Modified teleparallel gravity with higher-derivative torsion terms
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We construct F(T,(T)2,□T) gravitational modifications, which are novel classes of modified theories arising from higher-derivative torsional terms in the action and are different than their curvature analogue. Applying them in a cosmological framework, we obtain an effective dark energy sector comprised of the novel torsional contributions. We perform a detailed dynamical analysis for two specific examples, extracting the stable late-time solutions and calculating the corresponding observables. We show that the thermal history of the Universe can be reproduced, and it can result in a dark-energy-dominated, accelerating universe, where the dark-energy equation-of-state parameter lies in the quintessence regime, or may exhibit the phantom-divide crossing during the cosmological evolution. Finally, the scale factor behaves asymptotically, either as a power law or as an exponential, in agreement with observations.