Scientific Programme

Abstract:

A summary of the initial stages of the investigative career of Pedro González-Díaz is made. After his university formation in Granada (1970) he came to the “Instituto de Optica of the CSIC” to carry out his Ph.D. degree in Chemistry by studying the composition and structure of different phosphate apatites. Soon, and after obtaining a position as an investigator in the staff of the Institute (1975), he continued working in this field for a few years and directed several theses which gave place to a number of publications.  In 1982 he applied for and obtained a Project to establish at the Instituto a new line of research: The study of infrared multiphoton dissociation of polyatomic molecules, both from experimental and theoretical point of view. Pedro devoted a significant part of his scientific work to this field for more than ten years.

Abstract:

Gravity manifests itself as curvature of spacetime, and its strength can be measured by considering the deficit of the volume of tiny balls with respect to their flat spacetime counterparts. The area deficit of the enclosing boundaries can actually be put in relation, via the Einstein field equations, with the energy density of matter at the ball’s centre. In this talk we consider what happens when the matter energy density vanishes. The area still feels the effect of pure gravity, and this change should be related to the gravitational strength. This might lead to a quasilocal definition of gravitational energy, a goal pursued for many years by many researchers. A discussion of the successes, and the difficulties, arising will be given.

Abstract:

Pedro started shifting the focus of his research from optics to general relativity in the early 1980's, with his early contributions being to bounds on black hole entropy and the connection between black holes and elementary particles. I shall survey some of this early work, up to the stage where Pedro became interested in traversable wormholes.

Abstract:

From their inception, lasers have been considered as highly versatile sources of light. However, ultralong lasers implemented in optical fiber can also be seen as unique transmission media, paving the way to a new outlook on information transmission and secure communications. Ultralong lasers, first proposed and demonstrated in the 2000s, have been shown to induce virtual transparency in optical fiber, offering quasi-lossless transmission conditions which are ideal for the implementation of soliton-based systems and signal processing. Moreover, they allow us to explore some of the fundamental limits of lasing operation, resulting in the discovery of interesting new physical regimes, such as Random distributed feedback lasing. In this presentation I will provide a short review of the field.

Abstract:

I outline the two simplest classes of phenomenological models of slow-roll inflation in the early Universe based either on scalar fields in General Relativity or on modified f(R) gravity, their relation and basic assumptions necessary for their realization. At the present state-of-the-art, the most economic inflationary models from these classes producing the best fit to all existing astronomical data requires one (in f(R) gravity) or two (in GR) dimensionless parameters taken from observations only. Also, because the most cited and influential papers by Prof. Pedro González Díaz were devoted to the investigation of possible phantom regimes both in the early and present Universe in which the space-time curvature grows with the Universe expansion, I discuss if such regimes may appear in models of primordial and present dark energy based on modified gravity without ghosts. In existing cosmological models being in agreement with present observational data, such regimes are typically transient and can occur either during the early stochastic period of inflation when the amplitude of generated scalar metric perturbations exceeds unity, or in the remote future in the course of oscillatory approach to a stable de Sitter state.

Abstract:

The inflationary paradigm, a period of accelerated expansion in the early evolution of the universe, has become practically part of the Standard Model of Cosmology. It can solve the horizon and flatness problem of the Standard Cosmological Model, and it provides the seeds for large scale structure. However, what we still lack is a concrete realization of this paradigm within a particle physics model. The search for the model of inflation has been a very active field of research since inflation was first proposed, and it has evolved over the years with the feedback of data from the observations of the Cosmic Microwave Background radiation, mainly those of WMAP and PLANCK. In this talk I will make a (somehow personal and incomplete) review of inflationary model building, and where we are today after the final release by PLANCK last summer.

Abstract:

With the creative and unconventional spirit that Pedro transmitted to us, we review some of his viewpoints and proposals about the quantum state of the Universe and about the role that the perturbations around the vacuum can have in the generation of cosmological structures and even in the emergence of matter. We take a journey from his ideas about the wavefunction of low-energy fields to our current investigations in Quantum Cosmology, trying to extract predictions that might be confronted with observations.

Abstract:

Among his many interests, quantum gravity played a crucial role in Pedro's work. This included, in particular, the study of conceptual issues in quantum cosmology.

In my talk, I give an introduction to this topic. I explain why quantum gravity is needed to understand the Universe as a whole and show which conceptual problems arise when dealing with quantum cosmology. I focus on the topics boundary conditions, concept of time, and quantum-to-classical transition. The discussion is mainly based on quantum geometrodynamics with the Wheeler-DeWitt equation as its central equation.

After a brief introduction to the formalism, I explain the new features of boundary conditions in quantum cosmology. I critically evaluate the Hartle-Hawking ("no-boundary") and Vilenkin conditions. I then emphasize the timeless nature of quantum cosmology at the fundamental level and show how the standard notion of time emerges at an approximate level. The quantum-to-classical transition is based on the process of decoherence known from quantum mechanics. I explain this concept by examples. Finally I show how the observed arrow of time follows from quantum cosmology.

