Schedule

Reception: brunch and coffee 13:00 - 15:30

Talk by María Pérez

Title: The Unruh effect reviewed 

Abstract: In this talk, I analyze the Unruh effect, in which a uniformly accelerated observer perceives the vacuum state in flat spacetime as a thermal bath at a temperature proportional to its acceleration. In quantum field theory, the perception of the vacuum state in Minkowski quantization depends on the observer. To study this phenomenon, I employ a two-level system, called a detector, coupled to a massless free scalar field. Furthermore, the interaction between the detector and the field can be well approximated by a linear coupling. On the other hand, I study the regulating role of the so-called detector's switching function in the detector's response, and the effects of considering the detector as point-like. Finally, I also present a generalization of thermal states valid in systems with an infinite number of degrees of freedom, known as Kubo-Martin-Schwinger (KMS) conditions.

Discussion: Measuring the Unruh effect. Proposals and experiments. (coord. José Polo-Gómez)

The Unruh effect is one of the central predictions of QFT in curved spacetimes, but its experimental confirmation remains so far elusive, due to the large accelerations needed to reach the threshold of our available experimental precission. In this session we will review the alternative proposals to measure this effect that have been put forward in the last decades. The goal of the session is to discuss 1) To what extent these proposals should be understood as Unruh effect, 2) What features they share with the standard formulation and how they differ from it, and 3) Whether their experimental confirmation would in any case be as valuable for QFT in CS as that of the standard Unruh effect. If time permits, we would also review the main points of a claim of experimental confirmation of the Unruh effect [Phys. Rev. D 104, 025015 (2021)], whether this is satisfactory or not, and why.

Discussion: BH/BH mimickers. (coord. Julio Arrechea and Valentin Boyanov)

In this session we will briefly present the current state of the art of research into black holes and black hole mimickers, from their fundamental structure to potential observational signatures. We will then open the floor to a general discussion focused on the topics: - How long do trapped regions live - Do stable black hole mimickers exist - What are the observational signatures which can help us distinguish between the two types of object - How could the effectively classical description of spacetime and matter change when curvature is large.

Discussion: The hitchhikers guide to gravitational physics. (coord. Ana Alonso)

The physics of the last century has raised new conceptual issues into the developments of our theories, exacerbated by the distance between theory and experiments in most of the cases. In this session we will discuss the main aims, assumptions and beliefs on the current gravitational theories and try to approach our different views and proposals.

Discussion: On the dynamics of perturbed Black Holes: Applications and Implications. (coord. Carlos Sopuerta)

We have recently seen that dynamics of perturbated Black Holes (BH) has a very rich mathematical structure from which some new powerful tools emerge. In this session we will discuss these tools and potential applications to physical problems like: BH quasinormal modes, BH scattering, semiclassical processes around BHs or BH-like objects, etc.

Talk by Álvaro Torres

Title: Uniqueness of the Fock quantization of a massless scalar fieldin Kantowski-Sachs.

Abstract: In this talk, I discuss a criterion to guarantee theuniqueness of the Fock quantization of a massless free scalar fieldin a Kantowski-Sachs background. In general spacetimes, the infiniteambiguity of choosing a set of annihilation and creation operatorsleads to non-equivalent Fock representations, a fact that is due tothe unavailability of a privileged vacuum state in the theory. In thecase of a Kantowski-Sachs spacetime, we show that the problem can beovercome by imposing invariance under the background's spatialsymmetries and requiring a quantum dynamics that admits a unitaryimplementation. This result may find applications in the quantizationof matter fields and perturbations in the interior of nonrotatingblack hole spacetimes.

Talk by Antonio Vicente

Title: Primordial Power Spectrum in modified gravity: from thermodynamics of spacetime to LQC

Abstract: In this talk, I present a derivation of the Primordial Power Spectrum (PPS) from a phenomenological model of quantum gravity based on the thermodynamics of spacetime which coincides with the PPS in the dressed metric prescription in Loop Quantum Cosmology (LQC). Starting from general gravitational equations of motion which encode low-energy quantum gravity effects, we found the dynamics of the system by assuming a homogenous and isotropic universe with a free scalar field content. These dynamics coincides with the dynamics in the Dressed metric prescription in LQC when a free parameter is fixed. Then, I present the perturbations of this cosmological model where an approximative time-dependent effective mass should be imposed to obtain the perturbations and the vacuum choice and its oscillatory impact in the PPS. Lastly, I also discuss here the main distinctions in the PPS between the hybrid and dressed metric prescriptions in LQC.

Talk by Álvaro García

Title: Spinorial formalism for loop gravity: classical dynamics for a simple model 

Abstract: Loop quantum gravity (LQG) is a quantum theory of gravity that seeks to describe the geometry of spacetime at the Planck scale. Traditionally, LQG uses a Hilbert space constructed from wave functions on the SU(2) group. Recently, an equivalent formulation in terms of spinors (spinorial formalism) has been introduced, which opens a new way to explore the open problems of the theory. In this talk we will introduce the spinorial formalism and twisted geometries, which allow us to interpret spin networks geometrically. We will study the dynamics of a simple (but non-trivial and cosmologically interpreted) model. We will explore the evolution of the volume of the discrete polyhedra associated with this model and its semiclassical limit, looking for a connection with general relativity.

Discussion: The early Universe up against the classical ropes. (coord. Mercedes Martín-Benito)

The aim of this session is to establish a discussion on cosmology, and in particular on how quantum effects should be considered in the dynamics of the early Universe. I will start reviewing some of the different avenues to understand cosmology from a quantum perspective, and set the main questions that I would like to see discussed during the round table, including: - Are we ignoring main quantum gravity effects by focusing on homogeneous solutions to the Einstein equations? - Are the predictions of our quantum cosmological models meaningful? - Could we cure the problems of the standard cosmological model by just resorting to semiclassical gravity? - Are we close to detecting primordial gravitational waves, and if so, what can we learn about its possible quantum origin? To motivate the dialogue on these topics, the round table will count with Javier Olmedo, Carlos Sopuerta and Antonio Vicente-Becerril, but we will also encourage the assistants to contribute to the discussion.

Discussion: Time and Quantum Gravity. (coord. Lucía Menéndez-Pidal)

In this session we will discuss what rôle time plays when quantising gravity. This round table will consist in two parts. In the first part, I will give an introduction about how time has been incorporated in different approaches to quantum gravity, with a particular emphasis in Geometrodynamical quantisation, both in its canonical and its path integral formulation, mentioning briefly other approaches to quantum gravity. In particular, I will define what people often refer as "the problem of time" and the different methods that have been used to get around it. This introduction will hopefully kick start the subsequent discussion. The second part will be a debate about the physical and philosophical implications of the problem of time and its possible resolutions within different quantum gravity theories. This debate will be initially directed towards three main axes, although the overall conversation might lead elsewhere. The axes are the following: The problem of time in the canonical quantisation (both in geometrodynamics and LQG), the problem of time in the path integral quantisation (again both in geometrodynamics and LQG), and finally, the rôle of time in other approaches to quantum gravity (for example causal set theory, or QFTCS). The main aim of this session is to highlight the special rôle of time in quantum gravity, clarify what are its different interpretations, and discuss what consequences these have regarding the construction of a theory of quantum gravity.