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Leonard E Parker

Center for Gravitation, Cosmology & Astrophysics

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Leonard E Parker Center for Gravitation, Cosmology and Astrophysics

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Acknowledgement

The Leonard E Parker Center for Gravitation, Cosmology and Astrophysics is supported by NASA, the National Science Foundation, UW-Milwaukee College of Letters and Science, and UW-Milwaukee Graduate School. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of these organizations.

Past Seminars

August 26, 2013

TBD
Megan DeCesar , University of Maryland

Abstract: TBA

May 24, 2013

Solving Einstein's Equation Numerically on Manifolds with Arbitrary Spatial Topologies
Lee Lindblom , California Institute of Technology

Abstract: General relativity theory represents gravity as curvature of the geometry of spacetime. Unlike the rigid structure of flat Euclidean space, curved geometries can in principle have a wide range of global topological structures. Little is know, however, about the properties of the solutions to Einstein's equation on manifolds having non-trivial topologies. This talk will discuss recent work on developing flexible practical methods for solving Einstein's equation numerically on manifolds with arbitrary spatial topologies. Examples will be given to illustrate the use of these methods for solving simple elliptic and hyperbolic differential equations numerically on manifolds with non-trivial spatial topologies. Extending these methods to general relativity also required the development of a new fully covariant symmetric-hyperbolic representation of the Einstein equation. This new representation will be discussed, along with some simple numerical tests of Einstein evolutions on manifolds with non-trivial spatial topologies.

May 17, 2013

The spacetime geometry of an electromagnetic wave
Charles Torre , Utah State University

Abstract: Since the 1920's it has been known how to characterize almost all solutions to the Einstein-Maxwell equations in terms of geometric conditions built solely from the spacetime metric. These conditions are known as the "Rainich conditions"; they provide a generalization to electrovacuum spacetimes of the geometry of vacuum (Ricci-flat) spacetimes. With the aid of modern computer algebra systems, the Rainich conditions also provide a novel approach to solving the Einstein-Maxwell equations. The Rainich conditions fail to describe solutions of the Einstein-Maxwell equations which have a null electromagnetic field, e.g., electromagnetic plane waves. In this talk I will review Rainich geometry and then describe geometric conditions on a spacetime which are necessary and sufficient for the existence of a solution to the Einstein-Maxwell equations with a null electromagnetic field. These conditions can be viewed as the analog of the Rainich conditions for null electrovacua, and they are equally amenable to computer implementation.

May 10, 2013

Can Black Hole--Neutron Star Binary Mergers Produce Gamma-Ray Bursts?
Zach Etienne , University of Illinois - Urbana-Champaigne

Abstract: Gravitational waves sap orbital angular momentum and energy from a black hole--neutron star (BH-NS) binary, driving it to inspiral and merge. In the violence of merger, the NS may tidally disrupt and form a hot accretion disk with the collimated magnetic fields necessary to launch jets, providing the central engine for one of the most energetic phenomena in the Universe: a gamma-ray burst (GRB). We assess the feasibility of this scenario with numerical relativity simulations of magnetized BH-NS binary mergers, seeding the NS with magnetic fields and exploring their effects on the remnant disk and the gravitational waves. We find that the gravitational waves are likely to be detectable by Advanced LIGO if the merger occurs within ~100Mpc, though the effects of magnetic fields on the waveforms are likely negligible. Further, we find that a GRB central engine may form if large-scale poloidal magnetic fields anchored in the disk are accreted onto the BH after the NS disrupts.

May 3, 2013

Zooming in on Galactic Nuclei at the Dawn of Gravitational Wave Astronomy
Bence Kocsis , Harvard-Smithsonian Center For Astrophysics

Abstract: The rapid development of gravitational wave instruments will open a completely new window on the Universe. In this talk, I will describe how this may revolutionize our understanding of galactic nuclei. Galactic nuclei host a central supermassive black hole, a dense population of stars and compact objects, and in many cases a bright gaseous disk feeding the central supermassive black hole. These systems may be a treasure trove of gravitational wave sources. Recent electromagnetic observations revealed interesting structures including counterrotating disks and an isotropic central cluster of young stars. I will demonstrate that these structures can be naturally explained by methods commonly used in condensed matter physics. Stars and compact objects collectively resemble a gigantic liquid crystal, which can exhibit phase transitions. Gravitational wave observations may tell us if there is a central dark cluster or dark disk of compact objects. Ground-based gravitational wave observatories will be able to constrain the distribution and mass function of black holes. Pulsar timing can provide a detailed map of this region with unprecedented resolution and locate intermediate mass black holes, if present.

