Castellano

Past seminars


Loop gravity without spin networks

Speaker: Wolfgang Wieland
Affiliation: Perimeter Institute
Abstract: It is arguably one of the main achievements of loop quantum gravity to have demonstrated that space itself may have an atomic structure. One of the key open problems of the theory is to reconcile the fundamental loop quantum gravity discreteness of space with general relativity in the continuum. In this talk, I present recent progress regarding this issue: I will show that the loop gravity discreteness of space can be understood from a conventional Fock quantisation of gravitational boundary modes on a null surface. These boundary modes are found by considering a quasi-local Hamiltonian analysis, where general relativity is treated as a Hamiltonian system in domains with inner null boundaries. The presence of such null boundaries requires then additional boundary terms in the action. Using Ashtekar’s original SL(2,C) self-dual variables, I will explain that the natural such boundary term is nothing but a kinetic term for a spinor (defining the null flag of the boundary) and a spinor-valued two-form, which are both intrinsic to the boundary. Finally, I will show that in quantum theory, the cross-sectional area two-form turns into the difference of two number operators (unless the Barbero—Immirzi parameter is sent to infinity). The resulting area spectrum is discrete without ever introducing spin networks or triangulations of space.
Date and time: 3 May 2018, 15:00 h.
Venue: CSIC, CFMAC, conference room (121 Serrano St.)


Taming the beast: initial conditions and degrees of freedom in non-local gravity

Speaker: Gianluca Calcagni
Affiliation: IEM, CSIC
Abstract: Non-local quantum gravity is a class of fundamental theories whose classical and quantum dynamics is specified by "form factors", operators with infinitely many derivatives. After briefly reviewing this paradigm and its role in the resolution of big-bang and black-hole singularities, for the first time we count the number of nonpertubative field degrees of freedom as well as the number of initial conditions to be specified to solve the Cauchy problem. In particular, in four dimensions and for the string-related form factor, there are 8 degrees of freedom (2 graviton polarization mode and 6 rank-2 tensor modes) and 4 initial conditions. The method to obtain this result is first illustrated in a pedagogical way for the case of a non-local scalar field. Consequences for cosmology and astrophysics are discussed.
Date and time: 19 April 2018, 16:00 h.
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)


Towards new constraints in extended theories of gravity: large-scale structures, neutron stars and SKA

Speaker: Álvaro de la Cruz-Dombriz
Affiliation: U. of Cape Town and SKA Cosmology
Abstract: Model-independent methods in cosmology, N-body simulations and theoretical predictions in strong-gravity regimes have become essentials tool in order to deal with an increasing number of theoretical alternatives for explaining several open issues in the cosmic history. In principle, this provides a way of testing the ΛCDM cosmological paradigm under different assumptions and to rule out whole classes of competing theories. In this talk I will present some of the latest progress and shortcomings in a recent line of full N-body simulation technique modifying g-evolution to accommodate effects from higher-order theories. Then, I will provide a flavour on the phenomenology of neutron stars in scalar-tensor gravity theories and the potentiality of SKA to detect large-scale structure effects beyond the Einsteinian paradigm.
Date and time: 5 April 2018, 15:00 h.
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)



Testing the Dark Universe with Gravitational Waves

Speaker: Jose María Ezquiaga
Afffiliation: IFT, CSIC-UAM
Abstract: Gravitational wave (GW) astronomy has come to revolutionize our understanding of astrophysics, cosmology and fundamental physics. GWs from binary black-hole (BH) mergers allow us to learn about the population of BHs, their origin and their role through the history of the universe. If these BHs have a primordial origin and are abundant enough, they could comprise a large fraction of the Dark Matter (DM). As a case of study, I will present the primordial black-hole production in Critical Higgs Inflation, a particle physics motivated model in which the SM Higgs is both responsible of inflation and DM, and discuss its GWs signatures. Moreover, GWs detected with an associated counterpart can probe the evolution of the universe and Dark Energy (DE). The recent measurement of the GW speed following GW170817 is an excellent example of the immense potential available to GWs tests of gravity. I will present the conditions for anomalous GW speed and classify the theories for DE that remain viable after GW170817. Finally, I will discuss how other propagation effects could also shed light on the quest for the nature of DE.
Date and time: 8 March 2018, 15:00 h.
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)


Primordial fluctuations in Loop Quantum Cosmology

Speaker: Mercedes Martín-Benito
Affiliation: UCM
Abstract: A lot of attention has been devoted lately to the consideration of quantum corrections to the power spectrum of primordial fluctuations, as a possible window to unveil quantum geometry phenomena in the Early Universe. In this talk I will review the derivation of these corrections in the framework of hybrid Loop Quantum Cosmology (LQC). Homogeneous LQC provides a canonical quantization for FLRW models free of singularities: the classical big bang singularity is replaced by a quantum bounce, the quantum dynamics is everywhere well-defined, and physical observables never diverge. On the other hand, hybrid LQC extends the LQC quantization to systems with inhomogeneities. For the particular case of cosmological perturbations in flat FLRW supplemented with inflation, I will analyze the quantum dynamics of states that verify a Born-Oppenheimer ansatz. They describe a regime where the cosmological perturbations can be regarded as a field propagating over a homogeneous quantum geometry. From those states, we will recover a Mukhanov-Sasaki equation for cosmological perturbations that incorporates the mentioned quantum corrections. Such corrections are encoded in the expectation value of operators of the homogeneous geometry. I will discuss how these corrections might lead to observational imprints in the primordial power spectra.
Date and time: 22 February 2018, 15:00 h.
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)


From LISA Pathfinder to LISA: Enabling the Science of the Gravitational-Wave Low-Frequency Band

Speaker: Carlos Sopuerta
Affiliation: ICE-CSIC, Universidad Autónoma de Barcelona
Abstract: LIGO and Virgo have inaugurated the era of Gravitational Wave Astronomy by observing several binary black hole mergers and the merger of a neutron star binary that has made possible multimessenger astronomy with gravitational waves. Apart from the initial discovery, these detections have produced revolutionary discoveries.  At the same time, there are ongoing efforts to detect gravitational waves beyond the high-frequency band of the ground-based detectors.  Indeed, after the success of the LISA Pathfinder mission, the European Space Agency has selected LISA (Laser Interferometer Space Antenna) as its third large-classe mission (L3).  The main goal of LISA is to carry out the science proposed in the white paper "The Gravitational Universe", consisting in low-frequency gravitational-wave astronomy.  In this talk I will describe the main scientific goals of the mission and its potential for new discoveries in astrophysics,
cosmology, and fundamental physics.
Date and time:
Venue:  CSIC, CFMAC, meeting room (113 bis Serrano St., 2nd Floor)


