UC Santa Cruz

We study unparametrized conformal geodesics, or called conformal circles, and study local diffeomorphisms mapping conformal geodesics to conformal geodesics in pseudo-Riemannian conformal manifolds. We show that such local diffeomorphisms are conformal local diffeomorphisms. Our result extends the result of Yano and Tomonaga. We also present a holographic interpretation for our result on Poincaré-Einstein manifolds. The proofs take suitable variations of conformal geodesics.

Working with data is a fundamental and essential aspect of computer science, particularly in machine learning (ML), data science, AI applications, scientific analysis, and decision-making. Efficiency in data collection is crucial, as many scientific investigations, including those in computer science, rely on large volumes of data. Additionally, data quality significantly influences the overall effectiveness and performance of systems and algorithms. Citizen science facilitates public participation in scientific research, contributing to data collection, analysis, and reporting. This dissertation addresses two main challenges in the data collection process: improving efficiency and ensuring data quality. To tackle these challenges, I propose an approach that integrates ML with citizen science to enhance data collection. This synergy can improve data collection efficiency and quality, as ML algorithms assist citizen science participants in accurately identifying relevant data, filtering out label noise, and validating gathered data. Primarily, I focus on the potential of using computer vision ML models to guide and automate the collection process of visual data, such as images and videos. In this dissertation, I introduce a set of systems designed to improve the data collection process, including SmartCS, a platform for creating ML-powered citizen science applications without writing code; RipFinder, a mobile application that uses ML to guide the collection of rip current data; and RipScout, a drone-based system for the automated collection of rip current data. These systems address data quality earlier in the collection pipeline, rather than gathering and cleaning data afterward. Another contribution of my dissertation is engaging the general public in scientific research, demonstrated through my work on involving young students in research through these systems. Overall, my approach and developed systems advance the state of the art in modern data collection processes by uniquely combining citizen science and ML, demonstrating their significance in enhancing data quality and efficiency.

The Big Gay is an ongoing project intended to interrogate and update the visual culture of gay, leather, and queer kink community spaces, specifically bars. The first iteration featured a collaboration and curation with 8 different artists, each expressing their own vision of what erotic masculinity could be on wooden cutouts created at a superhuman scale. I traveled throughout California assisting in the creation of these pieces, the majority of which went on to be featured in my MFA thesis exhibition, Eagle UCSC.For the exhibition, a small wooden bar was installed in the center of the installation. The pieces were suspended in the air, giving viewers a sense of a space while also allowing them to feel as though the pieces were erotic participants in the communal physical experience. On the bar were physical magazines and flickering projections expressing the history of the visuals in these spaces, as well as a small television displaying documentation of the manufacturing of the pieces around the bar. I placed myself as a bartender in this space, serving drinks but also explaining the purpose of the project, the history of leather spaces, and the manufacturing of the pieces suspended around the viewers. The first iteration of The Big Gay will be completed once the pieces are installed in queer kink spaces, in dialogue with existing artwork and directly intervening into the historical and rarely updated visual culture of these spaces.

This dissertation considers counter-institutional archives as social and political gathering places and interweaves spatial analyses of the forces of gentrification affecting the surrounding geographic context of these archival spaces. I developed a method called “queer hanging out” which addresses many related activities of knowledge production with varying stakes. Queer hanging out takes place across political affinities, within and while maintaining the movement spaces discussed in the chapters to follow. Chapter One traces three archival projects in Park Slope Brooklyn, New York, Interference Archive, the Lesbian Herstory Archives, and Moncada Library. I analyze the development of these archives as political movement infrastructure resisting processes of displacement. As movement infrastructure I discuss archival preservation strategies, and programming as political education. Chapter Two discusses several projects in Barcelona to consider political labor, common vs. public space, archival histories, and futurity. In this chapter I think about two contemporary art pieces that were exhibited at the Centre d’Art Santa Mónica, alongside a social center in the Sants Neighborhood called Can Battló and two archives housed there, Fundacio Salvador Segui and Centre de Documentació de Moviment Socials Mercè Grenzner. In each of these examples I focus on how forms of political skepticism toward institutions give way to a political orientation called prefigurative politics. In Chapter Three I write about a recently closed counter-institutional archive in San Francisco, the Center for Sex and Culture (CSC). Already concerned over the preceding years about their ability to maintain the physical space in light of rising rents in the surrounding South of Market neighborhood, CSC’s all-volunteer collective spent two years planning the relocation of the archival materials. In the end, instead of dividing the materials according to contemporary categories and redistributing them to other counter-institutions, CSC’s collective decided to relocate the archive as a whole to Harvard’s Schlesinger library. The political decisions CSC members made explores questions around community-based archives and the ways they have become circumscribed under neoliberal capitalism. Counter-institutional spaces offer us the opportunity to land our rage, grief and solidarity among friends and those we share political affinity, and with whom we want to hang out.

