PhD Thesis Defenses, 2015
 
Kyle Story, "Measuring the Temperature Anisotropy and Gravitational Lensing of the Cosmic Microwave Background with the South Pole Telescope"
March 5, 2015 | 1:30 PM | LASR conference room
Scientific Advisor: John E. Carlstrom

PhD Committee members: Stephen Meyer, Scott Dodelson, Edward Blucher

Thesis Abstract: Over the past two decades, measurements of the cosmic microwave background (CMB) have provided profound insight into the nature of the universe. The CMB, which is comprised of thermal radiation that originated in the early universe, contains detailed information about the composition and evolution of the universe that is encoded in the temperature and polarization anisotropy. Measurements of this CMB anisotropy have enabled powerful tests of cosmological theory.

In this talk, I will present two studies of the CMB anisotropy using data from the South Pole Telescope (SPT). First, I will present a measurement of the power spectrum of the CMB temperature anisotropy from the 2500 square degree SPT-SZ survey using data from the first camera mounted on the SPT. This measurement has significant implications for cosmological models; I will discuss constraints on the standard cosmological model, neutrinos, and cosmic inflation. Second, I will present a measurement of the CMB gravitational lensing potential and its power spectrum using data from the polarization-sensitive camera SPTpol, the second camera installed on the SPT. The CMB lensing potential probes the amount of matter and the growth of large-scale structure in the universe. Additionally, this technique is used to make maps of the projected mass in the universe, which will be powerful for cross-correlating with other tracers of large-scale structure as well as in the search for inflationary gravity waves.

Related Links:
KICP Members: John E. Carlstrom; Scott Dodelson; Stephan S. Meyer
KICP Students: Kyle Story
Scientific projects: South Pole Telescope (SPT)
 
Youngsoo Park, "Combined Probes Analysis with Galaxy Clustering and Galaxy-Galaxy Lensing in the Dark Energy Survey"
April 30, 2015 | 2:00 PM | LASR conference room
Scientific Advisor: Scott Dodelson

PhD Committee members: Henry Frisch, Stephan Meyer, Michael Turner

Thesis Abstract:
Combining galaxy-galaxy lensing and galaxy clustering, a type of the so-called combined probes analysis methods, is a promising method for inferring the growth rate of large scale structure in current and future cosmological surveys. Measuring the growth of structure in the universe will shed light on the mechanism driving the acceleration of the universe. The Dark Energy Survey (DES) is a prime candidate for such an analysis, with its measurements of both the distribution of galaxies on the sky and the tangential shears of background galaxies induced by these foreground lenses. By constructing an end-to-end analysis that combines large-scale galaxy clustering and small-scale galaxy-galaxy lensing, we forecast the potential of a combined probes analysis on DES datasets. In particular, we develop a realistic approach to a DES combined probes analysis by jointly modeling the assumptions and systematics affecting the different components of the combined observable, employing a shared halo model, parametrized halo occupation distribution, photometric redshift uncertainties, and shear measurement errors. We also study the effect of external priors on different subsets of these parameters. We conclude that data from the first year of DES will provide powerful constraints on the evolution of structure growth in the universe, constraining the growth function to better than 8%.

Related Links:
KICP Members: Scott Dodelson; Stephan S. Meyer; Michael S. Turner
KICP Students: Youngsoo Park
Scientific projects: Dark Energy Survey (DES)
 
Yin Li, "Cosmic Void Abundance in a Spherical Boundary Model"
June 2, 2015 | 9:30 AM | ACC 211
Scientific Advisor: Wayne Hu

Ph.D. Committee Members: LianTao Wang, Daniel Holz, John Carlstrom

Thesis Abstract: Cosmic voids are large and underdense structures that fill up most of the volume of the universe, and are potentially a powerful probe of dark energy and modified gravity. By developing a more complete void model that accounts for both the boundary and interior evolution, we discover that shell-crossing is an inaccurate description for void formation, correcting a misconception that has accompanied void studies for decades. In addition, the model directly incorporate selection criterion acting on the significance of voids, making it possible to relate the theoretical prediction to voids identified in observations/simulations. The new model is also compatible with the existence of the largest voids observed in galaxy surveys, which is forbidden in the previous widely used model.