April 26, 2013

The road to multimessenger astronomy with Advanced LIGO
Leo Singer , Caltech

Abstract: Compact binary mergers are among the most promising sources for advanced, ground-based, gravitational wave detectors. They are also compelling progenitors for short, hard gamma-ray bursts and multiwavelength afterglows. One of the most hotly anticipated outcomes of the Advanced LIGO era is joint observations of a compact binary coalescence event in gravitational waves and optical. Yet there are formidable challenges to overcome in both channels to make this feasible. I will discuss ongoing work that I am involved in on several essential and adjacent steps along this path, from low-latency gravitational wave detection, to rapid and self-consistent Bayesian sky localization, to confronting optical transient hunting in multi-square-degree error regions.

April 19, 2013

Cosmic Rays at Ultra-High Energies
Tom Paul , UW-Milwaukee

Abstract: Cosmic rays with energies as high as 10^20 eV have been observed, but unveiling their origins and composition is a daunting challenge due to the rarity of events at this extreme end of the energy spectrum. I'll review some of the latest efforts on this front, focusing primarily on The Pierre Auger Observatory. This observatory was designed specifically to study such ultra-high energy cosmic rays using a giant array of particle detectors and fluorescence light telescopes. I'll describe the experimental techniques employed to infer cosmic ray properties from the extensive air showers they produce when they interact in Earth's atmosphere, and present the latest results concerning the energy spectrum, primary composition, and searches for the cosmic ray sources.

April 5, 2013

Tidal Synchronization, Heating, and Novae in Binary White Dwarfs
James Fuller , Cornell University

Abstract: In compact white dwarf (WD) binary systems (with periods ranging from minutes to hours), dynamical tides involving the excitation and dissipation of gravity waves play a dominant role in determining the physical conditions (such as rotation rate and temperature) of the WDs prior to mass transfer or binary merger. We calculate the amplitude of the tidally excited gravity waves as a function of the tidal forcing frequency for realistic WD models, under the assumption that the outgoing propagating waves are efficiently dissipated in outer layer of the star by nonlinear wave breaking. In our WD models, gravity waves are launched at composition gradients and propagate outwards. We study the effects of dynamical tides on the long-term evolution of WD binaries prior to mass transfer or merger. Above a critical orbital frequency, corresponding to an orbital period of about one hour (depending on WD models), dynamical tides efficiently drive the WD towards synchronous rotation, although a small, almost constant degree of asynchronization is maintained even at the smallest binary periods. We also investigate the effect of tidal heating on the WD (e.g., its internal structure, surface temperature, luminosity, etc.) by including tidal heating in stellar evolution calculations. We compare our predictions to observations of compact WD systems such as the 12.75 minute system SDSS J0651+2844, and we discuss the implications. Finally, we examine the possibility that tidal heating may trigger run-away hydrogen shell burning on the surface of a WD, leading to a nova-like event before the WD merging event.

March 15, 2013

Mass ejection from compact binary mergers and electromagnetic counterparts
Koutarou Kyutoku , UW-Milwaukee

Abstract: Coalescences of binary neutron stars and black hole-neutron star binaries are ones of the most promising gravitational-wave sources. Simultaneous detection of electromagnetic radiation (or counterparts) from these binaries is also desirable, because such counterparts confirm the gravitational-wave detection and maximize scientific returns from each event. In recent years, electromagnetic counterparts are getting a lot more attention and many theoretical emission models are proposed. Most of the models require mass ejection from the binary merger, and numerical relativity is the most reliable method to predict properties of ejecta such as the mass and velocity. In this talk, I will present results of recent numerical-relativity simulations performed focusing on dynamical mass ejection from binary mergers, and discuss possible electromagnetic radiation.

