Quantum Gravity

Inspired by the corpuscular model of gravity, I will discuss a simplified quantum description of neutral and electrically charged spherically symmetric black holes given by coherent states of toy gravitons. From such states I will derive effective geometries devoid of curvature singularities (replaced by integrable ones) and Cauchy horizons. Lastly, I will present a similar analysis for the Schwarzschild-de Sitter geometry, used as a model for the late Universe, showing that the “reaction” of the de Sitter background to the presence of localised matter sources induces a modified Newtonian dynamics at galactic scales and different values measured for the present Hubble parameter.

Zoom Link: https://pitp.zoom.us/j/93087108672?pwd=ekQrLysrcjRwVWFSZ2J5bndteXBtdz09

Considering the scale dependent effective spacetimes implied by the functional renormalization group in d-dimensional Quantum Einstein Gravity, we discuss the representation of entire evolution histories by means of a single, (d+1)-dimensional manifold furnished with a fixed (pseudo-) Riemannian structure.

We propose a universal form of the higher dimensional metric and discuss its properties. We show that, under precise conditions, this metric is always Ricci flat; if the evolving spacetimes are maximally symmetric, their (d+1)-dimensional representative has a vanishing Riemann tensor even. The non-degeneracy of the higher dimensional metric is linked to a monotonicity requirement for the running of the cosmological constant, which we test in the case of Asymptotic Safety.

Furthermore, we allow the higher dimensional manifold to be an arbitrary Einstein space, admitting the possibility that the spacetimes to be embedded have a Lorentzian signature, a prime example being a stack of de Sitter spaces. We “derive” the (A)dS/CFT correspondence by applying the gravitational Effective Average Action approach, by solving the corresponding functional RG and the effective Einstein equations, and finally embedding the 4D metrics into the one single 5-dimensional one. It is an intriguing possibility that in this way one might find a specific solution to the general equations which coincides with the 5D kinematic setting which forms the basis of the conjectured (A)dS/CFT correspondence.

Zoom Link: https://pitp.zoom.us/j/92268060878?pwd=M2E1S1RxcHBFbzBiblhpSUJCMWIzUT09

States of Low Energy (SLEs) have been constructed in cosmological spacetimes as the ones that minimize the mode contribution to the regularized energy density. These have been shown to be an adequate choice of vacuum state for primordial perturbations in models that include a period of kinetic domination prior to inflation. This is precisely the case in Loop Quantum Cosmology (LQC). In this talk we will review the construction and properties of SLEs for a general FLRW model and explore their application to LQC.

Zoom Link: https://pitp.zoom.us/j/98112698584?pwd=NkFhMWVNM3gvc1lYQ3cvVDFGL0laUT09

In this seminar, I will explore the effect of quantum gravity on matter couplings within a (Functional) Renormalization Group framework. I will mainly focus on gravitational contribution to the flow of gauge couplings. In particular, I will focus on results obtained from a class of interpolating regulators that allow us to extract certain universal pieces from non-universal quantities. I will argue that gravity might induce the UV completion of an Abelian-gauge sector, despite an apparent vanishing contribution to its flow when we consider only universal pieces. This result might offer a new perspective on the differences between perturbative studies and Functional Renormalization Group studies.

Zoom Link: https://pitp.zoom.us/j/97277305442?pwd=d240TXhrNnZ0TlErbGQ0bCtGU1NCZz09

In this talk, I intend to give a pedagogical, nonetheless biased, introduction to String Cosmology. After briefly reviewing the Lambda-CDM model and motivating inflation, we will remind ourselves that early universe cosmology remains singular and waiting for alternatives. That will be our cue to consider string theory to define our gravity sector and string thermodynamics to define our matter sector. We will learn how that doesn't work unless we consider more string corrections (in fact, an infinity of them!). Once we are stringy enough, we will be able to build a full cosmological model that is non-singular and already poses itself as an alternative to inflation at the background level.

