Braneworld localisation in hyperbolic spacetime

TL;DR

This paper constructs a supergravity background with hyperbolic geometry that localizes massless gravity near an NS5-brane, providing a new mechanism for gravity localisation in higher-dimensional models.

Contribution

It introduces a novel hyperbolic spacetime structure in type IIA supergravity that achieves gravity localisation via a specific wavefunction boundary condition.

Findings

Gravity is localized on the 5-brane due to a mass gap in the spectrum.

The transverse wave operator admits a self-adjoint extension ensuring massless gravity.

A continuum of massive states causes small corrections to Newton's law.

Abstract

We present a construction employing a type IIA supergravity and 3-form flux background together with an NS5-brane that localises massless gravity near the 5-brane worldvolume. The nonsingular underlying type IIA solution is a lift to 10D of the vacuum solution of the 6D Salam-Sezgin model and has a hyperbolic ${\cal H}^{(2,2)}\times S^1$ structure in the lifting dimensions. A fully back-reacted solution including the NS5-brane is constructed by recognising the 10D Salam-Sezgin vacuum solution as a "brane resolved through transgression." The background hyperbolic structure plays a key r\^ole in generating a mass gap in the spectrum of the transverse-space wave operator, which gives rise to the localisation of gravity on the 6D NS5-brane worldvolume, or, equally, in a further compactification to 4D. Also key to the successful localisation of gravity is the specific form of the corresponding transverse wavefunction Schr\"odinger problem, which asymptotically involves a $V=-1/(4\rho^2)$ potential, where $\rho$ is the transverse-space radius, and for which the NS5-brane source gives rise to a specific choice of self-adjoint extension for the transverse wave operator. The corresponding boundary condition as $\rho\to0$ ensures the masslessness of gravity in the effective braneworld theory. Above the mass gap, there is a continuum of massive states which give rise to small corrections to Newton's law.