arXiv subjects

Wayne Hu

Publications and source records attributed to Wayne Hu.

At least 199 records · Page 11Linked to original sources

arXiv2002

Benchmark Parameters for CMB Polarization Experiments

The recently detected polarization of the cosmic microwave background (CMB) holds the potential for revealing the physics of inflation and gravitationally mapping the large-scale structure of the universe, if so called B-mode signals below 10^{-7}, or tenths of a uK, can be reliably detected. We provide a language for describing systematic effects which distort the observed CMB temperature and polarization fields and so contaminate the B-modes. We identify 7 types of effects, described by 11 distortion fields, and show their association with known instrumental systematics such as common mode and differential gain fluctuations, line cross-coupling, pointing errors, and differential polarized beam effects. Because of aliasing from the small-scale structure in the CMB, even uncorrelated fluctuations in these effects can affect the large-scale B modes relevant to gravitational waves. Many of these problems are greatly reduced by having an instrumental beam that resolves the primary anisotropies (FWHM << 10'). To reach the ultimate goal of an inflationary energy scale of 3 \times 10^{15} GeV, polarization distortion fluctuations must be controlled at the 10^{-2}-10^{-3} level and temperature leakage to the 10^{-4}-10^{-3} level depending on effect. For example pointing errors must be controlled to 1.5'' rms for arcminute scale beams or a percent of the Gaussian beam width for larger beams; low spatial frequency differential gain fluctuations or line cross-coupling must be eliminated at the level of 10^{-4} rms.

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arXiv2002

Three-Dimensional Mapping of the Dark Matter

We study the prospects for three-dimensional mapping of the dark matter to high redshift through the shearing of faint galaxies images at multiple distances by gravitational lensing. Such maps could provide invaluable information on the nature of the dark energy and dark matter. While in principle well-posed, mapping by direct inversion introduces exceedingly large, but usefully correlated noise into the reconstruction. By carefully propagating the noise covariance, we show that lensing contains substantial information, both direct and statistical, on the large-scale radial evolution of the density field. This information can be efficiently distilled into low-order signal-to-noise eigenmodes which may be used to compress the data by over an order of magnitude. Such compression will be useful for the statistical analysis of future large data sets. The reconstructed map also contains useful information on the localization of individual massive dark matter halos, and hence the dark energy from halo number counts, but its extraction depends strongly on prior assumptions. We outline a procedure for maximum entropy and point-source regularization of the maps that can identify alternate reconstructions.

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arXiv2002

Mass Reconstruction with CMB Polarization

Weak gravitational lensing by the intervening large-scale structure of the Universe induces high-order correlations in the cosmic microwave background (CMB) temperature and polarization fields. We construct minimum variance estimators of the intervening mass distribution out of the six quadratic combinations of the temperature and polarization fields. Polarization begins to assist in the reconstruction when E-mode mapping becomes possible on degree-scale fields, i.e. for an experiment with a noise level of ~40 uK-arcmin and beam of ~7', similar to the Planck experiment; surpasses the temperature reconstruction at ~26 uK-arcmin and 4'; yet continues to improve the reconstruction until the lensing B-modes are mapped to l ~ 2000 at ~0.3 uK-arcmin and 3'. Ultimately, the correlation between the E and B modes can provide a high signal-to-noise mass map out to multipoles of L ~ 1000, extending the range of temperature-based estimators by nearly an order of magnitude. We outline four applications of mass reconstruction: measurement of the linear power spectrum in projection to the cosmic variance limit out to L ~ 1000 (or wavenumbers 0.002 < k < 0.2 in h/Mpc), cross-correlation with cosmic shear surveys to probe the evolution of structure tomographically, cross-correlation of the mass and temperature maps to probe the dark energy, and the separation of lensing and gravitational wave B-modes.

