2003 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/626

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Subject: search.filters.subject.Dark energy

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    Is the present expansion of the universe really accelerating?
    (2011-07-05) Vishwakarma, R. G.
    The current observations are usually explained by an accelerating ex- pansion of the present universe. However, with the present quality of the supernovae Ia data, the allowed parameter space is wide enough to accommodate the decelerating models as well. This is shown by considering a particular example of the dark energy equation-of-state wφ ≡ pφ/ρφ = −1/3, which is equivalent to modifying the geometrical curvature index k of the standard cosmology by shifting it to (k − α) where α is a constant. The resulting decelerating model is consistent with the recent CMB observations made by WMAP, as well as, with the high redshift supernovae Ia data including SN 1997ff at z = 1.755. It is also consistent with the newly discovered supernovae SN 2002dc at z = 0.475 and SN 2002dd at z = 0.95 which have a general tendency to improve the fit.
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    Can dark energy be decaying?
    (2011-07-05) Ujjaini, Alam; Sahni, Varun; Starobinsky, A. A.
    We explore the fate of the universe given the possibility that the density associated with ‘dark energy’ may decay slowly with time. Decaying dark energy is modeled by a homogeneous scalar field which couples minimally to gravity and whose potential has at least one local quadratic maximum. Dark energy decays as the scalar field rolls down its potential, consequently the current acceleration epoch is a transient. We examine two models of decaying dark energy. In the first, the dark energy potential is modeled by an analytical form which is generic close to the potential maximum. The second potential is the cosine, which can become negative as the field evolves, ensuring that a spatially flat universe collapses in the future. We examine the feasibility of both models using observations of high redshift type Ia supernovae. A maximum likelihood analysis is used to find allowed regions in the {m, φ0} plane (m is the tachyon mass modulus and φ0 the initial scalar field value; m ∼ H0 and φ0 ∼ MP by order of magnitude). For the first model, the time for the potential to drop to half its maximum value is larger than ∼ 8 Gyrs. In the case of the cosine potential, the time left until the universe collapses is always greater than ∼ 18 Gyrs (both estimates are presented for Ω0m = 0.3, m/H0 ∼ 1, H0 ≃ 70 km/sec/Mpc, and at the 95.4% confidence level).