after a topology-dependent modification of analytic regularization, yielding a pole and a sign flip that are interpreted as non-perturbative large-N effects.
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Correlators are simpler than wavefunctions
Equal-time correlators are simpler than wavefunctions because they come from full-spacetime integrals; this implies fewer poles, cleaner factorization, and a systematic pole expansion whose first subleading term vanishes.
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In-In EFT
An EFT for in-in correlators is defined by matching the full theory's real-time correlators, leading to operators with boundary terms and odd time derivatives.
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