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Author: search.filters.author.Sahni, VarunSubject: search.filters.subject.Universe

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    Recycling the universe using scalar fields
    (2001-07-02) Kanekar, Nissim; Sahni, Varun; Shtanov, Yuri
    We examine the behaviour of a closed oscillating universe filled with a homogeneous scalar field and find that, contrary to naive expectations, such a universe expands to larger vol- umes during successive expansion epochs. This intriguing be- haviour introduces an arrow of time in a system which is time- reversible. The increase in the maximum size of the universe s closely related to the work done on/by the scalar field dur- ng one complete oscillatory cycle which, in turn, is related to the asymmetry in the scalar field equation of state dur- ng expansion and collapse. Our analysis shows that scalar fields with polynomial potentials V (φ) = λφq , q > 1 lead to a growing oscillation amplitude for the universe: the increase n amplitude between successive oscillations is more signifi- cant for smaller values of q. Such behaviour allows for the effective recycling of the universe. A recycled universe can be quite old and can resolve the flatness problem. These results have strong bearing on cosmological models in which the role of dark matter is played by a scalar field. They are also rele- vant for chaotic inflationary models of the early universe since they demonstrate that, even if the universe fails to inflate the first time around, it will eventually do so during future oscil- atory cycles. Thus, the space of initial conditions favourable or chaotic inflation increases significantly.
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    Did the Universe loiter at high redshifts?
    (2005-04-01) Sahni, Varun
    We show that loitering at high redshifts (z > ∼ 6) can easily arise in braneworld models of dark energy which, in addition to being spatially flat, also accelerate at late times. Loitering is characterized by the fact that the Hubble parameter dips in value over a narrow redshift range which we shall refer to as the ‘loitering epoch’. During loitering, density perturbations are expected to grow rapidly. In addition, since the expansion of the universe slows down, its age near loitering dramatically increases. An early epoch of loitering is expected to boost the formation of high redshift gravitationally bound systems such as 109M⊙ black holes at z ∼ 6 and lower-mass black holes and/or Population III stars at z > 10, whose existence could be problematic within the LCDM scenario. Loitering models also help to reduce the redshift of reionization from its currently (high) value of zreion ≃ 17 in LCDM cosmology, thus alleviating a significant source of tension between observations of the high-redshift universe and theoretical model building. Currently a loitering universe accelerates with an effective equation of state w < −1 thus mimicking phantom dark energy. Unlike phantom, however, the late-time expansion of the universe in our model is singularity free, and a universe that loitered in the past will approach a LCDM model symptotically in the distant future.