Fractional Calculus as a Modelling Framework

Speaker: Luis Vázquez
Affiliation: Complutense University of Madrid
Abstract: The Fractional Calculus represents a natural instrument to model nonlocal phenomena either in space or time. From Physics and Chemistry to Biology and Engineering, there are many processes that involve different space / time scales. In many cases, the dynamics of such systems can be formulated by fractional differential equations which include the nonlocal effects. We give a panoramic view of the field with associated challenges and applications in the studies of metamaterials as well as in Martian studies.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)


Astrofísica gamma y física fundamental con MAGIC

Speaker: Marcos López Moya
Affiliation: Complutense University of Madrid
Abstract: La Astrofísica gamma nos abre una ventana a los fenómenos más violentos del universo, que suelen surgir en la vecindad de objetos cósmicos muy masivos y compactos, como agujeros negros, estrellas de neutrones o explosiones supernova, acelerando partículas hasta energías ultrarrelativistas. Entre los detectores actuales de rayos gamma destacan los telescopios MAGIC, situados en la isla de La Palma, con los que se han descubierto multitud de fuentes, tanto de origen galáctico como extragaláctico. Pero, además, estos telescopios permiten abordar cuestiones de física fundamental, como la búsqueda de materia oscura e incluso sondear la estructura microscópica del espacio-tiempo postulada por algunos modelos de gravedad cuántica. Según estos modelos, la velocidad de la luz debería depender ligeramente de la energía de los fotones. De ser el caso, tales cambios tendrían que plasmarse en los tiempos de llegada de los fotones de distintas energías detectados por los telescopios. En esta charla daremos una visión general del campo, destacando los logros conseguidos con MAGIC en cuestiones tales como los límites a la violación de la invariancia Lorentz.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)


Coincident General Relativity

Speaker: Tomi Koivisto
Affiliation: Nordita
Abstract: The Palatini formalism is applied to generate teleparallel and symmetric teleparallel theories of gravity without introducing frame elds. From the symmetric version an exceptional class is discovered, which is consistent with a completely vanishing ane connection. We use this remarkable property in this Letter to propose a simpler geometrical formulation of General Relativity that is oblivious to the structure of the spacetime, thus fundamentally depriving gravity from any inertial character. The resulting theory is described by the Einstein-Hilbert action purged from the boundary term and is more robustly underpinned by the spin-2 eld theory. We also show that this construction gives a natural starting point for modied gravity theories and present new and simple generalisations where analytical self-accelerating cosmological solutions arise naturally in the early and late time universe cosmology.
Date and time: 21 December 2017, 15:30 h.
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)


Computational Complexity and Holography

Speaker: José L.F. Barbón
Affiliation: IFT, UAM-CSIC
Abstract: I will review recent  ideas in holography, imported from the theory of quantum complexity, mainly in the context of AdS/CFT correspondences. In particular, I will discuss  various competing prescriptions to defineholographic versions of quantum complexity.
Date and time: 30 November 2017, 15:00 h.
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)


Gravitational waves. What's in for hadron physics?

Speaker: Felipe J. Llanes Estrada
Affiliation: Universidad Complutense de Madrid
Abstract: The finding of Gravitational Waves by the aLIGO scientific and VIRGO collaborations opens opportunities to better test and understand strong interactions, both nuclear-hadronic and gravitational. Assuming General Relativity holds, one can constrain hadron physics at a neutron star. But precise knowledge of the Equation of State and transport properties in hadron matter can also reverse the logic and be used to constrain the theory of gravity itself. I make an amateur review of a couple of these opportunities in the context of modified f(R) gravity, the maximum mass of neutron stars, and progress in the Equation of State of neutron matter from the effective theories of QCD.
Date and time: 16 November 2017, 15:00 h.
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)


Forecast and analysis of the cosmological redshift drift

Speaker: Ruth Lazkoz
Affiliation: University of the Basque Country (UPV/EHU), Leioa
Abstract: The cosmological redshift drift could lead to the next step in high-precision cosmic geometric observations, becoming a direct and irrefutable test for cosmic acceleration. In order to test the viability and possible properties of this effect, also called Sandage-Loeb (SL) test, we generate a model independent mock data set so as to compare its constraining power with that of the future mock data sets of Type Ia Supernovae (SNe) and Baryon Acoustic Oscillations (BAO). The performance of those data sets is analyzed by testing several cosmological models with Markov chain Monte Carlo (MCMC), both independently and combining all data sets. Final results show that, in general, SL data sets present allow for remarkable constrains on the matter density parameter today on every tested model, showing also a great complementarity with SNe and BAO data regarding dark energy (DE) parameters.
Date and time:
Venue: CSIC, CFMAC, meeting room (113 bis Serrano St., 2nd Floor)


Early Universe with a Born-Infeld taste

Speaker: Jose Beltrán
Affiliation: IFT, UAM-CSIC
Abstract: In order to regularize the energy of point-like charged particles, Born and Infeld introduced a modification of the Maxwell Lagrangian that naturally imposes an upper bound on electromagnetic fields. Similar ideas can be applied to gravity to resolve the GR singularities. I will discuss some general theoretical aspects of these theories (which are shared by more general metric-affine theories) as well as some cosmological applications, with emphasis in non-singular and inflationary solutions.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Lorentz invariance and the zero-point stress-energy

Speaker: Matt Visser
Affiliation: Victoria University of Wellington, New Zealand
Abstract: Some 65 years ago (1951) Wolfgang Pauli noted that the zero-point energy density could be set to zero by a carefully fine-tuned cancellation between bosons and fermions. In this seminar I will argue in a slightly different direction: The zero-point energy density is only one component of the zero-point stress energy tensor, and it is this tensor quantity that is in many ways the more fundamental object of interest. I shall demonstrate that Lorentz invariance of the zero-point stress energy tensor implies finiteness of the zero-point stress energy tensro, and viceversa.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Properties of Rotating Black Holes in Odd Dimensions

Speaker: Paco Navarro
Affiliation: Complutense University of Madrid, Spain
Abstract: On this talk, we will discuss about the properties of rotating Abelian black holes in odd-dimensional spacetimes, and show how the case when all angular momenta are equal becomes much simpler. This simplification also applies to theories with Chern-Simons terms, a cosmological constant, or scalar fields.

We will concentrate on the effects that do not appear in 4 dimensions and address subjects such as non-uniqueness, near-horizon geometry, radially excited solutions, etc.

