In this work we have studied the possibility of obtaining cosmic acceleration in Brans-Dicke theory with varying or constant ω (Brans-Dicke parameter) and with or without self-interacting potential, the background fluid being barotropic fluid or Generalized Chaplygin Gas. Here we take the power law form of the scale factor and the scalar field. We show that accelerated expansion can also be achieved for high values of ω for closed Universe.

In this letter we study a model of interaction between the scalar field and an inhomogeneous ideal fluid. We have considered two forms of the ideal fluid and a power law expansion for the scale factor. We have solved the equations for the energy densities. Also we show that besides being a dark energy model to explain the cosmic acceleration, this model shows a decaying nature of the scalar field potential and the interaction parameter. PACS numbers:Recent observations of type Ia Supernovae indicate that Universe is expanding with acceleration [1-5] and lead to the search for a new type of matter which violates the strong energy condition, i.e., ρ + 3p < 0. In Einstein's general relativity, an energy component with large negative pressure has to be introduced in the total energy density of the Universe in order to explain this cosmic acceleration. This energy component is known as dark energy [6 -8]. There are many candidates supporting this behaviour [9], scalar field or quintessence [10] being one of the most favoured candidates as it has a decaying potential term which dominates over the kinetic term thus generating enough pressure to drive acceleration. * writam1@yahoo.co.in † ujjaldebnath@yahoo.com

In this letter we present a new form of the well known Chaplygin gas model by introducing inhomogeneity in the EOS. This model explains ω = −1 crossing. Also we have given a graphical representation of the model using {r, s} parameters. We have also considered an interaction of this model with the scalar field by introducing a phenomenological coupling function and have shown that the potential decays with time. PACS numbers:Recent observations reveals [1,2] that the present Universe is subjected to an accelerated expansion, which can be explained in terms of some new type of matter which violates the strong energy condition ρ + 3p < 0. This type of matter is known as dark energy [3][4][5][6], which has the cosmological constant to be a strong candidate. However many models have been proposed to play the role of the dark energy, Quintessence [7] or the scalar field being one of the most favoured model because of its decaying potential term dominating the kinetic term so as to generate enough pressure to drive acceleration. Also one can try Chaplygin gas model [8] with equation of state (EOS), p = −B/ρ, as it generates negative pressure, where p and ρ are respectively the pressure and energy density and B is a positive constant. Subsequently this fluid has been modified to p = −B/ρ α with 0 ≤ α ≤ 1. andas generalized Chaplygin gas [9,10] and modified Chaplygin gas [11,12] respectively. Modified Chaplygin gas can explain the evolution of the Universe from radiation era to ΛCDM model. Later inhomogeneity has been introduced in the above EOS (1) by considering B to be a function of the scale factor a(t) [13,14]. This assumption is reasonable since B(a) is related to the scalar potential if we take the Chaplygin gas as a Born-Infeld scalar field [15].Interaction models where the dark energy weakly interacts with the dark matter have also been studied to explain the evolution of the Universe. This models describe an energy flow between the components. To obtain a suitable evolution of the Universe the decay rate should be proportional to the present value of the Hubble parameter for good fit to the expansion history of the Universe as determined by the Supernovae and CMB data. A variety of interacting dark energy models have been proposed and studied for this purpose [16][17][18][19].In this letter we study a new model by considering both A and B in the EOS (1) to be a function of the scale factor a(t) and thus introducing inhomogeneity in the EOS (1). We solve the EOS to get the energy density and show that the we can explain the evolution of the Universe suitably by choosing different values of the parameters. We then consider an interaction between the fluid and the scalar field by introducing a phenomenological interaction term which describes the energy flow between them, thus showing the effect of interaction in the evolution of the Universe. This kind of interaction term has been studied in ref. [20].The metric of a spatially flat homogeneous and isotropic universe in FRW model is ds 2 = dt 2 − a 2 (t) dr 2 + r...

In this letter, we have considered a model of the universe filled with modified Chaplygin gas and another fluid (with barotropic equation of state) and its role in accelerating phase of the universe. We have assumed that the mixture of these two fluid models is valid from (i) the radiation era to ΛCDM for −1 ≤ γ ≤ 1 and (ii) the radiation era to quiessence model for γ < −1. For these two fluid models, the statefinder parameters describe different phase of the evolution of the universe. PACS numbers:Recent measurements of redshift and luminosity-distance relations of type Ia Supernovae indicate that the expansion of the Universe is accelerating [1][2][3][4][5]. This implies that the pressure p and the energy density ρ of the Universe should violate the strong energy condition ρ + 3p < 0 i.e., pressure must be negative. The matter responsible for this condition to be satisfied at some stage of evolution of the universe is referred to as dark energy [6 -8]. There are different candidates to play the role of the dark energy. The most traditional candidate is a non-vanishing cosmological constant which can also be though of as a perfect fluid satisfying the equation of state p = −ρ. Negative pressure leading to an accelerating Universe can also be obtained in a Chaplygin gas cosmology [9], in which the matter is taken to be a perfect fluid obeying an exotic equation of state p = −B/ρ, (B > 0). The Chaplygin gas behaves as pressureless fluid for small values of the scale factor and as a cosmological constant for large values of the scale factor which tends to accelerate the expansion. Subsequently the above equation was generalized to the form p = −B/ρ α , 0 ≤ α ≤ 1 [10-12] and recently it was modified to the form p = Aρ − B/ρ α , (A > 0) [13,14], which is known as Modified Chaplygin Gas. This equation of state shows a radiation era (A = 1/3) at one extreme and a ΛCDM model at the other extreme.The metric of a homogeneous and isotropic universe in FRW model iswhere a(t) is the scale factor and k (= 0, ±1) is the curvature scalar.The Einstein field equations are (choosing 8πG = c = 1)The energy conservation equation (T ν µ;ν = 0) iṡ ρ + 3ȧ a (ρ + p) = 0For modified Chaplygin gas, equation (4) yields * writam1@yahoo.co.in † ujjaldebnath@yahoo.com

Recently, a tachyonic field was presented as a dark energy model to represent the present acceleration of the Universe. In this paper, we consider a mixture of tachyonic fluid with a perfect fluid. For this purpose we consider barotropic fluid and Generalized Chaplygin gas (GCG). We present a particular form of the scale factor. We solve the equations of motion to get exact solutions of the density, tachyonic potential and the tachyonic field. We introduce a coupling term to show that the interaction decays with time. We also show that the nature of the potentials vary, so the interaction term reduces the potential in both cases.