March 1, 2013

Mass ejection from compact binary mergers and electromagnetic counterparts
Abi Polin , NYU

February 22, 2013

Feedback in Faint Galaxies During the Peak Epoch of Star Formation
Dawn Erb , UW-Milwaukee

Abstract: Because faint, low mass galaxies are numerous at high redshifts, their impact on the Universe is expected to be significant. They may host a substantial fraction of global star formation, provide many of the energetic photons needed to reionize the universe, and contribute to the enrichment of the intergalactic medium through the expulsion of metals in galactic outflows. Because of their faintness, however, the properties of these galaxies are difficult to determine. I will discuss a variety of observations aimed at characterizing the physical conditions in low mass galaxies at redshifts z~2-3, the peak epoch of star formation in the Universe, with particular emphasis on the study of galactic outflows in faint galaxies.

January 18, 2013

Astronomy From The Stratosphere
Marc Berthoud , Yerkes Observatory

Abstract: The Stratospheric Observatory For Infrared Astronomy (SOFIA) is NASA's new airborne observatory. The telescope has a 2.7m diameter mirror mounted in a modified Boeing 747 aircraft. First light with SOFIA was achieved in May 2010. The High-resolution Airborne Wide-band Camera (HAWC) will be the facility far-infrared camera for SOFIA. Before flying in 2014 HAWC will be upgraded with dual 64 x 40 arrays and polarimetric capabilities. This talk will focus on the design, assembly and testing of the HAWC instrument.

December 7, 2012

Shedding Light on Distant Galaxies with 200,000 Quasars
Britt Lundgren , University of Wisconsin - Madison

November 30, 2012

Inferences From The Post-Merger Gravitational Wave Signal In Binary Neutron Star Coalescence: Status, Progress & Prospects
James Clark , University of Massachusetts - Amherst

November 16, 2012

Probing the Galactic and Extragalactic magnetic fields
Ann Mao , University of Wisconsin - Madison

Abstract: In this talk, I will give an summary of our current understanding of the Milky Way's magnetic field structure. I will focus on a study that uses Faraday rotation measures of polarized extragalactic sources to probe magnetic fields in the Milky Way halo. We show that existing Galactic halo magnetic field models cannot successfully reproduce extragalactic rotation measures at mid-Galactic latitudes in the second Galactic quadrant. We suggest that halo fields consist of magnetic spirals could potentially account for the observed rotation measure pattern. In addition, I will discuss on-going efforts to measure extragalactic magnetic fields, with the goal to understand the evolution of large-scale magnetic fields in galaxies.

November 9, 2012

AGN Variability, Tidal Disruptions, and Transients from Next Generation Radio Telescopes
Steve Croft , UWM/U.C-Berkeley

Abstract: The next generation of radio telescopes such as the Australian Square Kilometer Array Pathfinder (ASKAP) will provide radio variability information with a cadence of days for samples of hundreds of thousands of active galactic nuclei (AGNs). They will also potentially detect tens to hundreds of transient events per month, including radio afterglows from stellar explosions such as supernovae and gamma ray bursts; but the majority of these radio transients may arise from the tidal disruption of stars passing close to supermassive black holes. An early pioneer in the new generation of radio telescopes is the Allen Telescope Array (ATA), which undertook several systematic searches for radio transients and variability. I will present results from the Pi GHz Sky Survey (PiGSS), a survey which includes a wide field (~5000 sq. deg.) component, and also a total of 459 repeated observations of four 12 sq. deg. deep fields. PiGSS provides some of the most sensitive current upper limits on the transient rate in the radio, reaching sensitivities of a few milliJanskies over effective areas of thousands of square degrees. PiGSS also discovered several strongly variable sources, and provides variability measurements for hundreds of radio AGNs, for hundreds of epochs with a cadence of days. Steve is an Assistant Project Astronomer working with Geoff Bower in Berkeley and David Kaplan in Milwaukee on large radio surveys, transients, and variable sources. He helped commission the Allen Telescope Array for science operations and developed data analysis pipelines. He got his PhD from the University of Oxford, working with Steve Rawlings on radio galaxies in cluster environments. He then worked as a postdoc at the Lawrence Livermore National Laboratory with Wil van Breugel and Bob Becker, studying radio galaxy hosts and environments, high-redshift protoclusters, and jet-induced star formation, using data at a wide range of wavelengths from many different telescopes.

