Pengjie Zhang

Probing gravity at cosmological scales by measurements which test the relationship between gravitational lensing and matter overdensity

Pengjie Zhang, 2 Michele Liguori, Rachel Bean, and Scott Dodelson 6 Shanghai Astronomical Observatory, Chinese Academy of Science, 80 Nandan Road, Shanghai, China, 200030 Joint Institute for Galaxy and Cosmology (JOINGC) of SHAO and USTC Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge, CB3 0WA, United Kingdom Department of Astronomy, Cornell University, Ithaca, NY 14853 Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, IL 60510-0500 Department of Astronomy & Astrophysics, The University of Chicago, Chicago, IL 60637-1433

The influence of baryons on the clustering of matter and weak-lensing surveys

Future weak-lensing measurements of cosmic shear will reach such high accuracy that second-order effects in weak-lensing modeling, such as the influence of baryons on structure formation, become important. We use a controlled set of high-resolution cosmological simulations to quantify this effect by comparing pure N-body dark matter runs with corresponding hydrodynamic simulations, carried out both in nonradiative form and in dissipative form with cooling and star formation. In both hydrodynamic simulations, the clustering of the gas is suppressed while that of dark matter is boosted at scales k > 1 h Mpc(-1). Despite this counterbalance between dark matter and gas, the clustering of the total matter is suppressed by up to 1% at 1 h Mpc(-1) less than or similar to k less than or similar to 10 h Mpc(-1), while for k approximate to 20 h Mpc(-1) it is boosted, up to 2% in the nonradiative run and 10% in the run with star formation. The stellar mass formed in the latter is highly biased relative to the dark matter in the pure N-body simulation. Using our power spectrum measurements to predict the effect of baryons on the weak-lensing signal at scales corresponding to multipole moments 100 < l < 10,000, we find that baryons may change the lensing power spectrum by less than 0.5% at l < 1000, but by 1% to 10% at 1000 < l < 10,000. The size of the effect exceeds the predicted accuracy of future lensing power spectrum measurements and will likely be detected. Precise determinations of cosmological parameters with weak lensing, and studies of small-scale fluctuations and clustering, therefore rely on properly including baryonic physics.

Testing gravity against the early time integrated Sachs-Wolfe effect

A generic prediction of general relativity is that the cosmological linear density growth factor D is scale independent. But in general, modified gravities do not preserve this signature. A scale dependent D can cause time variation in gravitational potential at high redshifts and provides a new cosmological test of gravity, through early time integrated Sachs-Wolfe (ISW) effect-large scale structure (LSS) cross correlation. We demonstrate the power of this test for a class of f(R) gravity, with the form f(R) = {lambda}{sub 1}H{sub 0}{sup 2} exp(-R/{lambda}{sub 2}H{sub 0}{sup 2}). Such f(R) gravity, even with degenerate expansion history to {Lambda}CDM, can produce detectable ISW effect at z {approx}> 3 and l {approx}> 20. Null-detection of such effect would constrain {lambda}{sub 2} to be {lambda}{sub 2} > 1000 at > 95% confidence level. On the other hand, robust detection of ISW-LSS cross correlation at high z will severely challenge general relativity.

Imprint of the interaction between dark sectors in large scale cosmic microwave background anisotropies

Dark energy interacting with dark matter is a promising model to solve the cosmic coincidence problem. We study the signature of such interaction on large scale cosmic microwave background (CMB) temperature anisotropies. Based on the detail analysis in perturbation equations of dark energy and dark matter when they are in interaction, we find that the large scale CMB, especially the late integrated Sachs-Wolfe effect, is a useful tool to measure the coupling between dark sectors. We also discuss the possibility to detect the coupling by cross-correlating CMB maps with tracers of the large scale structure. We finally perform the global fitting to constrain the coupling by using the CMB power spectrum data together with other observational data. We find that in the 1 sigma range, the constrained coupling between dark sectors can solve the coincidence problem.

Behavior of f(R) gravity in the solar system, galaxies, and clusters

For cosmologically interesting f(R) gravity models, we derive the complete set of the linearized field equations in the Newtonian gauge, under environments of the solar system, galaxies, and clusters, respectively. Based on these equations, we confirm the previous gamma=1/2 solution in the solar system. However, f(R) gravity models can be strongly environment dependent and the high density (comparing to the cosmological mean) solar system environment can excite a viable gamma=1 solution for some f(R) gravity models. Although for f(R)proportional to-1/R, it is not the case; for f(R)proportional to-exp(-R/lambda H-2(0)2), such a gamma=1 solution does exist. This solution is virtually indistinguishable from that in general relativity (GR) and the value of the associated curvature approaches the GR limit, which is much higher than the value in the gamma=1/2 solution. We show that for some forms of f(R) gravity, this solution is physically stable in the solar system and can smoothly connect to the surface of the Sun. The derived field equations can be applied directly to gravitational lensing of galaxies and clusters. We find that, despite a significant difference in the environments of galaxies and clusters comparing to that of the solar system, gravitational lensing of galaxies and clusters can be virtually identical to that in GR, for some forms of f(R) gravity. Fortunately, galaxy rotation curve and intracluster gas pressure profile may contain valuable information to distinguish these f(R) gravity models from GR.

