Dynamical systems in Einstein Gauss-Bonnet gravity

In this work we explore the dynamical system phase space of Einstein-Gauss-Bonnet theory in the cosmological minisuperspace. This approach binds the main features of the theory through a system of autonomous differential equations, in the context of a flat Friedmann-Lemaître-Robertson-Walker spacetime. We analyze the critical points that feature in this system to assess their stability criteria. The phase space of this form of scalar-tensor gravity is very rich due to the fourth-order contributions of the Gauss-Bonnet invariant together with the second order contribution of the scalar field together with their coupling dynamics. We find additional critical points as compared with previous works in the literature which may be important for understanding the larger evolution of standard background cosmology within this class of gravitational models.peer-reviewe

Stable bouncing solutions in Teleparallel Horndeski gravity: violations of the no-go theorem

In order to have singularity-free solutions at the beginning of the Universe, we need to violate the null energy condition. In the general class of Horndeski gravity, there are healthy NEC-violating solutions, which however are plagued with instabilities or some kind of pathologies in the history of the Universe; this is known as the no-go theorem. In this paper, we study the possibility of stable bouncing solutions in the Teleparallel analog of Horndeski gravity and we show explicitly that there exist healthy violations of the no-go theorem.Comment: 18 figures, 12 page

Cosmological Perturbations in the Teleparallel analog of Horndeski gravity

In this work we study the cosmological perturbations in Bahamonde-Dialektopoulos-Levi Said (BDLS) theory, i.e. the teleparallel analog of Horndeski gravity. In order to understand the evolution of structure in a cosmological model, it is necessary to study its cosmology not only in the background but also perturbatively. Both Horndeski and its teleparallel analog have been analyzed a lot in the literature, but in order to study them quantitatively, we need to know their cosmological perturbations. That is why, we study here the scalar-vector-tensor decomposition of the theory and we also express the so-called alpha parameters in terms of the arbitrary functions of the theory, that designate the deviation from the {\Lambda}CDM model. We have explored tensor, vector and scalar perturbation of the action up to second order, which drastically opens up new possibilities on searches in the parameter space of scalar-tensor theories in the context of observations.Comment: 20 page

Strengthening extended gravity constraints with combined systems : ƒ (R) bounds from cosmology and the galactic center

Extended gravity is widely constrained in different astrophysical and astronomical systems. Since these different systems are based on different scales it is not trivial to get a combined constraint that is based on different phenomenology. Here, for the first time (to the best of our knowledge), we combine constraints for gravity from late time Cosmology and the orbital motion of the stars around the galactic center. gravity models give different potentials that are tested directly in the galactic center. The cosmological data set includes the type Ia supernova and baryon acoustic oscillations. For the galactic star center data set we use the published orbital measurements of the S2 star. The constraints on the universal parameter from the combined system give: for the Hu–Sawicki model, while for the Starobinsky dark energy model. These results improve on the cosmological results we obtain. The results show that combined constraint from different systems yields a stronger constraint for different theories under consideration. Future measurements from the galactic center and from cosmology will give better constraints on models with gravity.peer-reviewe

Classification of teleparallel Horndeski cosmology via Noether symmetries

Teleparallel Horndeski theory offers an avenue through which to circumvent the speed constraint of gravitational waves in an efficient manner. However, this provides an even larger plethora of models due to the increase in action terms. In this work we explore these models in the context of cosmological systems. Using Noether point symmetries, we classify the dynamical systems that emerge from teleparallel Horndeski cosmologies. This approach is very effective at selecting specific models in the general class of second-order teleparallel scalar–tensor theories, as well as for deriving exact solutions within a cosmological context. By iterating through the Lagrangians selected through the Noether symmetries, we solve for a number of cosmological systems which provides new cosmological systems to be studied