Relativistic stars in f(R) gravity, and absence thereof (original) (raw)
The realistic models of relativistic stars in f (R) = R + αR2 gravity
Classical and Quantum Gravity
In the context of f (R) = R + αR 2 gravity, we study the existence of neutron and quark stars for various α with no intermediate approximation in the system of equations. Analysis shows that for positive α the scalar curvature does not drop to zero at the star surface (as in General Relativity) but exponentially decreases with distance. Also the stellar mass bounded by star surface decreases when the value α increases. Nonetheless distant observers would observe a gravitational mass due to appearance of a so-called gravitational sphere around the star. The non-zero curvature contribution to the gravitational mass eventually is shown to compensate the stellar mass decrease for growing α's. We perform our analysis for several equations of state including purely hadronic configurations as well as hyperons and quark stars. In all cases, we assess that the relation between the parameter α and the gravitational mass weakly depends upon the chosen equation of state. Another interesting feature is the increase of the star radius in comparison with General Relativity for stars with masses close to maximal, whereas for intermediate masses 1.4 − 1.6M⊙ the radius of star depends upon α very weakly. Also the decrease in the mass bounded by star surface may cause the surface redshift to decrease in R 2-gravity when compared to Einsteinian predictions. This effect is shown to hardly depend upon the observed gravitational mass. Finally, for negative values of α our analysis shows that outside the star the scalar curvature has damped oscillations but the contribution of the gravitational sphere into the gravitational mass increases indefinitely with radial distance putting into question the very existence of such relativistic stars.
Existence of relativistic stars in f ( T ) gravity
Classical and Quantum Gravity, 2011
We examine the existence of relativistic stars in f (T ) modified gravity and explicitly construct several classes of static perfect fluid solutions. We derive the conservation equation from the complete f (T ) gravity field equations and present the differences with its teleparallel counterpart. Firstly, we choose the tetrad field in the diagonal gauge and study the resulting field equations. Some exact solutions are explicitly constructed and it is noted that these solutions have to give a constant torsion scalar. Next, we choose a non diagonal tetrad field which results in field equations similar to those of general relativity. For specific models we are able to construct exact solutions of these field equations. Among those new classes of solutions, we find negative pressure solutions, and an interesting class of polynomial solutions.
Theoretical and observational constraints of viablef(R)theories of gravity
Physical Review D, 2016
Modified gravity has attracted much attention over the last few years and remains a potential candidate for dark energy. In particular, the so-called viable f (R) gravity theories, which are able to both recover General Relativity (GR) and produce late-time cosmic acceleration, have been widely studied in recent literature. Nevertheless, extended theories of gravity suffer from several shortcomings which compromise their ability to provide realistic alternatives to the standard cosmological ΛCDM Concordance model. We address the existence of cosmological singularities and the conditions that guarantee late-time acceleration, assuming reasonable energy conditions for standard matter in the so-called Hu-Sawicki f (R) model, currently among the most widely studied modifications to General Relativity. Then using the Supernovae Ia Union 2.1 catalogue, we further constrain the free parameters of this model. The combined analysis of both theoretical and observational constraints sheds some light on the viable parameter space of these models and the form of the underlying effective theory of gravity.
Cosmological implications of a viable non-analytical f (R)-gravity model
2011
Power-law corrections (having the exponent strictly between 2 and 3) to the Einstein-Hilbert action yield an extended theory of gravity which is consistent with Solar-System tests and properly reproduces the main phases of the Universe thermal history. We find two distinct constraints for the characteristic length scale of the model: a lower bound from the Solar-System test and an upper bound by requiring the existence of the matter-dominated era. We also show how the extended framework can accommodate the existence of an early de Sitter phase. Within the allowed range of characteristic length scales, the relation between the expansion rate and the energy scale of inflation is modified, yielding a value of the rate several orders of magnitude smaller than in the standard picture. The observational implication of this fact is that a tiny value of the tensor-to-scalar ratio is expected in the extended framework. The suppression of primordial tensor modes also implies that the inflationary scale can be made arbitrarily close to the Planck one according to the current limits. Finally, an analysis of the propagation of gravitational waves on a Robertson-Walker background is addressed.
