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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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