Abstract. We determine the Hubble expansion and the general cosmic perturbation equations for a general system consisting of self-conserved matter, ρ m , and self-conserved dark energy (DE), ρ D . While at the background level the two components are non-interacting, they do interact at the perturbations level. We show that the coupled system of matter and DE perturbations can be transformed into a single, third order, matter perturbation equation, which reduces to the (derivative of the) standard one in the case that the DE is just a cosmological constant. As a nontrivial application we analyze a class of dynamical models whose DE density ρ D (H) consists of a constant term, C 0 , and a series of powers of the Hubble rate. These models were previously analyzed from the point of view of dynamical vacuum models, but here we treat them as self-conserved DE models with a dynamical equation of state. We fit them to the wealth of expansion history and linear structure formation data and compare their fit quality with that of the concordance ΛCDM model. Those with C 0 = 0 include the so-called "entropic-force" and "QCD-ghost" DE models, as well as the pure linear model ρ D ∼ H, all of which appear strongly disfavored. The models with C 0 = 0, in contrast, emerge as promising dynamical DE candidates whose phenomenological performance is highly competitive with the rigid Λ-term inherent to the ΛCDM.ArXiv ePrint: 1509.03298 arXiv:1509.03298v3 [gr-qc]

Despite the enormous significance of the Higgs potential in the context of the Standard Model of electroweak interactions and in Grand Unified Theories, its ultimate origin is fundamentally unknown and must be introduced by hand in accordance with the underlying gauge symmetry and the requirement of renormalizability. Here we propose a more physical motivation for the structure of the Higgs potential, which we derive from a generalized Brans-Dicke (BD) theory containing two interacting scalar fields. One of these fields is coupled to curvature as in the BD formulation, whereas the other is coupled to gravity both derivatively and non-derivatively through the curvature scalar and the Ricci tensor. By requiring that the cosmological solutions of the model are consistent with observations, we show that the effective scalar field potential adopts the Higgs potential form with a mildly time-evolving vacuum expectation value. This residual vacuum dynamics could be responsible for the possible time variation of the fundamental constants, and is reminiscent of former Bjorken's ideas on the cosmological constant problem.Here, in our search for alternative frameworks that could explain the origin of the Higgs sector, we consider the theoretical possibility that the very structure of the Higgs potential is dictated by a real feedback between the particle physics world and the gravitational interactions. Through this "communicative ansatz" we try to find a possible, more physical, explanation for the origin of the Higgs potential that is minimally satisfactory in the two large domains (Particle Physics and General Relativity/Cosmology) which have traditionally remained isolated from one another. Specifically, in this work we explore an extend Jordan-Fierz-Brans-Dicke ("BD" for short) type of gravitational theory [25,26], in which apart from including the usual non-minimal scalar-tensor interaction [27] for the BD-field, ψ, we introduce an interaction term between ψ and a second scalar, φ, which will play the role of Higgs boson after we determine self-consistently the form of its effective potential.Remarkably, this is possible if φ interacts non-minimally with curvature and if its kinetic term interacts with the Ricci tensor, thereby through a derivative interaction with gravity [28,29]. We do not address here higher order gravitational theories, as in our case we limit ourselves to generalized forms of Einstein's gravity involving only the first power of the Ricci tensor and scalar, but we extend our study to the case of Grand Unified Theories (GUT's), where more than one type of Higgs field is involved.The kind of cosmological solutions that we search for in order to fix self-consistently the Higgs potential are the simplest possible ones, namely the power-law solutions of the cosmological equations in a Friedmann-Lemaître-Robertson-Walker (FLRW) background.These scaling solutions are interpreted as representing asymptotic states of the different phases of the cosmic evolution [30][31][32][33][34][35][36][37][38][39]. The...

We investigate the interacting holographic dark energy (HDE) with Granda-Oliveros (GO) IRcutoff in the framework of Brans-Dicke (BD) cosmology. We obtain the equation of state (EoS) parameter of HDE, w D , the effective EoS parameter w eff , the deceleration parameter q and the squared of sound speed v 2 s in a flat FRW universe. We show that at late time the cosmic coincidence problem can be alleviated. Also we show that for non-interacting case, HDE can give a unified dark matter-dark energy profile in BD cosmology, except that it cannot solve the coincidence problem in the future. By studying the equation of state parameter, we see that the phantom divide may be crossed. Using the latest observational data, we calculate the best values of the parameters for interacting HDE in BD framework. Computing the deceleration parameter implies that the transition from deceleration to the acceleration phase occurred for redshift z ≥ 0.5. Finally, we investigate the sound stability of the model, and find that HDE with GO cutoff in the framework of BD cosmology can lead to a stable DE-dominated universe favored by observations, provided we take β = 0.44 and b 2 < 0.35. This is in contrast to HDE model in Einstein gravity which does not lead to a stable DE dominated universe.