2006 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/335

Browse

Author: search.filters.author.Sahni, Varun

Search Results

Now showing 1 - 5 of 5
  • No Thumbnail Available
    Item
    Smoothing supernova data to reconstruct the expansion history of the universe
    (2006-01-10) Shafieloo, Arman; Alam, Ujjaini; Sahni, Varun
    We propose a non-parametric method of smoothing supernova data over redshift using a Gaussian kernel in order to reconstruct important cosmological quantities including H(z) and w(z) in a model independent manner. This method is shown to be successful in discriminating between different models of dark energy when the quality of data is commensurate with that expected from the future SuperNova Acceleration Probe (SNAP). We find that the Hubble parameter is especially well-determined and useful for this purpose. The look back time of the universe may also be determined to a very high degree of accuracy ( < ∼ 0.2%) in this method. By refining the method, it is also possible to obtain reasonable bounds on the equation of state of dark energy. We explore a new diagnostic of dark energy– the ‘w-probe’– which can be calculated from the first derivative of the data. We find that this diagnostic is reconstructed extremely accurately for different reconstruction methods even if Ω0m is marginalized over. The w-probe can be used to successfully distinguish between ΛCDM and other models of dark energy to a high degree of accuracy.
  • No Thumbnail Available
    Item
    Reconstructing dark energy
    (2006-10-23) Sahni, Varun; Starobinsky, A. A.
    This review summarizes recent attempts to reconstruct the expansion history of the Uni- verse and to probe the nature of dark energy. Reconstruction methods can be broadly classified into parametric and non-parametric approaches. It is encouraging that, even with the limited observational data currently available, different approaches give consistent results for the reconstruction of the Hubble parameter H(z) and the effective equation of state w(z) of dark energy. Model independent reconstruction using current data allows for modest evolution of dark energy density with redshift. However, a cosmological constant (= dark energy with a constant energy density) remains an excellent fit to the data. Some pitfalls to be guarded against during cosmological reconstruction are summarized and future directions for the model independent reconstruction of dark energy are explored.
  • No Thumbnail Available
    Item
    Dark energy
    (2006-01-10) Sahni, Varun
    The cosmological constant problem as well as the case for dark energy are briefly reviewed and some theoretical models of dark energy are discussed in detail. These include: the cosmological constant, quintessence, the Chaplygin gas and Braneworld models. I also discuss model independent measures of dark energy and conclude by mentioning some properties of the Statefinder diagnostic which can successfully differentiate between different families of dark energy models.
  • No Thumbnail Available
    Item
    Confronting braneworld cosmology with supernova data and baryon oscillations
    (2006-03-18) Alam, Ujjaini; Sahni, Varun
    Braneworld cosmology has several attractive and distinctive features. For instance the effective equation of state in braneworld models can be both quintessence-like (w0 ≥ −1) as well as phantom like (w0 ≤ −1). Models with w0 ≥ −1 (w0 ≤ −1) are referred to as Brane 2 (Brane 1) and correspond to complementary embeddings of the brane in the bulk. (The equation of state in Brane 1 can successfully cross the ‘phantom divide’ at w = −1.) In this paper we compare the predictions of braneworld models to two recently released supernova data sets: the ‘Gold’ data (Riess et al., 2004) and the data from the Supernova Legacy Survey (SNLS) (Astier et al., 2005). We also incorporate the recent discovery of the baryon acoustic peak in the Sloan Digital Sky Survey (Eisenstein et al., 2005) into our analysis. Our main results are that braneworld models satisfy both sets of SNe data. Brane 1 (with w0 ≤ −1) shows very good agreement with data for values of the matter density bounded from below: Ω0m > ∼ 0.25 (Gold) and Ω0m > ∼ 0.2 (SNLS). On the other hand Brane 2 (with w0 ≥ −1) shows excellent agreement with data for values of the matter density which are bounded from above: Ω0m < ∼ 0.45 (Gold) and Ω0m < ∼ 0.35 (SNLS). The DGP model is excluded at 3σ by SNLS and at 1σ by the Gold dataset. Braneworld models with future ‘quiescent’ singularities (at which the Hubble parameter and the matter density remain finite but higher derivatives of the expansion factor diverge) are excluded by both datasets.
  • No Thumbnail Available
    Item
    Cosmic mimicry: Is LCDM a braneworld in disguise?
    (2006-01-01) Sahni, Varun; Shtanov, Yuri; Viznyuk, Alexander