David Brizuela

Bio: David Brizuela is an academic researcher from University of the Basque Country. The author has contributed to research in topics: Scalar field & Gravitational wave. The author has an hindex of 7, co-authored 17 publications receiving 230 citations. Previous affiliations of David Brizuela include Schiller International University & Spanish National Research Council.

xPert: Computer algebra for metric perturbation theory

Abstract: We present the tensor computer algebra package xPert for fast construction and manipulation of the equations of metric perturbation theory, around arbitrary backgrounds. It is based on the combination of explicit combinatorial formulas for the nth order perturbation of curvature tensors and their gauge changes, and the use of highly efficient techniques of index canonicalization, provided by the underlying tensor system xAct, for Mathematica. We give examples of use and show the efficiency of the system with timings plots: it is possible to handle orders n = 4 or n = 5 within seconds, or reach n = 10 with timings below 1 h.

Effective dynamics of the hybrid quantization of the Gowdy T^3 universe

Abstract: D.B. acknowledges financial support from the Spanish Ministry of Education through the Programa Nacional de Movilidad de Recursos Humanos of National Programme No. I-D+i2008-2011. This work was supported by the MICINN Project FIS2008-06078- C03-03 and the Consolider-Ingenio Program CPAN (CSD2007-00042) from Spain, by the Institute for Gravitation and the Cosmos (PSU), and by the Natural Sciences and Engineering Research Council of Canada.

Mode coupling of Schwarzschild perturbations: Ringdown frequencies

Abstract: Within linearized perturbation theory, black holes decay to their final stationary state through the well-known spectrum of quasinormal modes. Here we numerically study whether nonlinearities change this picture. For that purpose we study the ringdown frequencies of gauge-invariant second-order gravitational perturbations induced by self-coupling of linearized perturbations of Schwarzschild black holes. We do so through high-accuracy simulations in the time domain of first and second-order Regge-Wheeler-Zerilli type equations, for a variety of initial data sets. We consider first-order even-parity ($\ensuremath{\ell}=2$, $m=\ifmmode\pm\else\textpm\fi{}2$) perturbations and odd-parity ($\ensuremath{\ell}=2$, $m=0$) ones, and all the multipoles that they generate through self-coupling. For all of them and all the initial data sets considered we find that---in contrast to previous predictions in the literature---the numerical decay frequencies of second-order perturbations are the same ones of linearized theory, and we explain the observed behavior. This would indicate, in particular, that when modeling or searching for ringdown gravitational waves, appropriately including the standard quasinormal modes already takes into account nonlinear effects.

Interacting holographic tachyon model of dark energy

Abstract: We propose a holographic tachyon model of dark energy with interaction between the components of the dark sector. The correspondence between the tachyon field and the holographic dark energy densities allows the reconstruction of the potential and the dynamics of the tachyon scalar field in a flat Friedmann–Robertson–Walker universe. We show that this model can describe the observed accelerated expansion of our universe with a parameter space given by the most recent observational results.

A moment approach to compute quantum-gravity effects in the primordial universe.

Abstract: An approach to compute quantum-gravity corrections to the scalar and tensorial power spectra of the inflationary perturbations is presented. The analysis of the Wheeler-DeWitt equation is performed by a decomposition of the wave function into its infinite set of moments, which must obey certain system of (first-class) constraints. Considering a semiclassical approximation, the system is truncated at second order in moments and an appropriate gauge-fixing condition is introduced, which allows us to interpret the scale factor of the universe as an internal time. The evolution of the different fluctuations and correlations is then explicitly considered for a de Sitter universe. An approximate analytical solution is obtained for the corrections of the power spectra, which produces an enhancement of power for large scales. Remarkably, the result is in agreement with previous studies in the literature that made use of very different semiclassical approximations. Finally, the numerical implementation of the system is also considered to verify the validity of the analytical solution.

Loop Quantum Cosmology: A Status Report

Abstract: Loop quantum cosmology (LQC) is the result of applying principles of loop quantum gravity (LQG) to cosmological settings. The distinguishing feature of LQC is the prominent role played by the quantum geometry effects of LQG. In particular, quantum geometry creates a brand new repulsive force which is totally negligible at low spacetime curvature but rises very rapidly in the Planck regime, overwhelming the classical gravitational attraction. In cosmological models, while Einstein's equations hold to an excellent degree of approximation at low curvature, they undergo major modifications in the Planck regime: for matter satisfying the usual energy conditions, any time a curvature invariant grows to the Planck scale, quantum geometry effects dilute it, thereby resolving singularities of general relativity. Quantum geometry corrections become more sophisticated as the models become richer. In particular, in anisotropic models, there are significant changes in the dynamics of shear potentials which tame their singular behavior in striking contrast to older results on anisotropies in bouncing models. Once singularities are resolved, the conceptual paradigm of cosmology changes and one has to revisit many of the standard issues—e.g. the 'horizon problem'—from a new perspective. Such conceptual issues as well as potential observational consequences of the new Planck scale physics are being explored, especially within the inflationary paradigm. These considerations have given rise to a burst of activity in LQC in recent years, with contributions from quantum gravity experts, mathematical physicists and cosmologists. The goal of this review is to provide an overview of the current state of the art in LQC for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general and cosmologists who wish to apply LQC to probe modifications in the standard paradigm of the early universe. In this review, effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without loss of continuity.