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

The smooth and clumpy universe: present and asymptotic behaviour(s)

Speaker: Mariam Bouhmadi López
Affiliation: Universidad del País Vasco (UPV/EHU), IKERBASQUE
Abstract: On this talk, we will present a review on dark energy singularities and abrupt events. We will start with a classical description of several models, including their behaviour at the perturbative level. Then, we will contrast our results with observational data. On the second part of the talk, we will focus on the quantum analysis of these singularities/abrupt events. The analysis will encompass also modified theories of gravity. Finally, if time allows, we will as well show how phantom dark energy models might not at all imply dark energy singularities in Einstein-Cartan theory.​
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Deformed relativistic symmetries: decoherence and dimensional reduction at the Planck scale

Speaker: Michele Arzano
Affiliation: Sapienza - University of Rome, Italy
Abstract: I will start with a brief introduction to the framework of deformed relativistic symmetries based on Lie-group momentum spaces whose curvature sets a fundamental (Planckian) UV scale at the kinematical level. I will then show how these models can be related to a fundamental decoherence at the Planck scale and how they are relevant for the phenomenon of "dimensional reduction", ubiquitous in a variety of approaches to quantum gravity.
Date and time:
Venue: CSIC, CFMAC, conference room (113 bis Serrano St., 2nd Floor)

Tension and constraints on modified gravity parametrizations of Geff(z) from growth rate and Planck data

Speaker: Savvas Nesseris
Affiliation: IFT-CSIC, Madrid
Abstract: Some of the current cosmological data, i.e. weak lensing (WL), growth rate, seem to be in tension with Planck15/LCDM and indicate consistently that there is a lack of gravitational power in structures on intermediate-small cosmological scales. This lack of power may be expressed through different cosmological parameters in a degenerate manner. In this talk, after a brief overview of WL and Redshift Space Distortion (RSD), I will present an updated and extended compilation of growth rate data based on recent RSD measurements and use it to test its consistency with the best fit Planck15/LCDM.
Date and time:
Venue: CSIC, CFMAC, conference room (113 bis Serrano St., 2nd Floor)

Mapas de Galaxias Modernos, Espectros de Potencias de Galaxias y Forecast

Speaker: Miguel Aparicio Resco
Affiliation: Complutense University of Madrid, Spain
Abstract: La cosmología, entendida como rama de la física moderna, cuenta con tan solo cien años de historia desde su fundación en el contexto de la Teoría General de la Relatividad. Desde entonces las observaciones han ido aumentando a un ritmo vertiginoso, convirtiendo a la cosmología en una ciencia de precisión. Los últimos satélites encargados de medir el fondo cósmico de microondas, WMAP y Planck, así como el análisis de Supernovas Ia, nos han permitido conocer con precisión la composición del universo. Dicha composición requiere del enigmático fluido conocido como energía oscura, cuya dinámica es desconocida. Sin embargo, en los próximos años, las nuevas generaciones de mapas de galaxias (EUCLID, DESI, J-PAS) nos permitirán conocer mucho más acerca de la energía oscura. En esta charla se presentarán distintos mapas de galaxias. Se expondrán varios efectos observables en los mapas de galaxias que dependen del modelo cosmológico. Y finalmente, concluiremos con las técnicas para estimar errores en los parámetros cosmológicos.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Gravity at the horizon: new probes of gravity and cosmic acceleration

Speaker: Miguel Zumalacarregui
Affiliation: Nordita/UC Berkeley
Abstract: Gravity is core to many unsolved problems in physics, including the properties of black holes, the nature of space-time and the origin of the universe. The desire to better comprehend gravity and the mounting evidence for cosmic acceleration have led to the proliferation of both alternatives to Einstein's theory and efforts to test them. I will present recent progress to understand these theories and determine the properties of gravity using gravitational waves and the large scale structure of the universe: The speed of gravitational waves will provide the most stringent test for a large class of theories, while a new generation of galaxy surveys will be sensitive to new relativistic effects on the largest cosmological scales. I will also introduce /hi_class/ (www.hiclass-code.net), an accurate, fast and flexible code to compute cosmological predictions in a very large class of gravitational theories.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Relaciones universales I-LOVE-QNM para modos cuasi-normales de estrellas de neutrones que contienen materia exótica

Speaker: Luis Manuel González Romero
Affiliation: Complutense University of Madrid, Spain
Abstract: Se analizará el espectro de modos cuasi-normales de modelos realistas de estrellas de neutrones estudiando los modos axiales y polares. Consideraremos diferentes ecuaciones de estado que satisfacen la ligadura de 2 masas solares, la mayor parte de ellas conteniendo materia exótica. Se presentarán relaciones fenomenológicas nuevas entre la frecuencia y el tiempo de amortiguamiento de los modos. Estas nuevas relaciones son independientes de la ecuación de estado y relacionan el espectro de modos cuasi-normales con propiedades globales de la estrella. También, se presentarán relaciones universales entre las propiedades de los modos cuasi-normales y los parámetros I-LOVE-Q de estos modelos de estrellas de neutrones.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Gran explosión, constante cosmológica y gravedad cuántica: maravillas del siglo

Speaker: Gianluca Calcagni
Affiliation: Instituto de Estructura de la Materia (IEM-CSIC), Madrid, Spain
Abstract: Tres cuestiones han caracterizado la física moderna de las últimas décadas. La primera es si singularidades tales como las de dentro de los agujeros negros o la Gran Explosión inicial que dio origen al cosmos se pueden resolver. La segunda es sobre la determinación de la verdadera naturaleza de la componente principal y más misteriosa del Universo, la llamada energía oscura o constante cosmológica. Ni el problema de la Gran Explosión ni el de la constante cosmológica se pueden acomodar en la relatividad general de Einstein o en modelos ordinarios de interacciones de partículas. Una posibilidad es que dichos problemas se puedan resolver unificando física cuántica y gravitación, pero es muy difícil formular una teoría y verificarla experimentalmente. Así, la tercera cuestión es cómo cuantizar la gravedad y extraer predicciones observables. En esta charla introduciremos estos temas de forma accesible a un público general.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Primordial perturbations in Hybrid (Loop) Quantum Cosmology

Speaker: Laura Castelló Gomar
Affiliation: Instituto de Estructura de la Materia (IEM-CSIC), Madrid, Spain
Abstract: A lot of attention has been devoted recently to the consideration of quantum corrections to the power spectrum of primordial fluctuations, as a possible fundamental window on the Early Universe to explore quantum geometry phenomena. We present a framework for the quantum description of the evolution of cosmological scalar perturbations, starting from a formalism designed to preserve covariance, which allows us to extract physical predictions for those quantum modifications. We show how a Born-Oppenheimer ansatz leads to the recovery of a Mukhanov-Sasaki equation in which the coefficients are provided by expectation values on the background geometry. We also discuss technical tools and develop a procedure to deal with these quantum corrections beyond the most direct approximations. The case of hybrid Loop Quantum Cosmology is commented here. However, the presented scheme is general and aims at increasing the ability to discriminate between the predictions of different quantization proposals for cosmological perturbations.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Unruh-deWitt detectors: The Unruh effect and beyond

