arXiv subjects

Sudeep Das

Publications and source records attributed to Sudeep Das.

61 records · Page 4Linked to original sources

arXiv2008

Measuring Distance Ratios with CMB-Galaxy Lensing Cross-correlations

We propose a method for cosmographic measurements by combining gravitational lensing of the cosmic microwave background (CMB) with cosmic shear surveys. We cross-correlate the galaxy counts in the lens plane with two different source planes: the CMB at $z \sim 1100$ and galaxies at an intermediate redshift. The ratio of the galaxy count/CMB lensing cross-correlation to the galaxy count/galaxy lensing cross correlation is shown to be a purely geometric quantity, depending only on the distribution function of the source galaxies. By combining Planck, ADEPT and LSST the ratio can be measured to $\sim 4%$ accuracy, whereas a future polarization based experiment like CMBPOL can make a more precise ($\sim 1%$) measurement. For cosmological models where the curvature and the equation of state parameter are allowed to vary, the direction of degeneracy defined by the measurement of this ratio is different from that traced out by Baryon Acoustic Oscillation (BAO) measurements. Combining this method with the stacked cluster mass reconstruction cosmography technique as proposed by Hu, Holz and Vale (2007), the uncertainty in the ratio can be further reduced, improving the constraints on cosmological parameters. We also study the implications of the lensing-ratio measurement for early dark energy models, in context of the parameterization proposed by Doran and Robbers (2006). For models which are degenerate with respect to the CMB, we find both BAO and lensing-ratio measurements to be insensitive to the early component of the dark energy density.

astro-ph↗

arXiv2008

Efficient Power Spectrum Estimation for High Resolution CMB Maps

Estimation of the angular power spectrum of the Cosmic Microwave Background (CMB) on a small patch of sky is usually plagued by serious spectral leakage, specially when the map has a hard edge. Even on a full sky map, point source masks can alias power from large scales to small scales producing excess variance at high multipoles. We describe a new fast, simple and local method for estimation of power spectra on small patches of the sky that minimizes spectral leakage and reduces the variance of the spectral estimate. For example, when compared with the standard uniform sampling approach on a 8 degree X 8 degree patch of the sky with 2% area masked due to point sources, our estimator halves the errorbars at l=2000 and achieves a more than fourfold reduction in errorbars at l=3500. Thus, a properly analyzed experiment will have errorbars at l=3500 equivalent to those of an experiment analyzed with the now standard technique with ~ 16-25 times the integration time.

astro-ph↗

arXiv2008

Next Generation Redshift Surveys and the Origin of Cosmic Acceleration

Cosmologists are exploring two possible sets of explanations for the remarkable observation of cosmic acceleration: dark energy fills space or general relativity fails on cosmological scales. We define a null test parameter $ε(k,a) \equivΩ_m^{- γ} d \ln D / d \ln a - 1$, where $a$ is the scale factor, $D$ is the growth rate of structure, $Ω_m(a)$ is the matter density parameter, and $γ$ is a simple function of redshift. We show that it can be expressed entirely in terms of the bias factor, $b(a)$, (measured from cross-correlations with CMB lensing) and the amplitude of redshift space distortions, $β(k,a)$. Measurements of the CMB power spectrum determine $Ω_{m 0} H_0^2$. If dark energy within GR is the solution to the cosmic acceleration problem, then the logarithmic growth rate of structure $d \ln D / d \ln a = Ω_m^γ$. Thus, $ε(k,a) =0$ on linear scales to better than 1%. We show that in the class of Modified Gravity models known as $f(R)$, the growth rate has a different dependence on scale and redshift. By combining measurements of the amplitude of $β$ and of the bias, $b$, redshift surveys will be able to determine the logarithmic growth rate as a function of scale and redshift. We estimate the predicted sensitivity of the proposed SDSS III (BOSS) survey and the proposed ADEPT mission and find that they will test structure growth in General Relativity to the percent level.