Related Links:
KICP Members: John E. Carlstrom; Daniel E. Holz; Wayne Hu; Lian-Tao Wang
KICP Students: Yin Li
 
Vinicius Miranda, "Probing Inflation with the Cosmic Microwave Background"
June 2, 2015 | 1:30 PM | ACC 211
Scientific Advisor: Wayne Hu

PhD Committee members: Scott Dodelson, LianTao Wang, Steve Meyer

Thesis Abstract: The existence of a quasi-deSitter expansion in the early universe, known as inflation, generates the seeds of large-scale structures and is one of the foundations of the standard cosmological model. The main observational predictions of inflation include the existence of a nearly scale-invariant primordial power spectrum that is imprinted on the cosmic microwave background (CMB), that has been corroborated with remarkable precision in recent years. Generalizations of the vanilla single-field slow-roll inflation provide a wealth of observational signatures in the power spectrum and the non-Gaussianity of fluctuations of the CMB, and this motivates a technique that can evaluate predictions of inflation beyond the slow-roll approximation called the generalized slow-roll (GSR). I will describe the latest searches for signatures of slow-roll violations in the Planck data using the GSR formalism, which is an ideal framework to probe inflationary models in this regime.

Related Links:
KICP Members: Scott Dodelson; Wayne Hu; Stephan S. Meyer; Lian-Tao Wang
KICP Students: Vinicius Miranda
 
Jennifer Helsby, "Clustering-based redshifts for the Dark Energy Survey"
June 11, 2015 | 11:00 AM | TAAC 67
Scientific Advisor: Joshua A. Frieman

Ph.D. Committee members: Scott Dodelson, Stephan Meyer, Craig Hogan

"To constrain cosmology, and in particular to probe dark energy, from deep optical imaging surveys such as the Dark Energy Survey (DES), requires precise estimates of the redshifts of the distant galaxies they observe. Traditionally, these redshift estimates are made using galaxy colors, but this technique has known limitations and biases. Jennifer's thesis work involved the testing and implementation of a novel technique for estimating redshifts of galaxies, using the fact that they cluster in space with galaxies for which the redshifts may be known from spectroscopic measurements. Using simulations, Jen found that this "clustering redshift" technique accurately reconstructs the galaxy redshift distribution for a survey such as DES. She then applied this technique to determine the redshift distribution for several million galaxies in the first year of DES data, an important result that should prove extremely valuable for the cosmological analysis of these data."
- Joshua A. Frieman, PhD advisor

Thesis Abstract: Accurate determination of photometric redshifts and their errors is critical for large scale structure and weak lensing studies for constraining cosmology from deep, wide imaging surveys. Current photometric redshift methods suffer from bias and scatter due to incomplete training sets. Exploiting the clustering between a sample of galaxies for which we have spectroscopic redshifts and a sample of galaxies for which the redshifts are unknown can allow us to reconstruct the true redshift distribution of the unknown sample. Here we use this method in both simulations and early data from the Dark Energy Survey (DES) to determine the true redshift distributions of galaxies in photometric redshift bins. We find that cross-correlating with the spectroscopic samples currently used for training provides reliable estimates of the true redshift distribution in a photometric redshift bin. We discuss the use of the cross-correlation method in validating template- or learning-based approaches to redshift estimation and its future use in Stage IV surveys.

Related Links:
KICP Members: Scott Dodelson; Joshua A. Frieman; Craig J. Hogan; Stephan S. Meyer
KICP Students: Jennifer Helsby
Scientific projects: Dark Energy Survey (DES)
 
Benedikt Diemer, "On the (non-)universality of halo density profiles"
June 11, 2015 | 1:00 PM | LASR conference room
Scientific Advisor: Andrey V. Kravtsov

Ph.D. Committee members: Scott Dodelson, Joshua A. Frieman, Donald Q. Lamb

"In his PhD thesis Benedikt Diemer has shown that radial density profiles of dark matter halos cannot be characterized only as a function of halo mass, as was thought previously, but also depend on the mass accretion rate of halos. The work has resulted in a new model that accurately describes halo profiles in simulations from small radii out to 10 virial radii. Likewise, Benedikt has shown that halo concentrations depend not only on the halo mass (or more precisely on halo peak height), but also on the local slope of the power spectrum. Overall, this thesis showed that previously believed "universality" of the halo profiles is limited. Beyond just criticizing previous models, new models were developed that take into account the extra dependencies of halo profile parameters on the mass accretion rate and power law slope."
- Andrey V. Kravtsov, Ph.D. advisor