Speaker: José de Ramón Rivera
Affiliation: Complutense University of Madrid, Spain
Abstract: Model independent techniques are crucial to test both the present ΛCDM cosmological paradigm and alternative theories making the least possible number of assumptions about the Universe. Whether cosmography is able to distinguish between different gravitational theories or not has remained so far an open question. I will present the bounds on model parameters for three different extensions of General Relativity, namely quintessence, F(Torsion) and f(R) theories. Using extensive data from the Union2.1 SN catalogue, BAO data and H(z) differential age compilations, and through a Monte Carlo analysis using a Metropolis-Hastings algorithm with a Gelman-Rubin convergence criterion this technique is shown to be capable of providing 1-σ and 2-σ constraints. I shall compare these results to the truncated ΛCDM paradigm, showing that the permitted regions of coefficients are significantly modified and in general widened compared to values reported in the existing literature. Finally Bayesian selection criteria AIC and BIC will provide further insight on the limitations of the method.
Date and time:
Venue: CSIC, CFMAC, conference room (113 bis Serrano St., 2nd Floor)

Low-energy effects in super-renormalizable gravity models

Speaker: Breno Loureiro Giacchini
Affiliation: Centro Brasileiro de Pesquisas Físicas, Brazil
Abstract: Local gravitational theories with more than four derivatives have remarkable quantum properties. Namely, they are super-renormalizable and may be unitary in the Lee-Wick sense, if the massive poles of the propagator are complex. It is important, therefore, to explore also the IR limit of these theories and identify observable signatures of the higher derivatives. In this talk we present recent results in this direction. Specifically, we discuss the viability of a gravitational seesaw-like mechanism, which could be a mean of avoiding the Planck suppression of the higher derivatives' effects. Also, we show that the modified Newtonian potential in these theories is always a real quantity, besides being regular at the origin, even if complex poles are allowed in the propagator. Finally, we consider the bending of light within the general six-derivative model and discuss the possibility of the experimental detection of higher derivatives through a frequency-dependent gravitational scattering.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Loops for dummies

Speaker: Guillermo A. Mena Marugán
Affiliation: Instituto de Estructura de la Materia (IEM-CSIC)
Abstract: La Gravedad Cuántica de Lazos ("Loop Quantum Gravity" en inglés) es uno de los formalismos más sólidos como candidato para proporcionar una cuantización consistente de la Relativad General. Su aplicación a modelos que describen el origen y evolución del Universo ha dado lugar a una nueva disciplina física conocida como Cosmología Cuántica de Lazos. Su desarrollo está cambiando de raíz la visión que teníamos del Cosmos. En este coloquio comentaremos las ideas fundamentales en las que se basan la Gravedad y la Cosmología Cuántica de Lazos, y presentaremos en un tono informal (acorde a la filosofía de los "talks for dummies") algunos de sus avances más importantes.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Playing with the Misner space and the topology

Speaker: Juan Margalef Bentabol
Affiliation: Instituto de Estructura de la Materia (IEM-CSIC), Universidad Carlos III de Madrid
Abstract: Topology is essential in many branches of mathematics, however, in physics, it is often taken for granted as it usually plays no crucial role. I will try to show that on some cases topology turns out to be essential in order to understand some pathological behaviors that arise in general relativity. Namely I will talk about the so-called Misner space, which shows extremely pathological behaviors that can be neatly understood through topology. Besides I will introduce the g-boundary, a topological construction introduced by Hawking and Geroch in an attempt to define properly what a singularity is, and I will prove that in the case of the Misner space, one obtains the expected g-boundary.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Spontaneous symmetry breaking and the Unruh effect

Speaker: Antonio Dobado
Affiliation: Complutense University of Madrid, Spain
Abstract: In this work we consider the ontological status of the Unruh effect. Is it just a formal mathematical result? Or the temperature detected by an accelerating observer can lead to real physical effects such as phase transitions? In order to clarify this issue we use the Thermalization Theorem to explore the possibility of having a restoration of the symmetry in a system with spontaneous symmetry breaking of an internal continuous symmetry as seen by an accelerating observer. We conclude that the Unruh effect is an ontic effect rather than an epistemic one, giving rise, in the particular example considered here, to a phase transition (symmetry restoration) both for global and gauge symmetries (inverse Higgs mechanism) in the region close to the accelerating observer horizon.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

What model-independent reconstruction techniques really say about extended theories of gravity

Speaker: Álvaro de la Cruz-Dombriz
Affiliation: Cosmology and Gravity Group, University of Cape Town and National Institute for Theoretical Physics (NITheP)
Abstract: Model independent techniques are crucial to test both the present ΛCDM cosmological paradigm and alternative theories making the least possible number of assumptions about the Universe. Whether cosmography is able to distinguish between different gravitational theories or not has remained so far an open question. I will present the bounds on model parameters for three different extensions of General Relativity, namely quintessence, F(Torsion) and f(R) theories. Using extensive data from the Union2.1 SN catalogue, BAO data and H(z) differential age compilations, and through a Monte Carlo analysis using a Metropolis-Hastings algorithm with a Gelman-Rubin convergence criterion this technique is shown to be capable of providing 1-σ and 2-σ constraints. I shall compare these results to the truncated ΛCDM paradigm, showing that the permitted regions of coefficients are significantly modified and in general widened compared to values reported in the existing literature. Finally Bayesian selection criteria AIC and BIC will provide further insight on the limitations of the method.
Date and time:
Venue: CSIC, CFMAC, conference room (113 bis Serrano St., 2nd Floor)

Didáctica de Materia Oscura

Speaker: José A. R. Cembranos
Affiliation: Complutense University of Madrid, Spain
Abstract: De acuerdo con las observaciones actuales, la componente más abundante de la materia en el universo no se encuentra entre las partículas que conocemos dentro del modelo estándar de partículas e interacciones fundamentales. Este nuevo tipo de materia se conoce como materia oscura. Existen numerosas evidencias de su existencia, sin embargo, no se ha detectado a través de ningún proceso más allá de sus efectos gravitacionales. Por ello, la naturaleza de la materia oscura y sus características fundamentales continúan siendo un misterio, una de las cuestiones abiertas más importantes dentro de la física actual.
Date and time:
Venue: UCM, Faculty of Physical Sciences, lecture room 5A

