Abstract

Observations of the redshifted 21cm line of neutral hydrogen have the potential to probe the processes of structure formation and reionization in a unique way, complementing other techniques in cosmology. The high redshift means that observations have to be done at frequencies of 200 MHz and below, a requirement that presents interesting challenges. First-generation experiments are under construction and will be our first venture into this new area of study. Even the modest collecting areas of the first-generation experiments should be capable of detecting the power spectrum of density fluctuations and possibly the largest ionization "bubbles" around quasars. I will present the status of these experiments, and outline a potential path for future exploration of 21cm cosmology. The ultimate neutral hydrogen cosmology experiment, one that probes the smallest size scales and the highest redshifts, may need to be done from the far side of the Moon.

Bio

Jacqueline N. Hewitt, professor of physics, is the Director of MIT's Kavli Institute for Astrophysics and Space Research. The focus of her research is to apply the techniques of radio astronomy, interferometry, and signal processing to basic research in astrophysics and cosmology. Current interests are the epoch of reionization and the characterization of transient astronomical radio sources. After receiving her Ph.D. from MIT, she held positions at MIT's Haystack Observatory and at Princeton University, returning to MIT in 1989 to join the faculty. Her honors include the American Physical Society's Maria Goeppert-Mayer Award, the International Union of Radio Science's Henry G. Booker Prize, the National Science Foundation's Presidential Young Investigator Award, and the Annie Jump Cannon Award in astronomy. Hewitt is a fellow of the American Physical Society and has also been a David and Lucile Packard Fellow and an Alfred P. Sloan Research Fellow. Hewitt has served on a number of NASA, NSF and NRC advisory committees. She is currently a member of the Astronomy and Astrophysics Advisory Committee, a member of the Board of Trustees of Associated Universities, Incorporated, and she is chairing the Astro2010 Decadal Survey panel on Particle Astrophysics and Gravitation.

Abstract

Over the past several years, pulsar surveys with the Green Bank and Arecibo telescopes have revealed roughly 80 new Galactic pulsars. I will give an overview of the search algorithms used in these surveys, and will describe several interesting discoveries. These include transient pulsars detected only through their single pulses, a new double neutron star binary, a binary system that challenges conventional theories about binary evolution, and another binary system that spectacularly confirms these same theories. I will discuss the expected yields from these surveys and their importance for our understanding of pulsar emission mechanisms and pulsar populations and for the pulsar timing array for gravitational wave detection.

Abstract

Among the possible explanations for the observed acceleration of the universe, perhaps the boldest is the idea that new gravitational physics might be the culprit. In this colloquium I will discuss some of the challenges of constructing a sensible phenomenological extension of General Relativity, give examples of some candidate models of modified gravity and survey existing observational constraints on this approach. I will conclude by discussing how we might hope to distinguish between modifications of General Relativity and dark energy as competing hypotheses to explain cosmic acceleration

Bio

Mark Trodden is a Professor of Physics and Astronomy, and co-Director of the Center for Particle Cosmology at the University of Pennsylvania. Trodden has worked broadly in both cosmology and particle physics, in work ranging from the structure of inflationary spacetimes to the BPS structure of intersecting branes in supersymmetric theories. The majority of his work is firmly on the particle physics- cosmology border, and includes the development of the modified gravity approach to cosmic acceleration, approaches to dark energy and dark matter; extra dimensional models of particle physics and cosmology; the baryon asymmetry of the universe; inflation and its features; and topological defects in cosmology.

Trodden holds an MA in Mathematics and a Certificate of Advanced Study in Mathematics from Cambridge University. He also holds an M.Sc. and a Ph.D. in Physics from Brown University. He previously held the Alumni Professorship at Syracuse University, and has held visiting positions at Cornell University, the Kavli Institute for Theoretical Physics in Santa Barbara, and as a Sir Thomas Lyle Fellow at the University of Melbourne. Trodden is a Kavli Frontiers Fellow, a Cottrell Scholar of Research Corporation, and has chaired the National Academy of Sciences Kavli Frontiers of Science Symposium and the Working Group on Cosmological Connections of the American Linear Collider Physics Group. He sits on the editorial board of the New Journal of Physics, and of the Springer Multiversal Journeys Series.

Abstract

The Wilkinson Microwave Anisotropy Probe (WMAP) has made an accurate full-sky measurement of the microwave background temperature and polarization fluctuations. These measurements probe both the physics of the very early universe and the basic properties of the universe today. The WMAP measurements rigorously test our standard cosmological model and provide an accurate determination of basic cosmological parameters (the curvature of the universe, its matter density and composition). When combined with other astronomical measurements, the measurements constrain the properties of the dark energy and the mass of the neutrino. The observations also directly probe the physics of inflation: the current data imply that the primordial fluctuations were primarily adiabatic and nearly scale invariant.