November 2, 2012

Dynamical and Thermodynamic Stability of Perfect Fluid Stars
Stephen Green , University of Guelph

Abstract: We explore the stability of stationary axisymmetric perfect fluid configurations to axisymmetric perturbations in general relativity. We consider the class of perturbations which keep the particle number, entropy, and angular momentum of each fluid element fixed. We show that the condition for dynamical stability with respect to such perturbations is equivalent to positivity of the canonical energy. Additionally we show that, with respect to this class of perturbations, dynamical stability is equivalent to thermodynamic stability.

October 12, 2012

Searching for Gravitational Waves Using Pulsars
Justin Ellis , University of Wisconsin - Milwaukee

Abstract: The Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration aims to detect gravitational waves (GWs) through the precise timing of millisecond pulsars. GWs will come in the form of a stochastic background, continuous sources and burst sources. Here we will review recent progress on the development of data analysis pipelines aimed at the detection of a stochastic background as well as continuous sources. We will introduce the Optimal Statistic and F-Statistic methods that are used in the stochastic and continuous pipelines, respectively. Both pipelines are fully functional on real pulsar timing data and take into account the timing models for each pulsar. Finally, we will present the efficacy of each pipeline on simulated data as well as present preliminary results on real data. We then discuss future prospects of using GW measurements to gain astrophysical insight into supermassive black hole binary systems and the prospects of detecting electromagnetic counterparts.

September 7, 2012

Fundamental Physics with the LOFT, the Large Observatory For Timing
Luigi Stella , University of Wisconsin - Milwaukee

Abstract: High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, the equation of state of ultra-dense matter and black hole masses and spins. The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of four M3 candidate missions to undergo an assessment phase, is designed to exploit the relevant diagnostics and answer two of the fundamental questions of ESA's Cosmic Vision Theme "Matter under extreme conditions", namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars? Thanks to an innovative design and the development of large-area monolithic Silicon Drift Detectors, the Large Area Detector (LAD) on board LOFT will afford an effective area of ~10 m^2 (more than an order of magnitude larger than any spaceborne predecessor) and spectral resolution of <240 eV in the 2-30 keV. LOFT will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei, yielding unprecedented information on strongly curved spacetimes and matter under extreme conditions of density.

August 24, 2012

The Growth of Red Galaxies: Lessons from the ESO Distant Cluster Survey
Vandana Desai , Caltech

Abstract: Galaxies occupy three main regions in color-magnitude space: the red sequence, the blue cloud, and the green valley. These correspond respectively to regions where galaxies are passive, actively forming stars, or in transition. The total mass in typical field red sequence galaxies has doubled over the last 8 billion years, suggesting that galaxies can migrate from one region to the other, presumably through the cessation of star formation. Galaxy clusters provide a convenient laboratory for studying this transformation and its dependence on environment. I will present a summary of what we have learned about the formation of the galaxy red sequence from the ESO Distant Cluster Survey, a detailed study of galaxy clusters at intermediate redshift. I will also briefly describe a new observational effort to expand this cluster program to the lower density environments surrounding the cluster cores.

August 17, 2012

The Growth of Red Galaxies: Lessons from the ESO Distant Cluster Survey
Sarah Caudill , LSU/UWM

June 8, 2012

The VLT-Flames and Tarantula Survey: Rotational Mixing in Massive Stars, More Puzzling than Ever!
Ines Brott , University of Vienna

Abstract: The VLT-FLAMES Survey of Massive Stars (FLAMES-I) provided a homogeneous analysis of nitrogen abundances of B-type stars for a large sample (100+x stars) spanning a wide range of projected rotational velocities. This allows for the first time a statistically significant test of models of rotational mixing in massive stars. We have developed the detailed population synthesis code, STARMAKER, which we apply to simulate the FLAMES-I results in the Large Magellanic Cloud. This entailed the calculation of a dense grid of state-of-the-art stellar evolution models, with uncertain physics calibrated to the FLAMES-I observations and using initial conditions (chemistry, rotation) tailored to the FLAMES-I LMC sample. Our population synthesis results provide some remarkable surprises. Several groups of stars in the FLAMES-I sample imply that the current model of rotational mixing is too simple. We find that it is likely that additional fundamental physical processes need to be considered to understand the data. While binary physics may help to alleviate the problem, it is insufficient to fully resolve the issue. We emphasize indications that effects of magnetic fields in stars might be the root for the remaining discrepancies. Empirical constraints on this puzzle are one of the motivations behind the ongoing new multi-epoch VLT-FLAMES Tarantula Survey (FLAMES-II), of which the first highlights will be shown.

