I am a theoretical cosmologist at the National Nuclear Research Institute in Mexico, and presently on sabbatical leave for a year. I am currently working at the Berkeley Center for Cosmological Physics in the Physics Division at the Lawrence Berkeley National Laboratory. I received my PhD from the University of Konstanz in Germany in theoretical cosmology.
My research interests are: 1) Theoretical cosmology: inflationary models, quintessence with non-minimally coupling terms, and f(R) theories. Unified models of inflation, dark energy, and dark matter. 2) Theoretical astrophysics: Effects of alternative theories of gravity on stellar and galactic scales. 3) Theoretical aspects of alternative theories of gravity.
Abstract: We build a spherical halo model for galaxies using a general scalar-tensor theory of gravity in its Newtonian limit. The scalar field is described by a time-independent Klein-Gordon equation with a source that is coupled to the standard Poisson equation of Newtonian gravity. Our model, by construction, fits both the observed rotation velocities of stars in spirals and a typical luminosity profile. As a result, the form of the new Newtonian potential, the scalar field, and dark matter distribution in a galaxy are determined. Taking into account the constraints for the fundamental parameters of the theory (lambda,alpha), we analyze the influence of the scalar field in the dark matter distribution, resulting in shallow density profiles in galactic centers.
Abstract: We present some exact solutions and a phase space analysis of metric f(R)-gravity models of the type R/sup n/. We divide our discussion in n ne 2 and n = 2 models. The later model is a good approximation, at late times to the f(R) = (2/ pi )R tan/sup -1/(R/ beta /sup 2/) gravity model, being this an example of a non-singular case. For n ne 2 models we have found power law solutions for the scale factor that are attractors and that comply with WMAP 5-years data if n < -2.55 or 1.67 < n < 2. On the other hand, the quadratic model has the de Sitter solution as an attractor, that also complies with WMAP 5-years data.
Abstract: We present Lambda CDM N-body cosmological simulations in the framework of of a static general scalar-tensor theory of gravity. Due to the influence of the non-minimally coupled scalar field, the gravitational potential is modified by a Yukawa type term, yielding a new structure formation dynamics. We present some preliminary results and, in particular, we compute the density and velocity profiles of the most massive group.
Abstract: We study the cosmic evolution in recent times within the induced gravity theory. Taking into account the experimental constraints on the parameter of the theory, we study the quintessence dynamics. We obtain the possible models and discuss the physical consequences in cosmology and particle physics.
Abstract: We computed flat rotation curves from scalar-tensor theories in their weak field limit. Our model, by construction, fits a flat rotation profile for velocities of stars. As a result, the form of the scalar field potential and DM distribution in a galaxy are determined. By taking into account the constraints for the fundamental parameters of the theory ( lambda , alpha ), it is possible to obtain analytical results for the density profiles. For positive and negative values of alpha , the DM matter profile is as cuspy as NFW's.
Abstract: We study bar formation in galactic disks as a consequence of the collision of two spiral galaxies under the influence of a potential which is obtained from the Newtonian limit of a scalar-tensor theory of gravity. We found that dynamical effects depend on parameters (a, A) of the theory. In particular, we observe that the bar is shorter for weaker tidal perturbations, which in turn corresponds to smaller values of A used in our numerical experiments.
Abstract: We use the Newtonian limit of a general scalar-tensor theory around a background field to study astrophysical effects. The gravitational theory modifies the standard Newtonian potential by adding a Yukawa term to it, which is quantified by two theoretical parameters: lambda , the lengthscale of the gravitational interaction and its strength, alpha . Within this formalism we firstly present a numerical study on the formation of bars in isolated galaxies. We have found for positive a that the modified gravity destabilizes the galactic discs and leads to rapid bar formation in isolated galaxies. Values of lambda in the range ap 8-14 kpc produce strongest bars in isolated models. Then, we extent this work by considering tidal effects due to interacting galaxies. We send two spirals to collide and study the bar properties of the remnant. We characterize the bar kinematical properties in terms of our parameters ( lambda , alpha ).
Abstract: The joint influence of numerical parameters such as the number of particles N, the gravitational softening length e and the time-step Delta t is investigated in the context of galaxy simulations. For isolated galaxy models we have performed a convergence study and estimated the numerical parameters ranges for which the relaxed models do not deviate significantly from its initial configuration. By fixing N, we calculate the range of the mean interparticle separation lambda(r) along the disc radius. Uniformly spaced values of lambda are used as epsilon in numerical tests of disc heating. We have found that in the simulations with N = 1 310 720 particles lambda varies by a factor of 6, and the corresponding final Toomre's parameters Q change by only about 5 per cent. By decreasing N, the. and Q ranges broaden. Large e and small N cause an earlier bar formation. In addition, the numerical experiments indicate, that for a given set of parameters the disc heating is smaller with the Plummer softening than with the spline softening. For galaxy collision models we have studied the influence of the selected numerical parameters on the formation of tidally triggered bars in galactic discs and their properties, such as their dimensions, shape, amplitude and rotational velocity. Numerical simulations indicate that the properties of the formed bars strongly depend upon the selection of N and e. Large values of the gravitational softening parameter and a small number of particles result in the rapid formation of a well defined, slowly rotating bar. On the other hand, small values of e produce a small, rapidly rotating disc with tightly wound spiral arms, and subsequently a weak bar emerges. We have found that by increasing N, the bar properties converge and the effect of the softening parameter diminishes. Finally, in some cases short spiral arms are observed at the ends of the bar that change periodically from trailing to leading and vice-versa-the wiggle.
