1,720,992 research outputs found
Anisotropic fluid spheres in Hořava gravity and Einstein-æther theory with a nonstatic æther
No full textIn this paper we consider spherically symmetric interior spacetimes filled by anisotropic fluids in the context of Hořava gravity and Einstein-æther theory. We assume a specific nonstatic configuration of the æther vector field and show that the field equations admit a family of exact analytical solutions which can be obtained if one of the two metric coefficients is assigned. We study as an illustrative example the case in which the metric of the interior spacetime reproduces the Newtonian potential of a fluid sphere with constant density
Gravity beyond General Relativity: New Proposals and their Phenomenology
No full textThis Thesis is devoted to the study of phenomenologically viable gravitational theories, in order to address the most pressing open issues both at very small and very large energy scales. Lovelock’s theorem singles out General Relativity as the only theory with second-order field equations for the metric tensor. So, two possible ways to circumvent it and modify the gravitational sector are taken into account. The first route consists in giving up diffeomorphism invariance, which generically leads to extra propagating degrees of freedom. In this framework Hořava gravity is discussed, presenting two restrictions, called respectively “projectability” and “detailed balance”, which are imposed in order to reduce the number of terms in the full theory. We introduce a new version of the theory assuming detailed balance but not projectability, and we show that such theory is dynamically consistent as both the spin-0 and spin-2 gravitons have a well behaved dynamics at low-energy.
Moreover three-dimensional rotating black hole solutions are found and fully studied in the context of Hořava gravity, shedding light on its causal structure. A new concept of black hole horizon, dubbed “universal horizon”, arises besides the usual event horizon one, since in Lorentz-violating gravity theories there can be modes propagating even at infinite speed. The second route which is considered, consists in adding extra fields to the gravitational action while diffeomorphism invariance is preserved. In this respect we consider the less explored option that such fields are auxiliary fields, so they do not satisfy dynamical equations but can be instead algebraically eliminated. A very general parametrization for these theories is constructed, rendering also possible to put on them very tight, theory-independent constraints. Some insight about the cosmological implications of such theories is also given. Finally in the conclusions we discuss about the future challenges that the aforementioned gravity theories have to face
Relativistic polytropic equations of state in Hořava gravity and Einstein-æther theory
No full textThe equations of state for a characteristic spacetime are studied in the context of the spherically symmetric interior exact and analytical solutions in Hořava gravity and Einstein-æther theory in which anisotropic fluids are considered. In particular, for a given anisotropic interior solution, the equations of state relating the density to the radial and tangential pressure are derived, by means of a polynomial best fit. Moreover, the well-known relativistic polytropic equations of state are used in order to obtain the profile of the thermodynamical quantities inside the stellar object as provided by the specific exact solution considered. It is then shown that these equations of state need to be modified in order to account for the profiles of density and pressures
Covariant Tolman-Oppenheimer-Volkoff equations. I. The isotropic case
No full textWe construct a covariant version of the Tolman-Oppenheimer-Volkoff equations in the case of isotropic sources. The new equations make evident the mathematical problems in the determination of interior solutions of relativistic stellar objects. Using a reconstruction algorithm, we find two physically interesting generalizations of previously known stellar interior solutions. The variables that we use also allow an easier formulation of known generating theorems for solutions associated to relativistic stellar objects
On the anisotropic interior solutions in Hořava gravity and Einstein-æther theory
No full textWe find a reconstruction algorithm able to generate all the static spherically symmetric interior solutions in the framework of Hořava gravity and Einstein-æther theory in the presence of anisotropic fluids. We focus for simplicity on the case of a static æther finding a large class of possible viable interior star solutions which present a very rich phenomenology. We study one illustrative example in more detail
Covariant Tolman-Oppenheimer-Volkoff equations. II. The anisotropic case
No full textWe generalize the covariant Tolman-Oppenheimer-Volkoff equations proposed in Carloni and Vernieri [Phys. Rev. D 97, 124056 (2018).PRVDAQ0556-282110.1103/PhysRevD.97.124056]. to the case of static and spherically symmetric spacetimes with anisotropic sources. The extended equations allow a detailed analysis of the role of the anisotropic terms in the interior solution of relativistic stars and lead to the generalization of some well-known solutions of this type. We show that, like in the isotropic case, one can define generating theorems for the anisotropic Tolman-Oppenheimer-Volkoff equations. We also find that it is possible to define a reconstruction algorithm able to generate a double infinity of interior solutions. Among these, we derive a class of solutions that can represent "quasi-isotropic" stars
On the anisotropic interior solutions in Hořava gravity and Einstein-æther theory
No full textWe find a reconstruction algorithm able to generate all the static spherically symmetric interior solutions in the framework of Hořava gravity and Einstein-æther theory in the presence of anisotropic fluids. We focus for simplicity on the case of a static æther finding a large class of possible viable interior star solutions which present a very rich phenomenology. We study one illustrative example in more detail
Hor̃ava-Lifshitz gravity with detailed balance
No full textHor̃ava-Lifshitz gravity with "detailed balance" but without the projectability assumption is discussed. It is shown that detailed balance is quite efficient in limiting the proliferation of couplings in Hoava-Lifshitz gravity, and that its implementation without the projectability assumption leads to a theory with sensible dynamics. However, the (bare) cosmological constant is restricted to be large and negative
Erratum: Hořava-Lifshitz gravity: Detailed balance revisited (Physical Review D - Particles, Fields, Gravitation and Cosmology (2012) 85 (064003)
No full textHořava-Lifshitz gravity: Detailed balance revisited
No full textWe attempt a critical reconsideration of "detailed balance" as a principle that can be used to restrict the proliferation of couplings in Hořava-Lifshitz gravity. We reexamine the shortcomings that have been usually associated with it in the literature and we argue that easy remedies can be found for all of them within the framework of detailed balance, and that the most persistent of them are actually related to projectability. We show that, once projectability is abandoned, detailed balance reduces the number of independent couplings by roughly an order of magnitude and imposes only one restriction that constitutes a phenomenological concern: the size of the (bare) cosmological constant is unacceptably large. Remarkably, this restriction (which is present in the projectable version as well) has been so far underappreciated in the literature. Optimists might prefer to interpret it as a potential blessing in disguise, as it allows one to entertain the idea of a miraculous cancellation between the bare cosmological constant and the (still poorly understood) vacuum energy contribution
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