Gia Dvali, Cesar Gomez, Nico Wintergerst

We suggest that holography can be formulated in terms of the information capacity of the St\"uckelberg degrees of freedom that maintain gauge invariance of the theory in the presence of an information boundary. These St\"uckelbergs act as qubits that account for a certain fraction of quantum information. Their information capacity is measured by the ratio of the inverse St\"uckelberg energy gap to the size of the system. Systems with the smallest gap are maximally holographic. For massless gauge systems this information measure is universally equal to the inverse coupling evaluated at the systems' length scale. In this language it becomes very transparent why the St\"uckelberg information capacity of black holes saturates the Bekenstein bound and accounts for the entire information of the system. The physical reason is that the strength of quantum interaction is bounded from below by the gravitational coupling, which scales as area. Observing the striking similarity between the scalings of the energy gap of the boundary St\"uckelberg modes and the Bogoliubov modes of critical many-body systems, we establish a connection between holography and quantum criticality through the correspondence between these modes.

Anupam Mazumdar, Spyridon Talaganis

In this paper, we will discuss how in Einstein's theory a graviton can be enforced to probe fewer space-time dimensions in the deep ultraviolet (UV) as compared to far infrared (IR), and vice-versa. In particular, from a $D$ dimensional gravitational action in the IR we can project the $D$ dimensional graviton to probe only $2\leq N\leq D-1$ dimensions in the deep UV. Such projections of a graviton propagator can be thought of as an alternative to compactification. We will briefly explain the physical interpretation and consequences of such a dimensional transmutation.

David M. Jacobs

Boundary conditions on non-relativistic wavefunctions are generally not completely constrained by the basic precepts of quantum mechanics, so understanding the set of possible self-adjoint extensions of the Hamiltonian is required. For real physical systems, non-trivial self-adjoint extensions have been used to model contact potentials when those interactions are expected a priori. However, they must be incorporated into the effective description of any quantum mechanical system in order to capture possible short-distance physics that does not decouple in the low energy limit. Here, an approach is described wherein an artificial boundary is inserted at an intermediate scale on which boundary conditions may encode short-distance effects that are hidden behind the boundary. Using this approach, an analysis is performed of the free particle, harmonic oscillator, and Coulomb potential in three dimensions. Requiring measurable quantities, such as spectra and cross sections, to be independent of this artificial boundary, renormalization group-type equations are derived that determine how the boundary conditions flow with the scale of the boundary. Generically, observables differ from their canonical values and symmetries are anomalously broken. Connections are made to well-studied physical systems, such as the deuteron and condensed matter systems that employ Feshbach resonances.

William T. Emond, Paul M. Saffin

We extend a previous self-tuning analysis of the most general scalar-tensor theory of gravity in four dimensions with second order field equations by considering a generalized coupling to the matter sector. Through allowing a disformal coupling to matter we are able to extend the Fab Four model and construct a new class of theories that are able to tune away the cosmological constant on Friedmann-Lemaitre-Robertson-Walker backgrounds.

James H. C. Scargill, Johannes Noller

In this paper we consider how the strong-coupling scale, or perturbative cutoff, in a multi-gravity theory depends upon the presence and structure of interactions between the different fields. This can elegantly be rephrased in terms of the size and structure of the `theory graph' which depicts the interactions in a given theory. We show that the question can be answered in terms of the properties of various graph-theoretical matrices, affording an efficient way to estimate and place bounds on the strong-coupling scale of a given theory. In light of this we also consider the problem of relating a given theory graph to a discretised higher dimensional theory, a la dimensional deconstruction.

José Luis Bernal, Licia Verde, Antonio J. Cuesta

We perform an empirical consistency test of General Relativity/dark energy by disentangling expansion history and growth of structure constraints. We replace each late-universe parameter that describes the behavior of dark energy with two meta-parameters: one describing geometrical information in cosmological probes, and the other controlling the growth of structure. If the underlying model (a standard wCDM cosmology with General Relativity) is correct, that is under the null hypothesis, the two meta-parameters coincide. If they do not, it could indicate a failure of the model or systematics in the data. We present a global analysis using state-of-the-art cosmological data sets which points in the direction that cosmic structures prefer a weaker growth than that inferred by background probes. This result could signify inconsistencies of the model, the necessity of extensions to it or the presence of systematic errors in the data. We examine all these possibilities. The fact that the result is mostly driven by a specific sub-set of galaxy clusters abundance data, points to the need of a better understanding of this probe.

Sebastian Garcia-Saenz, Kurt Hinterbichler, Austin Joyce, Ermis Mitsou, Rachel A. Rosen

There are various no-go results forbidding self-interactions for a single partially massless spin-2 field. Given the photon-like structure of the linear partially massless field, it is natural to ask whether a multiplet of such fields can interact under an internal Yang-Mills like extension of the partially massless symmetry. We give two arguments that such a partially massless Yang-Mills theory does not exist. The first is that there is no Yang-Mills like non-abelian deformation of the partially massless symmetry, and the second is that cubic vertices with the appropriate structure constants do not exist.

M. Maniatis

We emphasize that scattering amplitudes of a wide class of models to any order are constructible by solely on-shell amplitudes. This follows from the Feynman-tree theorem combined with BCFW on-shell recursion relations. In contrast to the usual Feynman diagrams, there appear no virtual particles.

Pierre Fleury

The standard model of cosmology is based on the hypothesis that the Universe is spatially homogeneous and isotropic. When interpreting most observations, this cosmological principle is applied stricto sensu: the light emitted by distant sources is assumed to propagate through a Friedmann-Lema\^itre spacetime. The main goal of the present thesis was to evaluate how reliable this assumption is, especially when small scales are at stake. After having reviewed the laws of geometric optics in curved spacetime, and the standard interpretation of cosmological observables, the dissertation reports a comprehensive analysis of light propagation in Swiss-cheese models, designed to capture the clumpy character of the Universe. The resulting impact on the interpretation of the Hubble diagram is quantified, and shown to be relatively small, thanks to the cosmological constant. When applied to current supernova data, the associated corrections tend however to improve the agreement between the cosmological parameters inferred from the Hubble diagram and from the cosmic microwave background. This is a hint that the effect of small-scale structures on light propagation may become non-negligible in the era of precision cosmology. This motivated the development of a new theoretical framework, based on stochastic processes, which aims at describing small-scale gravitational lensing with a better accuracy. Regarding the isotropy side of the cosmological principle, this dissertation addresses, on the one hand, the potential effect of a large-scale anisotropy on light propagation, by solving all the equations of geometric optics in the Bianchi I spacetime. On the other hand, possible sources of such an anisotropy, namely scalar-vector models for inflation or dark energy, are analysed. Most of them turn out to be excluded as physically viable theories.

Martin B Einhorn, D R Timothy Jones

We show that classically scale invariant gravity coupled to a single scalar field can undergo dimensional transmutation and generate an effective Einstein-Hilbert action for gravity, coupled to a massive dilaton. The same theory has an ultraviolet fixed point for coupling constant ratios such that all couplings are asymptotically free. However the catchment basin of this fixed point does not include regions of coupling constant parameter space compatible with locally stable dimensional transmutation. We believe that the desirable outcome may obtain in more complicated theories with non-Abelian gauge interactions.