1,720,968 research outputs found
Implications of the Holst term in a f (R) theory with torsion
No full textWe analyze a modified f(R) theory of gravity in the Palatini formulation, when a Holst term endowed with a dynamical Immirzi field is included. We study the basic features of the model, especially in view of eliminating the torsion field via the Immirzi field and the scalar-tensor degrees of freedom of the f(R) model. The main task of this study is the investigation of the morphology of the gravitational wave polarization when their coupling to a circle of test particles is considered. We first observe that the dynamics of the scalar mode of the f(R) Lagrangian is frozen out, since its first order term identically vanishes. This allows a detailed characterization of the linearized theory, which outlines the emergence of a modified Newtonian potential in the static limit, and when time independence is relaxed a standard gravitational wave plus the scalar wave associated to the Immirzi field. Investigating the effect of the coupling of this scalar-tensor wave on a circle of test particles, we arrive to define two effective gravitational polarizations, corresponding to an equivalent phenomenological wave, whose morphology is anomalous with respect the standard case of general relativity. In fact, the particle circle suffers modifications as it was subjected to modified plus and cross modes, whose specific features depend on the model free parameters and are, in principle, detectable via a data analysis procedure
Generalized Ashtekar variables for Palatini f(R) models
No full textWe consider special classes of Palatini f(R) theories, featured by additional Loop Quantum Gravity inspired terms, with the aim of identifying a set of modified Ashtekar canonical variables, which still preserve the SU(2) gauge structure of the standard theory. In particular, we allow for affine connection to be endowed with torsion, which turns out to depend on the additional scalar degree affecting Palatini f(R) gravity, and in this respect we successfully construct a novel Gauss constraint. We analyze the role of the additional scalar field, outlining as it acquires a dynamical character by virtue of a non vanishing Immirzi parameter, and we describe some possible effects on the area operator stemming from such a revised theoretical framework. Finally, we compare our results with earlier studies in literature, discussing differences between metric and Palatini approaches. It is worth noting how the Hamiltonian turns out to be different in the two cases. The results can be reconciled when the analysis is performed in the Einstein frame
Big bounce cosmology for Palatini R 2 gravity with a Nieh–Yan term
Full text linkWe analyze the cosmological implementation of Palatini f(R) theories, constructed with a Nieh–Yan term and solved with respect to the torsion. We consider the relevant case of the quadratic correction to the Hilbert–Palatini action in the Ricci scalar, mimicking the Starobinsky model of the metric formulation. We point out the emergence of peculiar cosmological scenarios, depending on the sign of such correction, able to reproduce bouncing settings and to restore the standard Universe dynamics in the late asymptotic limit. Furthermore, we outline the settling of Little-Rip dynamics, which calls for a deeper investigation in order to be regularized via matter creation. Finally, we also show that in our model the Immirzi field is asymptotically frozen in time, resembling the morphology of Loop Quantum Gravity standard formulation
Gauge invariant formulation of metric f (R) gravity for gravitational waves
Full text linkWe analyze the propagation of gravitational waves in metric f(R) theories of gravity, on the special setting of flat background geometry (Minkowski spacetime). In particular, adopting a gauge invariant formalism, we clearly establish that the exact number of propagating degrees of freedom is three, consisting of the standard tensorial modes along with an additional massive scalar field. Then, investigating their effects on test masses via the geodesic deviation equation, we show that the additional dynamical degree contained in such extended formulations is actually detectable as a superposition of longitudinal and breathing stresses, which even though in principle correspond to distinct pure polarizations turn out to be never separable in the wave dynamics and cannot be interpreted as a proper independent excitations
Gravitational Landau damping for massive scalar modes
Full text linkWe establish the possibility of Landau damping for gravitational scalar waves which propagate in a non-collisional gas of particles. In particular, under the hypothesis of homogeneity and isotropy, we describe the medium at the equilibrium with a Jüttner–Maxwell distribution, and we analytically determine the damping rate from the Vlasov equation. We find that damping occurs only if the phase velocity of the wave is subluminal throughout the propagation within the medium. Finally, we investigate relativistic media in cosmological settings by adopting numerical techniques
