# Cosmic branes and asymptotic structure

## TL;DR

This paper investigates how including cosmic branes in higher-dimensional asymptotically flat spacetimes affects their boundary conditions and asymptotic symmetries, with implications for soft theorems and memory effects.

## Contribution

It extends the understanding of asymptotic structures by deriving boundary conditions for cosmic branes in dimensions greater than four and analyzing their impact on asymptotic symmetries.

## Key findings

- Cosmic (d-3)-branes are Riemann flat near the brane.
- Derived boundary conditions for asymptotically locally flat spacetimes with branes.
- Found polyhomogenous expansion with logarithmic terms in d=5 case.

## Abstract

Superrotations of asymptotically flat spacetimes in four dimensions can be interpreted in terms of including cosmic strings within the phase space of allowed solutions. In this paper we explore the implications of the inclusion of cosmic branes on the asymptotic structure of vacuum spacetimes in dimension d > 4. We first show that only cosmic (d-3)-branes are Riemann flat in the neighbourhood of the brane, and therefore only branes of such dimension passing through the celestial sphere can respect asymptotic local flatness. We derive the asymptotically locally flat boundary conditions associated with including cosmic branes in the phase space of solutions. We find the asymptotic expansion of vacuum spacetimes in d=5 with such boundary conditions; the expansion is polyhomogenous, with logarithmic terms arising at subleading orders in the expansion. The asymptotically locally flat boundary conditions identified here are associated with an extended asymptotic symmetry group, which may be relevant to soft scattering theorems and memory effects.