Entropic accelerating universe

To accommodate the observed accelerated expansion of the universe, one popular idea is to invoke a driving term in the Friedmann–Lemaître equation of dark energy which must then comprise 70% of the present cosmological energy density. We propose an alternative interpretation which takes into account the entropy and temperature intrinsic to the horizon of the universe due to the information holographically stored there. Dark energy is thereby obviated and the acceleration is due to an entropic force naturally arising from the information storage on the horizon surface screen. We consider an additional quantitative approach inspired by surface terms in general relativity and show that this leads to the entropic accelerating universe.     

Modified holographic dark energy in DGP brane world

In this Letter, the cosmological dynamics of a modified holographic dark energy which is derived from the UV/IR duality by considering the black hole mass in higher dimensions as UV cutoff, is investigated in Dvali–Gabadadze–Porrati (DGP) brane world model. We choose Hubble horizon and future event horizon as IR cutoff respectively. And the two branches of the DGP model are both taken into account. When Hubble horizon is considered as IR cutoff, the modified holographic dark energy (HDE) behaves like an effect dark energy that modification of gravity in pure DGP brane world model acts and it can drive the expansion of the universe speed up at late time in ?=−1$?=-1$ branch which in pure DGP model cannot undergo an accelerating phase. When future event horizon acts as IR cutoff, the equation of state parameter of the modified HDE can cross the phantom divide.     

Statefinder and Om Diagnostics for Interacting New Holographic Dark Energy Model and Generalized Second Law of Thermodynamics

In this work, we have considered that the flat FRW universe is filled with the mixture of dark matter and the new holographic dark energy. If there is an interaction, we have investigated the natures of deceleration parameter, statefinder and Om diagnostics. We have examined the validity of the first and generalized second laws of thermodynamics under these interactions on the event as well as apparent horizon. It has been observed that the first law is violated on the event horizon. However, the generalized second law is valid throughout the evolution of the universe enveloped by the apparent horizon. When the event horizon is considered as the enveloping horizon, the generalized second law is found to break down excepting at late stage of the universe.     

Some studies on dark energy related problems

In this work we perform some studies related to dark energy. Firstly, we propose a dynamical approach to explain the dark energy contents of the universe. We assume that a massless scalar field couples to the Hubble parameter with some Planck-mass suppressed interactions. This scalar field develops a Hubble parameter-dependent (thus time-dependent) vacuum expectation value, which renders a time-independent relative density for the dark energy and thus can explain the coincidence of the dark energy density of the universe. Furthermore, we assume that the dark matter particle is metastable and decays very late into the dark energy scalar field. Such a conversion of matter to dark energy can give an explanation for the starting time of the accelerating expansion of the universe. Secondly, we introduce multiple Affleck-Dine fields to the landscape scenario of dark energy in order to have the required baryon-asymmetrical universe. PACS: 95.36. + x, 95.35. + d     

Evolution of the Horizons for Dark Energy Universe

Recent observational evidences of accelerating phase of the universe strongly demand that the dominating matter in the universe is in the form of dark energy. In this work, we study the evolution of the apparent and event horizons for various dark energy models and examine their behavior across phantom barrier line.     

Leptoquark effects on ν τ propagation in the Earth

The proposal of pilgrim dark energy is based on the idea that phantom dark energy possesses enough resistive force to preclude black hole formation. We work on this proposal by choosing an interacting framework with cold dark matter and three cutoffs such as Hubble as well as event horizon and conformal age of the universe. We present a graphical analysis and focus our study on the pilgrim dark energy as well as interacting parameters. It is found that these parameters play an effective role on the equation of state parameter for exploring the phantom region of the universe. We also make the analysis of ω–ω′ and point out freezing region in the ω–ω′ plane. Finally, it turns out that the ΛCDM is achieved in the statefinders plane for all models.     

Holographic dark energy in Brans–Dicke cosmology with chameleon scalar field

We study a cosmological implication of holographic dark energy in the Brans–Dicke gravity. We employ the holographic model of dark energy to obtain the equation of state for the holographic energy density in non-flat (closed) universe enclosed by the event horizon measured from the sphere of horizon named L. Our analysis shows that one can obtain the phantom crossing scenario if the model parameter α (of order unity) is tuned accordingly. Moreover, this behavior is achieved by treating the Brans–Dicke scalar field as a Chameleon scalar field and taking a non-minimal coupling of the scalar field with matter. Hence one can generate phantom-like equation of state from a holographic dark energy model in non-flat universe in the Brans–Dicke cosmology framework.     

Generalized Second Law of Thermodynamics for?Interacting Dark Energy in the DGP Braneworld

In this paper, we investigate the validity of the generalized second law of thermodynamics (GSLT) in the DGP braneworld when the universe is filled with interacting two fluid system: one in the form of cold dark matter and other is holographic dark energy. The boundary of the universe is assumed to be enclosed by the dynamical apparent horizon or the event horizon. The universe is chosen to be homogeneous and isotropic FRW model and the validity of the first law has been assumed here.     

