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Dark energy might interact with dark matter in a direct, non-gravitational way, which can help remedy several theoretical defects. In order to find out the properties of interacting dark energy models, it is necessary to investigate the…
General Relativity (GR) exists in different formulations, which are equivalent in pure gravity. Once matter is included, however, observable predictions generically depend on the version of GR. In order to quantify the resulting ambiguity,…
Dark matter is a fundamental constituent of the universe, which is needed to explain a wide variety of astrophysical and cosmological observations. Although the existence of dark matter was first postulated nearly a century ago and its…
Although well established observations on cosmological, cluster, and galactic scales strongly suggest the existence of dark matter (DM), our understanding of its non-gravitational properties is still lacking. I review basic aspects of…
Shape Dynamics is a metric theory of pure gravity, equivalent to General Relativity, but formulated as a gauge theory of spatial diffeomporphisms and local spatial conformal transformations. In this paper we extend the construction of Shape…
We propose a new cosmological framework in which the strength of the gravitational force acted on dark matter at late time can be weaker than that on the standard matter fields without introducing extra gravitational degrees of freedom. The…
We consider generic freeze-in processes for generation of Dark Matter, together with the consequent re-thermalization of the Standard Model fluid. We find that Dark Matter inherits the Standard Model adiabatic inhomogeneities on the…
The reconstruction scheme is developed for modified $f(R)$ gravity with realistic matter (dark matter, baryons, radiation). Two versions of such theory are constructed: the first one describes the sequence of radiation and matter…
In this study, we investigate a scenario that dark matter (DM) has only gravitational interaction. In the framework of effective field theory of gravity, we find that DM is still stable at tree level even if there is no symmetry to protect…
$\tilde{\delta}$ Gravity is a gravitational field model, where the geometry is governed by two symmetric tensors, $g_{\mu \nu}$ and $\tilde{g}_{\mu \nu}$, and new matter fields ($\tilde{\delta}$ Matter fields) are added to the original…
Modified gravity can be considered as an alternative to dark energy. In a generalized theory of gravity, the universe may accelerate while containing only baryonic and dark matter. We study, in particular, the evolution of matter…
I examine how two specific examples of modified gravity explanations of cosmic acceleration help us understand some general problems confronting cosmological tests of gravity: how do we distinguish modified gravity from dark energy if they…
Galactic rotation curves are often considered the first robust evidence for the existence of dark matter. However, even in the presence of a dark matter halo, other galactic-scale observations, such as the Baryonic Tully-Fisher Relation and…
We use linear and quasi-linear perturbation theory to analyse cold dark matter models of structure formation in spatially flat models with a cosmological constant. Both a tilted spectrum of density perturbations and a significant…
We study the stability of a recently proposed model of scalar-field matter called mimetic dark matter or imperfect dark matter. It has been known that mimetic matter with higher derivative terms suffers from gradient instabilities in scalar…
We discuss ghost free models of the recently suggested mimetic dark matter theory. This theory is shown to be a conformal extension of Einstein general relativity. Dark matter originates from gauging out its local Weyl invariance as an…
By relaxing the conventional assumption of a purely gravitational interaction between dark energy and dark matter, substantial alterations to the growth of cosmological structure can occur. In this work we focus on the homogeneous transfer…
We investigate the metric perturbations of the restricted f(R) theory of gravity in the cosmological context and explore the phenomenological implications of this model. We show that it is possible to construct a restricted model of…
We construct new classes of modified theories in which the matter sector couples with the Einstein tensor, namely we consider direct couplings of the latter to the energy-momentum tensor, and to the derivatives of its trace. We extract the…
We present a bi-metric theory of gravity containing a length scale of galactic size. For distances less than this scale the theory satisfies the standard tests of General Relativity. For distances greater than this scale the theory yields…