Yong-Seon Song

Large scale structure of f(R) gravity

We study the evolution of linear cosmological perturbations in f(R) models of accelerated expansion in the physical frame where the gravitational dynamics are fourth order and the matter is minimally coupled. These models predict a rich and testable set of linear phenomena. For each expansion history, fixed empirically by cosmological distance measures, there exists two branches of f(R) solutions that are parametrized by B{proportional_to}d{sup 2}f/dR{sup 2}. For B 0 branch, f(R) models can reduce the large-angle CMB anisotropy, alter the shape of the linear matter power spectrum, and qualitatively change the correlations between the CMB and galaxy surveys. All of these phenomena are accessible with current and future data and provide stringent tests of general relativity on cosmological scales.

Reconstructing the history of structure formation using Redshift Distortions

Measuring the statistics of galaxy peculiar velocities using redshift-space distortions is an excellent way of probing the history of structure formation. Because galaxies are expected to act as test particles within the flow of matter, this method avoids uncertainties due to an unknown galaxy density bias. We show that the parameter combination measured by redshift-space distortions, fσ8mass provides a good test of dark energy models, even without the knowledge of bias or σ8mass required to extract f from this measurement (here f is the logarithmic derivative of the linear growth rate, and σ8mass is the root-mean-square mass fluctuation in spheres with radius 8h−1Mpc). We argue that redshift-space distortion measurements will help to determine the physics behind the cosmic acceleration, testing whether it is related to dark energy or modified gravity, and will provide an opportunity to test possible dark energy clumping or coupling between dark energy and dark matter. If we can measure galaxy bias in addition, simultaneous measurement of both the overdensity and velocity fields can be used to test the validity of equivalence principle, through the continuity equation.

Forecasting cosmological constraints from redshift surveys

Observations of redshift-space distortions in spectroscopic galaxy surveys offer an attractive method for observing the build-up of cosmological structure, which depends both on the expansion rate of the Universe and our theory of gravity. In this paper, we present a formalism for forecasting the constraints on the growth of structure which would arise in an idealized survey. This Fisher matrix based formalism can be used to study the power and aid in the design of future surveys.

Determining neutrino mass from the cosmic microwave background alone

Distortions of Cosmic Microwave Background (CMB) temperature and polarization maps caused by gravitational lensing, observable with high angular resolution and high sensitivity, can be used to measure the neutrino mass. Assuming two massless species and one with mass m_nu we forecast sigma(m_nu) = 0.15 eV from the Planck satellite and sigma(m_nu)=0.04 eV from observations with twice the angular resolution and about 20 times the sensitivity. A detection is likely at this higher sensitivity since the observation of atmospheric neutrino oscillations require mass-squared differences greater than about (0.04 eV)^2.

Distance-redshift and growth-redshift relations as two windows on acceleration and gravitation : Dark energy or new gravity?

Small distortions in the observed shapes of distant galaxies, a cosmic shear due to gravitational lensing, can be used to simultaneously determine the distance-redshift relation,

Cosmological Constraints on f(R) Acceleration Models

r(z)

Probing modifications of general relativity using current cosmological observations

, and the density contrast growth factor,

Large-scale tests of the Dvali-Gabadadze-Porrati model

g(z)

The Far-Infrared Background Correlation with Cosmic Microwave Background Lensing

. Both of these functions are sensitive probes of the acceleration. Their simultaneous determination allows for a consistency test and provides sensitivity to physics beyond the standard dark energy paradigm.

Velocities as a probe of dark sector interactions

Models which accelerate the expansion of the universe through the addition of a function of the Ricci scalar f(R) leave a characteristic signature in the large-scale structure of the universe at the Compton wavelength scale of the extra scalar degree of freedom. We search for such a signature in current cosmological data sets: the WMAP cosmic microwave background (CMB) power spectrum, Supernovae Legacy Survey supernovae distance measures, the Sloan Digital Sky Survey luminous red galaxy power spectrum, and galaxy-CMB angular correlations. Because of theoretical uncertainties in the nonlinear evolution of f(R) models, the galaxy power spectrum conservatively yields only weak constraints on the models despite the strong predicted signature in the linear matter power spectrum. Currently the tightest constraints involve the modification to the integrated Sachs-Wolfe effect from growth of gravitational potentials during the acceleration epoch. This effect is manifest for large Compton wavelengths in enhanced low multipole power in the CMB and anticorrelation between the CMB and tracers of the potential. They place a bound on the Compton wavelength of the field to be less than of order the Hubble scale.