arXiv subjects

Dhiraj Kumar Hazra

Publications and source records attributed to Dhiraj Kumar Hazra.

61 records · Page 4Linked to original sources

arXiv2013

BINGO: A code for the efficient computation of the scalar bi-spectrum

We present a new and accurate Fortran code, the BI-spectra and Non-Gaussianity Operator (BINGO), for the efficient numerical computation of the scalar bi-spectrum and the non-Gaussianity parameter f_{NL} in single field inflationary models involving the canonical scalar field. The code can calculate all the different contributions to the bi-spectrum and the parameter f_{NL} for an arbitrary triangular configuration of the wavevectors. Focusing firstly on the equilateral limit, we illustrate the accuracy of BINGO by comparing the results from the code with the spectral dependence of the bi-spectrum expected in power law inflation. Then, considering an arbitrary triangular configuration, we contrast the numerical results with the analytical expression available in the slow roll limit, for, say, the case of the conventional quadratic potential. Considering a non-trivial scenario involving deviations from slow roll, we compare the results from the code with the analytical results that have recently been obtained in the case of the Starobinsky model in the equilateral limit. As an immediate application, we utilize BINGO to examine of the power of the non-Gaussianity parameter f_{NL} to discriminate between various inflationary models that admit departures from slow roll and lead to similar features in the scalar power spectrum. We close with a summary and discussion on the implications of the results we obtain.

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arXiv2013

Probing primordial non-Gaussianity: The 3D Bispectrum of Ly-alpha forest and the redshifted 21-cm signal from the post reionization epoch

We explore possibility of using the three dimensional bispectra of the Ly-alpha forest and the redshifted 21-cm signal from the post-reionization epoch to constrain primordial non-Gaussianity. Both these fields map out the large scale distribution of neutral hydrogen and maybe treated as tracers of the underlying dark matter field. We first present the general formalism for the auto and cross bispectrum of two arbitrary three dimensional biased tracers and then apply it to the specific case. We have modeled the 3D Ly-alpha transmitted flux field as a continuous tracer sampled along 1D skewers which corresponds to quasars sight lines. For the post reionization 21-cm signal we have used a linear bias model. We use a Fisher matrix analysis to present the first prediction for bounds on f_{NL} and the other bias parameters using the three dimensional 21-cm bispectrum and other cross bispectra. The bounds on f_{NL} depend on the survey volume, and the various observational noises. We have considered a BOSS like Ly-alpha survey where the average number density of quasars \bar{n} = 10^{-3} Mpc^{-2} and the spectra are measured at a 2-sigma level. For the 21-cm signal we have considered a 4000 hrs observation with a futuristic SKA like radio array. We find that bounds on f_{NL} obtained in our analysis (6 < Δf_{NL} < 65) is competitive with CMBR and galaxy surveys. We also find that by combining the various cross-bispectrum estimators it is possible to constrain f_{NL} at a level Δf_{NL} ~ 4.7. For the equilateral and orthogonal template we obtain Δf_{NL}^{equ}~17 and Δf_{NL}^{orth}~13 respectively for the combined estimator. This shall be important in the quest towards understanding the mechanism behind the generation of primordial perturbations.

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arXiv2013

Changes in the halo formation rates due to features in the primordial spectrum

Features in the primordial scalar power spectrum provide a possible roadway to describe the outliers at the low multipoles in the WMAP data. Apart from the CMB angular power spectrum, these features can also alter the matter power spectrum and, thereby, the formation of the large scale structure. Carrying out a complete numerical analysis, we investigate the effects of primordial features on the formation rates of the halos. We consider a few different inflationary models that lead to features in the scalar power spectrum and an improved fit to the CMB data, and analyze the corresponding imprints on the formation of halos. Performing a Markov Chain Monte Carlo analysis with the WMAP seven year data and the SDSS halo power spectrum from LRG DR7 for the models of our interest, we arrive at the parameter space of the models allowed by the data. We illustrate that, inflationary potentials, such as the quadratic potential with sinusoidal modulations and the axion monodromy model, which generate certain repeated, oscillatory features in the inflationary perturbation spectrum, do not induce a substantial difference in the number density of halos at their best fit values, when compared with, say, a nearly scale invariant spectrum as is generated by the standard quadratic potential. However, we find that the number density and the formation rates of halos change by about 13-22% for halo masses ranging over 10^4-10^14 solar mass, for potential parameters that lie within 2-sigma around the best fit values arrived at from the aforesaid joint constraints. We briefly discuss the implications of our results.

astro-ph.CO↗

arXiv2012

Oscillations in the inflaton potential: Complete numerical treatment and comparison with the recent and forthcoming CMB datasets

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 a complete 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 on to arc minute 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.

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arXiv2012

Primordial Non-Gaussianity in the Forest: 3D Bispectrum of Ly-alpha Flux Spectra Along Multiple Lines of Sight

We investigate the possibility of constraining primordial non-Gaussianity using the 3D bispectrum of Ly-alpha forest. The strength of the quadratic non-Gaussian correction to an otherwise Gaussian primordial gravitational field is assumed to be dictated by a single parameter fnl. We present the first prediction for bounds on fnl using Ly-alpha flux spectra along multiple lines of sight. The 3D Ly-$α$ transmitted flux field is modeled as a biased tracer of the underlying matter distribution sampled along 1D skewers corresponding to quasars sight lines. The precision to which fnl can be constrained depends on the survey volume, pixel noise and aliasing noise (arising from discrete sampling of the density field). We consider various combinations of these factors to predict bounds on fnl. We find that in an idealized situation of full sky survey and negligible Poisson noise one may constrain fnl ~ 23 in the equilateral limit. Assuming a Ly-alpha survey covering large parts of the sky (k_{min} = 8 * 10^{-4} Mpc^{-1}) and with a quasar density of \bar n = 5 * 10^{-3} Mpc^{-2} it is possible to constrain fnl ~ 100 for equilateral configurations. The possibility of measuring fnl at a precision comparable to LSS studies maybe useful for joint constraining of inflationary scenarios using different data sets.

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arXiv2012

The scalar bi-spectrum during preheating in single field inflationary models

In single field inflationary models, preheating refers to the phase that immediately follows inflation, but precedes the epoch of reheating. During this phase, the inflaton typically oscillates at the bottom of its potential and gradually transfers its energy to radiation. At the same time, the amplitude of the fields coupled to the inflaton may undergo parametric resonance and, as a consequence, explosive particle production can take place. A priori, these phenomena could lead to an amplification of the super-Hubble scale curvature perturbations which, in turn, would modify the standard inflationary predictions. However, remarkably, it has been shown that, although the Mukhanov-Sasaki variable does undergo narrow parametric instability during preheating, the amplitude of the corresponding super-Hubble curvature perturbations remain constant. Therefore, in single field models, metric preheating does not affect the power spectrum of the large scale perturbations. In this article, we investigate the corresponding effect on the scalar bi-spectrum. Using the Maldacena's formalism, we analytically show that, for modes of cosmological interest, the contributions to the scalar bi-spectrum as the curvature perturbations evolve on super-Hubble scales during preheating is completely negligible. Specifically, we illustrate that, certain terms in the third order action governing the curvature perturbations which may naively be expected to contribute significantly are exactly canceled by other contributions to the bi-spectrum. We corroborate selected analytical results by numerical investigations. We conclude with a brief discussion of the results we have obtained.

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arXiv2010

Primordial features due to a step in the inflaton potential

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 $\ell=22$ 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, $r\simeq 0.1$. 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 $\ell=22$ and 40) in all these cases. We point out that, if the tensors prove to be small (say, $r\lesssim 0.01$), 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.

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