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Search Results: 1 - 10 of 761507 matches for " J. A. Belinchón "
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Scalar-Tensor Bianchi VI Models
J. A. Belinchón
Journal of Gravity , 2013, DOI: 10.1155/2013/491905
Abstract: We study how may vary the gravitational and the cosmological “constants,” ( and ) in several scalar-tensor theories with Bianchi III, , and symmetries. By working under the hypothesis of self-similarity we find exact solutions for two different theoretical models, which are the Jordan-Brans-Dicke (JBD) with and the usual JBD model with potential (that mimics the behaviour of . We compare both theoretical models, and some physical and geometrical properties of the solutions are also discussed putting special emphasis on the study of the isotropization of the solutions. 1. Introduction Current observations of the large scale Cosmic Microwave Background (CMB) suggest to us that our physical universe is expanding in an accelerated way. Such observations [1–3] indicate that the universe is dominated by an unidentified “dark energy” (DE) and suggest that this unidentified dark energy has a negative pressure [4–6]. This last characteristic of the dark energy points to the vacuum energy or cosmological constant , as a possible candidate for dark energy. From the theoretical point of view, it is convenient to consider the cosmological constant as a dynamical quantity in order to solve the so-called coincidence and fine tuning problems. In the same way other observations have pointed out a possible variation of the gravitational constant [7, 8]. For example, observations of Hulse-Taylor binary pulsar [9, 10], and type Ia supernova observations [11]. For an extensive review see Uzan [12]. We have several theoretical models that consider both constants as variable with respect to the cosmic time. Such theories are modified general relativity (MGR), modified scalar cosmological models (MST), and several scalar-tensor theories (STT). The MGR and MST have a drawback, since in them the variations of and are introduced in an ad hoc manner. Nevertheless we consider that the STT are the best models to study the variation of and , since they have been deduced form variational principles and where the time dependence can occur in a natural way, without any new assumption or modification of the theory. This class of models has received a renewed interest in recent times, for two main reasons. Firstly, the new inflationary scenario as the extended inflation has a scalar field that solves several problems present in the old theories. Secondly, string theories and other unified theories contain a scalar field which plays a similar role to the scalar field of the STT. The scalar-tensor theories started with the work of Jordan in 1950 [13]. A prototype of such models was proposed
Generalized Self-similar Scalar-Tensor Theories
J. A. Belinchón
Physics , 2011, DOI: 10.1140/epjc/s10052-012-1866-8
Abstract: We study through symmetry principles the form of the functions in the generalizated scalar-tensor theories under the self-similar hypothesis. The results obtained are absolutely general and valid for all the Bianchi models and the flat FRW one. We study the concrete example of the Kantowsky-Sach model finding some exact self-similar solutions.
Bianchi {VI}$_{0}$ in Scalar and Scalar-Tensor Cosmologies
J. A. Belinchón
Physics , 2011, DOI: 10.2478/s11534-012-0087-y
Abstract: We study several cosmological models with Bianchi \textrm{VI}$_{0}$ symmetries under the self-similar approach. In order to study how the \textquotedblleft constants\textquotedblright\ $G$ and $\Lambda$ may vary, we propose three scenarios where such constants are considered as time functions. The first model is a perfect fluid. We find that the behavior of $G$ and $\Lambda$ are related. If $G$ behaves as a growing time function then $\Lambda$ is a positive decreasing time function but if $G$ is decreasing then $\Lambda$ is negative. For this model we have found a new solution. The second model is a scalar field, where in a phenomenological way, we consider a modification of the Klein-Gordon equation in order to take into account the variation of $G$. Our third scenario is a scalar-tensor model. We find three solutions for this models where $G$ is growing, constant or decreasing and $\Lambda$ is a positive decreasing function or vanishes. We put special emphasis on calculating the curvature invariants in order to see if the solutions isotropize.
Perfect Fluid LRS Bianchi I with Time Varying Constants
J. A. Belinchón
Physics , 2004, DOI: 10.1007/s10509-005-9023-6
Abstract: It is investigated the behaviour of the ``constants'' $G,$ $c$ and $\Lambda $ in the framework of a perfect fluid LRS Bianchi I cosmological model. It has been taken into account the effects of a $c-$variable into the curvature tensor. Two exact cosmological solutions are investigated, arriving to the conclusion that if $q<0$ (deceleration parameter) then $G,$ $c$ are growing functions on time $t$ while $\Lambda $ is a negative decreasing function on time.
Full causal bulk viscous LRS Bianchi I with time varying constants
J. A. Belinchón
Physics , 2004, DOI: 10.1007/s10509-005-3424-4
Abstract: In this paper we study the evolution of a LRS Bianchi I Universe, filled with a bulk viscous cosmological fluid in the presence of time varying constants "but" taking into account the effects of a c-variable into the curvature tensor. We find that the only physical models are those which ``constants'' $G$ and $c$ are growing functions on time $t$, while the cosmological constant $\Lambda$ is a negative decreasing function. In such solutions the energy density obeys the ultrastiff matter equation of state i.e. $\omega=1$.