Zoom Link: https://pitp.zoom.us/j/98491655811?pwd=ZzArNjFVMmZIdE1STjd2TVNWSlZtZz09

I’ll review a new, simpler explanation for the large scale geometry of spacetime, presented recently by Latham Boyle and me in arXiv:2201.07279. The basic ingredients are elementary and well-known, namely Einstein’s theory of gravity and Hawking’s method of computing gravitational entropy. The new twist is provided by the boundary conditions we proposed for big bang-type singularities, respecting CPT symmetry and analyticity at the bang with finite Weyl curvature. These boundary conditions allow gravitational instantons for universes with positive Lambda, massless (conformal) radiation and positive or negative space curvature. Using these new instantons, we are able to infer the gravitational entropy for a complete set of quasi-realistic, four-dimensional cosmologies. If the total entropy in radiation exceeds that of Einstein’s static universe, the gravitational entropy exceeds the de Sitter entropy. As the total entropy in radiation is increased further,  the most probable large-scale geometry for the universe becomes increasingly flat, homogeneous and isotropic. I’ll also briefly summarize recent progress towards elaborating this picture into a fully predictive cosmological theory.

Zoom Link: https://pitp.zoom.us/j/95474226765?pwd=clN5UHBnSTFiSVAzM0p3dEdDMzV2Zz09

The analogue gravity framework uses condensed matter systems to simulate phenomena characteristics of quantum field theory in curved spacetimes (e.g. cosmological particle production or black hole evaporation). In this seminar, I will review the state of this field and explore its extension towards the simulation of the emergent spacetime paradigm. In doing so I will discuss three lessons that we can draw from this framework about long standing puzzles in black hole thermodynamics and cosmology.

Zoom Link: https://pitp.zoom.us/j/98714282554?pwd=QnAxRkk4SVg0TnVneDFySFJYTDJoQT09

In this talk, I will explain the construction of a (1+1) spacetime with a wormhole and its conversion into a time-machine spacetime. Then, I will describe a massless scalar quantum field theory in such spacetime. I focus in comparing this quantization of the field in the time-machine spacetime with the quantization on the two-dimensional Einstein cylinder, the emergence of a zero mode and its regularization.

Zoom link:  https://pitp.zoom.us/j/94856731951?pwd=MnFNZjlkUHVVWkRRQTIxR0o0TDFnUT09

In this talk, we first introduce the basic idea of asymptotic safety and discuss its application to quantum gravity. We compute non-perturbative flow equations for the couplings of quantum gravity in fourth order of a derivative expansion. It is shown that the beta functions admit two possible fixed points: One is the asymptotically safe fixed point, and the other is the asymptotically free one. The corresponding critical exponents to these fixed points are evaluated.

Next, we argue that asymptotically safe gravity could be an effective theory emerging from the spontaneous symmetry breaking in an ultraviolet (UV) theory. We consider a UV theory invariant under SO(4) local Lorentz symmetry and diffeomorphism. In particular, we impose the degenerate limit (zero eigenvalues of vierbein) on the action and then show that only spinor fields can be dynamical. We will discuss prospects whether or not this UV theory could be a fundamental theory underlying asymptotically safe gravity.

Zoom Link: https://pitp.zoom.us/j/94422371986?pwd=a25pS3cyUmJkNWJHL3hic1NhNlozQT09

By considering the photon-graviton scattering amplitude at low-energy including loops from the Standard Model, I will argue how we can infer constraints on the Regge behaviour of the gravitational scattering amplitude. I will also comment on implications to other gauge fields in four and higher dimensions and application to the Weak Gravity Conjecture and connections with causality considerations.

Zoom Link: https://pitp.zoom.us/j/91414100213?pwd=Z3g5SmRPb2xjUXEweGtjUC83WCtKdz09

We discuss the minimal coupling of gauge fields to the dual graph lattice of Causal Dynamical Triangulations (CDT) as a preliminary step towards the investigation of more realistic systems of gravity coupled to bosonic and fermionic matter. We first introduce the CDT approach and how the gravity and gauge fields are discretized, for general dimensions and gauge groups, focusing on some of the algorithmic complications which arise. Next, we discuss some results from 2D simulations of CDT coupled to U(1) and SU(2) gauge groups, where we studied both gravity and gauge-related observables and the ones related to gauge fields, comparing them to analogous simulations in the flat case.