Related papers: On hyperbolicity violations in cosmological models…
Modified theories of gravity usually present new degrees of freedom, as well as higher order derivatives, wrong signs in certain terms and complicated couplings already present in the Lagrangian from the beginning or originated by the field…
We discuss the appearance of chaos in time-periodic perturbations of reversible vector fields in the plane. We use the normal forms of codimension~$1$ reversible vector fields and discuss the ways a time-dependent periodic forcing term of…
The energy density of a scalar field $\phi$ with potential $V(\phi) \propto \phi^{-\alpha}$, $\alpha > 0$, behaves like a time-variable cosmological constant that could contribute significantly to the present energy density. Predictions of…
The tension between the Hubble constant obtained from the local measurements and from cosmic microwave background (CMB) measurements motivated us to consider the cosmological model beyond $\Lambda$CDM one. We investigate the cosmology in…
We present a detailed study of the cosmological evolution in general vector-tensor theories of gravity without potential terms. We consider the evolution of the vector field throughout the expansion history of the universe and carry out a…
We test the hypothesis that in models with an explicit breaking of Lorentz invariance (LIV models) the problem with the cosmological constant may, if not already be solved, then at least may partially be mitigated by improving its UV…
We study whether a violation of the null energy condition necessarily implies the presence of instabilities. We prove that this is the case in a large class of situations, including isotropic solids and fluids relevant for cosmology. On the…
The evolution of a class of inhomogeneous spherically symmetric universe models possessing a varying cosmological term and a material fluid, with an adiabatic index either constant or not, is studied.
We derive a set of equations monitoring the evolution of covariant and gauge-invariant linear scalar perturbations of Friedman-Lema\^itre-Robertson-Walker models with multiple interacting non-linear scalar fields. We use a dynamical…
A class of vector-tensor theories arises naturally in the framework of quadratic gravity in spacetimes with linear vector distortion. Requiring the absence of ghosts for the vector field imposes an interesting condition on the allowed…
The notion of Lorentz violation in four dimensions is extended to a 5-dimensional brane-world scenario by utilizing a dynamical vector field assumed to point in the bulk direction, with Lorentz invariance holding on the brane. The…
We have investigated the accelerating behaviour of the universe in $f(Q,T)$ gravity in an isotropic and homogeneous space-time. We have initially derive the dynamical parameters in the general form of $f(Q,T)=\alpha Q^{m}+\beta T$ [Xu et…
Chameleon fields may modify gravity on cluster scales while recovering general relativity locally. This article reviews signatures of chameleon modifications in the nonlinear cosmological structure, comparing different techniques to model…
In loop quantum cosmology nonperturbative modification to a scalar matter field at short scales implies inflation which also means a violation of the strong energy condition. In the framework of effective Hamiltonian we discuss the issue of…
In this work, a cosmological model is considered having two scalar fields minimally coupled to gravity with a mixed kinetic term. The model is characterized by the coupling function and the potential function which are assumed to depend on…
We study the dynamics of a cosmological model with a perfect fluid and $\mathcal{N}$ fields on a hyperbolic field space interacting via a symmetric potential. We list all late-time solutions, investigate their stability and briefly discuss…
We search for novel Lorentz- and CPT-violating field theories, beyond those contained in the superficially renormalizable standard model extension. We find a new class of scalar field self-interactions which are nonpolynomial in form,…
Gravitational vector degrees of freedom typically arise in many examples of modified gravity models. We start to systematically explore their role in these scenarios, studying the effects of coupling gravitational vector and scalar degrees…
We construct supersymmetric K field theories (i.e., theories with a non-standard kinetic term) in 1+1 and 2+1 dimensions such that the bosonic sector just consists of a nonstandard kinetic term plus a potential. Further, we study the…
Field theories with spontaneous Lorentz violation involving an antisymmetric 2-tensor are studied. A general action including nonminimal gravitational couplings is constructed, and features of the Nambu-Goldstone and massive modes are…