Session S02: Developing Constraints on Quantum Gravity

Causality in Holographic Approaches to Quantum Gravity

The development of holographic approaches to quantum gravity introduces a new ingredient into the longstanding question of which metrics should be included in a Lorentzian gravitational path integral. In the most well developed example, quantum gravity with asymptotically anti-de Sitter boundary conditions is equivalent to a quantum field theory (QFT) living on the boundary at infinity. The QFT must be causal with respect to the boundary metric, but there are bulk metrics that violate boundary causality. In other words, two spacelike separated boundary points can nevertheless be timelike related with respect to some bulk metrics. Must these "boundary causality violating" metrics be excluded from the Lorentzian gravitational path integral? I will show that the answer is no: these metrics can be included and still keep the QFT  causal.   

Gravitational Instabilities of Spacetime

Theories with compact extra dimensions can be unstable to decay into a bubble of nothing -- an instability that results in the destruction of spacetime. In this talk, I will discuss whether bubbles of nothing can exist in realistic theories where the moduli field responsible for setting the size of the extra dimensions is stabilized at a positive value of the vacuum energy. Using bottom-up methods, and focusing on a five-dimensional toy model, I will argue that four-dimensional de Sitter vacua admit bubbles of nothing for a wide class of stabilizing potentials. I will show that, unlike ordinary Coleman-De Luccia tunneling, the corresponding decay rate remains non-zero in the limit of vanishing vacuum energy, and that it can be faster than decay channels previously discussed in the literature. I will discuss potential implications on the physics of vacuum selection, as well as on the possibility of creating a Universe from "nothing".   

Swampland Constraints on the Particle Spectrum with Positive Vacuum Energy

The Swampland approach to quantum gravity research aims to identify the universal features that quantum gravity may have in low-energy physics. One of the most well-known statements, the Weak Gravity Conjecture (WGC), claims that in any consistent quantum theory of gravity with long-range electromagnetic interactions, there is a particle whose mass is larger than its charge in Planck units. This is supported by evidence coming from string theory compactifications, as well as universal black hole arguments. Repeating these arguments in the context of models with positive vacuum energy, such as de Sitter space, one arrives to a second bound on the spectrum: the Festina Lente (FL) bound, which implies a lower bound on the mass of every charged particle.  I will present this bound, derivation, and phenomenological implications for particle physics, models of de Sitter space in string theory, and inflation. Some of these phenomenological implications include bounds in the shape of the Higgs potential, spectrum of milli-charged dark matter, constraints on the matter sector during inflation, and implications for the electroweak hierarchy and comsological hierarchy problems.