Abstract
The paper deals with a spatially homogeneous and isotropic FRW space-time filled with perfect fluid and dark energy components. The two sources are assumed to interact minimally, and therefore their energy momentum tensors are conserved separately. A special law of variation for the Hubble parameter proposed by Berman (1983) has been utilized to solve the field equations. The Berman’s law yields two explicit forms of the scale factor governing the FRW space-time and constant values of deceleration parameter. The role of dark energy with variable equation of state parameter has been studied in detail in the evolution of FRW universe. It has been found that dark energy dominates the universe at the present epoch, which is consistent with the observations. The physical behavior of the universe is discussed in detail.
SOME FRW MODELS OF ACCELERATING UNIVERSE WITH DARK ENERGY
Suresh Kumar
Department of Applied Mathematics,
Delhi Technological University (Formerly Delhi College of Engineering),
Bawana Road, Delhi-110 042, India. E-mail: sukuyd@gmail.com
Keywords: FRW space-time, Hubble parameter, Deceleration parameter, Dark energy.
PACS number: 98.80.Cq, 04.20.-q, 04.20.Jb
1 Introduction
Recent observations of type Ia supernovae (SN Ia) [1-5], galaxy redshift surveys [6], cosmic microwave background radiation (CMBR) data [7, 8] and large scale structure [9] strongly suggest that the observable universe is undergoing an accelerated expansion. Observations also suggest that there had been a transition of the universe from the earlier deceleration phase to the recent acceleration phase [10]. The cause of this sudden transition and the source of the accelerated expansion is still unknown. Measurements of CMBR anisotropies, most recently by the WMAP satellite, indicate that the universe is very close to flat. For a flat universe, its energy density must be equal to a certain critical density, which demands a huge contribution from some unknown energy stuff. Thus, the observational effects like the cosmic acceleration, sudden transition, flatness of universe and many more, need explanation. It is generally believed that some sort of ‘dark energy’ (DE) is pervading the whole universe. It is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe [11]. The most recent WMAP observations indicate that DE accounts for 72% of the total mass energy of the universe [12]. However, the nature of DE is still a mystery.
Many cosmologists believe that the simplest candidate for the DE is the cosmological constant () or vacuum energy since it fits the observational data well. During the cosmological evolution, the -term has the constant energy density and pressure , where the superscript stands for DE. However, one has the reason to dislike the cosmological constant since it always suffers from the theoretical problems such as the “fine-tuning” and “cosmic coincidence” puzzles [13]. That is why, the different forms of dynamically changing DE with an effective equation of state (EoS), , have been proposed in the literature. Other possible forms of DE include quintessence ()[14], phantom () [15] etc. While the possibility is ruled out by current cosmological data from SN Ia (Supernovae Legacy Survey, Gold sample of Hubble Space Telescope) [5, 16], CMBR (WMAP, BOOMERANG) [17, 18] and large scale structure (Sloan Digital Sky Survey) [19] data, the dynamically evolving DE crossing the phantom divide line (PDL) is mildly favored. SN Ia data collaborated with CMBR anisotropy and galaxy clustring statistics suggest that (see, Tegmark et al. [20]).
In 1983, Berman [21] proposed a special law of variation of Hubble parameter in FRW space-time, which yields a constant value of deceleration parameter (DP). Such a law of variation for Hubble’s parameter is not inconsistent with the observations and is also approximately valid for slowly time-varying DP models. The law provides explicit forms of scale factors governing the FRW universe and facilitates to describe accelerating as well as decelerating modes of evolution of the universe. Models with constant DP have been extensively studied in the literature in different contexts (see, Kumar and Singh [22] and references therein). Most of the models with constant DP have been studied by considering perfect fluid or ordinary matter in the universe. But the ordinary matter is not enough to describe the dynamics of an accelerating universe as mentioned earlier. This motivates the researchers to consider the models of the universe filled with some exotic type of matter such as the DE along with the usual perfect fluid. Recently, some dark energy models with constant DP have been investigated by Akarsu and Kilinc [23, 24, 25], Yadav [26] and Yadav and Yadav [27].
In this paper, we have considered minimally interacting perfect fluid and DE energy components with constant DP within the framework of a FRW space-time in general relativity. The paper is organized as follows. In Sect. 2, the model and field equations have been presented. Sect. 3 deals with the exact solutions of the field equations and physical behavior of the model. Finally, concluding remarks have been given in Sect. 4.
2 Model and field equations
In standard spherical coordinates , a spatially homogeneous and isotropic FRW line element has the form (in units )
(1) |
where is the cosmic scale factor, which describes how the distances (scales) change in an expanding or contracting universe, and is related to the redshift of the 3-space; is the curvature parameter, which describes the geometry of the spatial section of space-time with closed, flat and open universes corresponding to , respectively. The coordinates , and in the metric (1) are ‘comoving’ coordinates. The FRW models have been remarkably successful in describing the observed nature of universe.
The Einstein’s field equations in case of a mixture of perfect fluid and DE components, in the units , read as
(2) |
where is the overall energy momentum tensor with and as the energy momentum tensors of ordinary matter and DE, respectively. These are given by
(3) | |||||
and
(4) | |||||
where and are, respectively the energy density and pressure of the perfect fluid component or ordinary baryonic matter while is its EoS parameter. Similarly, and are, respectively the energy density and pressure of the DE component while is the corresponding EoS parameter.
In a comoving coordinate system, the field equations (2), for the FRW space-time (1), in case of (3) and (4), read as
(5) |
(6) |
The energy conservation equation yields
(7) |
where is the Hubble parameter.
3 Solution of Field Equations
The field equations (5) and (6) involve five unknown variables, viz., , , , and . Therefore, to find a deterministic solution of the equations, we need three suitable assumptions connecting the unknown variables.