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arXiv2002

Sample Variance Considerations for Cluster Surveys

We present a general statistical framework for describing the effect of sample variance in the number counts of virialized objects and examine its effect on cosmological parameter estimation. Specifically, we consider effects of sample variance on the power spectrum normalization and properties of dark energy extracted from current and future local and high-redshift samples of clusters. We show that for future surveys that probe ever lower cluster masses and temperatures, sample variance is generally comparable to or greater than shot noise and thus cannot be neglected in deriving precision cosmological constraints. For example, sample variance is usually more important than shot variance in constraints on the equation of state of the dark energy from z < 1 clusters. Although we found that effects of sample variance on the sigma_8-Omega_m constraints from the current flux and temperature limited X-ray surveys are not significant, they may be important for future studies utilizing the shape of the temperature function to break the sigma_8-Omega_m degeneracy. We also present numerical tests clarifying the definition of cluster mass employed in cosmological modelling and accurate fitting formula for the conversion between different definitions of halo mass (e.g., virial vs. fixed overdensity).

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arXiv2002

Second Order Corrections to Weak Lensing by Large-Scale Structure

We calculate corrections to the power spectrum predictions of weak lensing by large scale structure due to higher order effects in the gravitational potential. Using a perturbative approach to third order in transverse displacements, we calculate a second order correction to the angular power spectra of E and B mode shear and convergence resulting from dropping the so-called Born approximation, where one integrates along the unperturbed photon path. We also consider a correction to the power spectra from the coupling between lenses at different redshifts. Both effects generate B-mode shear and the latter also causes a net rotation of the background galaxy images. We show all these corrections are at least two orders of magnitude below the convergence or E-mode power and hence relevant only to future ultra high precision measurements. These analytical calculations are consistent with previous numerical estimates and validate the use of current large scale structure weak lensing predictions for cosmological studies and future use of B-modes as a monitor of systematic effects.

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arXiv2001

Cosmic Microwave Background Anisotropies

Cosmic microwave background (CMB) temperature anisotropies have and will continue to revolutionize our understanding of cosmology. The recent discovery of the previously predicted acoustic peaks in the power spectrum has established a working cosmological model: a critical density universe consisting of mainly dark matter and dark energy, which formed its structure through gravitational instability from quantum fluctuations during an inflationary epoch. Future observations should test this model and measure its key cosmological parameters with unprecedented precision. The phenomenology and cosmological implications of the acoustic peaks are developed in detail. Beyond the peaks, the yet to be detected secondary anisotropies and polarization present opportunities to study the physics of inflation and the dark energy. The analysis techniques devised to extract cosmological information from voluminous CMB data sets are outlined, given their increasing importance in experimental cosmology as a whole.

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arXiv2001

Measuring Angular Diameter Distances through Halo Clustering

Current and upcoming wide-field surveys for weak gravitational lensing and the Sunyaev-Zel'dovich effect will generate mass-selected catalogues of dark matter halos with internal or followup photometric redshift information. The angular power spectrum of these halos provides additional information that complements the previously-studied number counts. In particular, using the shape of the linear power spectrum as a standard ruler that is calibrated by cosmic microwave background measurements, we find that a survey of 4000 sqr. deg. and a mass threshold of 10^14 M_sun can be used to determine the comoving angular diameter distance as a function of redshift. In principle, this test also allows an absolute calibration of the distance scale and measurement of the Hubble constant. This test is largely insensitive to the details of halo mass measurements, mass function, and halo bias. Determination of these quantities would further allow a measurement of the linear growth rate of fluctuations.