SELF-CALIBRATION OF GRAVITATIONAL SHEAR-GALAXY INTRINSIC ELLIPTICITY CORRELATION IN WEAK LENSING SURVEYS

The galaxy intrinsic alignment is a severe challenge to precision cosmic shear measurement. We propose self-calibrating the induced gravitational shear-galaxy intrinsic ellipticity correlation (the GI correlation) in weak lensing surveys with photometric redshift measurements. (1) We propose a method to extract the intrinsic ellipticity-galaxy density cross-correlation (I-g) from the galaxy ellipticity-density measurement in the same redshift bin. (2) We also find a generic scaling relation to convert the extracted I-g correlation to the necessary GI correlation. We perform a concept study under simplified conditions and demonstrate its capability to significantly reduce GI contamination. We discuss the impact of various complexities on the two key ingredients of the self-calibration technique, namely the method for extracting the I-g correlation and the scaling relation between the I-g and the GI correlation. We expect that none of them will likely be able to completely invalidate the proposed self-calibration technique.

Self-calibration of photometric redshift scatter in weak-lensing surveys

Photometric redshift (photo-z) errors, especially catastrophic errors, are a major uncertainty for precision weak-lensing cosmology. We find that the shear (galaxy number) density and density-density cross-correlation measurements between photo-z bins, available from the same lensing surveys, contain valuable information for self-calibration of the scattering probabilities between the true redshift and photo-z bins. The self-calibration technique we propose does not rely on cosmological priors nor parameterization of the photo-z probability distribution function, and preserves all of the cosmological information available from shear-shear measurement. We estimate the calibration accuracy through the Fisher matrix formalism. We find that, for advanced lensing surveys such as the planned Stage IV surveys, the rate of photo-z outliers can be determined with statistical uncertainties of 0.01-1 per cent for z < 2 galaxies. Among the several sources of calibration error that we identify and investigate, the galaxy distribution bias is likely the most dominant systematic error, whereby photo-z outliers have different redshift distributions and/or bias than non-outliers from the same bin. This bias affects all photo-z calibration techniques based on correlation measurements. Galaxy bias variations of O(0.1) produce biases in photo-z outlier rates similar to the statistical errors of our method, so this galaxy distribution bias may bias the reconstructed scatters at several-Sigma level, but is unlikely to completely invalidate the self-calibration technique.

Spherical Collapse in f(R) Gravity

We use one-dimensional numerical simulations to study spherical collapse in the f(R) gravity models. We include the nonlinear self-coupling of the scalar field in the theory and use a relaxation scheme to follow the collapse. We find an unusual enhancement in density near the virial radius which may provide observable tests of gravity. We also use the estimated collapse time to calculate the critical overdensity delta(c) c used in calculating the mass function and bias of halos. We find that analytical approximations previously used in the literature do not capture the complexity of nonlinear spherical collapse.

Angular Correlations of the MeV Cosmic Gamma-Ray Background

The measured cosmic gamma-ray background (CGB) spectrum at MeV energies is in reasonable agreement with the predicted contribution from Type Ia supernovae (SNe Ia). However, the characteristic features in the SN Ia gamma-ray spectrum, weakened by integration over source redshifts, are difficult to measure, and the contributions from other sources in the MeV range are uncertain, so that the SN Ia origin of the MeV CGB remains unproven. Since different CGB sources have different clustering properties and redshift distributions, by combining the CGB spectrum and angular correlation measurements the contributions to the CGB could be identified and separated. The SN Ia CGB large-scale structure follows that of galaxies. Its rms fluctuation at degree scales has a characteristic energy dependence ranging from ~1% to on the order of unity and can be measured to several percent precision by proposed future satellites such as the Advanced Compton Telescope. With the identification of the SN Ia contribution, the SN Ia rate could be measured unambiguously as a function of redshift up to z ~ 1 by combining the spectrum and angular correlation measurements, yielding new constraints on the star formation rate to even higher redshifts. Finally, we show that the gamma-ray and neutrino backgrounds from supernovae should be closely connected, allowing an important consistency test from the measured data. Identification of the astrophysical contributions to the CGB would allow much greater sensitivity to an isotropic high-redshift CGB contribution arising in extra dimension or dark matter models.

Weak lensing bispectrum

Weak gravitational lensing of background galaxies offers an excellent opportunity to study the intervening distribution of matter. While much attention to date has focused on the two-point-function of the cosmic shear, the three-point-function, the bispectrum, also contains very useful cosmological information. Here, we compute three corrections to the bispectrum which are nominally of the same order as the leading term. We show that the corrections are small, so they can be ignored when analyzing present surveys. However, they will eventually have to be included for accurate parameter estimates from future surveys.