Many key cosmological questions remain unanswered: what happened during the first moments of the big bang? what is the dark energy? what were the properties of the first stars? I will discuss results from our on-going ground based observations in Chile (Atacama Cosmology Telescope) and describe the role of upcoming measurements in addressing these key cosmological questions and describe how the combination of large-scale structure, supernova and CMB data can be used to address these questions.

Bio

David Spergel is the Charles Young Professor of Astronomy at Princeton University. His research interests range from extrasolar planets to the physics of the microwave background. Spergel is a member of the WMAP science team and has just finished leading the THEIA mission concept study that proposed a 4-meter class optical/UV telescope capable of detecting extrasolar planets. Spergel is currently chairing the NAS Cosmology and Fundamental Physics panel for the decadal survey and is leading the theoretical analysis for the Atacama Cosmology Telescope.

Abstract

Understanding the formation and evolution of structure in the Universe constitutes one of the fundamental goals of astrophysics. Over the past two decades, the LCDM cosmological model of hierarchical structure formation has emerged as the dominant paradigm in this pursuit owing to its remarkable ability to explain a plethora of observations on large scales and at various cosmic epochs. Supercomputer simulations are the ideal means by which to relate theoretical models with observational data, and advances in algorithms and supercomputer technology have provided the platform for increasingly realistic astrophysical modeling. Using high-resolution numerical simulations set within the LCDM paradigm I will investigate the response of large-scale galactic disks to the hierarchical assembly of structure, the transformation of typical disky dwarfs to the most dark matter dominated galaxies in the Universe, and the role of small-scale nuclear disks in the fueling of supermassive black holes (SMBHs) and the formation of SMBH binaries in galaxy mergers. Utilizing these results I will emphasize the importance of astrophysical disks in comparing theory with observations and highlight the predictive power of the LCDM theory on all scales.

Bio

Stelios Kazantzidis is a Long-Term Fellow at the Center for Cosmology and Astro-Particle Physics (CCAPP) at The Ohio State University. Prior to joining CCAPP, he received a Bachelor of Science in Physics in 2000 from the University of Athens in Greece and a Ph.D. in Theoretical Physics in 2005 from the University of Zurich. Kazantzidis was then an Institute Fellow at the Kavli Institute for Cosmological Physics at The University of Chicago, and at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University.

Kazantzidis' research focuses on various aspects of theoretical astrophysics and cosmology, from large-scale structure and galaxy formation to near-field cosmology and galactic structure, to supermassive black holes. To investigate these topics, he employs numerical simulations on large parallel supercomputers. Kazantzidis' work on galaxy formation and supermassive black holes has received world-wide coverage in a number of major media outlets.

Abstract

Estimates of the spins of massive black holes can be obtained by a variety of "traditional" electromagnetic observations. In the future LISA will provide extremely precise measurements of masses and spins through gravitational wave observations of the in spiral, merger and ringdown of black hole binaries. I will discuss the role of spins in the dynamics of black hole binaries and in their gravitational wave signature. I will argue that the combination of gravitational and electromagnetic observations will provide an excellent way of discriminating between different mechanisms of black hole growth. Finally, I will illustrate the potential of gravitational wave observations to rule out exotic alternatives to the massive black hole paradigm, such as boson stars or gravastars.

Bio

Emanuele Berti is an Assistant Professor at the University of Mississippi. After receiving his Ph.D. from the University of Rome "La Sapienza" he was a postdoc at the Aristotle University of Thessaloniki, at the Institut d'Astrophysique de Paris and at Washington University in Saint Louis. Prior to his appointment in Mississippi he was a NASA ORAU Senior Postdoctoral Fellow at JPL/Caltech. His work focuses on experimental verifications of classical general relativity and on astrophysical sources of gravitational waves. His research interests include the structure and dynamics of relativistic stars and black holes, the analytical and numerical modeling of gravitational waves from compact binary inspirals and LISA observations of massive black hole mergers.

Abstract

I will describe recent progress in our understanding of gamma-ray bursts. The focus will be on the mechanism by which low energy (less than ~ 1 MeV) and high energy (>100 MeV) photons are generated in these explosions. After a brief overview of observational properties including data from Swift, Fermi and AGILE, I will describe what we have learned about the radiation process. Swift and Fermi data suggest that the source for sub-MeV photons in GRBs is short lived whereas the high energy photon source continues to operate for a relatively much longer duration of time. Some of the recent multi-wavelength data are very puzzling and call into question the current paradigm of internal shocks and radiation mechanisms.

Bio

Abstract

Eta Carinae, with its very massive interacting winds, is an observer's and a modeler's dream, but a theoretician's nightmare.

