## Jain, Bhuvnesh

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Publication Observational Tests of Modified Gravity(2008-09-02) Jain, Bhuvnesh; Zhang, PengjieModifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the Universe. Modified gravity theories have richer observational consequences for large scale structures than conventional dark energy models, in that different observables are not described by a single growth factor even in the linear regime. We examine the relationships between perturbations in the metric potentials, density and velocity fields, and discuss strategies for measuring them using gravitational lensing, galaxy cluster abundances, galaxy clustering/dynamics, and the integrated Sachs-Wolfe effect. We show how a broad class of gravity theories can be tested by combining these probes. A robust way to interpret observations is by constraining two key functions: the ratio of the two metric potentials, and the ratio of the gravitational ‘‘constant’’ in the Poisson equation to Newton’s constant. We also discuss quasilinear effects that carry signatures of gravity, such as through induced three-point correlations. Clustering of dark energy can mimic features of modified gravity theories and thus confuse the search for distinct signatures of such theories. It can produce pressure perturbations and anisotropic stresses, which break the equality between the two metric potentials even in general relativity. With these two extra degrees of freedom, can a clustered dark energy model mimic modified gravity models in all observational tests? We show with specific examples that observational constraints on both the metric potentials and density perturbations can in principle distinguish modifications of gravity from dark energy models. We compare our result with other recent studies that have slightly different assumptions (and apparently contradictory conclusions).Publication Three-point Correlations in ƒ(R) Models of Gravity(2009-05-07) Borisov, Alexander; Jain, BhuvneshModifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the universe. We calculate quasilinear effects in the growth of structure in ƒ(R) models of gravity using perturbation theory. We find significant deviations in the bispectrum that depend on cosmic time, length scale and triangle shape. However the deviations in the reduced bispectrum Q for ƒ(R) models are at the percent level, much smaller than the deviations in the bispectrum itself. This implies that three-point correlations can be predicted to a good approximation simply by using the modified linear growth factor in the standard gravity formalism. Our results suggest that gravitational clustering in the weakly nonlinear regime is not fundamentally altered, at least for a class of gravity theories that are well described in the Newtonian regime by the parameters Geff and Φ/Ψ. This approximate universality was also seen in the N-body simulation measurements of the power spectrum by Stabenau & Jain (2006), and in other recent studies based on simulations. Thus predictions for such modified gravity models in the regime relevant to large-scale structure observations may be less daunting than expected on first principles. We discuss the many caveats that apply to such predictions.Publication Tests of Gravity from Imaging and Spectroscopic Surveys(2010-01-06) Guzik, Jacek; Jain, Bhuvnesh; Takada, MasahiroTests of gravity on large scales in the Universe can be made using both imaging and spectroscopic surveys. The former allow for measurements of weak lensing, galaxy clustering and cross correlations such as the integrated Sachs-Wolfe effect. The latter probe galaxy dynamics through redshift-space distortions. We use a set of basic observables, namely, lensing power spectra, galaxy-lensing and galaxyvelocity cross-spectra in multiple redshift bins (including their covariances), to estimate the ability of upcoming surveys to test gravity theories. We use a two-parameter description of gravity that allows for the Poisson equation and the ratio of metric potentials to depart from general relativity. We find that the combination of imaging and spectroscopic observables is essential in making robust tests of gravity theories. The range of scales and redshifts best probed by upcoming surveys is discussed. We also compare our parametrization to others used in the literature, in particular, the y parameter modification of the growth factor.Publication N-Body Simulations of Alternative Gravity Models(2006-10-05) Stabenau, Hans F.; Jain, BhuvneshTheories in which gravity is weaker on cosmological scales have been proposed to explain the observed acceleration of the universe. The nonlinear regime in such theories is not well studied, though it is likely that observational tests of structure formation will lie in this regime. A class of alternative gravity theories may be approximated by modifying Poisson’s equation. We have run N-body simulations of