String-inspired Teleparallel cosmology
The present paper represents an attempt for a very generic string inspired theory of gravitation, based on a stringy action in the teleparallel gravity which includes a specific functional which depends on the scalar field and its kinetic energy, as well as the torsion and boundary terms, embedding also possible effects from the teleparallel Gauss–Bonnet invariants. We focus our study in FLRW cosmology. After we deduce the cosmological equations for the associated generic theory of gravitation, we focus on string inspired couplings which are studied by considering different analytical techniques. The first analytical technique is based on the linear stability theory, by introducing proper dimensionless variables which enables us to study the structure of the phase space and the associated physical effects. In this case we have obtained different cosmological solutions which corresponds to matter and dark energy dominated solutions, achieving a possible transition between matter and dark energy dominated epochs. For each type of cosmological solutions we have discussed the corresponding physical features, attaining viable constraints for the coupling constants due to dynamical effects. The dynamical study of the physical features included also a numerical analysis by fine–tuning the initial conditions deep into the matter era, obtaining possible trajectories for the effective equation of state for specific coupling functions. ; This article is based upon work from CANTATA COST (European Cooperation in Science and Technology) action CA15117, EU Framework Programme Horizon 2020. SB is supported by Mobilitas Pluss N◦MOBJD423 by the Estonian government. PR acknowledges the Inter University Centre for Astronomy and Astrophysics (IUCAA), Pune, India for granting visiting associateship. MM would like to thank V. Baran for support. SDO is partially supported by Russian Ministry of Science and High Education, project No FEWF-2020-003. The authors thank Tomi S. Koivisto for spotting an important typo in the manuscript.
