Asantha Cooray

Uncorrelated estimates of dark energy evolution

Type Ia supernova data have recently become strong enough to enable, for the first time, constraints on the time variation of the dark energy density and its equation of state. Most analyses, however, are using simple two or three-parameter descriptions of the dark energy evolution, since it is well known that allowing more degrees of freedom introduces serious degeneracies. Here we present a method to produce uncorrelated and nearly model-independent band power estimates of the equation of state of dark energy and its density as a function of redshift. We apply the method to recently compiled supernova data. Our results are consistent with the cosmological constant scenario, in agreement with other analyses that use traditional parametrizations, though we find marginal (2-sigma) evidence for w(z)<-1 at z<0.2. In addition to easy interpretation, uncorrelated, localized band powers allow intuitive and powerful testing of the constancy of either the energy density or equation of state. While we have used relatively coarse redshift binning suitable for the current set of ~150 supernovae, this approach should reach its full potential in the future, when applied to thousands of supernovae found from ground and space, combined with complementary information from other cosmological probes.

Direct detection of the inflationary gravitational-wave background

Inflation generically predicts a stochastic background of gravitational waves over a broad range of frequencies, from those accessible with cosmic microwave background (CMB) measurements, to those accessible directly with gravitational-wave detectors, like NASAs Big-Bang Observer (BBO) or Japans Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), both currently under study. Here we investigate the detectability of the inflationary gravitational-wave background at BBO/DECIGO frequencies. To do so, we survey a range of slow-roll inflationary models consistent with constraints from the CMB and large-scale structure (LSS). We go beyond the usual assumption of power-law power spectra, which may break down given the 16 orders of magnitude in frequency between the CMB and direct detection, and solve instead the inflationary dynamics for four classes of inflaton potentials. Direct detection is possible in a variety of inflationary models, although probably not in any in which the gravitational-wave signal does not appear in the CMB polarization. However, direct detection by BBO/DECIGO can help discriminate between inflationary models that have the same slow-roll parameters at CMB/LSS scales.

What is L⋆? Anatomy of the Galaxy Luminosity Function

Using the empirical relations between the central galaxy luminosity and the halo mass, and between the total galaxy luminosity in a halo and the halo mass, we construct the galaxy luminosity function (LF). To the luminosity of the central galaxy in a halo of a given mass, we assign lognormal scatter with a mean calibrated against the observations. In halos where the total galaxy luminosity exceeds that of the central galaxy, satellite galaxies are distributed as a power law in luminosity. Combined with the halo mass function, this description reproduces the observed characteristics of the galaxy LF, including a shape consistent with the Schechter function. The L in the LF is the luminosity above which the central galaxy luminosity-halo mass relation begins to flatten in halos above ~1013 M☉. In surveys in which central galaxies in massive clusters are neglected, either by design or because of the cosmic variance, L is simply the mean luminosity of central galaxies in halos at the upper end of the selected mass range. The smooth, exponential decay of the Schechter function toward high luminosities reflects the intrinsic scatter in the central galaxy luminosity-halo mass relation. In addition to the LF, the model successfully reproduces the observed dependence of galaxy clustering bias on luminosity.

First sources in infrared light: stars, supernovae and miniquasars

The cosmic infrared background (IRB) at wavelengths between 1 and 3 μm provides a useful probe of early star formation prior to and during reionization. To explain the high optical depth to electron scattering, as measured by the Wilkinson Microwave Anisotropy Probe (WMAP), one requires significant star formation activity at redshifts of 10 and higher. In addition to massive stars, the IRB flux may be contributed by a population of early miniquasars. We study the relative contributions from first stars, supernovae and quasars to the IRB for reasonable star formation rates at high redshift. If miniquasars radiate efficiently at the Eddington limit, current background measurements limit the fraction of mass in first stars that is converted to seed black holes to be roughly less than 10 per cent. In the case of supernovae, though an individual supernova is much brighter than the progenitor star, due to the shorter lifetime (of the order of a few months), the fractional contribution to the IRB remains at a level of 10 per cent and below when compared to the same contribution from stars. The bright supernovae may, however, be directly detectable by future large ground-based and space telescopes.

Dissipationless Merging and the Assembly of Central Galaxies

We reanalyze the galaxy-mass correlation function measured by the Sloan Digital Sky Survey to obtain host dark matter halo masses at galaxy and galaxy-group scales. We extend the data to galaxy clusters in the 2MASS catalog and study the relation between central galaxy luminosity and halo mass. While the central galaxy luminosity scales as ~M0.7 to M0.8 at low masses, the relation flattens to shallower than ~M0.3 above ~4 × 1013 M☉. The total luminosity of galaxies in the halo, however, continues to grow as a power law ~M0.8-M0.9. Starting from the hypothesis that the central galaxies grow by hierarchical merging (galactic cannibalism), we develop a simple model for the evolution of their luminosities as a consequence of the accretion of satellite galaxies, tracking the merging of dark matter halos. The luminosity-mass relation flattens when the timescale on which dynamical friction induces orbital decay in the satellite galaxies exceeds the age of the dark matter halo. Then the growth of the central galaxy is suppressed, as it can cannibalize only the rare, massive satellite galaxies. The model takes the dependence of the total luminosity of galaxies in a halo on its mass and the global galaxy luminosity function as input and reproduces the observed central galaxy luminosity-mass relation over 3 decades in halo mass, 1012-1015 M☉. The success of the model suggests that gas cooling and subsequent star formation did not play an important role in the final assembly of central galaxies from sub-L precursors.

