Forum Schedule

Fridays 3:45pm - 4:45pm

Date Speaker Topic (click down-arrow to see abstract)
Jan 23

Jan 30 Prof. Fulvio Melia
The University of Arizona
host: Dr. Bing Zhang
The Zero Active Mass Condition in FRW Cosmologies

Abstract: The standard model of cosmology is based on the Friedmann-Robertson-Walker (FRW) metric. Often written in terms of co-moving coordinates, this elegant and highly practical solution to Einstein's equations is based on the Cosmological principal and Weyl's postulate. But not all of the physics behind such symmetries has yet been recognized. We invoke the fact that the co-moving frame also happens to be in free fall to demonstrate that the FRW metric is valid only for a medium with zero active mass. In other words, the application of FRW appears to require an equation-of-state rho+3p = 0, in terms of the total energy density rho and total pressure p. Though the standard model is not framed in these terms, the optimization of its parameters brings it ever closer to this constraint as the precision of the observations continues to improve.

Feb 6

Feb 13

Feb 20

Feb 27

Mar 6 (10:00AM) Jean-Michel Desert
CU Boulder
host: Dr. Rebecca Martin
New Frontiers in Exoplanetary Science

The field of exoplanetology has recently transitioned from the investigation of individual objects to statistical studies. However, answers to key questions in exoplanetary science come not only from the statistics of discovery surveys, but also from the detailed characterisation of individual systems. I argue that the study of exoplanet atmospheres and their diversity is the next step in leveraging their detections. This is because a planet's atmosphere provides a fossil record of its primordial origins and controls its fate, size and appearance. The study of exoplanet atmospheres thus is crucial to answer fundamental questions in planetary formation and exoplanetary physics. In this context, I present new results from ongoing comparative exoplanetology programs that aim to characterise planetary systems transiting nearby stars through the observations of their atmospheres. This is achieved by combining ground- and space-based multi-wavelength observations secured over wide spectral regions. The results on the atmospheric composition and physical properties provide insights into the formation and evolution of planetary systems and enhance our understanding of our own Solar System's formation. Finally, I also present strategies for probing rocky exoplanet atmospheres orbiting in the habitable zone of their parent stars, and for searching for bio-signatures with future facilities.

Mar 6 Samaya Nissanke
Radboud University, the Netherlands
host: Dr. Rebecca Martin
Follow the chirp: seeing and listening to the transient Universe

The mergers of binary compact objects (black holes, neutron stars, white dwarfs) are amongst some of the most violent events in the Universe. The physics driving these events in strong field gravity are extremely complex, rich but still remain elusive. These cosmic laboratories present us now with both a challenge and an opportunity. The challenge is to explain the physics at play in strong-field gravity in Universe. The opportunity is to detect the accompanying radiation and panoply of multi-messenger particles (high energy neutrinos, cosmic rays and gravitational waves) for the first time with a suite of time-domain telescopes and experiments. In this pivotal new era of multi-messenger astronomy, the most compelling astrophysical sources are neutron star binary mergers, which should emit both in electromagnetic (EM) and gravitational waves (GW). I will first review the most recent advances in this blossoming field of EM+GW astronomy, which combines two active disciplines: time-domain astronomy and general relativity. I will discuss the promises of this new convergence by illustrating the wealth of astrophysical information that a combined EM+GW measurement would immediately bring. I will then outline the main challenge that lies ahead for this new field in pinpointing the sky location of neutron star mergers using GW detectors and EM wide-field synoptic surveys.

Mar 10 (Tuesday 3:30) Jason Steffen
Northwestern University
host: Dr. George Rhee
Properties of exoplanets and systems with Kepler

NASA's Kepler mission has revolutionized the field of exoplanets and its discoveries give new insights into our theories of planet formation and dynamical evolution. With over 4000 planet candidates and 1000 confirmed planets, the variety of systems and planets shows the breadth of properties that planet formation models must encompass. I present some of the landmark results of the Kepler mission, especially relating to the planet masses and orbital architectures of the planetary systems. I discuss how these results affect our understanding of the solar system and of planets in general.

