Forum Schedule Fall 2022

In the coming years, several different experiments are poised to make measurements relevant to understanding Cosmic Dawn and the Epoch of Reionization. They Hydrogen Epoch of Reionization Array (HERA) is currently making measurements of the 21cm signal. The Simons Observatory will soon being observing the cosmic microwave background, including the kinetic Sunyaev-Zel’dovich (kSZ) effect. The James Webb Space Telescope, currently in flight, and the Nancy Grace Roman Space Telescope, scheduled to fly later this decade, will provide exquisite surveys of high-redshift galaxies. Taken together, these observatories will provide us with key insight on astrophysics and cosmology from the high-redshift universe, such as the global ionization fraction and the optical depth to the CMB tau. In this talk, I provide updates on measurements made by HERA, as well as explore how these measurements can be combined in cross-correlation to infer some of these parameters. I look in detail at the kSZ^2–galaxy cross-correlation, as well as the 21cm–galaxy cross-correlation. I show that these statistics are likely detectable with data from upcoming experiments, though there are still several observational challenges that must be overcome when combining these measurements in practice.

Materials are generally classified into crystalline and non-crystalline types, depending on whether long-range lattice periodicity is present in the material. In this talk, I will introduce a paracrystalline state of materials, as exemplified by the synthesis of paracrystalline diamond [H. Tang et al., Nature 599, 605, 2021] and a new type of oxide glass. The paracrystalline state of diamond is structurally distinct from either crystalline or amorphous diamond, consisting of sub-nanometer-sized clusters with well-characterized crystalline medium-range order (MRO). The paracrystalline diamond was synthesized under high-pressure and high-temperature conditions, with fullerene being its precursor, and was characterized through high-resolution X-ray diffraction, transmission electron microscopy, and other spectroscopy techniques. The paracrystalline diamond exhibits different properties from the conventional diamond we are familiar with. Theoretically, I will focus on how to identify the structural characteristics of the paracrystalline diamond as well as its formation mechanism through advanced computer simulation, including classical molecular dynamics incorporating a dynamics-acceleration technique, first-principles simulation, and the development of order parameters appropriate for characterizing its crystalline medium-range order. At last, I will shed light on our recent discovery of paracrystalline grossular, which possesses a record-high fracture toughness among all known oxide glasses. The discovery of paracrystalline materials provides new strategies for materials design but also unveils a new structural type linking nanomaterials and amorphous materials, revealing how the structural modification on the atomic level can profoundly affect the properties.

Active galactic nucleus (AGN) disks may be important sites for stellar-mass binary black hole (BBH) mergers, but the detailed processes that lead to a BBH merger in an AGN disk are not yet well-constrained. Binary formation in AGN disks could be efficient due to the so-called migration trap in AGN disks. Dynamical encounters in the migration trap could play a critical role in merging binaries in the AGN channel. I will show via numerical experiments with the high-accuracy, high-precision code SpaceHub that broken symmetry in dynamical encounters in AGN disks can lead to an asymmetry between prograde and retrograde BBH mergers. Asymmetric distribution of mass-weighted projected spin of the BBH mergers unlikely to be predicted in other merger channels will show in the AGN merger channel. I will also present the open-source few-body gravity integration toolkit SpaceHub. SpaceHub offers a variety of algorithmic methods, which we show outperform other methods in the literature and allow for fast, precise, and accurate computations to deal with few-body problems ranging from interacting black holes to planetary dynamics. With algorithmic regularization, chain algorithm, active round-off error compensation, and a symplectic kernel implementation, SpaceHub is the fastest and most accurate tool to treat black hole dynamics with extreme mass ratios, extreme eccentricities, and very close encounters.

Jupiter is the largest planet in the Solar System. It is composed of hydrogen and helium with heavier species such as water. Jupiter is radiating an internal energy flux to space much larger than the absorbed solar flux. It was believed that the heat flow was transported to the photosphere via vigorous convection, in which warm air rises and cold air sinks. In this talk, I will show this traditional picture is not correct using both observations and simulations. The convective heat transport is significantly inhibited in the stably stratified region in Jupiter's weather layer where water condenses. Instead, we argue that Jupiter's atmosphere behaves like an air conditioner using water as the refrigerant. The active hydrologic cycle with water condensation and evaporation produces intermittent moist storms that transport the latent heat across the stable layer. Our new theory explains mysterious observations from the Galileo and Cassini spacecraft and potentially reconciles the dilemma from the recent gravity measurement by the Juno spacecraft. Finally, I will talk about the impact of this new framework to understand other planets, exoplanets, and brown dwarfs with exciting discoveries from recent James Webb Space Telescope Early Release Science Programs.

With the development of multi-messenger astronomy and the detection of astrophysical neutrinos, we are in a golden age of new cosmic ray (CR) measurements and studies. Supernova remnants are believed as the leading candidate for the source of Galactic CRs. Recent observations, however, challenge the well-developed theory of diffusive shock acceleration of CRs in supernova remnants. I will reexamine the assumptions in the standard shock acceleration model and discuss the key missing physics for shocks propagating in the turbulent and inhomogeneous interstellar medium. A more realistic shock acceleration model holds the promise to understand the observational puzzles and the origin of Galactic CRs.

Neutron stars offer us an excellent testbed to explore extreme gravity and dense-matter physics that is difficult to access with terrestrial experiments. Recent multimessenger observations through radio, x-ray, and gravitational waves have advanced our understanding of the nature of gravity and supranuclear matter. In this talk, I will present certain universal relations among neutron star observables and nuclear parameters that are highly correlated and are insensitive to the underlying equations of state of nuclear matter. I will then explain how one can use such universal relations to perform a unique strong-field gravity test and infer nuclear parameters through multimessenger observations of neutron stars.

Neutrinos are the ideal messenger for high-energy astrophysics. Weakly interacting and uncharged, they propagate undeterred and unabsorbed through the universe. In the last decade, we have observed a flux of high-energy (TeV-scale) neutrinos and through a multi-messenger lens — the combined observations of neutrinos and other messengers like photons — we are starting to see hints of energetic neutrino sources for the first time. At higher energies still, beyond the PeV scale, we can probe the most energetic sources of both neutrinos and cosmic rays, but current neutrino experiments become too small to observe a sizable flux. Instead, we can use embedded radio experiments which detect the coherent radio emission from neutrino interactions in ice using a sparse array of detectors to build up enormous neutrino targets. In this talk, will describe in-ice detectors, focusing especially on the Radio Neutrino Observatory in Greenland (RNO-G), an experiment currently being built in Greenland and how it can serve as a testbed for the radio array for IceCube-Gen2.