Talks & Events
From Large to Small: Symmetries and the Origins of Structure in the Universe
What was the universe like in its first moments? Remarkably, we can gain insight into the infancy of universe by looking at the largest scales today, using subtle correlations imprinted at very early times. This striking connection between the very large the very small is an opportunity to use cosmological observations to probe high energies, and also to bring modern theoretical tools to bear on the deep questions that cosmology presents us with. I will discuss recent progress in both directions, highlighting the power of effective field theory and symmetries as guiding principles. As an example, I will explain how the symmetry-oriented viewpoint can help unravel the origins of structure in the universe by enabling us to derive powerful model-independent tests of the simplest inflationary paradigm. Violations of these relations can signal the presence of new heavy particles, allowing us to use the primordial universe as the highest-energy particle accelerator. I will also describe recent theoretical advances in understanding the signatures of these heavy particles, and will explain how these advances could help shed light on fundamental questions, like the emergence of time in cosmology.
Bridging Galaxy Evolution Across Cosmic Time: Tracing the Interplay between Massive Stars and the Interstellar Medium with Spectroscopy
The first stars and galaxies initiated the epoch of reionization (EoR) and provided the seeds from which all galaxy evolution grew. Knowledge of the properties of these galaxies are needed to understand ionizing photon production and escape, andwill provide the crucial missing link needed to weave a coherent picture of galaxy evolution.I will present several programs that are establishing the needed framework to interpret the spectra of galaxies from z~0‒10, bridging the present-day and early universe. These programs use multi-wavelength spectroscopy to disentangle the spectral signatures that characterize the interplay between massive stars and their surroundings, and allow us to interpret how radiative processes shape galaxies. I will show how precise measures of the stellar and nebular properties of both nearby and distant lensed galaxies directly link the ionizing stellar populations with the baryon+metal feedback cycle and the conditions of ionizing photon production and escape. My studies provide a detailed foundation of the diversity of local star-forming galaxies with which to interpret cosmic evolution, as well as unique laboratories of nearly pristine gas in which to test conditions analogous to the first galaxies. In preparation for the coming UV window onto the early universe with the advent of the James Webb Space Telescope and the Giant Magellan Telescope, I will introduce the COS Legacy Archival Spectroscopic SurveY (CLASSY) - an upcoming large HST program that will produce the first high-resolution UV spectral atlas of star-forming galaxies. CLASSY will calibrate new tools that will allow us to completely describe the stars and interstellar medium in galaxies across redshift, setting the stage to study cosmic origins, ionizing production, and the evolution of galaxies in a unified framework.
Scintillating Bubble Chambers
Moderately superheated bubble chambers have proven to be an excellent method for WIMP hunting thanks to their world-leading electron-recoil discrimination, easy scalability, and diversity of potential WIMP targets. While the PICO Collaboration continues to increase the size and sensitivity of these devices, the successes of the past decade have also enabled a new bubble chamber variant where the superheated target is also a liquid scintillator. In these Scintillating Bubble Chambers, the nuclear recoil from a WIMP interaction simultaneously nucleates a bubble and creates a flash of scintillation light. On paper, this technique combines the electron recoil discrimination of a bubble chamber with the event-by-event energy reconstruction of a scintillator. In practice, these two signals conspire to allow scintillating bubble chambers to run at much lower thresholds than can be achieved in a standard PICO chamber. Superheated noble liquids, in particular, may be completely insensitive to electron recoils even when running at thresholds as low as 100 eV. I will describe our current understanding of why scintillating bubble chambers can reach these low thresholds, review the unique dark matter and neutrino physics open to a detector capable of electron/nuclear recoil discrimination at sub-keV energies, and update our progress on the first physics-scale scintillating bubble chamber, a 10-kg argon detector now under construction at Fermilab.
Accessing the First Stars and Galaxies with Near-Field Cosmology
In the first billion years of the universe, stars and galaxies formed in the smallest dark matter halos, produced high-energy photons that reionized the intergalactic medium, and polluted the universe with the first heavy elements. Near-field cosmology probes this early era by observing nearby relic galaxies that have survived from ancient times. In particular, the elemental abundances of their old, metal-poor stars encode otherwise inaccessible information about the first stellar populations and first galaxy formation histories. Decoding these abundances requires connecting nuclear and stellar astrophysics to galaxy formation and hierarchical assembly. In this talk, I will use elements synthesized in the rapid neutron-capture process (r-process) to illustrate how I have used stellar abundances in dwarf galaxies to study the first stars and galaxies. My work has shaped our current understanding of the origin of r-process elements, informed future multi-messenger observations of neutron star mergers, produced unique constraints on gas dynamics in the first galaxies, and now enables reconstruction of the hierarchical assembly of our Milky Way's stellar halo. I will conclude with a blueprint for how to measure the old stellar populations and early star formation histories of galaxies across the Local Group, making near-field cosmology an observational pillar for accessing the high-redshift universe.