Cornell University - arXiv, 2012
We consider a class of metric f (R) modified gravity theories, analyze them in the context of a Friedmann-Robertson-Walker cosmology and confront the results with some of the known constraints imposed by observations. In particular, we focus in correctly reproducing the matter and effective cosmological constant eras, the age of the Universe, and supernovae data. Our analysis differs in many respects from previous studies. First, we avoid any transformation to a scalar-tensor theory in order to be exempted of any potential pathologies (e.g. multivalued scalar potentials) and also to evade any unnecessary discussion regarding frames (i.e. Einstein .vs. Jordan). Second, based on a robust approach, we recast the cosmology equations as an initial value problem subject to a modified Hamiltonian constraint. Third, we solve the equations numerically where the Ricci scalar itself is one of the variables, and use the constraint equation to monitor the accuracy of the solutions. We compute the "equation of state" (EOS) associated with the modifications of gravity using several inequivalent definitions that have been proposed in the past and analyze it in detail. We argue that one of these definitions has the best features. In particular, we present the EOS around the so called "phantom divide" boundary and compare it with previous findings.
Modified gravity with logarithmic curvature corrections and the structure of relativistic stars
Physical Review D, 2013
We consider the effect of a logarithmic f (R) theory, motivated by the form of the one-loop effective action arising from gluons in curved spacetime, on the structure of relativistic stars. In addition to analysing the consistency constraints on the potential of the scalar degree of freedom, we discuss the possibility of observational features arising from a fifth force in the vicinity of the neutron star surface. We find that the model exhibits a chameleon effect that completely suppresses the effect of the modification on scales exceeding a few radii, but close to the surface of the neutron star, the deviation from General Relativity can significantly affect the surface redshift that determines the shift in absorption (or emission) lines. We also use the method of perturbative constraints to solve the modified Tolman-Oppenheimer-Volkov equations for normal and self-bound neutron stars (quark stars).
A New Class of Cosmologically `Viable' f(R)f(R)f(R) Models
arXiv: General Relativity and Quantum Cosmology, 2016
Instead of assuming a form of gravity and demand cosmology fit with LambdaCDM\Lambda CDMLambdaCDM, a potentially `viable' f(R)f(R)f(R) gravity model is derived assuming an alternative model of cosmology. Taking the `designer' approach to f(R)f(R)f(R), a new class of solutions are derived starting with linear coasting cosmology in which scale factor linearly increases with time during matter domination. The derived forms of f(R)f(R)f(R) are presented as result.
Novel Geometrical Models of Relativistic Stars. I. The General Scheme
Arxiv preprint astro-ph/0409456, 2004
In a series of articles we describe a novel class of geometrical models of relativistic stars. Our approach to the static spherically symmetric solutions of Einstein equations is based on a careful physical analysis of radial gauge conditions. It brings us to a two parameter family of relativistic stars without stiff functional dependence between the stelar radius and stelar mass. It turns out that within this family there do exist relativistic stars with arbitrary large mass, which are to have arbitrary small radius and arbitrary small luminosity. In addition, point particle idealization, as a limiting case of bodies with finite dimension, becomes possible in GR, much like in Newton gravity.
Singularity-free non-exotic compact star in f(R, T) gravity
Pramana, 2020
In the present work, we have searched the existence of anisotropic and non-singular compact star in the f (R, T) gravity by taking into account the non-exotic equation of state (EoS). In order to obtain the solutions of the matter content of compact object, we assume the well known barotropic form of EoS that yields the linear relation between pressures and energy density. We propose the existence of non-exotic compact star which shows the validation of energy conditions and stability with in the f (R, T) extended theory of gravity perspective. The linear material correction in the extended theory the matter content of compact star remarkably able to satisfies energy condition. We discuss various physical features of compact star and show that proposed model of stellar object satisfies all regularity conditions and is stable as well as singularity-free.