July 13, 2012

Implications of PSR J0737-3039B for the Galactic NS-NS Binary Merger Rate
Chunglee Kim , West Virginia University

Abstract: The Double Pulsar (PSR J0737-3039) is the only system in which both neutron stars (NSs) have been detectable as radio pulsars. The contribution of the Double Pulsar on the Galactic NS-NS merger rate (R_gal) has been only based on the recycled PSR J0737-3039A (or A). The 5-yr observations of the second-born, non-recycled PSR J0737-3039B (or B) by the Green Bank Telescope enable us to constrain B's beam geometry that is the least known ingredient in empirical modeling of this pulsar. In this work, we model the B pulsar using the best observational constraints. We find that the probability density function of R_gal based on both A and B, along with other merging NS-NS binaries, is peaked at around 20 per Myr, much smaller than what previously thought. Considering the merger rate estimates and the horizon distance of 445 Mpc with the advanced LIGO-Virgo network, the detection rate for NS-NS inspirals is most likely to be ~7 per yr. We expect, within several years, the observed gravitational-wave detection rate for NS-NS inspirals will allow us to put independent constraints on models of the pulsar population.

June 1, 2012

A New Probe of Dark Matter in Galaxies
Sukanya Chakrabarti , Florida Atlantic University

Abstract: The cold dark matter paradigm of structure formation is successful at recovering the basic skeletal structure of the universe -- the large-scale distribution of galaxies. However, the agreement between theory and observation is less secure when this model is applied to galactic (and sub-galactic) scales. Problems such as the missing satellites problem, the lack of massive dark satellites, and the unexpected distribution of galactic satellites in the Milky Way, suggest that the current paradigm is not complete in its description of galaxy evolution. The extended atomic hydrogen disks of galaxies provide an unique probe of galaxy evolution. They are ideal tracers of tidal interactions with satellites and the galactic gravitational potential well. We have recently developed a method whereby one can infer the mass, and relative position (in radius and azimuth) of satellites from analysis of observed disturbances in outer gas disks, without requiring knowledge of their optical light. I will present the proof of principle of this method by applying it to galaxies with known optical companions. I will end by presenting recent work that extends earlier our earlier results to constrain the density profile of dark matter in local spiral galaxies. I will also compare and contrast this method to gravitational lensing as a means of probing dark matter in galaxies.

May 18, 2012

A New Probe of Dark Matter in Galaxies
Lee Lindblom , Caltech

Abstract: The inverse stellar structure problem determines the equation of state of the stellar matter from a knowledge of the stars' observable macroscopic properties, like their masses and radii. This seminar will discuss different approaches to solving this problem, including some new results based on spectral methods.

May 11, 2012

A New Probe of Dark Matter in Galaxies
Shane Larson , Utah State University

May 4, 2012

Here Be Dragons: The Dynamic Radio Sky
Geoffrey Bower , UC Berkeley

Abstract: Radio variability probes a wide range of astrophysical phenomena from the solar system to the early Universe including black holes, neutron stars, gravitational wave sources, and relativistic shocks from collapsing stars. Radio follow-up of events discovered at optical, X-ray, and gamma-ray wavelengths demonstrates a rich phenomenology but we lack a systematic and sensitive view of radio variability. Our efforts in recent years with the Very Large Array (VLA) and the Allen Telescope Array (ATA) have provided the beginnings of an unbiased exploration on time scales from milliseconds to decades. Powered by development of unique digital instrumentation, new algorithms, and new survey strategies, our surveys have defined the shape of next generation radio telescopes and surveys. In particular, these surveys are shaping our understanding of radio supernovae, tidal disruption events, and gravitational-wave sources. Planned surveys with newly commissioned national facilities such as the Expanded Very Large Array (EVLA), the Australian SKA Pathfinder (ASKAP), and the South African MeerKAT telescopes coupled with the development of new instrumentation will extend our reach into new parameter space.