Measuring the Epoch of Reionization with Line Intensity Mapping using TIME
TIME is an instrument being developed to use line intensity mapping (LIM) to study emission from the faint objects in our universe. We will use this instrument to study the epoch of reionization, advancing our understanding of the first astronomical objects that ionized the neutral hydrogen in the universe. TIME is a mm-wavelength spectrometer using Transition Edge Sensor (TES) bolometers. The instrument spans the frequency range of 200-300 GHz with 60 spectral pixels and 16 spatial pixels. TIME will measure redshifted ionized carbon ( [CII] ) emission over the redshift range 5 to 9 in order to probe the evolution of our universe during the epoch of reionization. TIME will also detect low-redshift CO fluctuations and determine the cosmic history of molecular gas in the epoch of peak cosmic star formation, redshift 0.5 to 2. This new instrument and emerging technique will allow us to make complementary measurements to galaxy surveys that are probing these epochs. TIME was installed for an engineering test on the 12m ALMA prototype antenna in Spring of 2019 at the Arizona Radio Observatory on Kitt Peak and will return to the telescope for 3 seasons of science observations in ~ winter 2020/2021.
Fast and furious: magnetic reconnection in relativistic jets and black hole coronae
Relativistic jets of blazars and magnetized coronae of low-luminosity accretion flows, like Sgr A* at our Galactic Center, routinely display fast and bright flares of high-energy emission. Yet, the "engine" responsible for accelerating the emitting particles to ultra-relativistic energies is still unknown. With fully-kinetic particle-in-cell (PIC) simulations, we argue that magnetic reconnection - a process by which magnetic field lines of opposite polarity annihilate, releasing their energy to the particles - can satisfy all the basic conditions for the emission. In blazar jets, we show that reconnection can naturally explain the puzzling ultra-fast bright flares observed at GeV and TeV energies, whose duration can be even shorter than the light-travel time across the black hole that powers the jet. In low-luminosity accretion flows like Sgr A*, we show that reconnection - potentially seeded by turbulence - can power both thermal and non-thermal emission, and we produce physically-grounded synthetic images and spectra to be compared with infrared and X-ray observations and with the upcoming results of the Event Horizon Telescope.
SPHEREx: An All-sky Infrared Spectral Survey Explorer Satellite
SPHEREx, a mission in NASA's Medium Explorer (MIDEX) program, is an all-sky survey satellite designed to address all three science goals in NASA's astrophysics division, with a single instrument, a wide-field spectral imager. We will probe the physics of inflation by measuring non-Gaussianity by studying large-scale structure, surveying a large cosmological volume at low redshifts, complementing high-z surveys optimized to constrain dark energy. The origin of water and biogenic molecules will be investigated in all phases of planetary system formation - from molecular clouds to young stellar systems with protoplanetary disks - by measuring ice absorption spectra. We will chart the origin and history of galaxy formation through a deep survey mapping large-scale spatial power in two deep fields located near the ecliptic poles. Following in the tradition of all-sky missions such as IRAS, COBE and WISE, SPHEREx will be the first all-sky near-infrared spectral survey. SPHEREx will create spectra (0.75 '?'€'?' 3.8 um at R ~ 40, and 3.8 '?'€'?' 5 um at R ~ 120) with high sensitivity using a cooled telescope with a wide field-of-view for large mapping speed. During its two-year mission, SPHEREx will produce four complete all-sky maps that will serve as a rich archive for the astronomy community. With over a billion detected galaxies, hundreds of millions of high-quality stellar and galactic spectra, and over a million ice absorption spectra, the archive will enable diverse scientific investigations including studies of young stellar systems, brown dwarfs, high-redshift quasars, galaxy clusters, the interstellar medium, asteroids and comets.
