Browsing by Author "Shafieloo, Arman"

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Assumptions of the primordial spectrum and cosmological parameter estimation
(2009-01-01) Shafieloo, Arman; Souradeep, Tarun
The observables of the perturbed universe, CMB anisotropy and large structures, depend on a set of cosmological parameters, as well as, the assumed nature of primordial perturbations. In particular, the shape of the primordial power spectrum (PPS) is, at best, a well motivated assumption. It is known that the assumed functional form of the PPS in cosmological parameter estimation can aﬀect the best ﬁt parameters and their relative conﬁdence limits. In this letter, we demonstrate that a speciﬁc assumed form actually drives the best ﬁt parameters into distinct basins of likelihood in the space of cosmological parameters where the likelihood resists improvement via modiﬁcations to the PPS. The regions where considerably better likelihoods are obtained allowing free form PPS lie outside these basins. In the absence of a preferred model of inﬂation, this raises a concern that current cosmological parameters estimates are strongly prejudiced by the assumed form of PPS. Our results strongly motivate approaches toward simultaneous estimation of the cosmological parameters and the shape of the primordial spectrum from upcoming cosmological data. It is equally important for theorists to keep an open mind towards early universe scenarios that produce features in the PPS.
Cosmological parameter estimation with free-form primordial power spectrum
(IUCAA, 2015-02) Hazra, Dhiraj Kumar; Shafieloo, Arman; Tarun, Sauradeep
Estimation of Primordial Spectrum with post-WMAP 3 year data
(2007-07-14) Shafieloo, Arman; Souradeep, Tarun
In this paper we implement an improved (error sensitive) Richardson-Lucy deconvolution algorithm on the measured angular power spectrum from the WMAP 3 year data to determine the primordial power spectrum assuming diﬀerent points in the cosmological parameter space for a ﬂat ΛCDM cosmological model. We also present the preliminary results of the cosmological parameter estimation by assuming a free form of the primordial spectrum, for a reasonably large volume of the parameter space. The recovered spectrum for a considerably large number of the points in the cosmological parameter space has a likelihood far better than a ‘best ﬁt’ power law spectrum up to ∆χ2 eﬀ ≈ −30. We use Discrete Wavelet Transform (DWT) for smoothing the raw recovered spectrum from the binned data. The results obtained here reconﬁrm and sharpen the conclusion drawn from our previous analysis of the WMAP 1st year data. A sharp cut oﬀ around the horizon scale and a bump after the horizon scale seem to be a common feature for all of these reconstructed primordial spectra. We have shown that although the WMAP 3 year data prefers a lower value of matter density for a power law form of the primordial spectrum, for a free form of the spectrum, we can get a very good likelihood to the data for higher values of matter density. We have also shown that even a ﬂat CDM model, allowing a free form of the primordial spectrum, can give a very high likelihood ﬁt to the data. Theoretical interpretation of the results is open to the cosmology community. However, this work provides strong evidence that the data retains discriminatory power in the cosmological parameter space even when there is full freedom in choosing the primordial spectrum.
Features in the primordial power spectrum? A frequentist analysis
(2009-12-01) Hamann, Jan; Shafieloo, Arman; Souradeep, Tarun
Features in the primordial power spectrum have been suggested as an explanation for glitches in the angular power spectrum of temperature anisotropies measured by the WMAP satellite. However, these glitches might just as well be artifacts of noise or cosmic variance. Using the e ective 2 between the best- t power-law spectrum and a deconvolved primordial spectrum as a measure of \featureness" of the data, we perform a full Monte-Carlo analysis to address the question of how signi cant the recovered features are. We nd that in 26% of the simulated data sets the reconstructed spectrum yields a greater improvement in the likelihood than for the actually observed data. While features cannot be categorically ruled out by this analysis, and the possibility remains that simple theoretical models which predict some of the observed features might stand up to rigorous statistical testing, our results suggest that WMAP data are consistent with the assumption of a featureless power-law primordial spectrum.
Features in the primordial spectrum from WMAP: A wavelet analysis
(2006-01-01) Shafieloo, Arman; Souradeep, Tarun; Manimaran, P.