Thesis Abstract: We present a systematic study of the density profiles of dark matter halos in LCDM cosmologies, focusing on the question whether these profiles are "universal", i.e., whether they follow the same functional form regardless of halo mass, redshift, cosmology, and other parameters. The inner profile can be described as a function of mass and concentration, and we thus begin by investigating the universality of the concentration-mass relation. We propose a universal model in which concentration is a function only of a halo's peak height and the local slope of the matter power spectrum. This model matches the concentrations in LCDM and scale-free simulations, correctly extrapolates over 16 orders of magnitude in halo mass, and differs significantly from all previously proposed models at high masses and redshifts. Testing the universality of the outer regions, we find that the profiles are remarkably universal across redshift when radii are rescaled by R200m, whereas the inner profiles are most universal in units of R200c, highlighting that universality may depend upon the definition of the halo boundary. Furthermore, we discover that the profiles exhibit significant deviations from the supposedly universal analytic formulae previously suggested in the literature, such as the NFW and Einasto forms. In particular, the logarithmic slope of the profiles of massive or rapidly accreting halos steepens more sharply than predicted around ~R200m, where the steepness increases with increasing peak height or mass accretion rate. We propose a new, accurate fitting formula that takes these dependencies into account. Finally, we demonstrate that the profile steepening corresponds to the caustic at the apocenter of infalling matter on its first orbit. We call the location of the caustic the splashback radius, Rsp, and propose this radius as a new, physically motivated definition of the halo boundary. We discuss potential observational signatures of Rsp that would allow us to estimate the mass accretion rate of halos.

Related Links:
KICP Members: Scott Dodelson; Joshua A. Frieman; Andrey V. Kravtsov; Donald Q. Lamb
KICP Students: Benedikt Diemer
 
Ke Fang, "Newborn Pulsars as Highest-Energy Cosmic Accelerators"
June 11, 2015 | 3:00 PM | LASR conference room
Scientific Advisor: Angela V. Olinto

Ph.D. Committee members: Paolo Privitera, Scott Wakely, Scott Dodelson.

Thesis Abstract: The workings of the most energetic astrophysical accelerators in the Universe are encoded in the origin of ultrahigh energy cosmic rays (UHECRs). Current observations by the Auger Observatory, the largest UHECR observatory, show a spectrum that agrees with an extragalactic origin, as well as an interesting transition in chemical composition from light element to heavier element as energy increases. Candidate sources range from young neutron stars to gamma-ray bursts and events in active galaxies. In this talk, I will discuss newborn pulsars as the sources of ultrahigh energy cosmic rays. I will show that a newborn pulsar model naturally injects heavier elements and can fit the observed spectrum once propagation in the supernova remnant is taken into account. With the proper injection abundances, integrated cosmic rays from the extragalactic pulsar population can match observation in all aspects - energy spectrum, chemical composition, and anisotropy. I will then examine the fingerprints of their Galactic counterparts on cosmic ray spectrum. Furthermore, I will consider the multi-messenger smoking gun of this scenario - the detectability of high energy neutrinos from pulsars in the Local Universe.

Related Links:
KICP Members: Scott Dodelson; Angela V. Olinto; Paolo Privitera; Scott P. Wakely
KICP Students: Ke Fang
Scientific projects: Pierre Auger Observatory (AUGER)
 
Louis Abramson, "Assessing and Understanding Diversity in Galaxy Star Formation Histories"
July 13, 2015 | 2:00 PM | LASR conference room
Scientific Advisor: Michael D. Gladders

Ph.D. Committee members: Hsiao-Wen Chen, Andrey Kravtsov, Rich Kron

"Dr. Louis Abramson is an expert on the observation and phenomenological modeling of galaxy evolution, with a particular focus on the relationship between bulk statistical observables of galaxies, such as the distributions of star-formation-rate and mass over cosmic time, and the star formation histories of galaxies. His work during his Ph.D. has led to several new insights into the relationship between the passive (i.e., bulges) and actively star-forming components of galaxies, and led to a clear understanding that the scatter of galaxies across the so-called 'star forming main-sequence' is a critical observable to consider in further analyses, which he will continue as a postdoc at UCLA."
- Michael D. Gladders, Ph.D. advisor

