PHYSICS COLLOQUIUM Fall 2020

Thursdays 4:00 p.m.

The colloquia will be held via Zoom unless otherwise noted.

For the Zoom link, please contact the colloquium organizer Dr. Shun Saito

Title: Cosmology from galaxies and lensing in the Dark Energy Survey

Abstract: The interplay of the gravity of dark matter, and of dark energy driving the accelerated expansion of the Universe, determines the growth of large-scale structure: galaxies, clusters of galaxies, and even larger density fluctuations. The Dark Energy Survey has now measured structures in the evolved Universe using the distribution of galaxies and their gravitational lensing signal in its first-year and three-year data. I will give an overview of how to learn about cosmology from these analyses, and on the methodology of extracting gravitational lensing signals from astronomical images with small and quantified systematic uncertainties. From a range of complementary properties of the matter density field -- its two-point correlation, its full PDF, and its peaks -- DES Y1 provides the best picture yet of evolved structures on cosmological scales. DES data now available challenges our understanding of cosmological models and details of the connection of galaxies, clusters, and matter density, a glimpse of what LSST will see soon.

Title: Nuclear structure and astrophysics with TPC detectors and gamma beams

Abstract
An Optical TPC detector has been used in combination with quasi-monoenergetic gamma beams at
the HIgS facility (Duke University) to study alpha clustering and key reactions in nuclear astrophysics.
The 16O(ɣ,α) and the 12C(ɣ,3α) reactions were studied. The inverse of the former reaction is crucial
for determining the carbon-to-oxygen ratio after helium burning, which plays a large part in dictating a star's evolution. Exploring the inverse reaction in conjunction with the TPC has a number of
key advantages.
The latter 12C(ɣ,3α) reaction was studied to provide insight into the structure of the Hoyle state.
The reaction populated the 10 MeV 2+ state in 12C, which is proposed as a collective excitation of
the Hoyle state. Its various decay modes into three alpha-particles were studied and an upper limit
for the direct 3 alpha decay branching ratio was calculated. Theoretical calculations were then used
to extrapolate and calculate the corresponding branching ratio for the Hoyle state, which was found
to be lower than expected for certain alpha cluster configurations.

The beginnings of gravitational wave astronomy: current state and future

Abstract: The first detection of gravitational waves was made in September 2015 with the measurement of the coalescence of two ~30 solar mass black holes at a distance of about 1 billion light years from Earth. The talk will provide a review of more recent measurements of black hole events as well as the first detection of the coalescence of two neutron stars and the beginning of multi-messenger astrophysics. The talk will end with a discussion of some prospects for the field.

Searching for “Fifth-forces”, Dark Matter, and Quantum Gravity in the Lab

 
Abstract: We normally think of large accelerators and massive detectors when we consider the frontiers of elementary particle physics, pushing to understand the universe at higher and higher energy scales. However, several tabletop low-energy experiments are positioned to discover a wide range of new physics beyond the Standard model of particle physics, where feeble interactions require precision measurements rather than high energies. In high vacuum, optically-levitated dielectric nanospheres achieve excellent decoupling from their environment, making force sensing at the zeptonewton level (10-21N) achievable. In this talk I will describe our progress towards using these sensors for tests of the Newtonian gravitational inverse square law at micron length scales and the Casimir Force. Optically levitated dielectric objects and show promise for a variety
of other applications, including searches for gravitational waves and Dark Matter. Cooling these objects into the quantum regime of their motion paves the way for tests of quantum coherence in macroscopic objects and tests of whether gravity can destroy such coherence. Looking forward, these systems may enable us to examine the
role that gravity plays in quantum mechanical entanglement.

Dr. Marco Cavaglia and Dr. Shun Saito will present a lecture about the 2020 Nobel Prize in Physics, which was awarded to Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity” and to Reinhard Genzel and Andrea Ghez “for the discovery of a supermassive compact object at the centre of our galaxy”.
The lecture is intended for a general audience and will be suitable for interested community members.

 

A complete experiment on multi-photon ionization of ultra-cold and polarized atoms
Bishnu Acharya
Advisor:  Dr. Daniel Fischer

Phase boundary near a magnetic percolation transition
Gaurav Khairnar
Advisor:  Dr. Thomas Vojta

High-harmonic generation in Dirac metals:  Application of the semiconductor – Bloch formalism to topological systems
Jack Crewse
Advisor:  Dr. Thomas Vojta

Search for Gravitational Waves from Magnetar Bursts

Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are thought to be magnetars, neutron stars with very strong magnetic fields. They emit sporadic bursts of hard X-rays and soft gamma rays, including rare energetic events known as giant flares. Recently, a fast radio burst (FRB) was observed from a magnetar in the Milky Way, providing evidence that magnetars can likely produce at least some of the observed extra-galactic FRBs. Quasiperiodic oscillations (QPOs) in the X-ray tails of giant flares and some bursts may be due to excited non-radial modes in the star which could emit gravitational waves. Previous analyses on LIGO data have been performed targeting magnetar bursts and a period of time following for short and long-duration gravitational wave transients. This talk gives an overview of magnetars, their GW search history, with a focus on the recent long-duration burst searches, and considers the outlook for future searches.