Person: Cao García, Francisco Javier
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First Name
Francisco Javier
Last Name
Cao García
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Estructura de la Materia, Física Térmica y Electrónica
Area
Física Aplicada
Identifiers
15 results
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Now showing 1 - 10 of 15
Item Quantum slow-roll and quantum fast-roll inflationary initial conditions: CMB quadrupole suppression and further effects on the low CMB multipoles(Physical Review D (particles, fields, gravitation, and cosmology), 2008) Cao García, Francisco Javier; De Vega, H. J.; Sanchez, N. G.Quantum fast-roll initial conditions for the inflaton which are different from the classical fast-roll conditions and from the quantum slow-roll conditions can lead to inflation that lasts long enough. These quantum fast-roll initial conditions for the inflaton allow for kinetic energies of the same order of the potential energies and nonperturbative inflaton modes with nonzero wave numbers. Their evolution starts with a transitory epoch where the redshift due to the expansion succeeds to assemble the quantum excited modes of the inflaton in a homogeneous (zero mode) condensate, and the large value of the Hubble parameter succeeds to overdamp the fast roll of the redshifted inflaton modes. After this transitory stage the effective classical slow-roll epoch is reached. Most of the e-folds are produced during the slow-roll epoch, and we recover the classical slow-roll results for the scalar and tensor metric perturbations plus corrections. These corrections are important if scales which are horizon size today exited the horizon by the end of the transitory stage and, as a consequence, the lower cosmic microwave background (CMB) multipoles get suppressed or enhanced. Both for scalar and tensor metric perturbations, fast roll leads to a suppression of the amplitude of the perturbations (and of the low CMB multipoles), while the quantum precondensate epoch gives an enhancement of the amplitude of the perturbations (and of the low CMB multipoles). These two types of corrections can compete and combine in a scale dependent manner. They turn out to be smaller in new inflation than in chaotic inflation. These corrections arise as natural consequences of the quantum nonperturbative inflaton dynamics, and can allow a further improvement of the fitting of inflation plus the CMB model to the observed CMB spectra. In addition, the corrections to the tensor metric perturbations will provide an independent test of this model. Thus, the effects of quantum inflaton fast-roll initial conditions provide a consistent and contrastable model for the origin of the suppression of the quadrupole and for other departures of the low CMB multipoles from the slow-roll inflation- CMB model, which are to be contrasted with the TE and EE multipoles and with the forthcoming and future CMB data.Item Domain wall dynamics in expanding spaces(Physical review E (statistical, nonlinear, biological, and soft matter physics), 2006) Cao García, Francisco Javier; Zamora-Sillero, Elias; Quintero, Niurka R.We study the effects on the dynamics of kinks due to expansions and contractions of the space. We show that the propagation velocity of the kink can be adiabatically tuned through slow expansions and/or contractions, while its width is given as a function of the velocity. We also analyze the case of fast expansions and/or contractions, where we are no longer on the adiabatic regime. In this case the kink moves more slowly after an expansion-contraction cycle as a consequence of the loss of energy through radiation. All these effects are numerically studied in the nonlinear Klein-Gordon equations (both for the sine-Gordon and for the 𝜙4 potential), and they are also studied within the framework of the collective coordinate evolution equations for the width and the center of mass of the kink. These collective coordinate evolution equations are obtained with a procedure that allows us to consider even the case of large expansions and/or contractions.Item Nonequilibrium dynamics in quantum field theory at high density: The ‘‘tsunami’’(Physical Review D (particles, fields, gravitation, and cosmology), 2001) Cao García, Francisco Javier; De Vega, H.J.The dynamics of a dense relativistic quantum fluid out of thermodynamic equilibrium is studied in the framework of the 𝛷4 scalar field theory in the large N limit. The time evolution of a particle distribution in momentum space (the tsunami) is computed. The effective mass felt by the particles in such a high density medium equals the tree level mass plus the expectation value of the squared field. The case of negative tree level squared mass is particularly interesting. In such a case dynamical symmetry restoration as well as dynamical symmetry breaking can happen. Furthermore, the symmetry may stay broken with a vanishing asymptotic squared mass showing the presence of out of equilibrium Goldstone bosons. We study these phenomena and identify the set of initial conditions that lead to each case. We compute the equation of state which turns out to depend on the initial state. Although the system does not thermalize, the equation of state for asymptotically broken symmetry is of radiation type. We compute the correlation functions at equal times. The two point correlator for late times is the sum of different terms. One stems from the initial particle distribution. Another term accounts for the out of equilibrium Goldstone bosons created by spinodal unstabilities when the symmetry is asymptotically broken. Both terms are of the order of the inverse of the coupling for distances where causal signals can connect the two points. The contribution of the out of equilibrium Goldstones exhibits scaling behavior in a generalized sense.Item Feedback Control in a Collective Flashing Ratchet(Physical Review Letters, 2004) Cao García, Francisco Javier; Dinis Vizcaíno, Luis Ignacio; Rodríguez Parrondo, Juan ManuelAn ensemble of Brownian particles in a feedback controlled flashing ratchet is studied. The ratchet potential is switched on and off depending on the position of the particles, with the aim of maximizing the current. We study in detail a protocol which maximizes the instant velocity of the center of mass of the ensemble at any time. This protocol is optimal for one particle and performs better than any periodic flashing for ensembles of moderate size, but is defeated by a random or periodic switching for large ensembles.Item Rocking feedback-controlled ratchets(Physical review E (statistical, nonlinear, biological, and soft matter physics), 2009) Feito, M.; Baltanas, J. P.; Cao García, Francisco JavierWe