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arXiv2001

Dark Synergy: Gravitational Lensing and the CMB

Power spectra and cross-correlation measurements from the weak gravitational lensing of the cosmic microwave background (CMB) and the cosmic shearing of faint galaxies images will help shed light on quantities hidden from the CMB temperature anisotropies: the dark energy, the end of the dark ages, and the inflationary gravitational wave amplitude. Even with modest surveys, both types of lensing power spectra break CMB degeneracies and they can ultimately improve constraints on the dark energy equation of state w by over an order of magnitude. In its cross correlation with the integrated Sachs-Wolfe effect, CMB lensing offers a unique opportunity for a more direct detection of the dark energy and enables study of its clustering properties. By obtaining source redshifts and cross-correlations with CMB lensing, cosmic shear surveys provide tomographic handles on the evolution of clustering correspondingly better precision on the dark energy equation of state and density. Both can indirectly provide detections of the reionization optical depth and modest improvements in gravitational wave constraints which we compare to more direct constraints. Conversely, polarization B-mode contamination from CMB lensing, like any other residual foreground, darkens the prospects for ultra-high precision on gravitational waves through CMB polarization requiring large areas of sky for statistical subtraction. To evaluate these effects we provide fitting formula for the evolution and transfer function of the Newtonian gravitational potential.

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arXiv2001

Mapping the Dark Matter through the CMB Damping Tail

The lensing of CMB photons by intervening large-scale structure leaves a characteristic imprint on its arcminute-scale anisotropy that can be used to map the dark matter distribution in projection on degree scales or ~100 Mpc/h comoving. We introduce a new algorithm for mass reconstruction which optimally utilizes information from the weak lensing of CMB anisotropies in the damping tail. It can ultimately map individual degree scale mass structures with high signal-to-noise. To achieve this limit an experiment must produce a high signal-to-noise, foreground-free CMB map of arcminute scale resolution, specifically with a FWHM beam of < 5' and a noise level of < 15 (10^-6-arcmin) or 41 (uK-arcmin).

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arXiv2000

Power Spectrum Covariance of Weak Gravitational Lensing

Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters which govern them. At low redshifts, the non-linear gravitational evolution of large scale structure produces a non-Gaussian covariance in the shear power spectrum measurements that affects their translation into cosmological parameters. Using the dark matter halo approach, we study the covariance of binned band power spectrum estimates and the four point function of the dark matter density field that underlies it. We compare this semi-analytic estimate to results from N-body numerical simulations and find good agreement. We find that for a survey out to z ~ 1, the power spectrum covariance increases the errors on cosmological parameters determined under the Gaussian assumption by about 15%.

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arXiv2000

CMB Observables and Their Cosmological Implications

We show that recent measurements of the power spectrum of cosmic microwave background anisotropies by BOOMERanG and MAXIMA can be characterized by four observables, the position of the first acoustic peak l_1= 206 pm 6, the height of the first peak relative to COBE normalization H_1= 7.6 pm 1.4, the height of the second peak relative to the first H_2 = 0.38 pm 0.04, and the height of the third peak relative to the first H_3 = 0.43 pm 0.07. This phenomenological representation of the measurements complements more detailed likelihood analyses in multidimensional parameter space, clarifying the dependence on prior assumptions and the specific aspects of the data leading to the constraints. We illustrate their use in the flat LCDM family of models, where we find Omega_m h^{3.8} > 0.079 (or nearly equivalently, the age of the universe t_0 < 13-14 Gyr) from l_1 and a baryon density Omega_b h^2 > 0.019, a matter density Omega_m h^2 < 0.42 and tilt n>0.85 from the peak heights (95% CL). With the aid of several external constraints, notably nucleosynthesis, the age of the universe and the cluster abundance and baryon fraction, we construct the allowed region in the (Omega_m,h) plane; it points to high h (0.6< h < 0.9) and moderate Omega_m (0.25 < Omega_m < 0.6).

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arXiv2000

Power Spectra Estimation for Weak Lensing

We develop a method for estimating the shear power spectra from weak lensing observations and test it on simulated data. Our method describes the shear field in terms of angular power spectra and cross correlation of the two shear modes which differ under parity transformations. Two of the three power spectra can be used to monitor unknown sources of noise in the data. The power spectra are decomposed in a model independent manner in terms of ``band-powers'' which are then extracted from the data using a quadratic estimator to find the maximum of the likelihood and its local curvature (for error estimates). We test the method against simulated data from Gaussian realizations and cosmological N-body simulations. In the Gaussian case, the mean bandpowers and their covariance are well recovered even for irregular (or sparsely) sampled fields. The mild non-Gaussianity of the N-body realizations causes a slight underestimation of the errors that becomes negligible for scales much larger than several arcminutes and does not bias the recovered band powers.