First, we assume that the perfect fluid and DE components interact minimally. Therefore, the energy momentum tensors of the two sources may be conserved separately.
The energy conservation equation , of the perfect fluid leads to
(8) |
whereas the energy conservation equation , of the DE component yields
(9) |
Next, we assume that the EoS parameter of the perfect fluid to be a constant, that is,
(10) |
while has been allowed to be a function of time since the current cosmological data from SN Ia, CMBR and large scale structures mildly favor dynamically evolving DE crossing the PDL as discussed in Section 1.
Finally, we constrain the system of equations with a law of variation for the Hubble parameter proposed by Berman [21], which yields a constant value of DP. The law reads as
(12) |
where and are constants. In the following subsections, we discuss the DE cosmology for and by using the law (12).
3.1 DE Cosmology for
In this case, integration of (12) leads to
(13) |
where is a constant of integration. Therefore, the model (1) becomes
(14) |
The Hubble parameter (), energy density () of perfect fluid, DE density () and EoS parameter () of DE, for the model (14) are found to be
(15) |
(16) |
(17) |
(18) |
The above solutions satisfy the equation (9) identically, as expected.
The spatial volume () and expansion scalar of the model read as
(19) |
(20) |
The density parameter of perfect fluid and the density parameter of DE are given by
(21) |
(22) |
The value of DP () is found to be
(23) |
which is a constant. The sign of indicates whether the model inflates or not. A positive sign of , i.e., corresponds to the standard decelerating model whereas the negative sign of , i.e., indicates inflation. The expansion of the universe at a constant rate corresponds to , that is, . Also, recent observations of SN Ia [1-5, 28-30] reveal that the present universe is accelerating and value of DP lies somewhere in the range It follows that in the derived model, one can choose the values of DP consistent with the observations.
We observe that at , the spatial volume vanishes while all the parameters diverge. Therefore, the model has a big bang singularity at , which can be shifted to by choosing . The cosmological evolution in FRW space-time is expansionary, since the scale factor monotonically increases with the cosmic time. So, the universe starts expanding with a big bang singularity in the derived model. The parameters , , and start off with extremely large values, and continue to decrease with the expansion of the universe. The spatial volume grows with the cosmic time.
The EoS parameter of DE asymptotically approaches to provided . Thus, the dynamics of depends on for sufficiently large values of . Fig.1 and Fig. 2 show the variation of during the cosmic evolution of closed (), flat () and open () universes for and , respectively. From Fig.1, we observe that for closed and flat universes, starts in phantom region , crosses the PDL and finally varies in the quintessence region . However, it evolves within the quintessence region only for open universe. For (see Fig. 2), varies within the quintessence region throughout the evolution of the three universes. Thus, the nature of DE depends on the constants involved in the expression (18) of . Also, observations predict that (see, Tegmark et al. [20]). Therefore, Fig. 1 and Fig. 2 suggest that the derived model is consistent with the observations.
This shows that , and according as (closed universe), (flat universe) and (open universe) respectively, as expected. We find that in the cases and for sufficiently large values of provided . Thus, the model predicts a flat universe for large times. Since the present-day universe is very close to flat, so the derived model is consistent with the observations.
Fig. 3 demonstrates the behavior of density parameters in the evolution of a flat universe (). We observe that initially the ordinary matter density dominates the universe. But later on, the DE dominates the evolution, which is probably the possible cause of acceleration of the present universe.
3.2 DE Cosmology for
In this case, integration of (12) yields
(25) |
where is a positive constant of integration. Therefore, the model (1) becomes
(26) |
The Hubble parameter, energy density of perfect fluid, DE density and EoS parameter of DE, for the model (26) are obtained as
(27) |
(28) |
(29) |
(30) |
The above solutions satisfy the equation (9) identically, as expected.
The spatial volume and expansion scalar of the model read as
(31) |
(32) |
The density parameters of perfect fluid and DE are given by
(33) |
(34) |
The DP is given by
(35) |
Recent observations of SN Ia [1-5, 28-30] suggest that the universe is accelerating in its present state of evolution. It is believed that the way universe is accelerating presently; it will expand at the fastest possible rate in future and forever. For , we get ; incidentally this value of DP leads to , which implies the greatest value of Hubble’s parameter and the fastest rate of expansion of the universe. Therefore, the derived model can be utilized to describe the dynamics of the late time evolution of the actual universe. So, in what follows, we emphasize upon the late time behavior of the derived model.
Fig. 4 depicts the behavior of the EoS parameter of DE in the closed, flat and open universes. We observe that for sufficiently large time, in each case. Therefore, the so called cosmological constant is a suitable candidate to represent the behavior of DE in the derived model at late times.
Further, at late times, we have
and
This shows that the ordinary matter density becomes negligible whereas the accelerated expansion of the universe continues with non-zero and constant DE density at late times, as predicted by the observations. Fig. 5 illustrates the behavior of the density parameters during the evolution of the universe in the derived model. It is observed that the DE component dominates the universe at late times and
4 Concluding Remarks
The special law of variation for Hubble’s parameter proposed by Berman in FRW space-time yields constant value of DP given by , which provides accelerating models of the universe for and decelerating ones for . In the present work, we have emphasized that decelerating models can be described by the usual perfect fluid, while dynamics of accelerating universe can be described by considering some exotic type of matter such as the DE. We have investigated the role of DE with variable EoS parameter in the evolution of universe within the framework of a spatially homogeneous FRW space-time by taking into account the special law of variation of Hubble parameter. DE cosmologies have been discussed in two different cases, viz., power-law () and exponential-law (). The analysis of the models reveals that the present-day universe is dominated by DE, which can successfully describe the accelerating nature of the universe consistent with the observations.
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