Bubble dynamics: (nucleating) radiation inside dust

TL;DR

This paper analyzes the dynamics of radiation bubbles nucleating within dust-filled spacetimes, providing an analytical approach to understand their evolution and energy conditions, with implications for cosmology and phase transitions.

Contribution

It offers an analytical, perturbative description of bubble evolution in dust and radiation spacetimes, extending to various matter configurations and potential cosmological applications.

Findings

Radiation bubbles can expand within an expanding dust universe.

Nucleation is energetically favored when radiation collapses into dust.

The approach is adaptable to different matter combinations and surface densities.

Abstract

We consider two spatially flat Friedmann-Robertson-Walker spacetimes divided by a time-like thin shell in the nontrivial case in which the inner region of finite extension contains radiation and the outer region is filled with dust. We will then show that, while the evolution is determined by a large set of constraints, an analytical description for the evolution of the bubble radius can be obtained by formally expanding for short times after the shell attains its minimum size. In particular, we will find that a bubble of radiation, starting out with vanishing expansion speed, can be matched with an expanding dust exterior, but not with a collapsing dust exterior, regardless of the dust energy density. The former case can then be used to describe the nucleation of a bubble of radiation inside an expanding dust cloud, although the final configuration contains more energy than the initial dust, and the reverse process, with collapsing radiation transforming into collapsing dust, is therefore energetically favored. We however speculate a (small) decaying vacuum energy or cosmological constant inside dust could still trigger nucleation. Finally, our perturbative (yet analytical) approach can be easily adapted to different combinations of matter inside and outside the shell, as well as to more general surface density, of relevance for cosmology and studies of defect formation during phase transitions.