Indirect searches of TeV Dark Matter at the Galactic Center

Speaker: Viviana Gammaldi
Affiliation: SISSA, Italy
Abstract: I will discuss several aspects of the indirect search of Dark Matter (DM) at the Galactic Center (GC). I will show that the gamma-ray flux observed by HESS from the J1745-290 GC source is well fitted as the secondary gamma-rays photons generated from DM annihilating into Standard Model particles in combination with a simple power-law background. The model independent fits are performed for all the possible channels of annihilation. The best fits are obtained for WIMP masses above ∼ 10 TeV. They require an enhancement factor of ∼ 10^3 and a background spectral index compatible with the Fermi-LAT data. I will investigate the possibility that the expected enhancement may be related with hydrodynamics in N-body simulation or a possible DM spike induced by the Super Massive Black Hole. I will also discuss the possibility that the latter could be related with the spatial tail detected in the gamma-ray signal by HESS II in 2015. These TeV DM masses are practically unconstrained by direct detection searches or colliders experiments, but they can be tested with the observations of other cosmic-rays.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Observational imprints of the interacting multiverse

Speaker: Salvador Robles Pérez
Affiliation: Ecological Station for Biocosmology
Abstract: The most controversial feature of the multiverse may be its testability. We present a novel paradigm of the multiverse where non-local interactions and other quantum correlations may exist among the universes of the multiverse. The interactions among universes may: i) determine some global properties of a single universe without violating its notion of causal closure; ii) produce a landscape structure of vacuum states that might lead to different process of vacuum decay; and iii) induce a pre-inflationary stage in the evolution of the universe that cannot be reproduced by any known matter content. All these effects would leave distinctive and distinguishable imprints in the properties of the CMB, making the whole proposal (and the underlying theories) testable.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Unitary dynamics selecting a quantization of the Dirac field in cosmology

Speaker: Beatriz Elizaga de Navascués
Affiliation: Instituto de Estructura de la Materia (IEM-CSIC), Madrid, Spain
Abstract: It is well known that linear canonical transformations are not generally implemented as unitary operators in QFT. Such transformations include the dynamics that arises from linear field equations on the background spacetime. This evolution is specially relevant in nonstationary backgrounds, where there is no time-translational symmetry that can be exploited to select a quantum theory. We investigate whether it is possible to find a Fock representation for the canonical anticommutation relations of a Dirac field, propagating on a homogeneous and isotropic cosmological background, such that the field evolution is unitarily implementable. First, we restrict our attention to Fock representations that are invariant under the group of spatial isometries of the background. Then, we prove that there indeed exist Fock representations such that the dynamics is implementable as a unitary operator. Finally, once a convention for the notion of particles and antiparticles is set, we show that these representations are all unitarily equivalent.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Late-time cosmology in f(R) gravity

Speaker: João Morais
Affiliation: UPV
Abstract: Since the discovery of the current acceleration of the Universe by the Supernova Cosmology Project and the High-Z Supernova Search Team in 1998, several proposals have been put forward to explain the nature of the dark energy fluid that governs such late-time behaviour. Among these different proposals, the modified theories of gravity, which have since then been the focus of a renewed interest, try to obtain the late-time acceleration through modifications of the Einstein-Hilbert action. In this talk, we will focus on metric f(R) theories of gravity, a particular class of modified theories of gravity, which replace the Ricci scalar R in the action by a general function f(R). In a first step we review the "designer" or "reconstruction" approach, which tries to derive viable f(R) actions from cosmological models. We apply this methodology to the modified Generalized Chaplygin Gas and obtain the compatible f(R). This model is constrained, using the latest Planck 2015 data, within the cosmographic approach and through the computation of the effects on the matter power spectrum as observed today. The matter power spectrum is computed using the full theory of linear perturbations for metric f(R) theories without any approximation in the perturbed equations.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Spacetime-noncommutativity regime of Loop Quantum Gravity

Speaker: Michele Ronco
Affiliation: Sapienza - University of Rome, Italy
Abstract: A recent study by Bojowald and Paily provided a path toward the identication of an effective quantum-spacetime picture of Loop Quantum Gravity, applicable in the "Minkowski regime", the regime where the large-scale (coarse-grained) spacetime metric is flat.

A pivotal role in the analysis is played by Loop-Quantum-Gravity-based modications to the hypersurface deformation algebra, which leave a trace in the Minkowski regime. We here show that the symmetry-algebra results reported by Bojowald and Paily are consistent with a description of spacetime in the Minkowski regime given in terms of the kappa-Minkowski noncommutative spacetime, whose relevance for the study of the quantum-gravity problem had already been proposed for independent reasons.

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Hamiltonian formulation of parametrized field theories: incorporating boundaries and gauge symmetries

Speaker: Fernando Barbero
Affiliation: Instituto de Estructura de la Materia (IEM-CSIC), Madrid, Spain
Abstract: Parametrized field theories have been used as a test bed for polymer quantization techniques in Loop Quantum Gravity. In this talk I will review the Hamiltonian formulation for singular (constrained) systems and discuss several issues that play a relevant role in the Hamiltonian description of parametrized models when boundaries and internal gauge symmetries are present. For concreteness I will concentrate on the discussion of the scalar field subject to several types of boundary conditions and electromagnetism.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Quantum corrections to the Higgs potential on a curved background

Speaker: Franco D. Albareti
Affiliation: Autonomous University of Madrid, Spain
Abstract: I will present recent results on finite gravitational corrections to the Higgs effective potential. In particular, 1-loop corrections due to the Higgs self-interactions are computed in a perturbed FRW background by mode summation. For this aim, the Klein-Gordon equation is solved to first order in perturbations and to zero order in the adiabatic approximation.

Dimensional regularization techniques are used to compute not only the divergent parts of the effective potential but also the finite ones. The obtained results amount to the presence of inhomogeneities in the Higgs vacuum expectation value which translate into variations on the masses of elementary particles. Possible observable signatures within the Solar System and on cosmological scales will be discussed.

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Einstein-Yang-Mills-Lorentz Black Holes

Speaker: Jorge Gigante Valcárcel
Affiliation: Complutense University of Madrid, Spain
Abstract: Since the discovery of the first non-abelian solution to the Einstein-Yang-Mills (EYM) theory done by Bartnik and McKinnon, a large number of properties and different solutions have been obtained. As they manifestly violate the non-hair conjecture, they have attracted much attention and encouraged a deep search for other black hole geometries.

In this talk, first we will review the most relevant particle-like and black hole solutions for such a theory. In addition, we will focus on the EYM-SO(N) model and establish a particular equivalence between this case and a certain class of theories with torsion within Riemann-Cartan space-times, in order to simplify the problem of finding exact solutions to the EYM equations. Finally, solutions for the torsion-free and the non-vanishing torsion with rotation and reflection symmetries will be presented by the explicit use of this method.