The Cosmological constant problem

Observational evidence seems to indicate that the expansion of the universe is currently accelerating. Such an acceleration strongly suggests that the content of the universe is dominated by a non-clustered form of matter, the so-called dark energy. The cosmological constant, introduced by Einstein to reconcile General Relativity with a closed and static Universe, is the most likely candidate for dark energy although other options such as a weakly interacting field, also known as quintessence, have been proposed. The fact that the dark energy density is some one hundred and twenty orders of magnitude lower than the energy scales present in the early universe constitutes the cosmological constant problem. We review various aspects of the cosmological constant problem and some interesting scenarios using supersymmetry or extra-dimensions attempting to solve one of the most puzzling issues in physics.     

Interacting entropy-corrected holographic dark energy with apparent horizon as an infrared cutoff

In this work we consider the entropy-corrected version of interacting holographic dark energy (HDE), in the non-flat universe enclosed by apparent horizon. Two corrections of entropy so-called logarithmic ‘LEC’ and power-law ‘PLEC’ in HDE model with apparent horizon as an IR-cutoff are studied. The ratio of dark matter to dark energy densities u, equation of state parameter w _D and deceleration parameter q are obtained. We show that the cosmic coincidence problem is solved for interacting models. By studying the effect of interaction in EoS parameter of both models, we see that the phantom divide may be crossed and also understand that the interacting models can drive an acceleration expansion at the present and future, while in non-interacting case, this expansion can happen only at the early time. The graphs of deceleration parameter for interacting models, show that the present acceleration expansion is preceded by a sufficiently long period deceleration at past. Moreover, the thermodynamical interpretation of interaction between LECHDE and dark matter is described. We obtain a relation between the interaction term of dark components and thermal fluctuation in a non-flat universe, bounded by the apparent horizon. In limiting case, for ordinary HDE, the relation of interaction term versus thermal fluctuation is also calculated.     

Interacting three fluid system and thermodynamics of the universe bounded by the event horizon

The work deals with the thermodynamics of the universe bounded by the event horizon. The matter in the universe has three constituents namely dark energy, dark matter and radiation in nature and interaction between then is assumed. The variation of entropy of the surface of the horizon is obtained from unified first law while matter entropy variation is calculated from the Gibbss’ law. Finally, validity of the generalized second law of thermodynamics is examined and conclusions are written point wise.     

The Generalized Second Law for the Interacting New Agegraphic Dark Energy in a Non-flat FRW Universe Enclosed by the Dynamical Apparent Horizon

We investigate the validity of the generalized second law of gravitational thermodynamics on the dynamical apparent horizon in a non-flat FRW universe containing the interacting new agegraphic dark energy with dark matter. We show that for this model, the equation of state parameter can cross the phantom divide line. We also present that for the selected model under thermal equilibrium with the Hawking radiation, the generalized second law is always satisfied throughout the history of the universe. Whereas, the evolution of the entropy of the universe and dynamical apparent horizon, separately, depends on the equation of state parameter of the interacting new agegraphic dark energy model.     

Thermodynamics of Lema$$\hat{i}$$ tre−Tolman−Bondi Model

Here we consider our universe as inhomogeneous spherically symmetric Lema [^(i)]{\hat{i}} tre−Tolman−Bondi Model and analyze the thermodynamics of this model of the universe. The trapping horizon is calculated and is found to coincide with the apparent horizon. The Einstein field equations are shown to be equivalent with the unified first law of thermodynamics. Finally assuming the first law of thermodynamics validity of the generalized second law of thermodynamics is examined at the apparent horizon for the perfect fluid and at the event horizon for holographic dark energy.     

Effects of dark energy interacting with massive neutrinos and dark matter on universe evolution

In this paper we investigate the evolution of the cosmology model with dark energy interacting with massive neutrinos and dark matter. Using the numerical method to investigate the dynamical system, we find that the stronger the interaction between dark energy and dark matter, the lower the ratio of dark matter in the universe is; also, the stronger the interaction between dark energy and massive neutrinos, the lower the ratio of massive neutrinos in the universe is. On the other hand, the interaction between dark energy and dark matter or massive neutrinos has an effect on disturbing the universe's acceleration; we also find that our universe is still accelerating.     

On the Ricci dark energy model

We study the Ricci dark energy model (RDE) which was introduced as an alternative to the holographic dark energy model. We point out that an accelerating phase of the RDE is that of a constant dark energy model. This implies that the RDE is not a new model of explaining the present accelerating universe.     

Validity of generalised second law in holographic DGP brane

In this paper we study the validity of the generalised second law of thermodynamics (GSLT) of the universe bounded by the event horizon on the Dvali–Gabadadze–Porrati (DGP) brane world. The radius of the event horizon is calculated by establishing a correspondence between holographic dark energy and the effective energy density in the DGP brane world. It is shown that in the absence of cold dark matter (CDM), GSLT is always respected. In the presence of CDM, we take three different DGP models and find conditions under which GSLT holds.     

A new gravitational model for dark energy

A new gravitational model for dark energy is presented based on the model of de Sitter gauge theory of gravity. In the model, in addition to the cosmological constant, the homogeneous and isotropic torsion and its coupling with curvature play an important role for dark energy. The model may supply the universe with a natural transit from decelerating expansion to accelerating expansion.     

A new gravitational model for dark energy

A new gravitational model for dark energy is presented based on the model of de Sitter gauge theory of gravity.In the model,in addition to the cosmological constant,the homogeneous and isotropic torsion and its coupling with curvature play an important role for dark energy.The model may supply the universe with a natural transit from decelerating expansion to accelerating expansion.