arXiv subjects

Asantha Cooray

Publications and source records attributed to Asantha Cooray.

At least 307 records · Page 17Linked to original sources

arXiv2003

CMB-induced Cluster Polarization as a Cosmological Probe

Scattering of the temperature anisotropy quadrupole by free electrons in galaxy clusters leads to a secondary polarization signal in the cosmic microwave background (CMB) fluctuations. At low redshifts, the temperature quadrupole contains a significant contribution from the integrated Sachs-Wolfe (ISW) effect associated with the growth of density fluctuations. Using polarization data from a sample of clusters over a wide range in redshift, one can statistically establish the presence of the ISW effect and determine its redshift evolution. Given the strong dependence of the ISW effect on the background cosmology, cluster polarization can eventually be used as a powerful probe of dark energy. As a further application, we also discuss how it might be used to understand the potential lack of power on large scales.

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arXiv2003

Kuiper Belt Object Sizes and Distances from Occultation Observations

There are several observational campaigns under way to detect kilometer size foreground Kuiper Belt Objects (KBOs) through their occultation of background stars. The interpretation of an occultation light curve, unfortunately, is affected by a geometric degeneracy such that one is unable to determine the KBO size independent of its distance. This degeneracy can be broken through a precise measurement of the relative velocity obtained from simultaneous observations of individual events. While an array of telescopes spread over an area of few square kilometers can be employed, it is unlikely that the relative velocity can be measured to the required accuracy to help break this geometric degeneracy. The presence of diffraction fringes in KBO occultation light curves, when projected sizes of occulted stars are smaller than the Fresnel scale, improves determination of size and distance significantly. In this regard, we discuss the potential role of a dedicated satellite mission for KBO occultation observations. If dwarf stars at the V-band magnitudes of 14th and fainter can be monitored at time intervals of 0.1 seconds with normalized flux errors at the level of 1%, the occultation observations will allow individual KBO sizes and distances to be determined at the level of a few percent or better.

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arXiv2003

Growth Rate of Large Scale Structure as a Powerful Probe of Dark Energy

The redshift evolution of the growth rate of the gravitational potential, d(D/a)/dz, is an excellent discriminator of dark energy parameters and, in principle, more powerful than standard classical tests of cosmology. This evolution is directly observable through the integrated Sachs-Wolfe effect in cosmic microwave background (CMB) anisotropies. We consider the prospects of measuring the growth rate via a novel method employed through measurements of CMB polarization towards galaxy clusters. The potentially achievable errors on dark energy parameters are comparable and fully complementary to those expected from other upcoming tests of dark energy, making this test a highly promising tool of precision cosmology.

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arXiv2003

Sunyaev-Zeldovich Fluctuations from the First Stars?

WMAP's detection of high electron-scattering optical depth suggests substantial star formation at high redshift z~17 +/- 5. On the other hand, the recovered sigma_8 ~ 0.84 +/-0.04 disfavors a cluster Sunyaev-Zeldovich (SZ) origin for the observed small-scale-CMB fluctuation excess, which generally requires sigma_8 ~ 1.1. Here we consider the effects of high-redshift star formation on the CMB. We derive a fairly model-independent relation between the electron scattering optical depth and the number of ionizing photons emitted per baryon, and use this to calibrate the amount of high-redshift supernova activity. The resulting supernova remnants Compton cool against the CMB creating a Compton-y distortion y ~ few x 10^-6, within observational bounds. However they also create small-scale SZ fluctuations, which could be comparable with SZ fluctuations from unresolved galaxy clusters. This raises the exciting possibility that we have already detected signatures of the first stars not just once, but twice, in the CMB.

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arXiv2003

Lensing Reconstruction with CMB Temperature and Polarization

Weak gravitational lensing by intervening large-scale structure induces a distinct signature in the cosmic microwave background (CMB) that can be used to reconstruct the weak-lensing displacement map. Estimators for individual Fourier modes of this map can be combined to produce an estimator for the lensing-potenial power spectrum. The naive estimator for this quantity will be biased upwards by the uncertainty associated with reconstructing individual modes; we present an iterative scheme for removing this bias. The variance and covariance of the lensing-potenial power spectrum estimator are calculated and evaluated numerically in a $Λ$CDM universe for Planck and future polarization-sensitive CMB experiments.

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arXiv2003

Occultation Searches for Kuiper Belt Objects

The occultation of background stellar sources by foreground Kuiper Belt Objects (KBOs) can be used to survey physical properties of the KBO population. We discuss statistics related to a KBO occultation survey, such as the event duration distribution, and suggest that occultation searches can be effectively used to probe the KBO size distribution below 10 km. In particular, we suggest that occultation surveys may be best suited to search for a turnover radius in the KBO size distribution due to collisions between small-size objects. For occultation surveys that monitor stellar sources near the ecliptic over a few square degrees, with time sampling intervals of order 0.1 sec and sensitivity to flux variations of a few percent or more, a turnover radius between 0.1 and 1.0 km can be probed. While occultation surveys will probe the low-radius limit and imaging surveys will detect KBOs of size 100 km or more, statistics of objects with sizes in the intermediate range of around 1 km to 100 km will likely remain unattainable.

