Testing Λ\LambdaCDM at the lowest redshifts with SN Ia and galaxy velocities

Peculiar velocities of objects in the nearby universe are correlated due to the gravitational pull of large-scale structure. By measuring these velocities, we have a unique opportunity to test the cosmological model at the lowest redshifts. We perform this test, using current data to constrain the amplitude of the "signal" covariance matrix describing the velocities and their correlations. We consider a new, well-calibrated "Supercal" set of low-redshift SNe Ia as well as a set of distances derived from the fundamental plane relation of 6dFGS galaxies. Analyzing the SN and galaxy data separately, both results are consistent with the peculiar velocity signal of our fiducial $\Lambda$CDM model, ruling out the noise-only model with zero peculiar velocities at greater than $7\sigma$ (SNe) and $8\sigma$ (galaxies). When the two data sets are combined appropriately, the precision of the test increases slightly, resulting in a constraint on the signal amplitude of $A = 1.05_{-0.21}^{+0.25}$, where $A = 1$ corresponds to our fiducial model. Equivalently, we report an 11% measurement of the product of the growth rate and amplitude of mass fluctuations evaluated at $z_\text{eff} = 0.02$, $f \sigma_8 = 0.428_{-0.045}^{+0.048}$, valid for our fiducial $\Lambda$CDM model. We explore the robustness of the results to a number of conceivable variations in the analysis and find that individual variations shift the preferred signal amplitude by less than ${\sim}0.5\sigma$. We briefly discuss our Supercal SN Ia results in comparison with our previous results using the JLA compilation.Comment: 21 pages, 5 figures, minor changes to match the published versio

Solving the system of atomic rate equations during recombination

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (leaves 59-60).The recombination of hydrogen at z ~ 800 - 1800 induces distortions to the Cosmic Microwave Background (CMB) spectrum. We present a careful calculation and analysis of all of the main transitions occurring during this period in order to find the electron density throughout recombination and its dependence on each process. Our original motivation was to analyze the effects that Thomson scattering and resonance scattering will have on recombination. However, while working on the project, we found that first we had to thoroughly account for all of the atomic transitions. We present a new method for solving the system of equations throughout the period of recombination. This method allows us to show the effect of individual processes on the total ionization fraction.by Daniel Scolnic.S.B