• Hassan Amirhashchi

      Articles written in Pramana – Journal of Physics

    • String cosmology in LRS Bianchi type-II dusty Universe with time-decaying vacuum energy density 𝛬

      Hassan Amirhashchi Hosein Mohamadian

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      A model of a cloud formed by massive strings is used as a source of LRS Bianchi type-II with time-decaying vacuum energy density 𝛬. To construct string cosmological models, we have used the energy–momentum tensor for such strings as formulated by Letelier (1983). The high nonlinear field equations have been solved for two types of strings: (i) massive string and (ii) Nambu string. The expansion 𝜃 in the model is assumed to be proportional to the shear 𝜎. This condition leads to $A = \beta B^m$, where 𝐴 and 𝐵 are the metric coefficients, 𝑚 is a constant and 𝛽 is an integrating constant. Our models are in accelerating phase which is consistent with the recent observations of supernovae type-Ia. The physical and geometrical behaviour of these models are also discussed.

    • String cosmology in Bianchi type-VI0 dusty Universe with electromagnetic field

      Hassan Amirhashchi

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      In this paper, the effect of electromagnetic field in the string Bianchi type-VI0 Universe is investigated. Einstein’s field equations have been solved exactly with suitable physical assumptions for two types of strings: (i) massive strings and (ii) Nambu strings. It is found that when the Universe is dominated by massive strings, the existence of electromagnetic field is necessary as it accelerates the expansion of the Universe. But when our Universe is dominated by Nambu strings, the electromagnetic field does not have significant effect on the evolution of the Universe. We have also shown that the early massive string-dominated Universe got converted to Nambu string-dominated Universe later. Our models are derived from an early deceleration phase to an accelerating phase which is consistent with the recent observations of supernovae type-Ia. The physical and geometrical behaviour of these models are also discussed.

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