Date and time:
Venue: CSIC, CFMAC, conference room (113 bis Serrano St., 2nd Floor)

Clasificación completa de singularidades cosmológicas

Speaker: Leonardo Fernández-Jambrina
Affiliation: Universidad Politécnica de Madrid
Abstract: El descubrimiento de la expansión acelerada del universo ha motivado la consideración de singularidades cosmológicas más allá de los clásicos Big Bang y Big Crunch. La primera de ellas fue el Big Rip, pero, a medida que se han ido proponiendo nuevos modelos, han ido apareciendo muchas más posibilidades. Incluso se están proponiendo nuevos escenarios no singulares como estado final posible para el Universo, tales como el Little Big Rip o el Pseudorip.

En esta charla se propondrá un intento sistemático de derivar todas estas familias de singularidades y escenarios, y algunas nuevas más, a partir de desarrollos en serie generalizados del parámetro de deceleración del universo o del índice barotrópico y se relacionará con resultados anteriores para determinar la fuerza de estas singularidades, ya que en muchos caso son débiles y no podrán considerarse un final para el universo.

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Fast oscillating fields cosmology

Speaker: Santos J. Núñez Jareño
Affiliation: Complutense University of Madrid, Spain
Abstract: The ubiquity of scalar fields in cosmology is not only owed to its simplicity, but mainly because it intrinsically respects the large degree of isotropy observed in the universe. On the contrary, the evolution of a coherent vector field is clearly anisotropic. However, we will show that the energy-momentum tensor of homogeneous fields of arbitrary spin in an expanding universe is always isotropic in average provided the fields remain bounded and evolve rapidly compared to the rate of expansion. This property enlarge the plausible candidates to solve cosmological problems to higher spin theories.

In the second part of the talk we will discuss the perturbations of a fast oscillating scalar field under a power law potential, following an effective approach as well as the exact system solution in the sub/super-Hubble limits.

Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Colapso de materia cuántica autogravitante sobre una geometría cuántica

Speaker: Javier Olmedo
Affiliation: Louisiana State University, EEUU
Abstract: En esta charla voy a resumir los últimos avances en relación a la cuantización de espacio-tiempos esféricamente simétricos en gravedad cuántica de lazos. Resumiré brevemente el escenario en ausencia de materia. Después, consideraré el acoplo con una cáscara autogravitante, fina, esférica y sin masa. Introduciré una redefinición del álgebra de ligaduras de tal modo que la ligadura escalar conmute consigo misma. Mostraré que esto se puede llevar a cabo también en el nivel cuántico. Además proporcionaré los observables cuánticos del modelo y definiré las componentes de la métrica a partir de ellos como observables parametrizados. Finalmente discutiré las consecuencias fisicas.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Is Cosmography a useful tool to target extended theories of gravity?

Speaker: Álvaro de la Cruz-Dombriz
Affiliation: Complutense University of Madrid, Spain
Abstract: Model-independent methods in cosmology have become an essential tool in order to deal with an increasing number of theoretical alternatives for explaining the late-time acceleration of the Universe. In principle, this provides a way of testing the Cosmological Concordance model under different assumptions and to rule out whole classes of competing theories. One such model-independent method is the so-called cosmographic approach, which relies only in the homogeneity and isotropy of the Universe on large scales. We show that this method suffers from many shortcomings, providing biased results depending on the auxiliary variable used in the series expansion and is unable to rule out models or adequately reconstruct theories with higher-order derivatives in either the gravitational or matter sector. Consequently, in its present form, this method seems unable to provide reliable or useful results for cosmological applications.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Higher derivative actions and inflation in string theory

Speaker: Francisco Pedro
Affiliation: IFT-CSIC, Madrid
Abstract: In this talk I will explore some aspects of higher derivative (>2) supergravity actions. I will review how the vacua and scalar potential of the bosonic sector of type IIB string compactifications can get modified in the presence of such corrections and will use them to build a viable inflationary model, where the inflaton is a geometric modulus, parametrising the size of the compactification space.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Phase Space Noncommutativity: From Gravity to Quantum Mechanics

Speaker: Catarina Bastos
Affiliation: IST-Lisbon
Abstract: A phase-space noncommutativity (NC) is considered in the context of a Kantowski-Sachs cosmological model. Then, the interior of a Schwarzschild black hole is analyzed. One deduces the thermodynamics and show that the Hawking temperature and entropy exhibit an explicit dependence on the momentum noncommutativity parameter, η. Furthermore, the t=r=0 singularity is analysed in the noncommutative regime and it is shown that the wave function vanishes in this limit.

Quantumness and separability criteria for continuous variable systems are discussed for the case of a NC phase-space quantum mechanics. In particular, the quantum nature and the entanglement configuration of NC two-mode Gaussian states are examined. Finally, a more general measurement disturbance uncertainty principle is presented in a Robertson-Schrödinger formulation. It is shown that it is stronger and having nicer properties than Ozawa's uncertainty relations. In particular is invariant under symplectic transformations. One shows also that there are states of the probe (measuring device) that saturate the matrix formulation of measurement disturbance uncertainty principle.

Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Aplicaciones del grafeno en física fundamental. Renormalización, campos gauge y curvatura con fermiones de Dirac sin masa en dos dimensiones

Speaker: José González Carmona
Affiliation: IEM - CSIC
Abstract: El descubrimiento del grafeno ha supuesto toda una revolución, no solo por el considerable potencial de este material para aplicaciones tecnológicas, sino también por ofrecer un banco de pruebas donde experimentar con una gran variedad de fenómenos de física fundamental.

En esta charla repasaremos algunos de los efectos que derivan del comportamiento de las excitaciones electrónicas del grafeno como fermiones de Dirac sin masa, haciendo énfasis en cómo el grado de libertad adicional (pseudoespín) de las excitaciones conduce a signaturas no convencionales que se pueden medir en el sistema electrónico.