Cosmic shear of the microwave background : The curl diagnostic

Weak-lensing distortions of the cosmic-microwave-background (CMB) temperature and polarization patterns can reveal important clues to the intervening large-scale structure. The effect of lensing is to deflect the primary temperature and polarization signal to slightly different locations on the sky. Deflections due to density fluctuations, gradient-type for the gradient of the projected gravitational potential, give a direct measure of the mass distribution. Curl-type deflections can be induced by, for example, a primordial background of gravitational waves from inflation or by second-order effects related to lensing by density perturbations. Whereas gradient-type deflections are expected to dominate, we show that curl-type deflections can provide a useful test of systematics and serve to indicate the presence of confusing secondary and foreground non-Gaussian signals.

Cosmic 21 cm Delensing of Microwave Background Polarization and the Minimum Detectable Energy Scale of Inflation

We propose a new method for removing gravitational lensing from maps of cosmic microwave background (CMB) polarization anisotropies. Using observations of anisotropies or structures in the cosmic 21 cm radiation, emitted or absorbed by neutral hydrogen atoms at redshifts 10 to 200, the CMB can be delensed. We find this method could allow CMB experiments to have increased sensitivity to a background of inflationary gravitational waves (IGWs) compared to methods relying on the CMB alone and may constrain models of inflation which were heretofore considered to have undetectable IGW amplitudes.

Cross-correlation studies between CMB temperature anisotropies and 21 cm fluctuations

During the transition from a neutral to a fully reionized universe, scattering of cosmic microwave background (CMB) photons via free electrons leads to a new anisotropy contribution to the temperature distribution. If the reionization process is inhomogeneous and patchy, the era of reionization is also visible via brightness temperature fluctuations in the redshifted 21 cm line emission from neutral hydrogen. Since regions containing electrons and neutral hydrogen are expected to trace the same underlying density field, the two are (anti)correlated and this is expected to be reflected in the anisotropy maps via a correlation between arcminute-scale CMB temperature and the 21 cm background. In terms of the angular cross-power spectrum, unfortunately, this correlation is insignificant due to a geometric cancellation associated with second-order CMB anisotropies. The same cross correlation between ionized and neutral regions, however, can be studied using a bispectrum involving large-scale velocity field of ionized regions from the Doppler effect, arcminute-scale CMB anisotropies during reionization, and the 21 cm background. While the geometric cancellation is partly avoided, the signal-to-noise ratio related to this bispectrum is reduced due to the large cosmic variance related to velocity fluctuations traced by the Doppler effect. Unless the velocity field during reionization can be independently established, it is unlikely that the correlation information related to the relative distribution of ionized electrons and regions containing neutral hydrogen can be obtained with a combined study involving CMB and 21 cm fluctuations.

Large‐scale non‐Gaussianities in the 21‐cm background anisotropies from the era of reionization

The brightness temperature fluctuations in the 21-cm background related to the neutral hydrogen distribution provide a probe of the physics related to the era of reionization, when the intergalactic medium changed from being completely neutral to partially ionized. We formulate statistics of 21-cm brightness temperature anisotropies in terms of the angular power spectrum, the bispectrum, and the trispectrum. Using the trispectrum, we estimate the covariance related to the power spectrum measurements and show that correlations resulting from non-Gaussianities are below a per cent, at most. While all-sky observations of the 21-cm background at arcminute-scale resolution can be used to measure the bispectrum with a cumulative signal-to-noise ratio of the order of a few tens, in the presence of foregrounds and instrumental noise related to first-generation interferometers, the measurement is unlikely to be feasible. For most purposes, non-Gaussianities in 21-cm fluctuations can be ignored and the distribution can be described with Gaussian statistics. Because 21-cm fluctuations are significantly contaminated by foregrounds, such as galactic synchrotron or low-frequency radio point sources, the lack of significant non-Gaussianity in the signal suggests that any significant detection of non-Gaussianity could be the result of foregrounds. Similarly, in addition to the frequency information that is now proposed to separate 21-cm fluctuations from foregrounds, if the non-Gaussian structure of foregrounds is known a priori, this additional information could potentially be used to reduce the confusion further.

Gravitational‐wave background of neutron star–white dwarf binaries

We discuss the stochastic background of gravitational waves from ultra-compact neutron star-white dwarf (NS-WD) binaries at cosmological distances. Under the assumption that accreting neutron stars and donor white dwarf stars form most of the low-mass X-ray binaries (LMXBs), our calculation makes use of recent results related to the luminosity function determined from X-ray observations. Even after accounting for detached NS-WD binaries not captured in X-ray data, the NS-WD background is at least an order of magnitude below that due to extragalactic white dwarf-white dwarf binaries and below the detectability level of the Laser Interferometer Space Antenna (LISA) at frequencies between 10 -5 and 10 -1 Hz. While the extragalactic background is unlikely to be detected, we suggest that around one to 10 galactic NS-WD binaries may be resolved with LISA such that their positions are determined to an accuracy of several degrees on the sky.