Precise measurements of the anisotropies in the cosmic microwave background enable us to do an accurate study on the form of the primordial power spectrum for a given set of cosmological parameters. In a previous paper [1], we implemented an improved (error sensitive) Richardson-Lucy deconvolution algorithm on the measured angular power spectrum from the ﬁrst year of WMAP data to determine the primordial power spectrum assuming a concordance cosmological model. This recovered spectrum has a likelihood far better than a scale invariant, or, ‘best ﬁt’ scale free spectra (∆ln L ≈ 25 w.r.t. Harrison Zeldovich, and, ∆ln L ≈ 11 w.r.t. power law with ns = 0.95). In this paper we use Discrete Wavelet Transform (DWT) to decompose the local features of the recovered spectrum individually to study their eﬀect and signiﬁcance on the recovered angular power spectrum and hence the likelihood. We show that besides the infra-red cut oﬀ at the horizon scale, the associated features of the primordial power spectrum around the horizon have a signiﬁcant eﬀect on improving the likelihood. The strong features are localized at the horizon scale.
ISW effect as probe of features in the expansion history of the Universe
(IUCAA, 2015-02) Das, Santanu; Shafieloo, Arman; Tarun, Sauradeep
Model Independent Reconstruction of the Expansion History of the Universe and the Properties of Dark Energy
(2007-07-06) Shafieloo, Arman
We have improved upon the method of smoothing supernovae data to reconstruct the expansion history of the universe, h(z), using two latest datasets, Gold and SNLS. The reconstruction process does not employ any parameterisation and is independent of any dark energy model. The reconstructed h(z) is used to derive the distance factor A up to redshift 0.35 and the results are compared with the given value of A from detection of baryon acoustic oscillation peak (BAO). We ﬁnd very good agreement be- tween supernovae observations and the results from BAO for Ω0m ≈ 0.276±0.023. The estimated values of Ω0m are completely model-independent and are only based on observational data. The derived values of Ω0m are then used to reconstruct the equation of state of dark energy, w(z). Using our smoothing method we can demonstrate that while SNLS data are in very good agreement with ΛCDM, the Gold sample slightly prefers evolving dark energy. We also show that proper estimation of the equation of state of dark energy at the high redshifts would be impossible at the current status of observations
Primordial power spectrum from Planck
(IUCAA, 2015-02) Hazra, Dhiraj Kumar; Shafieloo, Arman; Tarun, Sauradeep
Primordial power spectrum from WMAP
(2011-07-05) Shafieloo, Arman; Souradeep, Tarun
The observed angular power spectrum of the cosmic microwave background temperature anisotropy, Cl, is a convolution of a cosmological radiative transport kernel with an assumed primordial power spectrum of inhomogeneities. Exquisite measurements of Cl over a wide range of multipoles from the Wilkinson Microwave Anisotropy Probe (WMAP) has opened up the possibility to deconvolve the primordial power spectrum for a given set of cosmological parameters (base model). We implement an improved (error sensitive) Richardson-Lucy deconvolution algorithm on the measured angular power spectrum from WMAP assuming the concordance cosmological model. The most prominent feature of the recovered P(k) is a sharp, infrared cutoff on the horizon scale. The resultant Cl spectrum using the recovered spectrum has a likelihood far better than a scale invariant, or, “best fit” scale free spectra (ΔlnL≈25 with respect to Harrison Zeldovich, and, ΔlnL≈11 with respect to power law with ns=0.95). The recovered P(k) has a localized excess just above the cutoff which leads to great improvement of likelihood over the simple monotonic forms of model infrared cutoff spectra considered in the post WMAP literature. The recovered P(k), in particular, the form of infrared cutoff is robust to small changes in the cosmological parameters. We show that remarkably similar form of infrared cutoff is known to arise in very reasonable extensions and refinements of the predictions from simple inflationary scenarios. Our method can be extended to other cosmological observations such as the measured matter power spectrum and, in particular, the much awaited polarization spectrum from WMAP.
Primordial power spectrum: a complete analysis with the WMAP nine-year data
(IUCAA, 2015-02) Hazra, Dhiraj Kumar; Shafieloo, Arman; Tarun, Sauradeep