Mar 12 (Thursday 3:30) Dr. Norbert Werner
Kavli Institute for Particle Astrophysics and Cosmology
Stanford University
host: Dr. George Rhee
How supermassive black holes and star-formation sculpt the visible Universe

In the course of structure formation, only a small fraction of the baryons turned into stars - most remain in a diffuse intergalactic medium. The growth of galaxies is regulated by feedback processes, such as energy and momentum input from supernovae, and jets and winds of accreting supermassive black holes. These processes, collectively called galactic feedback, can limit or even inhibit star formation, and thus a detailed knowledge of how they work is essential for our understanding of galaxy formation and evolution. I will start my talk by presenting recent observational results on the role of supermassive black holes in keeping the most massive galaxies 'red and dead'. Then, I will 'zoom out' to the outskirts of galaxy clusters where we also find hints that supermassive black holes played an important role in the distant past. X-ray observations with the Suzaku satellite reveal a remarkably homogeneous distribution of iron out to the virial radius of the nearby Perseus Cluster, requiring that most of the metal enrichment of the intergalactic medium occurred before the cluster formed, probably more than ten billion years ago, during the period of maximal star formation and black hole activity. Finally, I will talk about the upcoming ASTRO-H satellite which will revolutionize X-ray spectroscopy and our understanding of how feedback processes couple to the intergalactic medium.

Mar 13 Prof. Florian Rodler
Harvard Center for Astrophysics
host: Dr. George Rhee
High-resolution spectroscopy of exoplanet atmospheres is cool!

High-resolution spectroscopy is a powerful tool to investigate the atmospheres of transiting exoplanets as well as of non-transiting ones. I will briefly review the basics underlying that technique and present the most important results up to now. In the light of the three upcoming Extremely Large Telescopes (ELTs), i.e. the GMT, TMT and E-ELT, I will further present feasibility studies dedicated on the detection of oxygen in the atmospheres of Earth-like planets orbiting M-dwarfs.

Mar 17 (Tuesday 3:30) Jake Simon
Southwest Research Institute (SwRI)
CU Boulder
host: Dr. George Rhee
Probing the Nature of Accretion and Planet Formation in Protoplanetary disks: Connecting Theory with ALMA Observations

Protoplanetary disks play a key role in star and planet formation processes. Turbulence in these disks, which arises from the magnetorotational instability (MRI), not only causes accretion of mass onto the central star, but also sets the conditions for processes such as dust settling, planetesimal formation, and planet migration. However, the exact nature of this turbulence is still not very well constrained in these systems. In this talk, I will first present recent numerical simulations of magnetohydrodynamic (MHD) turbulence in protoplanetary disks that point to the importance of large scale, vertical magnetic fields in driving disk accretion through both turbulent processes and magnetic winds. I will then describe new work, utilizing both state-of-the-art numerical simulations and powerful new radio observations, to directly link numerical predictions for the turbulent velocity structure of protoplanetary disks to observations by the Atacama Large Millimeter Array (ALMA). ALMA’s unprecedented resolution and sensitivity will allow us to generate a three-dimensional map of disk turbulence by measuring the turbulent broadening component of molecular lines at different disk heights (i.e., optical depths) and radii. A direct comparison between the observed turbulence values and those obtained from simulations will strongly constrain our theoretical understanding of these disks. I will conclude with an outlook for protoplanetary disk studies, and in particular how our current results may influence studies of planet formation processes and the construction of exoplanetary systems.

Mar 18 (Wednesday 10:45) Wei Bao
Renmin University of China, Beijing
host: Dr. Andrew Cornelius
High-Pressure Single Crystal Neutron Scattering Study of the 245 Superconductor

The iron vacancy order and the block antiferromagnetic order exist in the new iron 245 superconductors [1,2]. The appearance of the superconductivity crucially depends on the perfectness of the vacancy order [3]. The magnetic and vacancy orders in superconducting (Tl,Rb)2Fe4Se5 (245) single-crystals were investigated using high-pressure neutron diffraction technique [4]. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous drop at the critical pressure Pc =8.3 Gpa. Combining with previously determined Pc for superconductivity, our phase diagram under pressure reveals an intimate connection among the block antiferromagnetic order, the Fe vacancy order and superconductivity for the 245 superconductor. Similar connection between the perfectness of crystalline order and superconductivity has been previously demonstrated in our neutron scattering study on related Fe based superconductors [5,6].

Mar 19 (Thursday 3:45) Ashkan Salamat
Harvard University
host: Dr. Andrew Cornelius
A liquid-liquid phase transition to Metallic Hydrogen

Hydrogen is the simplest and most abundant element in the Universe. It is estimated that more than 70% of the planetary mass in our solar system is in the form of dense fluid hydrogen, with Jupiter and Saturn being the largest reservoirs. These planets contain extreme pressure and temperature conditions which are predicted to lead to the formation of metallic fluid hydrogen. The pressure-induced transition from insulator to metal in solid hydrogen was predicted as early as 1935 by Wigner and Huntington, but to date has not been experimentally confirmed. Metallic hydrogen is predicted to have spectacular properties such as room temperature superconductivity and metastability (i.e. it remains metallic when the pressure is released). If metallic hydrogen is found to be metastable, its application could revolutionize rocketry and fusion technology. There are two thermodynamic pathways to metallic hydrogen: direct pressurization at low or modest temperatures to a solid metallic phase, and in the megabar pressure region, heating into the liquid metallic phase. In this talk, I will present my recent work in which the insulator to metal transition in dense liquid hydrogen was observed experimentally for the first time.