From the observer's viewpoint, Eta Car A (10^-3 solar masses/year at 500 km/s is in a 5.4 year orbit with Eta Car B (10^-5 solar masses/year at 3000 km/s). At a distance of 2300 parsecs, the strength of the wind-wind interactions has been monitored by RXTE for a dozen years, its X-ray spectrum has been probed by Chandra. Ground-based optical spectra exist for the past two decades, and recently HST/STIS resolved the outer wind-wind structure, demonstrating that the low excitation forbidden lines trace the primary wind and the high excitation forbidden lines trace the outer, ballistic wind-wind interaction structure.

From the modeler's viewpoint, these observations have led to 3D SPH modeling that define the tilt of the orbital plane, the position of periastron and constrain the orbital plane.

However, the theoretician's nightmare is explaining what has led to such massive winds, the origin of this binary system, and the path by which it will evolve: will it provide two plain vanilla SNs? Could one become a hypernova or something akin to a burster?

I will describe the current observations and models, but leave it up to the theoreticians to explain.

Bio

Ted Gull is one of a dozen physics majors of the class of '66 at MIT drawn to astronomy. He received his PhD at Cornell University under the guidance of Martin Harwit. After a postdoc at University of Chicago building an echelle spectrograph for Lear Jet observations, he helped instrument the 4-meter telescopes at KPNO and CTIO, then worked in the Astronaut Office at JSC on potential Spacelab astronomy missions. He joined Goddard in 1977, helping bring on IUE and led a number of Spacelab astronomy studies. He was Mission Scientist for Astro-1, Associate Chief of Laboratory for Astronomy and Solar Physics, and Deputy PI of Space Telescope Imagine Spectrograph. For the past decade he has enjoyed the physics of Eta Carinae (and is not allowed to discuss it over lunch).

Abstract

There is a growing consensus in the supernova theory community that instabilities above the proto-neutron star help to produce explosions in the collapse of massive stars. This consensus has raised a number of new questions and we are far from understanding the engines behind core-collapse supernovae. Supernova scientists will need to broaden both their theoretical toolsets, leveraging off of knowledge from the fluid dynamics and computational science communities, and their observational probes (taking greater advantage of the wealth of observational data). For example, observations in the ultraviolet and X-ray provide ideal probes into the progenitor structure and composition. Observations of abnormal or "failed" supernovae are also ideal probes of the supernova engine, in some cases providing cleaner systems to test our theoretical models. I will discuss a few of these "non-standard" techniques to better understand the supernova engine.

Bio

Chris Fryer received his PhD at the University of Arizona under Willy Benz studying neutron stars and supernovae. He then went to UCSC to work with Stan Woosley on supernovae and gamma-ray bursts before arriving at Los Alamos National Laboratory. He has worked on a broad range of astrophysics from studies of the Galactic center to star formation, but has focused primarily on the progenitors, remnants and explosions of supernovae.

Abstract

Over recent decades, there has been an increasing awareness of the importance of turbulence in governing the physical states and kinematics of the interstellar medium, a picture that advances us beyond the earlier simple concept of quiescent, dense clouds that are confined by the external pressure of a lower density warm medium. There are various ways to observe the strength and character of interstellar turbulence; one of them is to sense the distribution of thermal pressures, even though such pressures represent a small fraction of the total pressure in the medium. An effective way to examine this distribution is to measure the strengths of absorption features from interstellar neutral carbon atoms that are seen in the ultraviolet spectra of hot stars. The ground state of C I is split into three fine-structure levels; the relative populations of the upper two levels are governed by collisions with hydrogen atoms and thus reveal the local thermal pressures. An interpretation of high-resolution UV spectra in the HST archive for over 100 stars indicates that the mass-weighted pressure distribution function follows approximately a log-normal distribution centered on a value of p/k = 2600 cm^-3 K, with an rms dispersion in log(p/k) = 0.16. Accompanying almost all of the gas is about a 0.1% fraction that seems to have an anomalously high pressure of p/k > 105 cm^-3 K.

Bio

Dr. Jenkins performs research on the interstellar and intergalactic medium by analyzing absorption lines in the UV spectra of stars and quasars. Special interests include (1) measurements of the distribution of thermal pressures in the Galactic interstellar medium through the analysis of atomic fine-structure level populations, (2) Observations of O VI absorption features which reveal the presence of collisionally ionized gas at T = 300,000 K, (3) A generalized description of depletions of atomic gas-phase abundances of different elements as they condense into solid form onto dust grains, (4) The abundance of deuterium and its variability, and (5) the ionization and electron density of the Local Interstellar Medium (LISM). His past research has made use of the Copernicus satellite, the International Ultraviolet Explorer, the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. He was the principal investigator for the Interstellar Medium Absorption Profile Spectrograph (IMAPS), which obtained spectra of bright stars between 950 and 1150 A at a wavelength resolving power of about 100,000. This instrument flew on sounding rockets and also operated on two ORFEUS-SPAS missions launched into orbit by the Space Shuttle.