1,721,628 research outputs found
Invariance principles and extended gravity: Theory and probes
No full textThis new book presents information on the study of gravitational theories which can be seen as modifications or extensions of General Relativity. The basic principles that a gravitational theory should follow, and their geometrical interpretation, are analysed in a broad perspective which highlights the basic assumptions of General Relativity and suggests possible modifications. In particular, the invariance principles that are related to the gravitational field are presented, showing how gravity can emerge as a gauge theory. Also, the boundle approach to gravity and derive geometrical quantities which intervene in the construction of gravitational field are discussed
Extended gravity: State of the art and perspectives
No full textSeveral issues coming from Cosmology, Astrophysics and Quantum Field Theory suggest to extend the General Relativity in order to overcome several shortcomings emerging at conceptual and experimental level. From one hand, standard Einstein theory fails as soon as one wants to achieve a full quantum description of space-time. In fact, the lack of a final self-consistent Quantum Gravity Theory can be considered one of the starting points for alternative theories of gravity. Specifically, the approach based on corrections and enlargements of the Einstein scheme, have become a sort of paradigm in the study of gravitational interaction. On the other hand, such theories have acquired great interest in cosmology since they “naturally” exhibit inflationary behaviours which can overcome the shortcomings of standard cosmology. From an astrophysical point of view, Extended Theories of Gravity do not require to find candidates for dark energy and dark matter at fundamental level; the approach starts from taking into account only the “observed” ingredients (i.e., gravity, radiation and baryonic matter); it is in full agreement with the early spirit of General Relativity but one has to relax the strong hypothesis that gravity acts at same way at all scales. Several scalar-tensor and f(R)-models agree with observed cosmology, extragalactic and galactic observations and Solar System tests, and give rise to new effects capable of explaining the observed acceleration of cosmic fluid and the missing matter effect of self-gravitating structures. Despite these preliminary results, no final model addressing all the open issues is available at the moment, however the paradigm seems promising in order to achieve a complete and self-consistent theory working coherently at all interaction scales
Interpreting the Dark Side of the Universe as Curvature Effects
No full textAstrophysical observations are pointing out huge amounts of "dark matter" and "dark energy" needed to explain the observed large scale structures and cosmic accelerating expansion. Up to now, no final experimental evidence has been found, at fundamental level, to explain such mysterious components. The problem could be completely reversed considering dark matter and dark energy as "shortcomings" of General Relativity at infrared limit. Alternative theories of gravity, in particular f(. R) gravity, could explain, in principle, the accelerated expansion of the Universe and the clustering of structures without adding unknown matter ingredients but extending General Relativity by using more general functions of the curvature invariants. However, no final extended theory is capable, up to now, of fitting all the observations at any scales. In any case, this alternative "geometric" paradigm deserves a lot of attention since, in the near future, could give rise to a new self-consistent picture of the dark side of the Universe
Metric and connections in theories of gravity. the role of equivalence principle
No full textFundamental issues underlying gravitational physics and some of the shortcomings of Einstein's general relativity (GR) are discussed. In particular, after taking into account the role of the two main objects of relativistic theories of gravity, i.e. the metric and the connection fields, we consider the possibility that they are not trivially related so that the geodesic structure and the causal structure of the spacetime could be disentangled, as supposed in the Palatini formulation of gravity. In this perspective, the equivalence principle (EP), in its weak and strong formulations, can play a fundamental role in discriminating among competing theories. The possibility of its violation at quantum level could open new perspectives in gravitational physics and in unification with other interactions. We shortly debate the possibility of EP measurements by ground-based and space experiments
Rotating Black Hole Solutions in f(R)-Gravity
No full textWe present a strategy to get axially symmetric solutions in f(R) gravity by starting from spherically symmetric space-times. To do so, we assume the validity of a complex coordinate transformation, which acts on the spherically symmetric metric and permits one to infer the corresponding f(R) modification. The consequences of this recipe are here described, giving particular emphasis to define a class of compatible axially symmetric solutions, which fairly well describes the motion in cylindrical geometries in the field of f(R), in two different classes of coordinates. We demonstrate that our approach is general and may be applied for several cases of interest. We also show that our treatment is compatible with the standard approach of general relativity, evaluating the motion of a freely falling particle in the context of our metric
Cosmological double inflation in <i>F</i>(<i>R, G</i>) gravity
No full textCosmological inflation is discussed in the framework of F (R, G) gravity where F is a generic function of the curvature scalar R and the Gauss-Bonnet topological invariant G. The main feature that emerges in this analysis is the fact that this kind of theory can exhaust all the curvature budget related to curvature invariants without considering derivatives of R, Rμν, Rλσμν etc. in the action. Cosmological dynamics results driven by two effective masses (lenghts) related to the R scalaron and the G scalaron working respectively at early and very early epochs of cosmic evolution. In this sense, a double inflationary scenario naturally emerges
Noether symmetry approach for Dirac-Born-Infeld cosmology
No full textWe consider the Noether Symmetry Approach for a cosmological model derived from a tachyon scalar field T with a Dirac-Born-Infeld Lagrangian and a potential V(T). Furthermore, we assume a coupled canonical scalar field Φ with an arbitrary interaction potential B(T, Φ). Exact solutions are derived consistent with the accelerated behavior of cosmic fluid
Noether symmetry approach for teleparallel-curvature cosmology
No full textWe consider curvature-teleparallel F(R,T) gravity, where the gravitational Lagrangian density is given by an arbitrary function of the Ricci scalar R and the torsion scalar T. Using the Noether Symmetry Approach, we show that the functional form of the F(R,T) function can be determined by the presence of symmetries. Furthermore, we obtain exact solutions through the presence of conserved quantities and the reduction of cosmological dynamical system. Example of particular cosmological models are considered
Dark matter in galaxies: Leads to its nature
Full text linkRecent observations have revealed the structural properties of the dark and luminous mass distribution in spirals. These results led to the vision of a new and amazing scenario. The investigation of single and coadded objects has shown that the rotation curves of spirals follow, from their centers out to their virial radii, an universal profile that implies a tuned combination of their stellar disk and dark halo mass distributions. This, alongside with accurate mass modeling of individual galaxies, poses important challenges to the presently theoretically favored ΛCDM Cosmology
- …