Thesis Abstract: Galaxy star formation histories (SFHs) form a central thread of the cosmological narrative. Understanding them is therefore a central mission of the study of galaxy evolution. Although an ever-better picture is emerging of the build-up of the stellar mass of the *average* galaxy over time, the relevance of this track to the growth of *individual* galaxies is unclear. Largely, this ambiguity is due to the availability of only loose, ensemble-level constraints at any redshift appreciably greater than zero. In this talk, I outline how one of these constraints -- the the star formation rate/stellar mass relation -- shapes empirically based SFH models, especially in terms of the *diversity* of paths leading to a given end-state. I show that two models propose very different answers to this question -- galaxies grow *together* vs. galaxies grow *apart* -- corresponding (largely) to two different interpretations of the scatter in instantaneous galaxy growth rates at fixed stellar mass -- unimportant vs. essential. I describe how these interpretations affect one's stance on the profundity of galaxy "bimodality," the role of quenching mechanisms, and the influence of environment on galaxy evolution. Finally, after endorsing one of the models, I present some predictions that --- given upcoming observations --- should have the power to prove me right or wrong.

Related Links:
KICP Members: Hsiao-Wen Chen; Michael D. Gladders; Andrey V. Kravtsov; Richard G. Kron
KICP Students: Louis Abramson
 
Pierre Gratia, "Cosmology and Singularities in Massive Gravity"
July 20, 2015 | 10:00 AM | LASR conference room
Scientific Advisor: Wayne Hu

Ph.D. Committee members: Juan Collar, Carlos Wagner, Bob Wald

Thesis Abstract: In my thesis, I focus on the theory of massive gravity and its cosmological implications. I show that a massive graviton leads to self-acceleration, and that a whole class of solutions possess this property, including FRW universes. Subsequently, I investigate fluctuations around these solutions, and show that they are stable, at least for spherically symmetric fluctuations. Massive gravity also contains a coordinate-independent determinant singularity that shows up in specific examples with well-defined initial conditions, and that cannot be dynamically avoided. The vierbein formulation of massive gravity is particularly suited to address this issue. It can be seen that a commonly chosen convention is not supported by the vierbein formalism. Finally, I look at the appearance and properties of similar singularities in bimetric gravity, in which the second, fiducial metric is allowed to evolve independently of the spacetime metric. I find that while determinant singularities continue to show up in bimetric massive gravity theories, physical quantities such as curvature remain finite as they cross them.

Related Links:
KICP Members: Wayne Hu
KICP Students: Pierre Gratia
 
Alan Robinson, "Dark Matter Limits from a 2L C3F8 Filled Bubble Chamber"
August 31, 2015 | 10:00 AM | LASR conference room
Scientific Advisor: Juan I. Collar

Ph.D. Committee members: Luca Grandi, Dan Hooper, Philippe Guyot-Sionnest

"Alan's thesis goes beyond presenting new WIMP limits from our bubble chambers. In addition to that, he has provided the community of dark matter experimentalists with new tools that should generate a wide interest: revised cross-section libraries for neutron production and neutron scattering that can be employed to better assess the sensitivity of any WIMP detector."
- Juan I. Collar, Ph.D. advisor

Thesis Abstract: The PICO-2L C3F8 bubble chamber search for Weakly Interacting Massive Particle (WIMP) dark matter was operated in the SNOLAB underground laboratory at the same location as the previous CF3I filled COUPP-4kg detector. Neutrons calibration using photoneutron sources in C3F8 and CF3I filled calibration bubble chambers were performed to verify the sensitivity of these target fluids to dark matter scattering. This data was combined with similar measurements using a low-energy neutron beam at the University of Montreal and in situ calibrations of the PICO-2L and COUPP-4kg detectors. C3F8 provides much greater sensitivity to WIMP-proton scattering than CF3I in bubble chamber detectors. PICO-2L searched for dark matter recoils with energy thresholds below 10 keV. Radiopurity assays of detector materials were performed and the expected neutron recoil background was evaluated to be 1.6$^{+0.3}_{-0.9}$ single bubble events during the 211.5 kg-day exposure. Twelve single bubble dark matter candidate events were observed. These events were not uniformly distributed in time, and were likely caused by particulates in the active volume. Despite this background, PICO-2L sets a world-leading upper limit to the WIMP-proton spin dependent scattering cross-section.

