Effects of a generalized dark energy fluid is investigated on cosmic density fluctuations such as cosmic microwave background. As a general dark energy fluid, we take into consideration the possibility of the anisotropic stress for dark energy, which has not been discussed much in the literature. We comprehensively study its effects on the evolution of density fluctuations along with that of no...

Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations of cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are nearly all based on...

The cosmological constant is the most economical candidate for dark energy. No other approach really alleviates the difficulties faced by the cosmological constant because, in all other attempts to model the dark energy, one still has to explain why the bulk cosmological constant (treated as a low-energy parameter in the action principle) is zero. I argue that the until the theory is made invar...

In a companion paper [1], we have introduced a model of scalar field dark energy, Cuscuton, which can be realized as the incompressible (or infinite speed of sound) limit of a k-essence fluid. In this paper, we study how Cuscuton modifies the constraint sector of Einstein gravity. In particular, we study Cuscuton cosmology and show that even though Cuscuton can have an arbitrary equation of sta...

Aims. We use data from observational cosmology to put constraints on higherdimensional extensions of general relativity in which the effective four-dimensional dark-energy density (or cosmological “constant”) decays with time. Methods. In particular we study the implications of this decaying dark energy for the age of the universe, large-scale structure formation, big-bang nucleosynthesis and t...

The phenomena customly described with the standard ΛCDM model are broadly reproduced by an extremely simple model in TeVeS, Bekenstein’s (2004) modification of General Relativity motivated by galaxy phenomenology. Our model can account for the acceleration of the universe seen in SNeIa distances without a cosmological constant, and the accelerations seen rotation curves of nearby spiral galaxie...

Gravitational field equations in Randers-Finsler space of approximate Berwald type are investigated. A modified Friedmann model is proposed. It is showed that the accelerated expanding universe is guaranteed by a constrained RandersFinsler structure without invoking dark energy. The geodesic in Randers-Finsler space is studied. The additional term in the geodesic equation acts as repulsive forc...

We propose a class of actions for the spacetime metric that introduce corrections to the Einstein-Hilbert Lagrangian depending on the logarithm of some curvature scalars. We show that for some choices of these invariants the models are ghost free and modify Newtonian gravity below a characteristic acceleration scale given by a0 = cμ, where c is the speed of light and μ is a parameter of the mod...

The nature of the fuel that drives today’s cosmic acceleration is an open and tantalizing mystery. We entertain the suggestion that the acceleration is not the manifestation of yet another new ingredient in the cosmic gas tank, but rather a signal of our first real lack of understanding of gravitational physics. By requiring that the underlying gravity theory respects Birkhoff’s law, we derive ...

Recent observations on Type-Ia supernovae and low density (Ωm = 0.3) measurement of matter including dark matter suggest that the presentday universe consists mainly of repulsive-gravity type ‘exotic matter’ with negative-pressure often said ‘dark energy’ (Ωx = 0.7). But the nature of dark energy is mysterious and its puzzling questions, such as why, how, where and when about the dark energy, a...