This dissertation investigates how contemporary art in Oceania materializes the role of colonial history in understanding and addressing climate change. Future-centric representations and solutions to rising sea levels and dwindling biodiversity—such as apocalypse narratives and techno-fixes—allow for escapism from the root causes of ecological collapse. Colonially induced climate change in Oceania stems from the control and dispossession of Indigenous Pacific bodies and lands, urban and agricultural development, militarism, and resource extraction. This dissertation asks how contemporary art in Oceania and its diaspora helps global audiences understand the role of colonial history in these issues. Without a deeper engagement with colonialism, we risk repeating and perpetuating unequal structures of power into the future. How, then, does artistic practice make manifest the dialogic relationship between space and time in the context of climate change? By materializing the continually unfolding past on which the present and future depend, contemporary art has transformative potential for our responses to the climate crisis. The chapters investigate a range of artistic strategies, including: the use of archival photography and film to challenge historical narratives about nuclear testing in the Marshall Islands and phosphate mining in Banaba; digital media and site-specific installations that materialize alternative pasts and futures in an urbanized Hawaiʻi; and experimental films that make visible the ancestral present in the Northern Territory of Australia, resisting apocalypse narratives and ecological grief that climate change often engenders. These case studies investigate how the legacies of colonialism are contributing to climate change in site-specific, localized ways, while also identifying regional and global connections. The project contributes to the fields of climate justice, Pacific Studies, and art history by taking a sustained look at the material entanglement between ecological collapse, temporality (the passage of time), and historical consciousness (how we understand our relationships to the past). The dissertation concludes by outlining a framework for Indigenous Pacific Climate Change Studies that foregrounds the arts as a necessary mechanism by which climate justice in Oceania can be achieved.

This dissertation investigates the multifaceted roles of transfer RNAs (tRNAs) in gene expression regulation, extending beyond their traditional roles in protein translation. Chapter I explores tRNAs in human brain development, utilizing cerebral cortical organoids and specialized tRNA sequencing to reveal dynamic expression patterns of tRNAs and tRNA-derived small RNAs (tDRs) in the early human cerebral cortex. Notably, it identifies a variety of upregulated tDRs from diverse isodecoders, with sequence-specific conservation among neural-specific groups, suggesting a pivotal role in neural development. Chapter II examines the impact of complete telomere-to-telomere (T2T) assemblies of great ape (and human) genomes on tRNA gene discovery. This has led to the identification of nearly 100 new human cytosolic-tRNA gene loci, particularly in regions of chromosome 1 associated with neural gene regulation and implicated in the neural expansion of humans and primates. The chapter also discusses the dynamic nature of tRNA loci copy number variation across tandem repeats and their roles on adjacent neural genes. Chapter III introduces two novel bioinformatics tools, tRNAgraph, and tRNAmap, designed to address the challenges of analyzing tRNA sequencing data across multiple species and experimental conditions. tRNAgraph offers multi-variate analysis, automated clustering, and classification of tRNAseq data, while tRNAmap aligns the ‘tRNAnome’ of eukaryotic species for cross-species comparison. These tools enable advanced analysis of the increasingly common tRNA sequencing data, contributing significantly to our understanding of tRNAs in neural development and evolution. This dissertation’s exploration into the complex roles of tRNAs and the innovative tools developed for their analysis not only advances our understanding of neural development and evolution but also paves the way for future genomic research to uncover the intricate mechanisms of gene regulation.

Planets both inside and beyond the solar system have different atmospheres, making them display various phenomena. With the increasing number of exoplanets detected, in particular with detailed observations taken by the advanced JWST nowadays, more samples are available to widen such diversity. One of the key challenges is to understand their atmospheric components and physical properties via remote sensing (like images, spectra, and lightcurves) rather than in-situ detection of probes, especially for these distant objects. So applying physical models could offer additional assistance in explaining currently limited data, as well as give some guidance for conducting future observations. Moreover, the Bayesian retrieval approach makes it possible to seek the most probable solutions in a high-dimension parameter space efficiently.

Chapter One describes two possible scenarios of Pluto's atmospheric haze, directly based on the temperature-pressure profile acquired by the New Horizons spacecraft. A radiative-conductive-conductive model was used to examine the energy balance of gases and haze individually, suggesting that an additional mechanism of eddy heat transport is essential in Pluto's lower atmosphere. An icy haze, 20 times smaller than Titan's tholins in opacity, is also proposed and will be determined by infrared observations of JWST.

Chapter Two further studies Pluto's haze by rotational emission lightcurves indirectly. The total outgoing thermal emission comes from not only Pluto's and Charon's surfaces as previously believed, but also Pluto's atmospheric haze as learned in Chapter One. A two-dimensional surface model of heat conduction is developed to estimate the surface parameters and flux contribution more accurately. After removing surfaces, the remaining total flux should come from Pluto's haze, which is significant in the mid-infrared but may be neglected in the far-infrared. Predictions at other mid-infrared wavelengths are then given in view of current haze knowledge. JWST MIRI works at these wavelengths to prove or constrain such haze.

