MG15 - Talk detail

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We investigate the gravitational waves and their properties in various modified teleparallel theories, such as f(T), f(T,B) and f(T,TG) gravities. We find that the speed of gravitational waves within f(T) gravity is exactly equal to the light speed, and hence the constraints from GW170817 and GRB170817A are trivially satisfied. Nevertheless, by examining the dispersion relation and the frequency of cosmological gravitational waves, we observe a deviation from the results of General Relativity, quantified by a new parameter. For non-trivial f(T,B) gravity, by examining the geodesic deviation equations, we show that extra polarization models, namely the longitudinal and breathing modes, do appear at first-order perturbation level. Finally, for f(T,TG) gravity we show that at first-order perturbations the gravitational waves exhibit the same behavior to those of f(T) gravity. Since different modified teleparallel theories exhibit different gravitational wave properties, the advancing gravitational-wave astronomy would help to alleviate the degeneracy not only between curvature and torsional modified gravity but also between different subclasses of modified teleparallel gravities.

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We present a unified description of dark energy and dark matter sectors, in the framework of shift-symmetric generalized Galileon theories. Considering a particular combination of terms in the Horndeski Lagrangian, in which we have not introduced a cosmological constant or a matter sector, we obtain an effective unified cosmic fluid whose equation of state is zero during the whole matter era. Then at smaller redshifts it starts decreasing, passing the bound which marks the onset of acceleration at around z ∼ 0.5. At present times it acquires the value −0.7. Finally, it tends toward a de-Sitter phase in the far future. This behaviour is in excellent agreement with observations. Additionally, confrontation with Supernovae type Ia data leads to a very efficient fit. Examining the model at the perturbative level, we show that it is free from pathologies such as ghosts and Laplacian instabilities, at both scalar and tensor sectors, at all times.

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Session

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Time

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Abstract is for an invited talk for the parallel session CM4: "Tensions on LCDM cosmological model and model-independent constraints" (co-chaired by Joan Solà and Luca Amendola) Abstract: There is a long standing tension between the geometry and structure formation data. We review the problem and we provide its alleviation through two theoretical directions. The first direction is the interacting dark energy scenarios, in which the dark energy and dark matter sectors are allowed to interact. The second direction concerns scenarios in which the universe is governed by torsional modified gravity.

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