17495887
A Bethe Ansatz study of free energy and excitation spectrum for even spin Fateev–Zamolodchikov model
A Bethe–Ansatz study of a self-dual ZN spin model is undertaken for even spin system. One must solve a coupled system of Bethe–Ansatz equations (BAE) involving zeroes of two families of transfer matrices. A numerical study on finite size lattices is done for identification of elementary excitations over the ferromagnetic and antiferromagnetic ground states. The free energies for both ferromagnetic and antiferromagnetic ground states and dispersion relation for elementary excitations are found.
6715665
ISOSPIN LATTICE GAS MODEL AND NUCLEAR-MATTER PHASE DIAGRAM AND PRESSURE-VOLUME ISOTHERMS
We study a cubic lattice gas model for nuclear matter where each lattice site can be either occupied, by one proton or one neutron, or unoccupied. A nearest-neighbor interaction of the form - ∑ Jijτziτzj is assumed. Our model is an isospin-1 Ising model, with τz= (1, 0, –1) representing respectively (proton, vacancy, neutron). A kinetic-energy term has been included in our model. Under the Bragg-Williams mean-field approximation our model exhibits the existence of a dense phase (liquid-like) and a rare phase (gas-like). The nuclear-matter p−v isotherms given by our model are discussed.
119183907
A Dark Energy Model in Kaluza-Klein Cosmology
We study a dynamic Λ model with varying gravitational constant G under the Kaluza-Klein cosmology. Physical features and the limitations of the present model have been explored and discussed. Solutions are found mostly in accordance with the observed features of the accelerating universe. Interestingly, signature flipping of the deceleration parameter is noticed and the present age of the Universe is also attainable under certain stringent conditions. We find that the time variation of gravitational constant is not permitted without vintage Λ.
118736938
Magnetized dark energy and the late time acceleration
In the present work we have searched the existence of the late time acceleration of the Universe. The matter source that is responsible for the late time acceleration of the Universe consists of cosmic fluid with the equation of state parameter $\ensuremath \omega = p/\rho$ and uniform magnetic field of energy density $\ensuremath \rho_{B}$ . The study is done here under the framework of spatially homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi-I cosmological model in the presence of magnetized dark energy. To get the deterministic model of the Universe, we assume that the shear scalar ($ \sigma$) in the model is proportional to expansion scalar ($ \theta$) . This condition leads to $\ensuremath A=B^{n}$ , where A and B are metric functions and n is a positive constant giving the proportionality condition between shear and expansion scalar. It has been found that the isotropic distribution of magnetized dark energy leads to the present accelerated expansion of the Universe and the derived model is in good agreement with the recent astrophysical observations. The physical behavior of the Universe has been discussed in details.
121572152
Does accelerating Universe permit varying speed of light?
We investigate the possible variation of c in the context of the present accelerating Universe as discovered through SN Ia observations and show that variability of c is not permitted under the variable Λ models.
14627322
The dark energy equation of state
ABSTRACT We perform a study of cosmic evolution with an equation of state parameter ω(t) = ω 0 +ω 1 (tH/H˙ ) by selecting a phenomenological Λ model of the form, Λ˙ ∼ H 3 . This simpleproposition explains both linearly expanding and inﬂation ary Universes with a single set ofequations. We notice that the inﬂation leads to a scaling in t he equation of state parameter,ω(t), and hence in equation of state. In this approach, one of its two parameters have been pinpointed and the other have been delineated. It has been possible to show a connection betweendark energy and Higgs-Boson.Key words: gravitation - cosmological parameters - cosmology: theory- early Universe. 1 INTRODUCTIONCosmological research is mainly concerned with time (and in somecases space as well) evolution of various physical parameters likescale factor, Hubble parameter, matter-energy density etc. Alongwith these parameters, in recent years a new physical entity Λ hasresurrected in the foreground of cosmology. In fact, Λ has becomean essential part of the ﬁeld equations of Einstein after som e ob-servational results (Riess et al. 1998; Perlmutter et al. 1999) indi-cated towards an accelerating Universe. It is believed by most ofthe physicists that the cosmological parameter Λ is responsible fordriving the present acceleration because it can exert negative pres-sure. Moreover, due to some ﬁne-tuning problem (known as cos-mological constant problem), Λ is regarded as a variable quantityrather than a constant.Now, in order to specify exact time-dependence of the un-known physical quantities including Λ, one has to take recourse ofa relationship between cosmic pressure p and matter-energy den-sity ρ involving the equation of state parameter ω. Mathemati-cally speaking, one variable quantity can depend on the productof two other variable quantities. So, one may construct ω as a func-tion of time, red-shift or scale factor (Chervon & Zhuravlev2000;Zhuravlev 2001; Peebles & Ratra 2003). In fact, values of ω at dif-ferent stages of cosmic evolution suggest that it may evolve withtime. As an instance, for the present pressure-less Universe, thevalue of ω is considered as zero, whereas its value was 1/3 in theearly radiation dominated Universe. However, it is convenient toconsider ω as a constant quantity because observational data canhardly distinguish between a varying and a constant equation of
118535855
Singularity-free dark energy star
We propose a model for an anisotropic dark energy star where we assume that the radial pressure exerted on the system due to the presence of dark energy is proportional to the isotropic perfect fluid matter density. We discuss various physical features of our model and show that the model satisfies all the regularity conditions and is stable as well as singularity-free.