Sphere amplitudes and observing the universe's size

TL;DR

This paper explores how sine dilaton gravity relates to 2D quantum cosmology and DSSYK, using sphere amplitudes to predict universe size and addressing issues in dS JT gravity within a holographic framework.

Contribution

It demonstrates how to interpret sine dilaton as 2D quantum cosmology and uses dual matrix integrals to analyze universe size predictions.

Findings

Finite sphere amplitude matches dual matrix integral on-shell action.

No-boundary wavefunction norm predicts universe size, with issues in slow-roll inflation.

Normalizable states in sine dilaton gravity lead to a flat universe size distribution.

Abstract

Sine dilaton gravity is holographically related to DSSYK. We explain how to interpret sine dilaton as 2d quantum cosmology. This paves the way for using two copies of DSSYK as hologram for Big-Bang cosmologies. We study the most basic cosmological observable: the sphere amplitude. Via canonical quantization we find a finite answer that matches the on-shell action of a dual matrix integral. The sphere amplitude (or the norm of the no-boundary wavefunction) also gives a prediction for the universe's size. In the context of slow-roll inflation, the no-boundary state is non-normalizable, and predicts a small universe, in contradiction with experiments. We argue that an avatar of these issues exists in dS JT gravity. By considering sine dilaton as a UV completion of dS JT gravity, the state becomes normalizable. We then consider the observer's no-boundary state and show that this prefers neither small nor large universes. The resulting distribution is flat.