Abstract: We present a formulation for potential-density pairs to describe axi-symmetric galaxies in the Newtonian limit of scalar-tensor theories of gravity. The scalar field is described by a modified Helmholtz equation with a source that is coupled to the standard Poisson equation of Newtonian gravity. The net gravitational force is given by two contributions: the standard Newtonian potential plus a term stemming from massive scalar fields. General solutions have been found for axisymmetric systems and the multipole expansion of the Yukawa potential is given. In particular, we have computed potential-density pairs of galactic disks for an exponential profile and their rotation curves.
Abstract: A family of potential-density pairs has been found for spherical haloes and bulges of galaxies in the Newtonian limit of scalar-tensor theories of gravity. The scalar field is described by a Klein-Gordon equation with a source that is coupled to the standard Poisson equation of Newtonian gravity. The net gravitational force is given by two contributions: the standard Newtonian potential plus a term stemming from massive scalar fields. General solutions have been found for spherical systems. In particular, we compute potential-density pairs of spherical, galactic systems, and some other astrophysical quantities that are relevant to generate initial conditions for spherical galaxy simulations.
Abstract: We use scaled variables to deduce a set of coupled equations valid for isotropic and anisotropic Bianchi type I, V, IX models in some scalar-tensor theories. In particular, for the Brans-Dicke theory the equations decouple, then one is able to integrate the system for FRW and Bianchi I and V models. Additionally, we investigate, through the method of Lyapunov, the stability of our solutions; particular results are presented for BD theory. Furthermore, an extended Lyapunov theorem is demonstrated in the appendix.
Abstract: Using scaled variables we are able to integrate an equation valid for isotropic and anisotropic Bianchi type I, V, IX models in Brans-Dicke theory. We specialize our analysis for the case in which omega = -1 that corresponds to string effective theories. In these theories we analyze the possibility that anisotropic models asymptotically isotropize, and/or possess inflationary properties. Additionally, a new solution of curve (k not equal 0) Friedmann-Robertson-Walker (FRW) cosmologies is discussed.
Abstract: Using scaled variables we are able to integrate an equation valid for isotropic and anisotropic Bianchi type I, V, IX models in Brans-Dicke (BD) theory. We analyze known and now solutions for these models in relation with the possibility that anisotropic models asymptotically isotropize, and/or possess inflationary properties. In particular, anew solution of curved (k not equal 0) Friedmann-Robertson-Walker (FRW) cosmologies in Brans-Dicke theory is analyzed.
Abstract: We analyze if Bianchi type I, V, and IX models in the induced gravity (IG) theory can evolve to a Friedmann-Roberson-Walker (FRW) expansion due to the nonminimal coupling of gravity and the scalar field. The analytical results that we find for the Brans-Dicke (BD) theory are now applied to the IG theory which has omega much less than 1 (omega being the square ratio of the Higgs boson to Planck mass) in a cosmological era in which the IG potential is not significant. We find that the isotropization mechanism crucially depends on the value of omega. Its smallness also permits inflationary solutions. For the Bianchi type V model, inflation due to the Higgs potential takes place afterwards, and subsequently spontaneous symmetry breaking ends with an effective FRW evolution. The ordinary tests of successful cosmology are well satisfied. [S0556-2821(98)06624-7].
Abstract: We analyze if Bianchi type I, V, and IX models in the induced gravity (IG) theory can evolve to a Friedmann-Roberson-Walker (FRW) expansion due to the nonminimal coupling of gravity and the scalar field. The analytical results that we find for the Brans-Dicke (BD) theory are now applied to the IG theory which has omega Lt1 ( omega being the square ratio of the Higgs boson to Planck mass) in a cosmological era in which the IG potential is not significant. We find that the isotropization mechanism crucially depends on the value of omega . Its smallness also permits inflationary solutions. For the Bianchi type V model, inflation due to the Higgs potential takes place afterwards, and subsequently spontaneous symmetry breaking ends with an effective FRW evolution. The ordinary tests of successful cosmology are well satisfied
Abstract: We discuss the dynamics of anisotropic Bianchi type IX models in Jordan-Brans-Dicke cosmological theory rendering the evolution of a universe model with closed space near its beginning before inflation sets in. This paper displays how, when written in terms of reduced variables, the field equations allow straightforward partial integration. The mean expansion H, the scalar field, and the three scale factors are given in terms of the volume expansion.
