PhD Thesis Defenses
Ross Cawthon, "Effects of Redshift Uncertainty on Cross-Correlations of CMB Lensing and Galaxy Surveys"
July 18, 2018 | 2:00 PM | ERC 576
Ph.D. Committee members: Josh Frieman (PhD advisor), Scott Dodelson, Steve Kent, Dan Hooper

Thesis Abstract: Wide-field galaxy imaging surveys, such as the Dark Energy Survey (DES), and cosmic microwave background experiments, such as the South Pole Telescope (SPT), have great synergy in studying the large-scale structure of the Universe. One of the most important combined measurements from these types of surveys is the cross-correlation of galaxy positions with locations of gravitational lensing of the CMB.

In this thesis, I go in depth in projecting future cosmological constraints of this measurement. The key element of my work that has not been studied previously is the effect of redshift uncertainties on these constraints. I will show that discounting this effect can greatly overestimate the precision of these measurements. I will also show how some of this loss of precision can be mitigated, going from 50 times larger uncertainties than the perfectly known redshifts case, to only 2-3 times larger. A secondary goal of my work is to show that these measurements can simultaneously constrain the redshift information of samples of galaxies. I will show that these constraints on redshift information can be competitive with typical methods of estimating photometric redshifts. I will discuss projections of both the current DES/SPT era, and the exciting future era (2020s) of the Large Synoptic Survey Telescope (LSST) and the CMB-Stage 4 experiment.

Cameron Liang, "Multiphase Gaseous Halos around Galaxies"
June 15, 2018 | 1:00 PM | ERC 401
Ph.D. Committee members: Nickolay Y. Gnedin, Joshua A. Frieman, Daniel Fabrycky

Thesis Abstract: There is no doubt that our atmosphere is an integral part of the ecosystem of the Earth. Everyday weather and long-term climate of the atmosphere are directly linked to activities on the surface of the Earth and vice versa. Gaseous halos, known as the circumgalactic medium (CGM), are the equivalent atmospheres of galaxies.

In this thesis, I provide a major step towards the empirical constraints and theoretical modeling of the CGM and the co-evolution with their host galaxies. Using background quasars, I statistically map the spatial extent of multiphase gaseous halos in a sample of ~200 galaxies that span nearly five orders of magnitude in stellar mass, from dwarf to L*, and more massive galaxies. With these empirical constraints, I explore the effects of theoretical modeling of star formation and feedback processes using a set of high-resolution cosmological zoom-in simulations of a Milk-Way progenitor. To connect more closely with observations, I develop a synthetic absorption pipeline, as a virtual telescope, to observe the simulated galaxy and their CGM. This series of observational and theoretical studies have led to new insights of the cold gas in galactic winds and halos. I explore a new model, the circumgalactic mist, with a set of magneto-hydrodynamic simulations. I will present the model implications on the spatial distribution of multiphase gas around galaxies.

Pavel Motloch, "Topics in Gravitational Lensing of the Cosmic Microwave Background"
June 4, 2018 | 2:30 PM | ERC 401
Ph.D. Committee members: Wayne Hu (Ph.D. advisor), Stephan Meyer, Michael Turner Robert Wald

Thesis Abstract: Gravitational lensing of the cosmic microwave background (CMB) has recently started to gain importance as a cosmological probe. With growing detection significance of this effect, it is necessary to further develop theoretical understanding of its consequences. Such studies are the main topic of this work.

We start by looking at correlations that the gravitational lensing induces between CMB temperature, polarization and reconstructed lensing potential and investigate how neglecting them in an analysis impacts constraints on cosmological parameters. We find that for the planned CMB Stage 4 experiment, neglecting these correlations can significantly underestimate variance of certain combinations of cosmological parameters, as well as lead to an increased frequency of mistakenly rejecting the underlying cosmological model.

Then we discuss a method we developed to directly measure the gravitational lensing potential from the CMB data and explain how to practically perform such measurement. This method helps us understand why it is necessary to include the lensing-induced covariances to get correct constraints on cosmological parameters. Additionally, comparing direct measurements of the lensing potential from various subsets of data or across experiments allows for powerful consistency checks that can be used to search for residual systematics and exotic new physics. When assuming a particular cosmological model, this technique can also be used to probe internal consistency of lensing within a single data set.

In the final part of this work we apply this methodology to check lensing consistency of the Planck satellite data. We find that it is not possible to resolve the lensing anomalies seen in this data even when allowing for an arbitrary gravitational lensing potential, beyond the predictions of the standard cosmological model. Significances of these tensions are evaluated at above 2σ; one possible explanation are residual systematics in the Planck temperature power spectrum. Without large modifications, this technique can be applied to data from other current and especially future experiments, where its full power will become manifest.