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arXiv2000

Gravitational Time Delay Effects on CMB Anisotropies

We study the effect of gravitational time delay on the power spectra and bispectra of the cosmic microwave background (CMB) temperature and polarization anisotropies. The time delay effect modulates the spatial surface at recombination on which temperature anisotropies are observed, typically by ~1 Mpc. While this is a relatively large shift, its observable effects in the temperature and polarization fields are suppressed by geometric considerations. The leading order effect is from its correlation with the closely related gravitational lensing effect. The change to the temperature-polarization cross power spectrum is of order 0.1% and is hence comparable to the cosmic variance for the power in the multipoles around l~1000. While unlikely to be extracted from the data in its own right, its omission in modeling would produce a systematic error comparable to this limiting statistical error and, in principle, is relevant for future high precision experiments. Contributions to the bispectra result mainly from correlations with the Sachs-Wolfe effect and may safely be neglected in a low density universe.

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arXiv2000

Delayed Recombination

Under the standard model for recombination of the primeval plasma, and the cold dark matter model for structure formation, recent measurements of the first peak in the angular power spectrum of the cosmic microwave background temperature indicate the spatial geometry of the universe is nearly flat. If sources of Lya resonance radiation, such as stars or active galactic nuclei, were present at z ~ 1000 they would delay recombination, shifting the first peak to larger angular scales, and producing a positive bias in this measure of space curvature. It can be distinguished from space curvature by its suppression of the secondary peaks in the spectrum.

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arXiv2000

Weak Gravitational Lensing Bispectrum

Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters which govern them. The non-linear evolution of large scale structure produces a non-Gaussian signal which is potentially observable in galaxy shear data. We study the three-point statistics of the convergence, specifically the bispectrum, using the dark matter halo approach which describes the density field in terms of correlations between and within dark matter halos. Our approach allows us to study the effect of the mass distribution in observed fields, in particular the bias induced by the lack of rare massive halos (clusters) in observed fields. We show the convergence skewness is primarily due to rare and massive dark matter halos with skewness converging to its mean value only if halos of mass M > 10^15 M_\sun are present. This calculational method can in principle be used to correct for such a bias as well as to search for more robust statistics related to the two and three point correlations.

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arXiv2000

Cold and Fuzzy Dark Matter

Cold dark matter (CDM) models predict small-scale structure in excess of observations of the cores and abundance of dwarf galaxies. These problems might be solved, and the virtues of CDM models retained, even without postulating {\it ad hoc} dark matter particle or field interactions, if the dark matter is composed of ultra-light scalar particles (m ~ 10^{-22}eV), initially in a (cold) Bose-Einstein condensate, similar to axion dark matter models. The wave properties of the dark matter stabilize gravitational collapse providing halo cores and sharply suppressing small-scale linear power.

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arXiv2000

Weak Lensing by Large-Scale Structure: A Dark Matter Halo Approach

Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters which govern them but are currently limited to small fields and subject to selection biases. We show how the expected signal from large-scale structure arises from the contributions from and correlations between individual halos. We determine the convergence power spectrum as a function of the maximum halo mass and so provide the means to interpret results from surveys that lack high mass halos either through selection criteria or small fields. Since shot noise from rare massive halos is mainly responsible for the sample variance below 10', our method should aid our ability to extract cosmological information from small fields.

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arXiv2000

CMB Anisotropies: A Decadal Survey

We review the theoretical implications of the past decade of CMB anisotropy measurements, which culminated in the recent detection of the first feature in the power spectrum, and discuss the tests available to the next decade of experiments. The current data already suggest that density perturbations originated in an inflationary epoch, the universe is spatially flat, and baryonic dark matter is required. We discuss the underlying assumptions of these claims and outline the tests required to ensure they are robust. The most critical test - the presence of a second feature at the predicted location - should soon be available. Further in the future, secondary anisotropies and polarization should open new windows to the early and low(er) redshift universe.

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