Related Links:
KICP Members: Juan I. Collar; Luca Grandi; Daniel Hooper
KICP Students: Alan Robinson
Scientific projects: COUPP/PICO
 
Jing Zhou, "Direct Dark Matter Detection with the DAMIC experiment at SNOLAB"
September 1, 2015 | 10:00 AM | LASR conference room
Scientific Advisor: Paolo Privitera

Ph.D. Committee members: Luca Grandi, Liantao Wang, Sidney Nagel

"Jing has made fundamental contributions to the DAMIC experiment in its crucial R&D phase. Her measurements of radiogenic backgrounds in silicon include novel powerful methods which make use of the excellent spatial resolution of the CCDs. These measurements put stringent limits on the presence of uranium and thorium and provide a first evidence for sizeable cosmogenic silicon 32 in the bulk of high-purity silicon, an important discovery for the present and next generation of dark matter silicon detectors. Also, she has measured the nuclear recoil ionization efficiency in silicon below 3 keV, an energy range so far unexplored and fundamental for the search of low mass WIMPs. The impact of these results goes beyond their application in DAMIC, and will influence any WIMP detector based on silicon."
- Paolo Privitera, Ph.D. advisor

Thesis Abstract: DAMIC (Dark Matter in CCDs) is a novel experiment with unique sensitivity to dark matter particles of masses below 10 GeV/c2. It employs the bulk silicon of scientific-grade charge-coupled devices (CCDs) as the target for coherent WIMP-nucleus elastic scattering. The extremely low noise of the CCD readout results in an unprecedentedly low energy threshold to ionization of a few tens of eV. DAMIC was installed at SNOLAB at the end of 2012, and is now completing the R&D phase required for the construction of a 100g detector, DAMIC100.

Related Links:
KICP Members: Luca Grandi; Paolo Privitera; Lian-Tao Wang
KICP Students: Jing Zhou
Scientific projects: Dark Matter in CCDs (DAMIC)
 
Tyler Natoli, "A Search for Transient Sources in the First 100 deg2 of SPTpol Data"
September 29, 2015 | 4:15 PM | ERC 401
Scientific Advisor: John E. Carlstrom

Ph.D. Committee members: Brad Benson, Henry Frisch, Daniel Holz

Thesis Abstract: We will present the results from a systematic transient source search over a 100 deg2 field of millimeter wavelength data taken with the SPTpol camera on the South Pole Telescope (SPT). The SPTpol instrument was installed on the SPT in the Austral summer of 2011/2012 and features 1,536 polarization sensitive detectors and a resolution at 150 GHz of 1.1 arcminute. The transient data set presented here includes 9 months of SPTpol observations over the same 100 deg2 patch of sky attaining an average depth at 150 GHz of 7.4 mJy over 36 hours of observing. The technology developed for SPTpol detectors is described here along with the associated technology necessary to operate the detectors. We present the specifics of producing millimeter wavelength maps from raw detector timestreams including the quality cuts in the process. We describe the specific analysis and results of searching for transient sources with timescales of days to weeks in the first 100 deg2 of data taken by the SPTpol camera. Using the search results we place upper limits on the number of millimeter transient sources as a function of the transient duration above a flux of 20 mJy.

Related Links:
KICP Members: Bradford A. Benson; John E. Carlstrom; Daniel E. Holz
KICP Students: Tyler Natoli
Scientific projects: South Pole Telescope (SPT)
 
Lee McCuller, "Testing a Model of Planck-Scale Quantum Geometry With Broadband Correlation of Colocated 40m Interferometers"
October 30, 2015 | 3:00 PM | ERC 401
Scientific Advisor: Stephan S. Meyer

Ph.D. Committee members: Craig Hogan, Aaron Chou, Daniel Holz, Cheng Chin

"Lee has been a central contributor to the design, construction, operation and analysis of the Holometer experiment. Lee designed and implemented the control system that maintains the power-recycling cavity locked to the laser and the interferometer differential arm length constant to better than than 1/2 Angstrom over their 40 meter length. His analysis of the system has lead to precise understanding of the instrument state including knowledge of the calibration and constraints on systematics."
- Stephan Meyer, Ph.D. advisor

Thesis Abstract: The Holometer is designed to test for a Planck-diffraction scale uncertainty in resolving position over large baselines. The experiment overlaps two independent 40 meter optical Michelson inteferometers to detect the proposed uncertainty as a common broadband length fluctuation. This defense will present results from cross correlating two 2kW interferometers over 150 hours, including systematics studies to limit spurious sources of background correlation. The observing interval provides statistical power to test the underlying theory, where noncommutative geometry is normalized to relate position states against Black hole entropy bounds of the Holographic principle.

Related Links:
KICP Members: Craig J. Hogan; Daniel E. Holz; Stephan S. Meyer
KICP Students: Lee McCuller