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arXiv2002

After MAP: Next Generation CMB

We discuss several opportunities involving cosmic microwave background (CMB) observations during the post-MAP era. The curl-modes of CMB polarization allow a direct detection of inflationary gravitational waves and a measurement of the energy scale of inflation. While a significant source of confusion is expected from cosmic shear conversion of polarization related to density perturbations, higher resolution observations of CMB anisotropies can be used for a lensing reconstruction and to separate gravitational-wave polarization signature from that of lensing. With perfect all-sky maps, separations based on current lensing reconstruction techniques allow the possibility to probe inflationary energy scales down to 10^15 GeV, well below that of grand unified theories. Another aspect of future CMB studies will be related to large scale structure, such as wide-field imaging of Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. Here, we comment on a potentially interesting and unique application of the SZ effect involving a measurement of the absolute temperature of CMB at high redshifts. Finally, we address cluster polarization signals and the possibility to directly measure some aspect of the temperature quadrupole associated with time evolving potentials.

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arXiv2002

CMB Polarization towards Clusters as a Probe of the Integrated Sachs-Wolfe Effect

The scattering of temperature anisotropy quadrupole by free electrons in galaxy clusters leads to a now well-known polarization signal in the cosmic microwave background (CMB) fluctuations. Using multi-frequency polarization data, one can extract the temperature quadrupole and separate it from the contaminant polarization associated with the kinematic quadrupole due to transverse motion of clusters. At low redshifts, the temperature quadrupole contains a significant contribution from the integrated Sachs-Wolfe effect (ISW) associated with the growth of density fluctuations. Using polarization data from a sample of clusters over a wide range in redshift, one can statistically establish the presence of the ISW effect and determine the redshift dependence of the ISW contribution to the rms quadrupole. Given the strong dependence of the ISW effect on the background cosmology, the cluster polarization can eventually be used as a probe of the dark energy.

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arXiv2002

Kuiper Belt and Oort Cloud Objects: Microlenses or Stellar Occulters?

The occultation of background stars by foreground Solar system objects, such as planets and asteroids, has been widely used as an observational probe to study physical properties associated with the foreground sample. Similarly, the gravitational microlensing of background stellar sources by foreground mass concentrations has also been widely used to understand the foreground mass distribution. Though distinct, these two possibilities present two extreme cases during a transit; At the edge of the Solar system and beyond, the Kuiper belt and Oort cloud populations may present interesting foreground samples where combinations of occultation and lensing, and possibly both during the same transit, can be observed. To detect these events, wide-field monitoring campaigns with time sampling intervals of order tenths of seconds are required. For certain planetary occultation light curves, such as those involving Pluto, an accounting of the gravitational lensing effect may be necessary when deriving precise physical properties of the atmosphere through the associated refraction signal.

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arXiv2002

The Far-Infrared Background Correlation with CMB Lensing

The intervening large--scale structure distorts cosmic microwave background (CMB) anisotropies via gravitational lensing. The same large--scale structure, traced by dusty star--forming galaxies, also induces anisotropies in the far--infrared background (FIRB). We investigate the resulting inter--dependence of the FIRB and CMB with a halo model for the FIRB. In particular, we calculate the cross--correlation between the lensing potential and the FIRB. The lensing potential can be quadratically estimated from CMB temperature and/or polarization maps. We show that the cross--correlation can be measured with high signal--to--noise with data from the Planck Surveyor. We discuss how such a measurement can be used to understand the nature of FIRB sources and their relation to the distribution of dark matter.

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arXiv2002

Small-Scale Cosmic Microwave Background Polarization from Reionization

We discuss fluctuations in the cosmic microwave background (CMB) polarization due to scattering from reionized gas at low redshifts. Polarization is produced by re-scattering of the primordial temperature anisotropy quadrupole and of the kinematic quadrupole that arises from gas motion transverse to the line of sight. We show that both effects produce equal E- and B-parity polarization, and are, in general, several orders of magnitude below the dominant polarization contributions at the last scattering surface to E-modes or the gravitational-lensing contribution to B-modes at intermediate redshifts. These effects are also several orders of magnitude below the B polarization due to lensing even after subtraction with higher-order correlations, and are thus too small to constitute a background for searches for the polarization signature of inflationary gravitational waves.

