### AVVISI

Gravitazione ed estensioni della Relatività Generale

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Approvo

# Gravitazione ed estensioni della Relatività Generale

Persone

Arturo Stabile

Web

http://www.arturostabile.com/research-1.html

Attività

## The formation and especially the evolution of stars are other tests of the validity of ETGS and of their compatibility with current knowledge. Observed stellar structures are incompatible with the standard models of stellar structure. In particular, we refer to the neutron stars (magnetars) with mass larger than the Volkoff mass. It therefore seems that on particular length scales the gravitational force is larger or smaller than the corresponding value in GR. For example, a modification of the Hilbert-Einstein Lagrangian consisting of an R^2 term enables a major attraction while a R_{\alpha\beta} R^{\alpha\beta}term gives a repulsive contribution. Understanding on which scales the modifications to GR are activated or what is the weight of corrections to gravitational potential is a crucial point.

Massive states of gravitational waves in extended theories of gravity

A relevant aspect of higher order gravity theories is that, in the post-Minkowskian limit (i.e., with weak fields and arbitrary velocities), the propagation of the gravitational fields turns out to be characterized by waves with both tensorial and scalar modes. This feature represents a striking difference between GR and extended gravity since, in the standard Einstein scheme, only tensorial degrees of freedom are allowed. Gravitational waves represent a fundamental tool to discriminate between GR and alternative gravities. A graviton with non-zero mass produces several effects, such as extra degrees of polarization of gravitational wave modes and a frequency-dependent speed of propagation resulting in a non-trivial dispersion relation. Then, the graviton mass could be constrained using future observations of gravitational waves with the LIGO, Virgo, and the space-based LISA experiments. For example, dynamical binary systems emitting gravitational waves could constitute a good testbed to probe massive gravitons. During their dynamical evolution, the frequency of the binary orbit increases, ramping up rapidly in the late stages of the evolution just before coalescence. Then one proposes a systematic study of gravitational waves for a more general fourth order theory (with the addition of all curvature invariants). Such theories should be analyzed for given sources (for example binary systems) by building models of gravitational emission also for further massive modes of propagation. Today the propagation is studied as a perturbation of Minkowski space (weak gravitational waves), while we can investigate also what the propagation on a curved background (“hard” gravitational waves).

Energy and conservation laws in non-linear (extended) theories of gravity