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تسجيل مستخدم جديدCosmological evolution, future singularities, Little Rip and Pseudo-Rip in viable f(R) theories and their scalar-tensor counterpart
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تأليف
Diego Saez-Gomez
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Modified f(R) gravity is one of the most promising candidates for dark energy, and even for the unification of the whole cosmological evolution, including the inflationary phase. Within this class of theories, the so-called viable modified gravities represent realistic theories that are capable of reproducing late-time acceleration, and satisfy strong constraints at local scales, where General Relativity is recovered. The present manuscript deals with the analysis of the cosmological evolution for some of these models, which indicates that the evolution may enter into a phantom phase, but the behavior may be asymptotically stable. Furthermore, the scalar-tensor equivalence of f(R) gravity is considered, which provides useful information about the possibility of the occurrence of a future singularity. The so-called Little Rip and Pseudo-Rip are also studied in the framework of this class of modified gravities.
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One of the so-called viable modified gravities is analyzed. This kind of gravity theories are characterized by a well behavior at local scales, where General Relativity is recovered, while the modified terms become important at the cosmological level
, where the late-time accelerating era is reproduced, and even the inflationary phase. In the present work, the future cosmological evolution for one of these models is studied. A transition to the phantom phase is observed. Furthermore, the scalar-tensor equivalence of f(R) gravity is also considered, which provides important information concerning this kind of models.
We investigate the complete universe evolution in the framework of $f(T)$ cosmology. We first study the requirements at the kinematic level and we introduce a simple scale factor with the necessary features. Performing a detailed analysis of the phas
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A complete analysis of the dynamics of the Hu-Sawicki modification to General Relativity is presented. In particular, the full phase-space is given for the case in which the model parameters are taken to be n=1, c1=1, and several stable de Sitter equ
ilibrium points together with an unstable matter-like point are identified. We find that if the cosmological parameters are chosen to take on their Lambda CDM values today, this results in a universe which, until very low redshifts, is dominated by an equation of state parameter equal t1/3, leading to an expansion history very different from Lambda CDM. We demonstrate that this problem can be resolved by choosing Lambda CDM initial conditions at high redshifts and integrating the equations to the present day.
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g $Q$ with matter works to change the star radius in comparison to General Relativity, while the maximum allowed mass of neutron stars is hardly modified for both SLy and FPS equations of state. In Brans-Dicke theory with the massive potential $V(phi)=m^2 phi^2/2$, where $m^2$ is a positive constant, we show the difficulty of realizing neutron star solutions with a stable field profile due to the existence of an exponentially growing mode outside the star. As in $f(R)$ gravity with the $R^2$ term, this property is related to the requirement of extra boundary conditions of the field at the surface of star. For the self-coupling potential $V(phi)=lambda phi^4/4$, this problem can be circumvented by the fact that the second derivative $V_{,phi phi}=3lambdaphi^2$ approaches 0 at spatial infinity. In this case, we numerically show the existence of neutron star solutions for both SLy and FPS equations of state and discuss how the mass-radius relation is modified as compared to General Relativity.
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