Abstract:

The analogue gravity research area was born from the observation that some gravitational phenomena, difficult or even impossible to observe in its proper realm, could be simulated in laboratory systems. The bridge thus established has become an important source of enlightenment for both sides. Here, I concentrate on reviewing recent experimental advances with analogue horizons, aimed at observing quantum field theory effects such as Hawking radiation or superradiance. In addition, I will discuss some of the lessons one could take back to the gravitational realm. Before that, I will describe how I became interested on these issues under Pedro's atmosphere.

Abstract:

We will start reviewing some on the works of Prof. Pedro F. González-Díaz on the late-universe, in particular those focusing on phantom energy. We will afterwards summarise some of the recent works we have carried on this topic from a background and a perturbative point of view and how this has motivated in some cases a quantum cosmological approach of the late-universe within general relativity and beyond it.

Abstract:

I give the introduction to modified gravity. Mainly, F(R) gravity is considered but also other theories, like modified Gauss-Bonnet gravity are mentioned. It is shown the basic properties of such theory and how one can construct viable models from F(R) gravity. The possibility to unify the early-time inflation with late-time acceleration is described in detail following the line developed in the series of our works. Special attention is paid to construction of realistic inflation. Not only slow- roll inflation but also constant-roll inflation is discussed in F(R) gravity. The account of quantum gravity R^2-like contribution is done. The possibility to describe in realistic way the constant-roll F(R) epoch with exponential dark energy era is presented in detail. The correct autonomous dynamical system approach to F(R) gravity which gives stable and instable de Sitter vacuum is developed. The possibility of bounce cosmology in F(R) is briefly discussed. The lecture is based significally on recent review: S.Nojiri, S.D.Odintsov and V.K.Oikonomou, ``Modified Gravity Theories on a Nutshell: Inflation, Bounce and Late-time Evolution,'' [arXiv:1705.11098 [gr-qc]]

Abstract:

The General Theory of Relativity (GTR) has been an extremely successful theory, with a well-established experimental footing, at least for weak gravitational fields. Traditionally, the solutions of GTR have been obtained by first considering a plausible distribution of matter, and through the Einstein field equation, the spacetime metric of the geometry is determined. However, one may solve the Einstein field equation in the reverse direction, namely, one first considers an interesting and exotic spacetime metric, then finds the matter source responsible for the respective geometry. In this manner, it was found that some of these solutions possess a peculiar property, namely 'exotic matter', involving a stress-energy tensor that violates the null energy condition. These geometries also allow closed timelike curves, with the respective causality violations. These solutions are primarily useful as 'gedanken-experiments' and as a theoretician's probe of the foundations of general relativity, and include traversable wormholes and superluminal 'warp drive' spacetimes. In this talk, we review some of these exotic spacetimes, ranging from the Flamm-Einstein-Rosen bridge, the Wheeler geons, and the modern renaissance of traversable wormholes, focussing on Pedro’s contributions to these research topics.

Abstract:

En el universo de Pedro se pasaba de la ciencia al teatro y del teatro a la ciencia con naturalidad. Pedro practicaba un mestizaje fluido entre dos disciplinas aparentemente muy alejadas.

¿Y están alejadas? El teatro se percibe como pura intuición, como arte… la ciencia, sin embargo, parece fría y metódica. Ni lo uno ni lo otro es, en realidad, cierto. La ciencia necesita grandes dosis de imaginación, y la práctica teatral requiere de preparación y profundo estudio.

Entre bambalinas, con la luz de las candilejas como guía, transitaremos de la ciencia al teatro y del teatro a la ciencia a través de algunas propuestas escénicas. ¡Arriba el telón!

Abstract:

A lo largo de los años he podido comprobar que muchos de los científicos en activo con los que he tenido el gusto de entrar en contacto, destacadamente Pedro Félix González, tienen una cierta imagen acerca del método, fines y procedimientos que suelen aplicar en sus investigaciones. Los filósofos e historiadores de la Ciencia han investigado desde antiguo esos mismos asuntos llegando a conclusiones que no suelen ser populares entre la comunidad científica y que a menudo contradicen la visión ingenua que muchos de aquellos tienen acerca de su actividad. En esta charla analizaré algunos de estos puntos controvertidos partiendo de una conversación mantenida con Pedro acerca de la corroboración de las hipótesis científicas por métodos experimentales.

Abstract:

Until recent years the possibility of observing other universes seemed to be disregarded by the own definition of the universe. Even in frameworks where the theory predicts the existence of other universes, like in quantum cosmology, one was assumed to consider just one single solution as the representative of our universe and disregard the rest of them as being physically redundant or just account them for statistical measures. We now know that this is not necessarily the case. The existence of other universes may have an influence in the properties of our universe that could, in principle, be measured making thus testable the whole multiverse proposal and the underlying theories. In this talk we show how the creation of universes in entangled pairs, which is suggested by some symmetry and conservation principles, would leave an imprint on the primordial fields of each universe that would be in principle distinguishable from the case where the universes were created as single universes.