Naturalness in Cosmological Initial Conditions

TL;DR

This paper introduces a boundary term framework within effective field theory to classify and analyze the naturalness of cosmological initial conditions, including inflation and cyclic models, with implications for observability and fine-tuning.

Contribution

It develops a novel boundary term approach to classify and assess the naturalness of initial conditions in cosmology, linking effective field theory with boundary operators.

Findings

Boundary terms can support extra degrees of freedom.

Naturalness constraints can identify fine-tuned models.

Some non-Gaussian initial conditions are observable and natural.

Abstract

We propose a novel approach to the problem of constraining cosmological initial conditions. Within the framework of effective field theory, we classify initial conditions in terms of boundary terms added to the effective action describing the cosmological evolution below Planckian energies. These boundary terms can be thought of as spacelike branes which may support extra instantaneous degrees of freedom and extra operators. Interactions and renormalization of these boundary terms allow us to apply to the boundary terms the field-theoretical requirement of naturalness, i.e. stability under radiative corrections. We apply this requirement to slow-roll inflation with non-adiabatic initial conditions, and to cyclic cosmology. This allows us to define in a precise sense when some of these models are fine-tuned. We also describe how to parametrize in a model-independent way non-Gaussian initial conditions; we show that in some cases they are both potentially observable and pass our naturalness requirement.