Mar 20

Mar 24 (Tuesday 3:30) Kayhan Gultekin
University of Michigan
host: Dr. George Rhee
Supermassive Black Holes

Supermassive black holes are some of the most fascinating energetic objects in the Universe, and they play a key role in what we can see across cosmic time and a large range of critical astrophysical phenomena. Despite their importance, much is unknown about their basic physics including how they were formed, how they grow, how they appear in different wavelengths, and what kind of galaxies they live in. The answers to many of these basic questions are within reach. I will review my recent, current, and future research plans to find their solutions.

Mar 27 Kentaro Nagamine
Osaka University, Japan
host: Bing Zhang
Accelerated Structure Formation at High Redshift

I will present two topics that suggest an accelerated growth of structures in high-density regions of the early universe. First is the birth of supermassive black hole (SMBH), and second is the formation of massive disk galaxies, both at redshifts z>6. Recent discoveries of billion solar mass SMBH at z=6-7 suggests that the gas accretion was quite rapid in the early universe with a super-Eddington rate. I will discuss a scenario called the 'Direct Collapse' of BH seed at high redshift, which has been attracting significant attention lately. I will also present our cosmological SPH simulation results of high-redshift galaxies at z=6-12, and discuss their observability with ALMA.

Apr 3 Spring Break

Apr 7
(Tuesday 3:30)
LiDong Pan
Johns Hopkins University
host: Andrew Cornelius
How to weigh a monopole – studying complex material response with THz spectroscopy

An important theme of modern condensed matter physics is the realization of novel excitations in materials (e.g. quasiparticles). Although they are not fundamental particles, such quasiparticles do constitute the most basic description of the excited states of the "vacuum" they reside in. In this regard the magnetic textures of the excited states of spin ices, magnetic pyrochlore oxides with dominant Ising interactions, can be modeled as effective magnetic monopoles. Utilizing the unique phase sensitive capabilities of time domain terahertz spectroscopy and microwave cavity techniques, we study the complex dynamic magnetic susceptibility of quantum spin ice Yb2Ti2O7. We find strong evidence of inertial effects in the monopoles dynamics. From the spectral weight, an effective mass of the monopoles is also obtained. Our results establish the magnetic monopoles as true coherently propagating quasiparticles in quantum spin ice.

Apr 10 Joshua C. H. Lui
Massachusetts Institute of Technology
host: Andrew Cornelius
Shedding Light on Two-Dimensional Electrons in Graphene and Beyond

Graphene, a single layer of carbon atoms, has stimulated intense scientific interest due to its distinctive electronic and mechanical properties. Graphene exhibits strong interactions with light over a broad spectral range. This enables us to examine its electronic and vibrational properties through optical spectroscopy. In addition to gaining understanding of the properties of single-layer graphene, we can also probe the behavior of electrons in few-layer graphene. This reveals the unique electronic and vibrational properties for graphene of each layer thickness and stacking order, as well as their distinct capability to induce an electrically tunable band gap. I will also highlight recent development of 2D materials beyond graphene.

Apr 17

Apr 24 Jason Rhodes
host: Dr. George Rhee
Exploring The Dark Sector with Euclid and WFIRST-AFTA

Dark energy, the name given to the cause of the accelerating expansion of the Universe, is one of the most profound mysteries in modern science. Current cosmological models hold that dark energy is currently the dominant component of the Universe, but the exact nature of dark energy remains poorly understood. There are ambitious ground-based surveys underway that seek to understand dark energy and NASA is participating in the development of significantly more ambitious space-based surveys planned for the next decade. NASA is providing mission-enabling technology to the European Space Agency's (ESA) Euclid mission in exchange for US scientists to participate in the Euclid mission. NASA is also developing the Wide Field Infrared Survey Telescope-Astrophysics Focused Telescope Asset (WFIRST-AFTA) mission for possible launch in 2023. WFIRST was the highest ranked space mission in the Astro2010 Decadal Survey and the AFTA incarnation of the WFIRST design uses a 2.4m space telescope to go beyond what the Decadal Survey envisioned for WFIRST. Understanding dark energy is one of the primary science goals of WFIRST-AFTA. I'll discuss the status of Euclid and WFIRST and comment on the complementarity of the two missions. I'll also briefly discuss other, exciting science goals for WFIRST, including a search for exoplanets using both microlensing and a dedicated coronagraph for exoplanet imaging.

May 1

May 8

May 15 Study Week

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