Browsing by Author "Aich, Moumita"

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Evolution of primordial statistical isotropy violation
(2011-08-11) Aich, Moumita
Oscillation in the inflaton potential: Exact numerical analysis and comparison with the recent and forthcoming CMB datasets
(2011-06-15) Aich, Moumita; Hazra, Dhiraj Kumar; Sriramkumar, L.; et al.
Amongst the multitude of inﬂationary models currently available, models that lead to features in the primordial scalar spectrum are drawing increasing attention, since certain features have been found to provide a better ﬁt to the CMB data than the conventional, nearly scale invariant, primordial spectrum. In this work, we carry out an exact numerical analysis of two models that lead to oscillations over all scales in the scalar power spectrum. We consider the model described by a quadratic potential which is superposed by a sinusoidal modulation and the recently popular axion monodromy model. Since the oscillations continue even onto smaller scales, in addition to the WMAP data, we also compare the models with the small scale data from ACT. Though, both the models, broadly, result in oscillations in the spectrum, interestingly, we ﬁnd that, while the monodromy model leads to a considerably better ﬁt to the data in comparison to the standard power law spectrum, the quadratic potential superposed with a sinusoidal modulation does not improve the ﬁt to a similar extent. We also carry out forecasting of the parameters using simulated Planck data for both the models. We show that the Planck mock data performs better in constraining the model parameters as compared to the presently available CMB datasets.
Primordial features due to a step in the inflaton potential
(2010-05-12) Hazra, Dhiraj Kumar; et al.; Aich, Moumita; Jain, Rajeev Kumar
Certain oscillatory features in the primordial scalar power spectrum are known to provide a better ﬁt to the outliers in the cosmic microwave background data near the multipole moments of ℓ = 22 and 40. These features are usually generated by introducing a step in the popular, quadratic potential describing the canonical scalar ﬁeld. Such a model will be ruled out, if the tensors remain undetected at a level corresponding to a tensor-to-scalar ratio of, say, r ≃ 0.1. In this work, in addition to the popular quadratic potential, we investigate the eﬀects of the step in a small ﬁeld model and a tachyon model. With possible applications to future datasets (such as PLANCK) in mind, we evaluate the tensor power spectrum exactly, and include its contribution in our analysis. We compare the models with the WMAP (ﬁve as well as seven-year), the QUaD and the ACBAR data. As expected, a step at a particular location and of a suitable magnitude and width is found to improve the ﬁt to the outliers (near ℓ = 22 and 40) in all these cases. We point out that, if the tensors prove to be small (say, r . 0.01), the quadratic potential and the tachyon model will cease to be viable, and more attention will need to be paid to examples such as the small ﬁeld models.
Statistical isotropy violation of the CMB brightness fluctuations
(2010-03-22) Aich, Moumita; Souradeep, Tarun
Certain anomalies at large angular scales in the cosmic microwave background measured by WMAP have been suggested as possible evidence of breakdown of statistical isotropy(SI). SI violation of cosmological perturbations is a generic feature of ultra large scale structure of the cosmos and breakdown of global symmetries. Most CMB photons free-stream to the present from the surface of last scattering. It is thus reasonable to expect statistical isotropy violation in the CMB photon distribution observed now to have originated from SI violation in the baryon-photon ﬂuid at last scattering, in addition to anisotropy of the primordial power spectrum studied earlier in literature. We consider the generalized anisotropic brightness distribution ﬂuctuations, ∆(~ k, ˆ n, τ ) (at con-formal time τ ) in contrast to the SI case where it is simply a function of |~ k| and ˆ k ˆ n. The brightness ﬂuctuations expanded in Bipolar Spherical Harmonic (BipoSH) series, can then be written as ∆LM ℓ1ℓ2 (k, τ ) where L > 0 terms encode deviations from statistical isotropy. Violation of SI encoded in the present oﬀ-diagonal elements of the harmonic space correlation haℓmaℓ′m′ i, equivalently, the BipoSH coeﬃcients ALM ℓℓ′ , are then related to the generalized BipoSH brightness ﬂuctuation terms at present. We study the evolution of ∆LM ℓ1ℓ2 (k, τ ) from non-zero terms ∆LM ℓ3ℓ4 (k, τs) at last scattering, in the free streaming regime. We show that the terms with given BipoSH multipole, LM, evolve independently. Moreover, similar to the SI case, power at small spherical harmonic (SH) multipoles of ∆LM ℓ3ℓ4 (k, τs) at the last scattering, is transferred to ∆LM ℓ1ℓ2 (k, τ ) at larger SH multipoles. The structural similarity is more apparent in the asymptotic expression for large values of the ﬁnal SH multipoles. This formalism allows an elegant identiﬁcation of any SI violation observed today to a possible origin in SI violating physics present in the baryon-photon ﬂuid. This is illustrated for the known result of SI violating angular correlations due to the presence of a homogeneous magnetic ﬁeld in the baryon-photon ﬂuid.
WMAP anomaly : Weak lensing in disguise
(IUCAA, 2015-02) Rotti, Aditya; Aich, Moumita; Souradeep, Tarun