Maximal freedom at minimum cost: linear large-scale structure in general modifications of gravity

Abstract: We present a turnkey solution, ready for implementation in numerical codes, for the study of linear structure formation in general scalar-tensor models involving a single universally coupled scalar field. We show that the totality of cosmological information on the gravitational sector can be compressed — without any redundancy — into five independent and arbitrary functions of time only and one constant. These describe physical properties of the universe: the observable background expansion history, fractional matter density today, and four functions of time describing the properties of the dark energy. We show that two of those dark-energy property functions control the existence of anisotropic stress, the other two — dark-energy clustering, both of which are can be scale-dependent. All these properties can in principle be measured, but no information on the underlying theory of acceleration beyond this can be obtained. We present a translation between popular models of late-time acceleration (e.g. perfect fluids, f(R), kinetic gravity braiding, galileons), as well as the effective field theory framework, and our formulation. In this way, implementing this formulation numerically would give a single tool which could consistently test the majority of models of late-time acceleration heretofore proposed.

I-Love-Q Relations in Neutron Stars and their Applications to Astrophysics, Gravitational Waves and Fundamental Physics

Abstract: The exterior gravitational field of a slowly rotating neutron star can be characterized by its multipole moments, the first few being the neutron star mass, moment of inertia, and quadrupole moment to quadratic order in spin. In principle, all of these quantities depend on the neutron star's internal structure, and thus, on unknown nuclear physics at supranuclear energy densities, all of which is usually parametrized through an equation of state. We here find relations between the moment of inertia, the Love numbers and the quadrupole moment (I-Love-Q relations) that do not depend sensitively on the neutron star's internal structure. Such universality may arise for two reasons: (i) these relations depend most sensitively on the internal structure far from the core, where all realistic equations of state mostly approach each other; (ii) as the neutron star compactness increases, the I-Love-Q trio approaches that of a black hole, which does not have an internal-structure dependence. Three important consequences derive from these I-Love-Q relations. On an observational astrophysics front, the measurement of a single member of the I-Love-Q trio would automatically provide information about the other two, even when the latter may not be observationally accessible. On a gravitational-wave front, the I-Love-Q relations break the degeneracy between the quadrupole moment and the neutron star spins in binary inspiral waveforms, allowing second-generation ground-based detectors to determine the (dimensionless) averaged spin to $\mathcal{O}(10)%$, given a sufficiently large signal-to-noise ratio detection. On a fundamental physics front, the I-Love-Q relations allow for tests of general relativity in the neutron star strong field that are both theory and internal-structure independent. As an example, by combining gravitational-wave and electromagnetic observations, one may constrain dynamical Chern-Simons gravity in the future by more than six orders of magnitude more stringently than Solar System and table-top constraints.

The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations

Abstract: Using techniques from loop quantum gravity, the standard theory of cosmological perturbations was recently generalized to encompass the Planck era. We now apply this framework to explore pre-inflationary dynamics. The framework enables us to isolate and resolve the true trans-Planckian difficulties, with interesting lessons both for theory and observations. Specifically, for a large class of initial conditions at the bounce, we are led to a self consistent extension of the inflationary paradigm over the 11 orders of magnitude in density and curvature, from the big bounce to the onset of slow roll. In addition, for a narrow window of initial conditions, there are departures from the standard paradigm, with novel effects—such as a modification of the consistency relation between the ratio of the tensor to scalar power spectrum and the tensor spectral index, as well as a new source for non-Gaussianities—which could extend the reach of cosmological observations to the deep Planck regime of the early universe. Communicated by P Singh

Maximal freedom at minimum cost: linear large-scale structure in general modifications of gravity

Abstract: We present a turnkey solution, ready for implementation in numerical codes, for the study of linear structure formation in general scalar-tensor models involving a single universally coupled scalar field. We show that the totality of cosmological information on the gravitational sector can be compressed - without any redundancy - into five independent and arbitrary functions of time only and one constant. These describe physical properties of the universe: the observable background expansion history, fractional matter density today, and four functions of time describing the properties of the dark energy. We show that two of those dark-energy property functions control the existence of anisotropic stress, the other two - dark-energy clustering, both of which are can be scale-dependent. All these properties can in principle be measured, but no information on the underlying theory of acceleration beyond this can be obtained. We present a translation between popular models of late-time acceleration (e.g. perfect fluids, f (R), kinetic gravity braiding, galileons), as well as the effective field theory framework, and our formulation. In this way, implementing this formulation numerically would give a single tool which could consistently test the majority of models of late-time acceleration heretofore proposed.