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Sistema de dos niveles en las cercanías de un agujero negro de Schwarzschild

Speaker: Jose Sánchez
Affiliation: IEM - CSIC
Resumen: La búsqueda de agujeros negros no es tarea fácil. Mediante el estudio del espectro de emisión espontánea de un sistema de dos niveles, se propone una posibilidad para la misma. Se discuten aspectos básicos de la cuantización de campos cuando consideramos espacio tiempos no planos y se discuten algunos de los efectos derivados. Igualmente se discuten los aspecto básicos de la óptica cuántica para sistemas de dos niveles, llegando a la llamada ecuación maestra.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Effective field theory approach to modified gravity with applications to inflation and dark energy

Speaker: Shinji Tsujikawa
Affiliation: Tokyo University of Science, Japan
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Sparsity of the Hawking flux

Speaker: Matt Visser
Affiliation: Victoria University of Wellington, New Zealand
Abstract:The Hawking flux that reaches spatial infinity is extremely sparse and thin, with the Hawking quanta dribbling out of the black hole one-by-one. The typical time between quanta reaching infinity is larger than the timescale set by the energy of the quanta by factors of many hundreds. Among other things, this means that the Hawking evaporation of a black hole should be viewed as a sequential cascade of 3-body decays.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Constraining the early universe with primordial black holes

Speaker: Sam Young
Affiliation: Dept. of Physics and Astronomy, University of Sussex, UK
Abstract:Primordial black holes (PBHs) could have formed very early in the history of the universe after inflation ended. They are a viable dark matter candidate and represent a unique probe to study the small scales of the early universe, not visible by other means. I will discuss how PBHs have been used to constrain the small scales, and how this depends strongly on the non-Gaussianity parameters, as well as how PBHs can be used to constrain the large scales in the early universe.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

"The magnificent seven and friends": Horndeski theories self-tuning to de Sitter

Speaker: Prado Martín Moruno
Affiliation: Complutense University of Madrid, Spain
Abstract: The cosmological constant problem is not alleviated in the context of alternative theories of gravity generically. Focusing attention on theories with equations containing non-higher than second order derivatives, Horndeski theories, there is a dynamical adjustment mechanism able to screen any value of the cosmological constant. Under the assumption that the gravity's vacuum is described by a de Sitter geometry, the late time accelerated expansion of the universe can be understood as the result of that adjustment mechanism. I will present the Horndeski models able to carry out that kind of screening, which, therefore, have a de Sitter critical point for any type of material content. I will summarize the cosmological consequences of the resulting models, emphasizing the case with symmetry under shift redefinitions of the field.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Non-linear evolution of the BAO scale in alternative theories of gravity

Speaker: Miguel Zumalacárregui
Affiliation: ITP Heidelberg/Nordita
Abstract: The scale of Baryon Acoustic Oscillations (BAO) imprinted in the matter distribution is known to suffer sub-percent deviations from being a perfect standard ruler due to non-linear corrections. However, such corrections have never been computed in alternative theories of gravity. I will give an overview of historical and recent developments in alternative theories of gravity and present the first computation of the BAO shift based on perturbation theory (à la Bellini-Sawicki) and a peak-background split (à la Sherwin-Zaldarriaga), valid for any Horndeski theory. The BAO shift is modified by the enhanced growth of linear perturbations, as well as by novel, non-linear gravitational effects. The modified gravity result can range from a negligible to order-unity correction at z=0 depending on the model, and in some cases presents a characteristic time evolution. Although small, this effect might be within the reach of future galaxy surveys, and might have implications for BAO reconstruction in theories other than Einstein's.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Quantum cosmology: Games without frontiers

Speaker: Ana Alonso Serrano
Affiliation: IFF - CSIC, Madrid
Abstract: In classical General Relativity, the presence of horizons excises parts of the spacetime making inaccessible physical observations beyond them. But, is it so quantum mechanically? We develop a canonical quantization of a minisuperspace model whose classical solutions possess horizons, in order to check that this is not the case: Quantum gravitational states with support in spacetime configurations that exclusively describe either each region of the whole spacetime are not consistent, due to quantum correlations that appear across the horizon, making the physical states of the different regions not separable but entangle.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Modifying General Relativity: is it worth?

Speaker: Diego Sáez-Gómez
Affiliation: U. of Lisbon and ACGC - U. of Cape Town
Abstract: In this talk I will review some models of modified gravities and their features from a classical point of view. Over the last years, modified gravities have drawn much attention within the cosmological context since they can reproduce with great accuracy the late-time acceleration and even the inflationary epoch. I will show the pros and cons of some of the main proposals.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Graphene: Dirac fermions in curved spaces

Speaker: Alberto Cortijo
Affiliation: Instituto de Ciencia de Materiales de Madrid, CSIC
Abstract: It is well known that the low energy electronic excitations in graphene are described by the massless Dirac equation. It might seem natural in terms of effective field theories that when a graphene sheet is corrugated the description in terms of Dirac fermions can still be valid, but in a curved background, In this seminar we review the consequences of assuming such viewpoint and we compare it with the usual approach adopted in solid state physics.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Anomaly-induced effective action and Starobinsky model of inflation

Speaker: Ilya L. Shapiro
Affiliation: Federal University of Juiz de Fora, Brazil

Abstract: The effective action of gravity contains the main information about quantum effects, but in most cases it can not be calculated exactly. An important exception is the one-loop effective action for massless conformal invariant matter fields, which can be obtained by integrating trace anomaly. The integration constant is an arbitrary conformal functional of the background metric, but for the zero-order cosmology this functional is irrelevant and the solution becomes exact. The most important applications include systematic classification of vacuum states in the vicinity of the black hole and the Starobinsky model of inflation. The last is based on the non-local anomaly-induced metric, however the most relevant part is a local term which is just a square of the scalar curvature. The modified version of the model does not require special choice of initial conditions and is based on the interpolation between stable and unstable inflationary solutions.

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

A bridge between the microscopic structure of space-time and effective geometries: the crystal lessons

Speaker: Diego Rubiera-Garcia
Affiliation: Fudan University, Shanghai, China

Abstract: A striking mathematical and physical resemblance exists between quantum gravity effective geometries arising from a hypothetical microstructure of space-time pervaded by creation and annihilation of wormholes (space-time foam), and the continuized version of crystalline structures. If the latter contains defects on its microstructure, the macroscopic effective description must be done in terms of a metric-affine geometry with nonmetricity and torsion. Using a crystal-motivated action in a simplified scenario, we find the presence of Wheeler's geons: self-consistent topologically non-trivial gravito-electromagnetic entities, representing the gravitional analog of point defects.

What lessons can we extract from this analogy?

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

CMB: beyond the standard statistical scenario

Speaker: Thiago Dos Santos Pereira
Affiliation: Universidade Estadual de Londrina, Brasil

Abstract: In this talk I will review the basic hypotheses which motivate the statistical framework used to analyze the cosmic microwave background radiation (CMB), and discuss how that framework can be enlarged as we relax those hypotheses. In particular, I will try to separate as much as possible the questions of gaussianity, homogeneity and isotropy from each other, while giving particular emphasis to their signatures and the enhanced "cosmic variances" that become increasingly important as our putative Universe becomes less symmetric. After reviewing the basic formalism I will present a few model-independent statistical tests, including some new results, which can be applied to CMB data when searching for large scale "anomalies".

Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Topological Quantum Computation - From Concepts To Experiment

Speaker: Markus Müller
Affiliation: Theoretical Physics I Dept.,UCM, Spain

Abstract: Quantum computers hold the promise to allow one to solve problems that cannot be efficiently treated on classical computers. To date, the construction of a quantum computer remains a fundamental scientific and technological challenge, due the influence of unavoidable noise which affects the fragile quantum states.

In our talk, we begin with a general introduction to quantum computation and then introduce the basic idea of quantum error correction based on topological quantum codes, which allow one to protect quantum information during storage and processing. We then discuss a recent experimental realization of a quantum error correcting code in which a logical qubit was distributed over seven trapped-ion qubits. This encoding not only allowed us to detect arbitrary single-qubit errors, but also to realize for the first time quantum computations on an encoded qubit. This quantum error correcting code represents a fully functional instance of a topologically encoded qubit, or color code, and opens a route toward fault-tolerant quantum computing.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Vacuum energy (again), but now it seems dark matter

Speaker: Franco D. Albareti
Affiliation: Institute for Theoretical Physics (IFT-CSIC/UAM)

Abstract: A comoving cutoff is used to renormalize the vacuum stress-energy tensor of a scalar field in RW geometries in a covariant way. At the IR regime (late times) a matter-like contribution dominates. A phenomenological bound to the comoving cutoff is obtained from the current abundance of dark matter. It is also shown that this "dark matter" could support perturbations with a negligible speed of sound, thus it could seed the formation of structures.

Besides, I will present the latest results on the possible variation of the fine structure constant using SDSS-III/BOSS QSO spectra with [OIII] emission lines.

Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Supernovae as probes of cosmic parameters: estimating the bias from under-dense lines of sight

Speaker: Vinicius Busti
Affiliation: Astrophysics, Cosmology and Gravity Center, University of Cape Town, South Africa

Abstract: Correctly interpreting observations of sources such as type Ia supernovae (SNe Ia) require knowledge of the power spectrum of matter on AU scales -which is very hard to model accurately. Because under-dense regions account for much of the volume of the universe, light from a typical source probes a mean density significantly below the cosmic mean. The relative sparsity of sources implies that there could be a significant bias when inferring distances of SNe Ia, and consequently a bias in cosmological parameter estimation. While the weak lensing approximation should in principle give the correct prediction for this, linear perturbation theory predicts an effectively infinite variance in the convergence for ultra-narrow beams.

In this talk, I will adopt an observational viewpoint and discuss some approximations to see the effects of under-dense lines of sight might have in parameter estimation. Also, I will address tests to decide what is the best way to describe light propagation in the real Universe.

Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

The cosmological constant problem: a lesson from Helium-3

Speaker: Gil Jannes
Affiliation: Universidad Carlos III de Madrid, Spain
Abstract: The effective gravity emerging for relativistic quasiparticles in superfluid 3He-A provides a direct connection between the properly determined vacuum energy and the cosmological constant lambda. This connection is non-trivial, and requires a detour via the masses of the "gravitons" in the associated bi-metric gravity. Although the cosmological constant lambda turns out to be huge -it is determined by the effective Planck scale-, the cosmological term (the stress-energy tensor associated to lambda) is in fact naturally non-constant and vanishes in the equilibrium vacuum, as dictated by thermodynamics. This suggests that the equilibrium state of any condensed-matter system, and perhaps also the final state of the Universe, is not gravitating.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Junction conditions in gravity theories

Speaker: José M. M. Senovilla
Affiliation: Universidad del País Vasco (UPV/EHU), Bilbao, Spain

Abstract: I will present the method to obtain the proper junction conditions in general theories of gravity, highlighting the differences specific to General Relativity (GR) and its peculiarities. As an illustrative example I will consider F(R) Lagrangian theories explicitly.

The discussion will analyze two different junction levels: allowing for branes/thin shells or not. In the former case I will argue that the generic brane/shell construction is crucially different to that in GR. In the latter case I will show that properly matched solutions in GR are no longer solutions of F(R) and other theories.

(An exceptional case arises in theories with a Lagrangian quadratic in the curvature: gravitational double layers are feasible, and this leads to new Physics -but I will probably have no time to describe this)

Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Cosmología y gravedad cuántica

Speaker: Gianluca Calcagni
Affiliation: Instituto de Estructura de la Materia (IEM-CSIC), Madrid, Spain
Abstract: Se revisan los problemas cosmológicos de la Gran Explosión (¿El universo nació de una singularidad inicial como predice la relatividad clásica?) y de la energía oscura (¿Por qué el universo reciente está acelerando?) en algunos enfoques de gravedad cuántica y teorías que van más allá del Modelo Estándar. Aunque no haya todavía ninguna solución consolidada y satisfactoria de ambos problemas, las observaciones cosmológicas modernas pueden proporcionar una valiosa información sobre diversos modelos exóticos de física de partículas y de gravedad.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Averaging and modified gravity theories: An overview of tools able to explain the cosmological late-time acceleration

Speaker: Álvaro de la Cruz Dombriz
Affiliation: Complutense University of Madrid, Spain
Abstract: One possible explanation for the present observed acceleration of the Universe is the breakdown of homogeneity and isotropy due to the formation of non-linear structures. How the formation of non-linear structures and subsequent inhomogeneities affect the averaged cosmological expansion rate and may eventually lead to late-time acceleration is usually referred to as backreaction mechanism. General Relativity together with dust matter scenarios have until recently been considered as the sole ingredients for averaged calculations. We shall present the backreaction formalism in more general scenarios, including imperfect fluids as well as extended theories of gravity, and apply an averaging procedure to them in order to determine possible backreaction effects.
Date and time:
Venue: CSIC, CFMAC, conference room (121 Serrano St.)

Cosmological solutions in bigravity

Speaker: Prado Martín-Moruno
Affiliation: Instituto Superior Técnico, Lisboa, Portugal
Abstract: Bigravity theories, which are formulated using two mutually interacting dynamical metrics, were initially introduced by Isham, Salam, and Strathdee in the seventies. These theories have recently attracted considerable attention since Hassan and Rosen found a bigravity theory which is potentially stable, that is, free of the Boulware-Deser ghost which otherwise affects these theories. As bigravity can be interpreted to describe two different universes, I will present a simple way to study the cosmology of such a classical bi-universe in general situations. I will pay particular attention to the interplay of both gravitational sectors and how one can extract conclusions about the dynamics of one universe from the knowledge about the occurrence of violent events in the other universe.
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

Emergent electromagnetism: some training towards a theory of emergent gravity

Speaker: Carlos Barceló
Affiliation: Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada, Spain
Date and time:
Venue: UCM, Faculty of Physical Sciences, Theoretical Physics I Dept., seminar room (3rd floor)

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