Soner Albayrak, Paolo Benincasa, Carlos Duaso Pueyo
SciPost Phys. 16, 157 (2024) ·
published 20 June 2024

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Unitarity of time evolution is one of the basic principles constraining physical processes. Its consequences in the perturbative BunchDavies wavefunction in cosmology have been formulated in terms of the cosmological optical theorem. In this paper, we reanalyse perturbative unitarity for the BunchDavies wavefunction, focusing on: $i)$ the role of the $i\epsilon$prescription and its compatibility with the requirement of unitarity; $ii)$ the origin of the different ``cutting rules''; $iii)$ the emergence of the flatspace optical theorem from the cosmological one. We take the combinatorial point of view of the cosmological polytopes, which provide a firstprinciple description for a large class of scalar graphs contributing to the wavefunctional. The requirement of the positivity of the geometry together with the preservation of its orientation determine the $i\epsilon$prescription. In kinematic space it translates into giving a small negative imaginary part to all the energies, making the wavefunction coefficients welldefined for any value of their real part along the real axis. Unitarity is instead encoded into a nonconvex part of the cosmological polytope, which we name \textit{optical polytope}. The cosmological optical theorem emerges as the equivalence between a specific polytope subdivision of the optical polytope and its triangulations, each of which provides different cutting rules. The flatspace optical theorem instead emerges from the nonconvexity of the optical polytope. On the more mathematical side, we provide two definitions of this nonconvex geometry, none of them based on the idea of the nonconvex geometry as a union of convex ones.
SciPost Phys. 12, 192 (2022) ·
published 10 June 2022

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The way we organise perturbation theory is of fundamental importance both for computing the observables of relevance and for extracting fundamental physics out of them. If on one hand the different ways in which the perturbative observables can be written make manifest different features ({\it e.g.} symmetries as well as principles such as unitarity, causality and locality), on the other hand precisely demanding that some concrete features are manifest lead to different ways of organising perturbation theory. In the context of flatspace scattering amplitudes, a number of them are already known and exploited, while much less is known for cosmological observables. In the present work, we show how to systematically write down both the wavefunction of the universe and the flatspace scattering amplitudes, in such a way that they manifestly show physical poles only. We make use of the invariant definition of such observables in terms of {\it cosmological polytopes} and their {\it scattering facet}. In particular, we show that such representations correspond to triangulations of such objects through hyperplanes identified by the intersection of their facets outside of them. All possible triangulations of this type generate the different representations. This allows us to provide a general proof for the conjectured allloop causal representation of scattering amplitudes. Importantly, all such representations can be viewed as making explicit a subset of compatible singularities, and our construction provides a way to extend Steinmann relations to higher codimension singularities for both the flatspace scattering amplitudes and the cosmological wavefunction.