Research Papers (TP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/3
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Item Understanding the origin of CMB constraints on dark energy(Wiley-Blackwell, 2010-03-08) Jassal, H. K.; Bagla, J. S.; Padmanabhan, T.We study the observational constraints of cosmic microwave background (CMB) temperature and polarization anisotropies on models of dark energy, with special focus on models with variation in properties of dark energy with time. We demonstrate that the key constraint from CMB observations arises from the location of acoustic peaks. An additional constraint arises from the limits on ΩNR from the relative amplitudes of acoustic peaks. Further, we show that the distance to the last scattering surface is not how the CMB observations constrain the combination of parameters for models of dark energy. We also use constraints from supernova observations and show that unlike the gold and silver samples, the Supernova Legacy Survey (SNLS) sample prefers a region of parameter space that has a significant overlap with the region preferred by the CMB observations. This is a verification of a conjecture made by us in an earlier work. We discuss combined constraints from Wilkinson Microwave Anisotropy Probe 5-yr and SNLS observations. We find that models with w ~−1 are preferred for models with a constant equation-of-state parameters. In case of models with a time-varying dark energy, we show that constraints on evolution of dark energy density are almost independent of the type of variation assumed for the equation-of-state parameter. This makes it easy to get approximate constraints from CMB observations on arbitrary models of dark energy. Constraints on models with a time-varying dark energy are predominantly due to CMB observations, with supernova constraints playing only a marginal role.Item WMAP constraints on low redshift evolution of dark energy(Wiley-Blackwell, 2004-10-29) Jassal, H. K.; Bagla, J. S.; Padmanabhan, T.The conceptual difficulties associated with a cosmological constant have led to the investigation of alternative models in which the equation of state parameter, w = p/ρ,of the dark energy evolves with time. We show that combining the supernova type Ia observations with the constraints from WMAP observations restricts large variation of ρ(z)atlow redshifts. The combination of these two observational constraints is stronger than either one. The results are completely consistent with the cosmological constant as the source of dark energy.Item Theoretician’s analysis of the supernova data and the limitations in determining the nature of dark energy(Wiley-Blackwell, 2003-06-02) Padmanabhan, T.; Choudhury, T. RoyItem Response of distance measures to the equation of state(Wiley-Blackwell, 2003-04-05) Saini, Tarun Deep; Padmanabhan, T.; Bridle, SarahWe show that the distance measures (such as the luminosity and angular diameter distances) are linear functionals of the equation of state function w(z) of the dark energy to a fair degree of accuracy in the regimes of interest. That is, the distance measures can be expressed as a sum of (i) a constant and (ii) an integral of a weighting function multiplied by the equation of state parameter w(z). The existence of such an accurate linear response approximation has several important implications. (1) Fitting a weighted average of w(z). (2) Any polynomial (or other expansion coefficients) can also be expressed as weighted sums of the true w. (3) A replacement for the commonly used heuristic equation for the effective w, as determined by the cosmic microwave background, can be derived and the result is found to be quite close to the heuristic expression commonly used. (4) The reconstruction of w()z by Huterer & Starkman can be expressed as a matrix inversion. In each case the limitations of the linear response approximation are explored and found to be surprisingly small.