In this paper we study non-perturbative instabilities in Anti de-Sitter vacua arising from flux compactifications of string models with broken supersymmetry. In the semi-classical limit, these processes drive the vacua towards lower fluxes, which translate into higher curvatures and higher string couplings. In order to shed some light on this regime, we provide evidence for a description in terms of branes, which generate near-horizon AdS throats. To this end, we study the attractor properties of the geometries near the throat, and we also characterize their asymptotics away from it. We also describe the instability within a probe-brane picture, finding an agreement between low-energy (super)gravity and brane instanton estimates of the decay rates.

We explore self-similar collapse solutions to the Einstein-axion-dilaton system in arbitrary spacetime dimensions, which are invariant under spacetime dilation combined with internal SL(2, R) transformations. We set up a numerical formalism, and test it in four and five dimensions, discovering several new families of solutions in the three conjugacy classes of sl(2, R).

We explore the holographic properties of non-perturbative vacuum decay in Anti-de Sitter (AdS) geometries. To this end, we consider a gravitational theory in a metastable AdS 3 state, which decays into an AdS 3 of lower vacuum energy via bubble nucleation, and we employ the Ryu-Takayanagi conjecture to compute the entanglement entropy S ent in its alleged holographic dual. Our analysis connects the nucleation and growth of a vacuum bubble to a relevant deformation and a subsequent Renormalization Group (RG) flow in the boundary theory, with S ent a c-function. We provide some evidence for the claim and comment on the holographic interpretation of off-centred or multiple bubbles. We also frame the issue in the formalism of Holographic Integral Geometry, highlighting some consequences on the structure of the holographic RG flow and recovering the standard holographic RG as a limiting case.

We explore systematically perturbations of self-similar solutions to the Einstein-axion-dilaton system, whose dynamics are invariant under spacetime dilations combined with internal sl(2, R) transformations. The self-similar solutions capture the enticing behavior "critical" systems on the verge of gravitational collapse, in arbitrary spacetime dimensions. Our methods rest on a combination of analytical and numerical tools, apply to all three conjugacy classes of sl(2, R) transformations and allow accurate estimates of the corresponding Choptuik exponents. It is well known that these exponents depend on the spacetime dimension and on the matter content. Our main result is that they also attain different values, even within a given conjugacy class, for the distinct types of critical solutions that we recently identified in the Einstein-axion-dilaton system.

We consider the relativistic generalization of the problem of the “least uncomfortable” linear trajectory from point A to point B. The traditional problem minimizes the time-integrated squared acceleration (termed the “discomfort”), and there is a universal solution for all distances and durations. This universality fails when the maximum speed of the trajectory becomes relativistic, and we consider the more general case of minimizing the squared proper acceleration over a proper time. The least uncomfortable relativistic trajectory has a rapidity that evolves like the motion of a particle in a hyperbolic sine potential, agreeing with the classical solution at low velocities. We consider the special case of a hypothetical trip to Alpha Centauri and compare the minimal-discomfort trajectory to the one with uniform Earth-like acceleration.