A comment about the cosmology on a bubble wall

TL;DR

This paper discusses a model of an inflating universe represented by a bubble wall between different AdS vacua, highlighting a discrepancy in wavefunction predictions due to the effective gravity being scalar rather than tensorial.

Contribution

It clarifies that the effective lower-dimensional description of the bubble wall universe is scalar gravity, explaining the disagreement with Hartle-Hawking predictions.

Findings

The model predicts an inflating universe via a bubble wall interface.

Discrepancies in wavefunction predictions are due to scalar gravity effects.

Effective description is spin-0 (Nordström) gravity, not Einstein gravity.

Abstract

The interface between a big bubble of true AdS$_D$ vacuum expanding inside a false AdS$_D$ vacuum is a model of an inflating $D-1$ dimensional universe. It looks like an interesting setup to study fundamentals of inflation. A recent computation shows that the prediction of this model for the wavefunction of the universe disagrees with that of Hartle and Hawking. We show that this discrepancy is because the effective $D-1$ dimensional description of the model is spin-$0$ (Nordstr\"om) gravity rather than spin-2 (Einstein) gravity.