investigate the different regimes that emerge when a periodic driving force, the rocking force, acts on a collective feedback flashing ratchet. The interplay of the rocking and the feedback control gives a rich dynamics with different regimes presenting several unexpected features. In particular, we show that for both the one-particle ratchet and the collective version of the ratchet an appropriate rocking increases the flux. This mechanism gives the maximum flux that has been achieved in a ratchet device without an a priori bias.Item Nuevas tecnologías en la enseñanza universitaria(II Jornada Campus Virtual UCM: cómo integrar investigación y docencia en el CV-UCM, 2005) Cao García, Francisco Javier; Fernández-Valmayor Crespo, Alfredo; Fernández-Pampillón Cesteros, Ana María; Merino Granizo, JorgeLas nuevas tecnologías proporcionan recursos didácticos adicionales para facilitar el aprendizaje a los alumnos. Sin embargo, la creación de materiales docentes que aprovechen las potencialidades de las nuevas tecnologías no siempre es sencilla. Con el fin de simplificar la creación de nuevos materiales docentes estamos desarrollando varias aplicaciones informáticas, entre ellas una aplicación para la creación de test interactivos autoevaluables. Estos test incluyen la posibilidad de autocorregirse y la de mostrar la justificación de la respuesta. La aplicación permite crear test interactivos autoevaluables a partir de documentos en varios formatos (Word, LaTeX, HTML) que contengan la información necesaria. Así, la creación de test interactivos elaborados queda enormemente simplificada.Item Quantum inflaton, primordial perturbations, and CMB fluctuations(Physical Review D (particles, fields, gravitation, and cosmology), 2004) Cao García, Francisco Javier; De Vega, H.J.; Sánchez, N.G.We compute the primordial scalar, vector and tensor metric perturbations arising from quantum field inflation. Quantum field inflation takes into account the nonperturbative quantum dynamics of the inflaton consistently coupled to the dynamics of the (classical) cosmological metric. For chaotic inflation, the quantum treatment avoids the unnatural requirements of an initial state with all the energy in the zero mode. For new inflation it allows a consistent treatment of the explosive particle production due to spinodal instabilities. Quantum field inflation (under conditions that are the quantum analog of slow-roll) leads, upon evolution, to the formation of a condensate starting a regime of effective classical inflation. We compute the primordial perturbations taking the dominant quantum effects into account. The results for the scalar, vector and tensor primordial perturbations are expressed in terms of the classical inflation results. For a N-component field in a O(N) symmetric model, adiabatic fluctuations dominate while isocurvature or entropy fluctuations are negligible. The results agree with the current Wilkinson Microwave Anisotropy Probe observations and predict corrections to the power spectrum in classical inflation. Such corrections are estimated to be of the order of m(2)/N-2, where m is the inflaton mass and H the Hubble constant at the moment of horizon crossing. An upper estimate turns to be about 4% for the cosmologically relevant scales. This quantum field treatment of inflation provides the foundations to the classical inflation and permits to compute quantum corrections to it.Item Out of equilibrium nonperturbative quantum field dynamics in homogeneous external fields(Physical Review D (particles, fields, gravitation, and cosmology), 2002) Cao García, Francisco Javier; De Vega, H.J.The quantum dynamics of the symmetry broken lambda ((Phi) over right arrow (2))(2) scalar field theory in the presence of an homogeneous external field is investigated in the large N limit. We choose as the initial state the around state for a constant external field (T) over right arrow. The sign of the external field is suddenly flipped from (T) over right arrow to - (T) over right arrow at a given time and the subsequent quantum dynamics calculated. Spinodal instabilities and parametric resonances produce large quantum fluctuations in the field components transverse to the external field. This allows the order parameter to turn around the maximum of the potential for intermediate times. Subsequently, the order parameter starts to oscillate near the global minimum for the external field - (T) over right arrow, entering a novel quasiperiodic regime.Item Out of equilibrium quantum field dynamics of an initial thermal state after a change in the external field(Physical Review D (particles, fields, gravitation, and cosmology), 2006) Cao García, Francisco Javier; Feito, M.The effects of the initial temperature in the out of equilibrium quantum field dynamics in the presence of a homogeneous external field are investigated. We consider an initial thermal state of temperature T for a constant external field J ->. A subsequent sign flip of the external field, J ->->-J ->, gives rise to an out of equilibrium nonperturbative quantum field dynamics. The dynamics is studied here for the symmetry broken lambda(Phi ->(2))(2) scalar N component field theory in the large N limit. We find a dynamical effective potential for the expectation value that helps us to understand the dynamics. The dynamics presents two regimes defined by the presence or absence of a temporal trapping close to the metastable equilibrium position of the potential. The two regimes are separated by a critical value of the external field that depends on the initial temperature. The temporal trapping is shorter for larger initial temperatures or larger external fields. Parametric resonances and spinodal instabilities amplify the quantum fluctuations in the field components transverse to the external field. When there is a temporal trapping, this is the main mechanism that allows the system to escape from the metastable state for large N. Subsequently, backreaction stops the growth of the quantum fluctuations and the system enters a quasiperiodic regime.Item Thermodynamics of feedback controlled systems(Physical review E (statistical, nonlinear, biological, and soft matter physics), 2009) Cao García, Francisco Javier; Feito, M.We compute the entropy reduction in feedback controlled systems due to the repeated operation of the controller. This was the lacking ingredient to establish the thermodynamics of these systems, and in particular of Maxwell’s demons. We illustrate some of the consequences of our general results by deriving the maximum work that can be extracted from isothermal feedback controlled systems. As a case example, we finally study a simple system that performs an isothermal information-fueled particle pumping.