Chapter Three jumps out of the solar system and pays attention to the sub-Neptune planets that are intermediate in size between Earth and Neptune. Helium enrichment due to preferential hydrogen escape is suggested for their thin atmospheric envelopes, which requires a method to obtain its helium amount correctly. Transmission spectra featured by the mechanism of collision-induced absorption are applied to retrievals for assessing how accurate it can be when future JWST data arrive. The results are optimistic in the helium-enhanced situation with 30 ppm errors.

In summary, this dissertation thesis consists of three projects exploring the physics of planetary atmospheres, two of which are constraining Pluto's haze opacity but from different aspects. The other one is loosely related to icy planets (like Pluto) and is mainly focused on the helium abundances in sub-Neptune's atmosphere. These projects cover the flux calculations of emission and transmission, along with the time or wavelength sequences, from the surface or atmosphere. The physical modeling method is applied as a bridge between the physical understanding of atmospheric components and current or future observations, in the era of JWST.

This dissertation presents the development of innovative Bayesian nonparametric models tailored for the complex demands of analyzing time-to-event data, specifically for problems in survival analysis. These models offer flexibility and computational efficiency in estimating various functionals of the survival distribution. The first thesis component introduces a flexible Erlang mixture model for survival analysis, structured on a weighted combination of Erlang densities with integer shape parameters, and a common scale parameter. The mixture weights are constructed through increments of a distribution function on the positive real line, which is assigned a Dirichlet process prior. The model balances general inference for survival functionals with efficient posterior simulation. The modeling approach is extended to accommodate multiple experimental groups through a dependent Dirichlet process prior. Moving to the second part of the dissertation, a Dirichlet process mixture model with a log-logistic kernel is proposed. The model incorporates covariates through a density regression framework, allowing variations in mixture weights and mixing parameters as functions of covariates. The model yields flexible inference for density, survival, and hazard functions across the covariate space. The final dissertation component explores a joint modeling approach for recurrent events and survival time, relevant for medical studies where the recurrent events process and the risk of death are related. Here, the density functions for the survival times and the gap times of recurrent events are modeled by dependent Dirichlet process mixtures with a log-logistic kernel. This modeling approach builds dependence between survival times and recurrent events through bivariate random effects. The joint modeling framework aims to provide flexibility in inferring marginal and conditional functionals of survival and gap times. For all proposed models, we discuss model properties, prior specification, and posterior simulation techniques, illustrating their effectiveness through synthetic and real data examples.

Highly confined lead halide perovskite nanomaterials, like perovskite quantum dots (PQDs) and other novel nanoclusters (PNCLs) have attracted attention owing to their superior optoelectronic properties and have great potential in fields like light-emitting diodes (LEDs), photodetectors, solar cells, and even now, quantum information processing. Despite their encouraging potential, PQDs exhibit optical and spin-optical performance degradation that can be due to chemical and structural instability that is strongly correlated to the presence of defects and electron-phonon interactions. Although extensive research has been done on the degradation in correlation to performance, in-depth experimental studies of defect and electron-phonon interactions influences on physical processes, such as electron spin relaxation and photoinduced charge separation and recombination, are generally lacking detailed mechanisms. The primary goal of this thesis is to reveal the optical and spin-optical dynamics and the role they play on material performance of novel lead halide perovskite nanomaterials. Precise chemical tuning of PQD structure, high-quality synthesis, QD passivation, and isotope enrichment, is an appropriate way to intuitively enhance the materials optical and spin-optical properties. Since charge carrier processes occur on femtosecond to microsecond time scales, sensitive time-correlated single photon counting and ultrafast pump-probe spectroscopic methods, such as time-resolved photoluminescence and femtosecond transient absorption, will be used to interrogate these physical processes. This method of tracking the photogenerated charge carriers will collect valuable information, like population densities and trap states near the band edges, that will be used to construct a kinetic model. This dissertation research will establish charge carrier mechanisms, identify electron-phonon coupling, investigate the influence of nuclear spin-electron spin interaction, and provide a window into how these phenomena affect material performance. This dissertation aims to establish a deeper intuition of the charge and spin carrier kinetics and provide insight into the structure-function relationships of multifunctional PQDs and other perovskite variants.

Recent interest in the dynamics of stratified turbulence has led to the development of newmodels for quantifying vertical transport of momentum and buoyancy (Chini et al 2022, Shah et al 2024). These models are still incomplete as they do not yet include all the relevant dynamics often present in real physical settings such as rotation and magnetic fields. Here we expand on prior work by adding rotation. Variation of the Rossby number affects the mean flow and perturbation dynamics independently. We conduct 3D direct numerical simulations of rotating, stochastically forced, strongly stratified turbulence (F r ≪ 1), and vary the Rossby number. We find that rotation gradually suppresses small-scale dynamics and, therefore, inhibits vertical transport as Ro decreases towards F r. The effect is particularly pronounced within the cores of emergent cyclonic vortices. For sufficiently strong rotation, vertical motions are suppressed.