Parker Solar Probe: Understanding Coronal heating and Solar Wind Acceleration
The magnetic field is fundamental to solar activity and shapes the inter-planetary environment, as shown by the full three dimensional monitoring of the heliosphere provided by measurements from many past and present interplanetary and remote sensing spacecraft. Magnetic fields are also the source for coronal heating and the very existence of the solar wind; produced by the sun's dynamo and emerging into the corona, magnetic fields become a conduit for waves, act to store energy, and then propel plasma into the Heliosphere in the form of Coronal Mass Ejections (CMEs). Magnetic fields are also at the heart of the generation and acceleration of Solar Energetic Particle (SEPs) that modify the space weather environment of the Earth and other planets.
Parker Solar Probe (PSP) was launched in August 2018 to carry out the first in situ exploration of the outer solar corona and inner Heliosphere. Direct measurements of the plasma in the closest atmosphere of our star should lead to a new understanding of the questions of coronal heating, solar wind acceleration, and the generation, acceleration and propagation of SEPs.
In this lecture I will start with an introduction to our present knowledge of the magnetized solar corona and wind before describing the PSP scientific objectives, orbit, and instrument suites, and showing results from the first three orbits. Emphasis will be on how PSP will confirm or falsify present wind models as well as the potential new discoveries stemming from the first exploration of the space inside the orbit of Mercury. I will also discuss how synergies with Solar Orbiter might lead us to accurately understand the state of the solar wind all the way from the corona into interplanetary space, a stepping stone for understanding the dynamics of active magnetized plasmas throughout the universe.
The Cosmology Large Angular Scale Surveyor
The Cosmology Large Angular Scale Surveyor (CLASS) is a project to probe reionization and inflation by measuring the largest scales in the Cosmic Microwave Background (CMB) polarization. These scales are made accessible due to specialized front-end modulation technology and a survey strategy that covers 75% of the sky from Chile's Atacama Desert. Operating since 2016, CLASS observes at frequencies from 40 to 220 GHz to distinguish between CMB and Galactic emission. We are currently publishing results from the first two years of 40 GHz observations, which demonstrate recovery of large angular scale polarization. In this talk, I will give an overview and an update on CLASS as well as a discussion of the two-year results.
Cold Gas in Hot Halos: The Formation and Survival of Cold Gas in Galactic Halos
In recent years, observations of the circumgalactic medium has undercovered a large reservoir of T ~ 10^4 K, photoionized gas in the much hotter halos of galaxies. Inflowing cold gas in galactic halos helps fuel star formation, whilst outflowing cold gas is our primary observational marker of feedback. However, the formation and survival of dense cold gas in the atmospheres of galaxy halos is still poorly understood. For instance, we do not yet understand how cold gas can be entrained in a hot wind, as is observed; most simulations indicate it should be shredded by hydrodynamic instabilities. The small scale structure of the cold gas is also poorly understood; galaxy formation simulations show CGM properties which are not converged numerically, and it is not clear what scales need to be resolved to achieve convergence. In this talk I will highlight some recent progress on these questions.
New Approaches to Galaxy Clustering
All large-scale structure cosmologists are faced with the question: how do we robustly extract cosmological information, such as on dark energy, gravity, and inflation, from tracers such as galaxies whose astrophysics is extremely complex and incompletely understood? Based on advances in our theoretical understanding of large-scale structure, we now know how to absorb these complexities into free "nuisance" parameters on large scales. This opens up the possibility for physically robust cosmology inference from galaxy clustering. However, to really make use of the power of this approach, we have to go beyond current analyses based on the power spectrum. I will describe a novel approach that attempts to extract the information in the entire galaxy density field, rather than compressing it into summary statistics, while robustly marginalizing over the complexities of galaxy formation.
Thermal History of the Intergalactic Medium Since Reionization
The structure of the intergalactic medium (IGM) encodes information on the physics of structure formation and the late thermal history of baryons in the universe. The power of IGM observations, such as probes of the Lyman alpha forest of absorption lines tracing the cosmic web, to constrain the shape of the power spectrum has motivated in part new experiments like the Dark Energy Spectroscopic Instrument that seek to understand the development of cosmic structure. The properties of Lyman alpha forest are heavily influenced by its thermal history, which determines its pressure smoothing and small scale structure. A thorough understanding of the thermal history is therefore crucial to interpreting constraints on the power spectrum. In this talk, I'll show how we can leverage a new computational approach that enables high resolution throughout cosmological volumes to compute the thermal history of the IGM in fine detail. I'll show how the reionization history of the IGM influences the subsequent evolution of the Lyman alpha forest, and discuss how current and forthcoming experiments will teach us about both the thermal evolution and physical structure of the IGM after reionization.