Scalar modes in extended hybrid metric-Palatini gravity: Weak field phenomenology
No full textWe investigate the nature of additional scalar degrees of freedom contained in extended hybrid metric-Palatini gravity, outlining the emergence of two coupled dynamical scalar modes. In particular, we discuss the weak field limit of the theory, both in the static case and from a gravitational waves perspective. In the first case, performing an analysis at the lowest order of the postparametrized Newtonian structure of the model, we stress the settling of Yukawa corrections to the Newtonian potential. In this respect, we show that one scalar field can have long range interactions as used in the principle for mimicking dark matter effects. Concerning the gravitational waves propagation, instead, we demonstrate that it is possible to have well-defined physical degrees of freedom, provided by suitable constraints on model parameters. Moreover, the study of the geodesic deviation points out the presence of breathing and longitudinal polarizations due to these novel scalar waves, which on peculiar assumptions can give rise to beating phenomena during their propagation
Metric f(R) gravity with dynamical dark energy as a scenario for the Hubble tension
Full text linkWe introduce a theoretical framework to interpret the Hubble tension, based on the combination of a metric f(R) gravity with a dynamical dark energy contribution. The modified gravity provides the non-minimally coupled scalar field responsible for the proper scaling of the Hubble constant, in order to accommodate for the local SNIa pantheon+ data and Planck measurements. The dynamical dark energy source, which exhibits a phantom divide line separating the low redshift quintessence regime (−1 < w < −1/3) from the phantom contribution (w < −1) in the early Universe, guarantees the absence of tachyonic instabilities at low redshift. The resulting H0(z) profile rapidly approaches the Planck value, with a plateau behaviour for z ≿ 5. In this scenario, the Hubble tension emerges as a low redshift effect, which can be in principle tested by comparing SNIa predictions with far sources, like QUASARS and gamma ray bursts
Semiclassical and quantum analysis of the isotropic Universe in the polymer paradigm
No full textWe analyze the semiclassical and quantum dynamics of the isotropic universe in the framework of the polymer quantum mechanics in order to implement a cutoff physics on the initial singularity. We first identify in the Universe cubed scale factor (i.e., the spatial volume) the suitable configuration variable, providing a constant critical energy density, such that the bounce arises as intrinsic geometric feature. We then investigate the obtained semiclassical bounce dynamics for the primordial Universe, and we outline its impact on the resolution of cosmological paradoxes, as soon as the semi-classical evolution is extended (in the spirit of the Ehrenfest theorem) to the collapsing prebounce Universe. Finally, we validate the use of the semiclassical effective dynamics by investigating the behaviour of the expectation values of a proper semiclassical states. The present analysis has the merit to enforce the equivalence between the polymer quantization paradigm in the minisuperspace and the loop quantum cosmology approach. In fact, our study allows to define a precise correspondence between the polymer cutoff scale and the discrete geometric structure of LQG
Big bounce and future time singularity resolution in Bianchi I cosmologies: The projective invariant Nieh-Yan case
No full textWe extend the notion of the Nieh-Yan invariant to generic metric-affine geometries, where both torsion and nonmetricity are taken into account. Notably, we show that the properties of projective invariance and topologicity can be independently accommodated by a suitable choice of the parameters featuring this new Nieh-Yan term. We then consider a special class of modified theories of gravity able to promote the Immirzi parameter to a dynamical scalar field coupled to the Nieh-Yan form, and we discuss in more detail the dynamics of the effective scalar tensor theory stemming from such a revised theoretical framework. We focus, in particular, on cosmological Bianchi I models and we derive classical solutions where the initial singularity is safely removed in favor of a big bounce, which is ultimately driven by the nonminimal coupling with the Immirzi field. These solutions, moreover, turn out to be characterized by finite time singularities, but we show that such critical points do not spoil the geodesic completeness and wave regularity of these spacetimes
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