Abstract: Some exact solutions in the Brans-Dicke (BD) theory are shown for a Bianchi V metric having the properties of inflationary expansion, graceful exit and asymptotic evolution to a Friedmann-Robertson-Walker open model. It is remarkable that inflationary behaviour can occur, even without a cosmological potential or constant. However the horizon and flatness problems cannot be solved within the standard ED theory because the inflationary period is severely restricted by the value of the ED parameter omega.
Abstract: The cosmic, general analytic solutions of the Brans-Dicke theory for the hat Friedmann-Robertson-Walker (FRW) models containing perfect, barotropic, fluids are seen to belong to a wider class of solutions, which includes cosmological models with the open and the closed spaces of the FRW metric, as well as solutions for models with homogeneous but anisotropic spaces corresponding to the Bianchi-type metric classification, when all these solutions are expressed in terms of reduced variables. The existence of such a class lies in the fact that the scaled scalar-field psi = phi alpha(3(1-beta)) (with alpha(3) alpha(1) alpha(2) alpha(3) the ''volume element'' and beta the barotropic index, p = beta rho) can be written as a quadratic function of the scaled time and this solution is independent of the metrics here employed. This reduction procedure permits one to analyze if explicitly given anisotropic cosmological solutions ''isotropize'' in, the course of their time evolution. If this can happen, it could be claimed that there exists a subclass of solutions that is stable under anisotropic perturbations: This seems to be the case for the Bianchi type I, V, and IX.
Abstract: We are considering the cosmological consequences of an induced
gravity theory coupled to the minimal standard model of particle physics.
The non-minimal coupling parameter between gravity and the Higgs
field must then be very large, yielding some new cosmological consequences
for the early Universe and new constraints on the Higgs mass.
As an outcome, new inflation is only possible for very special initial
conditions producing first a short contraction era after which an inflationary
expansion automatically follows; a chaotic inflationary scenario
is successfully achieved. The contrast of density perturbations required
to explain the seed of astronomic structures are obtained for very large
values of the Higgs mass (MH >> G−1/2
F ), otherwise the perturbations
have a small amplitude; in any case, the spectral index of scalar perturbations
agrees with the observed one.
Notes: Related paper:
Induced gravity inflation in the SU(5) GUT.
J.L. Cervantes-Cota, H. Dehnen, (Konstanz U.) . Dec 1994. 18pp.
Published in Phys.Rev.D51:395-404,1995.
e-Print: astro-ph/9412032
Abstract: We show that the general analytic solution, obtained for the flat space Friedmann-Robertson-Walker (FRW) metric is also a solution for the Bianchi-Type I cosmological model and is, in the sense stable under anisotropic perturbations
Abstract: The dynamics of the production of relative high frequency gravitational waves by astrophysical events taking place within the galaxies in the cosmological context can be well represented by a phenomenological description of the generating processes through a ''radiation energy profile'' that contains two adjustable parameters. This profile gives a relation between the total pressure and energy density of the matter generating gravitational radiation together with the ''waves'' themselves, both treated as ordinary hydrodynamic fluids. The resulting cosmological models then represent, in a descriptive way only, universes filled with two interacting fluids: on the one hand gravitational radiation, and the matter which emits it but does not reabsorb it, on the other. It is shown that the dynamic effects of the model can be significant, even if the conversion rate of matter into gravitational radiation is relatively small.
Abstract: The space-space field equations of the Jordan Brans-Dicke theory describing a spatially homogeneous vacuum spacetime of the Bianchi type can be brought under transformation into a general set of non-linear differential equations, an all-encompassing general equation implicitly contained in each of the cosmological models studied, most of whose analytic solutions we have found. The scalar field displays a considerable influence in the cosmological context during the early, strongly relativistic stage of the universe's expansion where the main difference between the cosmologies of Einstein general relativity and Jordan Brans-Dicke theories lies in the possible existence of a sourceless phi-field which, since it is not generated by matter, is contrary to Mach's principle. Several solutions presented in this paper are reported for the first time.
Abstract: A short introduction to the Standard Big Bang model is provided, presenting its physical model, and emphasizing its long–standing problems such as the horizon, flatness, baryon asymmetry, among others. Next, an introduction to the inflationary cosmology is presented to elucidate a solution to some of the above–mentioned problems. It is shown that the inflationary scenario succeeds in explaining what the standard Big Bang model cannot, passing the tests of the high precision experimental constraints which have been performed since last decade. This contribution should serve as an introduction to the standard ideas and scenarios which will be used in the forthcoming lectures of this book.
Abstract: Spectacular experimental advances in observational cosmology have helped raise cosmology to the status of a genuine science, and it is now possible to test many speculative theoretical issues and to obtain reliable values for the key parameters defining our observable universe. This book has emerged from selected lectures given at the Mexican School on Gravitation and Mathematical Physics by leaders in their field. Conceived as both a broad survey and as topical coverage of the latest developments, it will benefit graduate students and newcomers to this field and provide researchers in the field with a modern source of reference