a set of such models to study the nonlinear clustering of matter on 1–100 Mpc scales. We find that nonlinear gravity enhances the deviations of the power spectrum of these models from standard gravity. This occurs due to mode coupling, so that models with an excess or deficit of large-scale power (at k < 0.2 Mpc-1) lead to deviations in the power spectrum at smaller scales as well (up to k ~ 1 Mpc-1), even though the linear spectra match very closely on the smaller scales. This makes it easier to distinguish such models from general relativity using the three-dimensional power spectrum probed by galaxy surveys and the weak lensing power spectrum. If the potential for light deflection is modified in the same way as the potential that affects the dark matter, then weak lensing constrains deviations from gravity even more strongly. Our simulations show that, even with a modified potential, gravitational evolution is approximately universal. Based on this, the Peacock-Dodds approach can be adapted to get an analytical fit for the nonlinear power spectra of alternative gravity models, though the recent Smith et al. formula is less successful. Our conclusions extend to models with modifications of gravity on scales of 1–20 Mpc. We also use a way of measuring projected power spectra from simulations that lowers the sample variance, so that fewer realizations are needed to reach a desired level of accuracy.Publication Galaxy-CMB and Galaxy-Galaxy Lensing on Large Scales: Sensitivity to Primordial Non-Gaussianity(2009-12-22) Jeong, Donghui; Komatsu, Eiichiro; Jain, BhuvneshA convincing detection of primordial non-Gaussianity in the local form of the bispectrum, whose amplitude is given by the ƒNL parameter, offers a powerful test of inflation. In this paper, we calculate the modification of two-point cross-correlation statistics of weak lensing—galaxy-galaxy lensing and galaxycosmic microwave background (CMB) crosscorrelation—due to ƒNL. We derive and calculate the covariance matrix of galaxy-galaxy lensing, including cosmic variance terms. We focus on large scales (l < 100) for which the shape noise of the shear measurement becomes irrelevant and cosmic variance dominates the error budget. For a modest degree of non-Gaussianity, ƒNL = ±50 modifications of the galaxy-galaxy-lensing signal at the 10% level are seen on scales R ~ 300 Mpc, and grow rapidly toward larger scales as ∝ R2. We also see a clear signature of the baryonic acoustic oscillation feature in the matter power spectrum at ~ 150 Mpc, which can be measured by next-generation lensing experiments. In addition, we can probe the local-form primordial non-Gaussianity in the galaxy-CMB lensing signal by correlating the lensing potential reconstructed from CMB with high-z galaxies. For example, for ƒNL = ±50, we find that the galaxy-CMB lensing cross-power spectrum is modified by ~ 10% at l ~ 40, and by a factor of 2 at l ~ 10, for a population of galaxies at z = 2 with a bias of 2. The effect is greater for more highly biased populations at larger z; thus, high-z galaxy surveys cross correlated with CMB offer a yet another probe of primordial non-Gaussianity.Publication Short GRB and Binary Black Hole Standard Sirens as a Probe of Dark Energy(2006-09-18) Dalal, Neal; Holz, Daniel E.; Hughes, Scott A.; Jain, BhuvneshObservations of the gravitational radiation from well-localized, inspiraling compact-object binaries can measure absolute source distances with high accuracy. When coupled with an independent determination of redshift through an electromagnetic counterpart, these standard sirens can provide an excellent probe of the expansion history of the Universe and the dark energy. Short γ-ray bursts, if produced by merging neutron star binaries, would be standard sirens with known redshifts detectable by ground-based gravitational wave (GW) networks such as Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), Virgo, and Australian International Gravitational Observatory (AIGO). Depending upon the collimation of these GRBs, the measurement of about 10 GW-GRB events (corresponding to about 1 yr of observation with an advanced GW detector network and an all-sky GRB monitor) can measure the Hubble constant h to ~ 2–3%. When combined with measurement of the absolute distance to the last scattering surface of the cosmic microwave background, this determines the dark energy equation of state parameter w to ~9%. Similarly, supermassive binary black hole inspirals will be standard sirens detectable by Laser Interferometer Space Antenna (LISA). Depending upon the precise redshift distribution, ~100 sources could measure w at the ~4% level.Publication Spherical Collapse in ƒ(R) Gravity(2012-03-23) Borisov, Alexander; Jain, Bhuvnesh; Zhang, PengjieWe use one-dimensional numerical simulations to study spherical collapse in the ƒ(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 δc 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.