Narrow Results

In this investigation report, we present the analysis of a stellar model formed by ordinary matter and quintessence-type matter in the frame of Rastall’s gravitation theory. The ordinary matter with a state equation for the radial pressure $P_r(\rho )=\frac{1}{2}(c^2\rho -4B_g)$, where $B_g$ is the so called MIT Bag constant and the quintessence-type matter, with density ${\rho }_q$ and tangential state equation of the form $P_{wt}=(1+3w){\rho }_q/2$, with $-1<w<-1/3$. Through an analytic and graphic analysis, it is shown that the proposal is consistent with the expected behavior of the hydrostatic functions and, in particular, it’s focused on the analysis taking the observational data of the mass $M=1.44_{-0.14}^{+0.15}{M}_{\odot }$ and radius $R=13.02_{-1.06}^{+1.24}$km corresponding to the star PSR $\mathrm{\text{J}}0030+45$. We obtain the central density of the ordinary matter ${\rho }_c\in [3.5479,7.1327]\times 10^{17}$kg/m³ and the MIT Bag constant $25.32159\mathrm{\text{MeV}}/{\mathrm{\text{fm}}}^3\le B_g\le 47.45455\mathrm{\text{MeV}}/{\mathrm{\text{fm}}}^3$ which changes depending on the compactness and the Rastall parameter but its values are independent from the parameter of the quintessence matter w.

Starting from a regular, static and spherically symmetric spacetime, we present a stellar model formed by two sources of ordinary and quintessence matter both with anisotropic pressures. The ordinary matter, with density $\rho$, is formed by a fluid with a state equation type Chaplygin $P_r(\rho )=\mu c^2\rho -\nu /(c^2\rho )$ for the radial pressure. And the quintessence matter, with density ${\rho }_q$, has a state equation $P_r({\rho }_q)=-c^2{\rho }_q$ for the radial pressure and $P_r({\rho }_q)=-(1+3w)c^2{\rho }_q/2$ for the tangential pressure with $-1<w<-\frac{1}{3}$. The model satisfies the required conditions to be physically acceptable and additionally the solution is potentially stable, i.e. $v_t^2-v_r^2<0$ according to the cracking concept, and it also satisfies the Harrison–Zeldovich–Novikov criteria. We describe in a graphic manner the behavior of the solution for the case in which the mass is $1.3M_{\odot }$ and radius $R=8.849$km which matches the star EXO 1785-248, from where we obtain the maximum density ${\rho }_c=1.206510^{18}\mathrm{\text{kg}}/{\mathrm{\text{m}}}^3$ for the values of the parameters $\mu =0.15535$, $w=-0.33334$.

In this paper, we explore the characteristics of two novel regular spacetimes that exhibit a nonzero vacuum energy term, under the form of a (quasi) anti-de Sitter phase. Specifically, the first metric is spherical, while the second, derived by applying the generalized Newman–Janis algorithm to the first, is axisymmetric. We show that the equations of state of the effective fluids associated with the two metrics asymptotically tend to negative values, resembling quintessence. In addition, we study test particle motions, illustrating the main discrepancies among our models and more conventional metrics exhibiting non-vanishing anti-de Sitter phase.

In this paper we highlight our recent work in arXiv:0803.4504. In that work, we proposed a new consistency test of quintessence models for dark energy. Our test gave a simple and direct signature if certain category of quintessence models was not consistent with the observational data. For a category that passed the test, we further constrained its characteristic parameter. Specifically, we found that the exponential potential was ruled out at the 95% confidence level and the power-law potential was ruled out at the 68% confidence level based on the current observational data. We also found that the confidence interval of the index of the power-law potential was between -2 and 0 at the 95% confidence level.

The quintessence is one of the several candidates which represents the dark energy responsible for the acceleration of the universe. The quintessence is described by a canonical scalar field minimally coupled to gravity. We study the timelike geodesic congruences in the background of a rotating black hole spacetime surrounded by quintessence in equatorial plane. The effect of equation of state (EOS) parameter for quintessence and the normalization factor on geodesics is investigated in detail in view of the structure of possible orbits, including the innermost stable circular orbits (ISCOs). The structure of photon orbits is also studied for the different values of parameters involved in. The results obtained are also then compared with those of the Kerr black hole spacetime and Schwarzschild black hole spacetime in GR with or without quintessence.

Quintessence fields, or scalar fields minimally coupled to gravity, are considered to be viable candidates for dark energy. It is well-known that some classes of modified theories of gravity can be recast as Einstein’s general relativity with a minimally coupled scalar field, through conformal transformation. The ‘universe’ described by the initial and the final actions are referred to as the Jordan and Einstein frames, respectively. Although these conformally connected frames are mathematically equivalent, the equations of motion in these two frames may describe drastically different physical scenarios. Depending upon the choice of the Jordan frame action (or equivalently the scalar field potential in Einstein frame) it is possible that while the Einstein frame expands, the Jordan frame collapses. We classify quintessence models in the Einstein frame that are dual to f(R) gravity theories in the Jordan frame, based on whether they possess such expansion-collapse duality. We derive a general condition for expansion-collapse duality, applicable to quintessence models with arbitrary time-dependent equations of state. The condition also takes into account the presence of other components in the Einstein frame universe. Such expansion-collapse duality between these conformally connected frames can lead to an effective description of a collapsing universe in terms of an expanding one, which is a topic of further exploration.

We proposes a phenomenological generalisation of the standard model with only one extra degree of freedom that parametrises the evolution of a scalar field responsible for the cosmic acceleration. The model also foresees an additional parameter in the form of a coupling between dark energy and dark matter. This model captures a large diversity of dark energy evolutions at low redshift and could usefully complement common CPL parametrisations widely used. In this context, we have been constraining the parametrisation with data from Planck and KiDS, bringing different results between the early and late universe observations.

The cosmology has made enormous progress after the construction of General Relativity (GR) in 1915. The theoretical predictions of GR — such as the existence of black holes and gravitational waves — have been directly/indirectly confirmed by observations. Now, we know that GR is sufficiently dependable to describe the gravitational law in the solar system.