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Astrophysics Seminars


The CASS Astrophysics Seminar features world-class astrophysicists from around the world speaking on current topics of research. Presentations are aimed at the graduate and post-graduate level, but are open to the general public. CASS seminars take place on Wednesdays from 3:00 - 4:00 p.m. in 383 SERF (Marlar Seminar Room), unless otherwise noted. You can watch a live stream of the talk or prior talks at the CASS Seminar YouTube Channel. The organizers are Prof. Quinn Konopacky and Dr. Alexei Kritsuk.

Upcoming Seminars

Spring 2019

March 27, 2019

NOTE: Spring Break - no seminar today

April 3, 2019

 "Engineering the Next Generation of Exoplanet Imagers"

Dmitry Savransky
Assistant Professor of Mechanical and Aerospace Engineering
Cornell University

 As exoplanet direct imaging has rapidly matured and is now providing invaluable data and greatly expanding our understanding of how planetary systems form and evolve, we are now well positioned to consider the ultimate goal of discovering and characterizing Earth-like planets with future, space-based exoplanet imaging instrumentation. In this talk, I will discuss a variety of projects currently being worked on at the Space Imaging and Optical Systems Lab at Cornell University in support of this goal. These projects span numerous disciplines and touch upon various aspects of a space mission, including optimization of the overall mission, linking predicted science yields to engineering decisions, as well as investigating data post-processing techniques and the control of optical systems with mechanical degrees of freedom.

I will present our software framework for the simulation of exoplanet imaging missions (EXOSIMS) and the particular challenges of metric-driven systems engineering in the context of missions of discovery. I will also discuss our recent work on applying common spatial pattern filtering to point source extraction in the weak signal-to-noise regime, as well as our applications of optimal control techniques to wavefront sensing. Finally, I will present a possible new path towards future, giant space-based observatories.

April 10, 2019

 "A Cosmic Test of Quantum Entanglement and Bell's Inequality: Choosing
Measurements with Light from High Redshift Quasars"

Andrew Friedman
Assistant Research Scientist

 In our recent experimental tests of Bell’s inequality, for the first time, we used observations of astronomical sources to randomly choose measurement settings for polarization-entangled photons sent through free space to two distant detectors. In our 2017 pilot test in Vienna, Austria, we used the color of light from Milky Way stars, and in 2018, we performed the first Cosmic Bell test in the Canary Islands using light from high redshift quasars. In both sets of tests, we observed statistically significant violation of Bell’s inequality, implying that John Bell’s very reasonable assumptions about the world cannot all be true in nature. These assumptions include realism/determinism, locality, and experimental freedom of choice. Our tests aim to put tension on the latter assumption, arguably the most subtle of Bell’s axioms, which holds that each detector’s measurement choices are completely free of any non-quantum degrees of freedom in the causal past of the experiment that could also affect the measurement outcomes. Since the nearest star in the pilot test was ~600 light years away, given our assumptions, the observed Bell violation implies that any non-quantum explanations for entanglement must have been in place prior to ~600 years ago, an improvement of ~16 orders of magnitude compared to previous tests. Similarly, since the nearest quasar in our best experimental run was 7.8 billion light years away, any such mechanism is relegated an additional ~6 orders of magnitude into the cosmically distant past, corresponding to excluding non-quantum explanations from 96% of the space-time volume in the causal past of the experiment. In addition to exploring how free our experimental choices are while investigating the fundamental nature of reality in the subatomic world, such foundational tests are relevant to whether practical quantum encryption schemes will ultimately be as secure as many researchers believe. Our other new theoretical work shows that relaxing freedom of choice by a relatively small amount can still reproduce the quantum predictions, making the associated loophole easier to exploit than other well known entanglement test loopholes.

April 17, 2019

 "The Hidden Baryons of the Milky Way"

Yong Zheng
Miller Postdoctoral Fellow
UC Berkeley

 Galaxies are not lonely islands floating in the Universe. They host large gaseous envelopes of baryons, a.k.a., the circum-galactic medium (CGM), that exist far beyond a galaxy’s visible extent. Baryonic inflows from CGM replenish star-forming fuel in galaxies, whereas outflows from galaxies enrich the CGM. In this talk, I will describe the theoretical and observed distribution and flows of baryons in the MW’s CGM, including how the MW’s disk hides up to half of its CGM from direct observation. I will then describe new techniques to generate synthetic observations of the CGM using the Enzo & FOGGIE cosmological simulations, and show how these can be used to reveal the hidden baryons in the MW’s CGM. Finally, I will briefly highlight the connections between low- and high-redshift CGM studies, including new applications that rely on fast radio bursts (FRBs).

April 24, 2019

 "Gravitational wave signal from primordial hydro-magnetic turbulence"

Tina Kahniashvili
Associate Research Professor
Carnegie Mellon University

 The first direct detections of gravitational waves from the mergers of binary black holes and binary neutron stars by the LIGO and VIRGO experiments have electrified the physics and astronomy communities. A clear next experimental step is an interferometer in space, which can detect lower frequency signals than a ground-based detector, including supermassive black hole binary coalescences from early galaxy mergers and a known stochastic background from confusion-limited white-dwarf binaries. An even more intriguing signal is the stochastic background from early-universe physics. In this talk I will present our recent work (in collaboration with Axel Brandenburg, Arthur Kosowsky, Sayan Mandal, and Alberto Roper Pol). Using direct numerical simulations of early universe hydromagnetic turbulence with energy densities of up to 10% of the radiation energy density, we show that gravitational waves with energy densities of about 10-10 times the critical energy density of the Friedmann universe today were produced. Their characteristic strain today is found to be about 10-20 and should be observable with the Laser Interferometer Space Antenna (LISA) in the mHz range. The gravitational waves have positive (negative) circular polarization if the magnetic field has positive (negative) magnetic helicity. The gravitational wave energy reaches a constant value after the turbulent energy (kinetic or magnetic) has reached its maximum. Compressive modes are found to produce about 10 times stronger gravitational waves than solenoidal ones. Finally, I will discuss the range of phase transition energy scales and properties that may be detectable with the envisioned space-based interferometer configurations such as LISA.

May 15, 2019

Richard Lingenfelter
Research Physicist Emeritus

May 29, 2019

Arun Surya
Postdoctoral Scholar

June 5, 2019

Greg Mace
Research Associate
University of Texas, Austin

June 12, 2019

Melville Ulmer
Northwestern University