Department of Physics

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Primordial Non-Gaussianity in the Forest: 3D Bispectrum of Lyman-α Flux Spectra along Multiple Lines of Sight
(APS, 2012-09) Sarkar, Tapomoy Guha
We investigate the possibility of constraining primordial non-Gaussianity using the 3D bispectrum of the Lyman (Ly)-α 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-α 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 quasar 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-α survey covering large parts of the sky (kmin=8×10−4 Mpc−1) and with a quasar density of ¯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 large scale structure studies maybe useful for joint constraining of inflationary scenarios using different data sets.
Probing primordial non-Gaussianity: the 3D Bispectrum of Ly-α forest and the redshifted 21-cm signal from the post reionization epoch
(IOP, 2013) Sarkar, Tapomoy Guha
We explore possibility of using the three dimensional bispectra of the Ly-α 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-α 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 fNL and the other bias parameters using the three dimensional 21-cm bispectrum and other cross bispectra. The bounds on fNL depend on the survey volume, and the various observational noises. We have considered a BOSS like Ly-α survey where the average number density of quasars bar n = 10−3Mpc−2 and the spectra are measured at a 2-σ 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 fNL obtained in our analysis (6 ⩽ ΔfNL ⩽ 65) is competitive with CMBR and galaxy surveys and may prove to be an important alternative approach towards constraining primordial physics using future data sets. Further, we have presented a hierarchy of power of the bispectrum-estimators towards detecting the fNL. Given the quality of the data sets, one may use this method to optimally choose the right estimator and thereby provide better constraints on fNL. We also find that by combining the various cross-bispectrum estimators it is possible to constrain fNL at a level ΔfNL ∼ 4.7. For the equilateral and orthogonal template we obtain ΔfNLequ ∼ 17 and ΔfNLorth ∼ 13 respectively for the combined estimator. This shall be important in the quest towards understanding the mechanism behind the generation of primordial perturbations.