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arXiv2002

Weak Lensing of the CMB: Cumulants of the Probability Distribution Function

We discuss the real-space moments of temperature anisotropies in the cosmic microwave background (CMB) due to weak gravitational lensing by intervening large-scale structure. We show that if the probability distribution function of primordial temperature anisotropies is Gaussian, then it remains unchanged after gravitational lensing. With finite resolution, however, non-zero higher-order cumulants are generated both by lensing autocorrelations and by cross-correlations between the lensing potential and secondary anisotropies in the CMB such as the Sunayev-Zel'dovich (SZ) effect. Skewness is produced by these lensing-SZ correlations, while kurtosis receives contributions from both lensing alone and lensing-SZ correlations. We show that if the projected lensing potential is Gaussian, all cumulants of higher-order than the kurtosis vanish. While recent results raise the possibility of detection of the skewness in upcoming data, the kurtosis will likely remain undetected.

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arXiv2002

Small Angular Scale CMB Anisotropies from CBI and BIMA: Early Universe or Local structures ?

The advent of high resolution cosmic microwave background (CMB) experiments now allows studies on the temperature fluctuations at scales corresponding to few arcminutes and below. Though the reported excess power at $\ell \sim 2000 - 6000$ by CBI and BIMA is roughly consistent with a secondary contribution resulting from the Sunyaev-Zeldovich effect, this requires a higher normalization for the matter power spectrum than measured by other means. In addition to a local red-shift contribution, another strong possibility for anisotropies at very small scales involve non-standard aspects of inflationary models. To distinguish between contributions from early universe and local structures, including a potential point source contribution, and to understand the extent to which structures at low red-shifts contribute to small scale temperature anisotropies, it maybe necessary to perform a combined study involving CMB and the large scale structure. We suggest a cross-correlation of the temperature data with a map of the large scale structure, such as the galaxy distribution. For next generation small angular scale CMB experiments, multi-frequency observations may be a necessary aspect to allow an additional possibility to distinguish between these different scenarios.

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arXiv2002

Halo Models of Large Scale Structure

We review the formalism and applications of the halo-based description of nonlinear gravitational clustering. In this approach, all mass is associated with virialized dark matter halos; models of the number and spatial distribution of the halos, and the distribution of dark matter within each halo, are used to provide estimates of how the statistical properties of large scale density and velocity fields evolve as a result of nonlinear gravitational clustering. We first describe the model, and demonstrate its accuracy by comparing its predictions with exact results from numerical simulations of nonlinear gravitational clustering. We then present several astrophysical applications of the halo model: these include models of the spatial distribution of galaxies, the nonlinear velocity, momentum and pressure fields, descriptions of weak gravitational lensing, and estimates of secondary contributions to temperature fluctuations in the cosmic microwave background.

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arXiv2002

Second Moment of Halo Occupation Number

The halo approach to large scale structure provides a physically motivated model to understand clustering properties of galaxies. An important aspect of the halo model involves a description on how galaxies populate dark matter halos or what is now called the halo occupation distribution. We discuss a way in which clustering information, especially in the non-linear regime, can be used to determine moments of this halo occupation number. We invert the non-linear part of the real space power spectrum from the PSCz survey to determine the second moment of the halo occupation distribution in a model independent manner. The precise measurement of higher order correlations can eventually be used to determine successive higher order moments of this distribution.

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arXiv2002

Weak Lensing Probes of the Galaxy-Mass Cross Power Spectrum

The angular correlation function of the background shear-foreground galaxy distribution probes the three dimensional cross power spectrum between mass and galaxies. The same cross power spectrum is also probed when foreground galaxy distribution is cross-correlated with a distribution of background sources disjoint in redshift space. The kernels that project three dimensional clustering to the two dimensional angular space is different for these two probes. When combined, they allow a study of the galaxy-mass cross power spectrum from linear to non-linear scales. By inverting the background shear-foreground galaxy correlation function measured by the Sloan Digital Sky Survey, we present a first estimate of the cross power spectrum between mass and galaxies at low redshifts.

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arXiv2002

A Lensing Reconstruction of Primordial Cosmic Microwave Background Polarization

We discuss a possibility to directly reconstruct the CMB polarization field at the last scattering surface by accounting for modifications imposed by the gravitational lensing effect. The suggested method requires a tracer field of the large scale structure lensing potentials that deflected propagating CMB photons from the last scattering surface. This required information can come from a variety of observations on the large scale structure matter distribution, including convergence reconstructed from lensing shear studies involving galaxy shapes. In the case of so-called curl, or B,-modes of CMB polarization, the reconstruction allows one to identify the distinct signature of inflationary gravitational waves.

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arXiv2002

Is the Cosmic Microwave Background Circularly Polarized?

The primordial anisotropies of the cosmic microwave background (CMB) are linearly polarized via Compton-scattering. The Faraday conversion process during the propagation of polarized CMB photons through regions of the large-scale structure containing magnetized relativistic plasma, such as galaxy clusters, will lead to a circularly polarized contribution. Though the resulting Stokes-V parameter is of order 10^-9 at frequencies of 10 GHz, the contribution can potentially reach the total Stokes-U at low frequencies due to the cubic dependence on the wavelength. In future, the detection of circular polarization of CMB can be used as a potential probe of the physical properties associated with relativistic particle populations in large-scale structures.

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