astro-ph↗

arXiv2008

A Large Sky Simulation of the Gravitational Lensing of the Cosmic Microwave Background

Large scale structure deflects cosmic microwave background (CMB) photons. Since large angular scales in the large scale structure contribute significantly to the gravitational lensing effect, a realistic simulation of CMB lensing requires a sufficiently large sky area. We describe simulations that include these effects, and present both effective and multiple plane ray-tracing versions of the algorithm, which employs spherical harmonic space and does not use the flat sky approximation. We simulate lensed CMB maps with an angular resolution of ~0.9 arcmin. The angular power spectrum of the simulated sky agrees well with analytical predictions. Maps generated in this manner are a useful tool for the analysis and interpretation of upcoming CMB experiments such as PLANCK and ACT.

astro-ph↗

arXiv2007

Probing Light Pseudoscalars with Light: Propagation, Resonance and Spontaneous Polarization

Radiation propagating over cosmological distances can probe light weakly interacting pseudoscalar (or scalar) particles. The existence of a spin-0 field changes the dynamical symmetries of electrodynamics. It predicts spontaneous generation of polarization of electromagnetic waves due to mode mixing in the presence of background magnetic field.We illustrate this by calculations of propagation in a uniform medium, as well as in a slowly varying background medium, and finally with resonant mixing. Highly complicated correlations between different Stokes parameters are predicted depending on the parameter regimes. The polarization of propagating waves shows interesting and complex dependence on frequency, the distance of propagation, coupling constants, and parameters of the background medium such as the plasma density and the magnetic field strength. For the first time we study the resonant mixing of electromagnetic waves with the scalar field, which occurs when the background plasma frequency becomes equal to the mass of the scalar field at some point along the path. Dynamical effects are found to be considerably enhanced in this case. We also formulate the condition under which the adiabatic approximation can be used consistently, and find caveats about comparing different frequency regimes.

hep-ph↗

arXiv2006

Testing a new analytic model for gravitational lensing probabilities

We study gravitational lensing with a multiple lens plane approach, proposing a simple analytical model for the probability distribution function (PDF) of the dark matter convergence, kappa, for the different lens planes in a given cosmology as a function of redshift and smoothing angle, theta. The model is fixed solely by the variance of kappa, which in turn is fixed by the amplitude of the power spectrum, sigma_8. We test the PDF against a high resolution Tree-Particle-Mesh simulation and find that it is far superior to the Gaussian or the lognormal, especially for small values of theta << 1 arcmin and at large values of kappa relevant to strong lensing. With this model, we predict the probabilities of strong lensing by a single plane or by multiple planes. We find that for theta ~ 10 arcsec, a single plane accounts for almost all (~ 98%) of the strong lensing cases for source redshift unity. However, for a more typical source redshift of 4, about 12% of the strong lensing cases will result from the contribution of a secondary clump of matter along the line of sight, introducing a systematic error in the determination of the surface density of clusters, typically overestimating it by about 2-5%. We also find that matter inhomogenieties introduce a dispersion in the value of the angular diameter distance about its cosmological mean. The probable error relative to the mean increases with redshift to a value of about 8% for z ~ 6 and theta ~ 10 arcsec.

astro-ph↗

arXiv2004

The Dynamical Mixing of Light and Pseudoscalar Fields

We solve the general problem of mixing of electromagnetic and scalar or pseudoscalar fields coupled by axion-type interactions ${\cal L}_{int} =g_ϕ ϕε_{μν\a \b}F^{μν}F^{\a \b} $. The problem depends on several dimensionful scales, including the magnitude and direction of background magnetic field, the pseudoscalar mass, plasma frequency, propagation frequency, wave number, and finally the pseudoscalar coupling. We apply the results to the first consistent calculations of the mixing of light propagating in a background magnetic field of varying direction, which shows a great variety of fascinating resonant and polarization effects.

hep-ph↗