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- An LRS Bianchi I model is considered with constant deceleration parameter, q = α−1, where α ≥ 0 is a constant. The physical and kinematical behaviour of the models for α = 0 and α = 0 is studied in detail. The model with α = 0 describes late time acceleration, but eternal inflation demands a violation of the NEC and WEC. The acceleration is caused by phantom matter which approaches a cosmological constant at late times. The solutions with a scalar field also show that the model is compatible with a phantom field only. A comparison with the observational outcomes indicates that the universe has entered into the present accelerating phase in recent past somewhere between 0.2 z 0.5. The model obeys the “cosmic no hair conjecture”. The models with 0 <α< 1 describe late time acceleration driven by quintessence dark energy. A violation of the NEC and WEC is required to accommodate the early inflationary epoch caused by phantom matter. The models with 1 <α< 3 describe decelerating phases which are usually occur in the presence of dust or radiation. These models are also found anisotropic at early times and attain isotropy at late times. The model for α = 3 represents a stiff matter era which also has shear at early stages and becomes shear free at late times, but it evolves with an insignificant ceaseless anisotropy. The models with α > 3 violate the DEC and the corresponding scalar field models have negative potential which is physically unrealistic.
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- Locally-rotationally-symmetric Bianchi-I space time model is studied with constant Hubble parameter in f (R,T ) = R + 2λT gravity. Although a single (primary) matter source is considered, an additional matter source appears due to the coupling between matter and f (R,T ) gravity. The constraints are obtained for a realistic cosmological scenario. The solutions are also extended to the case of a scalar field (normal or phantom) model, and it is found that the model is consistent with a phantom scalar field only. The coupled matter also acts as phantom matter. The study shows that if one expects an accelerating universe from an anisotropic model, then the solutions become physically relevant only at late times when the universe enters into an accelerated phase. Placing some observational bounds on the present equation of state of dark energy, ω0, the behavior of ω(z) is depicted, which shows that the phantom field starts dominating very recently, somewhere between 0.2 < z < 0.5. The geometrical behavior of the model remains identical to the one in general relativity
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- A plane symmetric Bianchi-I model filled with strange quark matter (SQM) was explored in f(R,T)=R+2λT gravity, where R is the Ricci scalar, T is the trace of the energy-momentum tensor, and λ is an arbitrary constant. Three different types of solutions were obtained. In each model, comparisons of the outcomes in f(R,T) gravity and bag constant were made to comprehend their roles. The first power-law solution was obtained by assuming that the expansion scalar is proportional to the shear scalar. This solution was compared with a similar one obtained earlier. The second solution was derived by assuming a constant deceleration parameter q. This led to two solutions: one power-law and the other exponential. Just as in the case of general relativity, we can obtain solutions for each of the different eras of the universe, but we cannot obtain a model which shows transitional behavior from deceleration to acceleration. However, the third solution is a hybrid solution, which shows the required transition. The models start off with anisotropy, but are shear free at late times. In general relativity, the effect of SQM is to accelerate the universe, so we expect the same in f(R,T) gravity.
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- The general solution of the field equations in LRS Bianchi-I spacetime with perfect fluid equation-of-state (EoS) is presented. The models filled with dust, vacuum energy, Zel’dovich matter and disordered radiation are studied in detail. A unified and systematic treatment of the solutions is presented, and some new solutions are found. The dust, stiff matter and disordered radiation models describe only a decelerated universe, whereas the vacuum energy model exhibits a transition from a decelerated to an accelerated phase.
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