On the equation of state of a flat FRW model filled with a bulk viscous fluid
J. A. Belinchón
Physics , 2004,
Abstract: In this paper, we study the equation of state admissible for a flat FRW models filled with a bulk viscous fluid by using the Lie group method. It is founded that the model admits scaling symmetries iff the bulk viscous parameter $\gamma =1/2$. In this case, it is found that the main quantities follow a power law solution and in particular the bulk viscous pressure $\Pi $ has the same order of magnitude as the energy density $\rho ,$ in such a way that it is possible to formulate the equation of state $\Pi =\varkappa \rho ,$ where $\varkappa \in \mathbb{R}^{-}$ (i.e. is a negative numerical constant)$.$ If we assume such relationship we find again that the model is scale invariant iff $\gamma =1/2.$ We conclude that the model accepts a scaling symmetry iff $\gamma =1/2$ and that for this value of the viscous parameter, $\Pi =\varkappa \rho ,$ but the hypothesis $\Pi =\varkappa \rho $ does not imply $\gamma =1/2,$ and that the model is scale invariant.
Bulk viscous FWR with time varying constants revisited
J. A. Belinchón
Physics , 2005,
Abstract: We study a full causal bulk viscous cosmological model with flat FRW symmetries and where the ``constants'' $G,c$ and $\Lambda $ vary. We take into account the possible effects of a $c-$variable into the curvature tensor in order to outline the field equations. Using the Lie method we find the possible forms of the ``constants'' $G$ and $c$ that make integrable the field equations as well as the equation of state for the viscous parameter. It is found that $G,c$ and $\Lambda $ follow a power law solution verifying the relationship $G/c^{2}=\kappa .$ Once these possible forms have been obtained we calculate the thermodynamical quantities of the model in order to determine the possible values of the parameters that govern the quantities, finding that only a growing $G$ and $c$ are possible while $% \Lambda $ behaves as a negative decreasing function.
Perfect Fluid FRW with Time-varying Constants Revisited
J. A. Belinchón,J. L. Caramés
Physics , 2004,
Abstract: In this paper we revise a perfect fluid FRW model with time-varying constants \textquotedblleft but\textquotedblright taking into account the effects of a \textquotedblleft$c$-variable\textquotedblright into the curvature tensor. We study the model under the following assumptions, $div(T)=0$ and $div(T)\neq0,$ and in each case the flat and the non-flat cases are studied. Once we have outlined the new field equations, it is showed in the flat case i.e. K=0, that there is a non-trivial homothetic vector field i.e. that this case is self-similar. In this way, we find that there is only one symmetry, the scaling one, which induces the same solution that the obtained one in our previous model. At the same time we find that \textquotedblleft constants" $G$ and $c$ must verify, as integration condition of the field equations, the relationship $G/c^{2}=const.$ in spite of that both \textquotedblleft constants" vary. We also find that there is a narrow relationship between the equation of state and the behavior of the time functions $G,c$ and the sign of $\Lambda$ in such a way that these functions may be growing as well as decreasing functions on time $t,$ while $\Lambda$ may be a positive or negative decreasing function on time $t.$ In the non-flat case it will be showed that there is not any symmetry. For the case $div(T)\neq0,$ it will be studied again the flat and the non-flat cases. In order to solve this case it is necessary to make some assumptions on the behavior of the time functions $G,c$ and $\Lambda.$ We also find the flat case with $div(T)=0,$ is a particular solution of the general case $div(T)\neq0.$
Exact solutions of a Flat Full Causal Bulk viscous FRW cosmological model through factorization
O. Cornejo-Pérez,J. A. Belinchón
Physics , 2012, DOI: 10.1142/S0218271813500314
Abstract: We study the classical flat full causal bulk viscous FRW cosmological model through the factorization method. The method shows that there exists a relationship between the viscosity parameter $s$ and the parameter $\gamma$ entering the equations of state of the model. Also, the factorization method allows to find some new exact parametric solutions for different values of the viscous parameter $s$. Special attention is given to the well known case $s=1/2$, for which the cosmological model admits scaling symmetries. Furthermore, some exact parametric solutions for $s=1/2$ are obtained through the Lie group method.
Lesiones craneales secundarias a parto asistido con forceps Cranial lesions due to forceps delivery
F. Villarejo,J.M. Belinchón,F. Carceller,A. Gómez-Sierra
Neurocirugía , 2009,
Abstract: Los partos asistidos con forceps o vacuum aumentan la incidencia de lesiones craneoencefálicas fetales, siendo la tendencia actual a realizar cesáreas en partos que se prevén difíciles. Presentamos una serie de tres casos de lesiones craneales secundarias a parto asistido con forceps, dos casos de fracturas deprimidas y una fractura deprimida con hematoma epidural subyacente. El diagnóstico se realiza con la clínica y técnicas de imagen como TAC o IRM. El tratamiento es quirúrgico en la mayoría de casos, con elevación de la fractura y evacuación del hematoma. La forma correcta de aplicar los forceps resulta esencial para prevenir lesiones craneales fetales, especialmente en partos difíciles. Deliveries with forceps or vacuum-extraction increase the incidence of perinatal craneoencephalic lesions, for which reason cesarean sections are performed more frequently. We report 3 cases of cranial lesions due to forceps deliveries, 2 with depressed skull fractures and 1 with a depressed fracture and an associated epidural hematoma. Diagnosis is made on clinical and radiological founds with CT scan or MRI. Treatment is surgical and consists of elevation of the depressed fracture and evacuation of the hematoma. The correct use of forceps is very important to avoid this kind of lesions in the newborn, especially in cases of difficult delivery.
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