April 27, 2012

Jets from Neutron Stars and Black Holes as Weather Vanes and Probes of Accretion Physics
Sebastian Heinz , UW-Madison

Abstract: Accreting compact objects often generate powerful, collimated outflows from very close to the inner regions of the accretion flow. These jets can transport a significant fraction of the accretion energy away from the compact object and deposit it at large distances, when the jets interact with the cosmic environment. In the case of stellar mass black holes and neutron stars, this interaction occurs in the interstellar medium, and observations of signs of this interaction provide us with sensitive tools to study the outflows themselves, the environment they are interacting with, and even allow some deeper inferences about the compact objects at their heart. I will review some of the recent advances in modeling and observations of Galactic jets, and what we have learned from them.

March 16, 2012

Search for Gravitational Waves from Intermediate Mass Binary Black Holes
Chris Pankow , UW-Milwaukee

Abstract: In the realm between solar mass and supermassive black holes lies a yet undiscovered class of objects dubbed intermediate mass black holes (IMBH). With little dynamical evidence for their existence, astrophysicists have struggled to understand how IMBH form and influence their environment. However, it is thought that their birth and growth are intrinsically linked to mergers of very massive objects, possibly tens to hundreds of solar masses. The merger of lower mass IMBH binaries could potentially form an exciting source for current interferometric gravitational-wave detectors. This talk will briefly outline some theories behind IMBH formation, evolution, and current observational evidence. Then I will describe the motivation and results from the first search for gravitational waves from the merger of IMBH binaries with total mass between 100 and 450 solar masses. This search used nearly a year of data from the joint run between LIGO and Virgo called S5/VSR1 taken between 2005 and 2007. Since no detections were made, I will end with a discussion with the rate upper limits that we established with our results and progress made in improving and extending the search.

March 9, 2012

Chromo-Natural Inflation: Natural Inflation on a steep potential with classical non-Abelian gauge fields.
Mark Wyman , University of Chicago

Abstract: I will describe a new model for inflation consisting of an axionic scalar field coupled to a set of three non-Abelian gauge fields. What's new in this model is that it relies on the gauge fields starting inflation with a rotationally invariant vacuum expectation value (VEV) that is preserved through identification of SU(2) gauge invariance with rotations in three dimensions. The gauge VEV interacts with the background value of the axion, leading to an attractor solution that exhibits slow roll inflation even when the axion decay constant has a natural value (<MPl). Assuming a sinusoidal potential for the axion, we find that inflation continues until the axionic potential vanishes. The speed at which the axion moves along its potential is modulated by its interactions with the gauge VEV, rather than being determined by the slope of its potential. For sub-Plankian axion decay constants vanishingly small tensor to scalar ratios are predicted, a direct consequence of the Lyth bound. Of the four free parameters in our theory, only one appears tuned: The parameter that controls the interaction strength between the axion and the gauge fields must be O(100).

March 8, 2012 (Thursday)

Cosmic-Ray Acceleration in Supernova Remnants
Pat Slane , Harvard University

Abstract: The Milky Way is infused with a population of energetic particles whose energy density rivals that of the magnetic fields and starlight in the Galaxy. Such "cosmic rays" are observed at energies far in excess of those attainable in terrestrial particle accelerators, yet their exact origin remains poorly understood. At energies up to the "knee" in the spectrum at about 1 PeV, cosmic rays are thought to originate from within the Galaxy. Based on overall energetics as well as the presence of a viable mechanism for shock acceleration of particles, it has long been suspected that supernova remnants may be a major source of cosmic rays. Results from a wide range of multiwavelength studies of supernova remnants have now provided strong evidence for acceleration of particles to extremely high energies. Here I review the observational signatures for such particle acceleration, and summarize recent X-ray and gamma-ray studies, as well as modeling efforts, that are providing a multi-faceted approach to our understanding of such particle acceleration in supernova remnants, and the source of Galactic cosmic rays.

March 2, 2012

Evidence for Neutron Superfluidity in Neutron Star Cores
Craig Heinke , University of Alberta

Abstract: Massive stars explode as supernovae at the end of their lives, leaving neutron stars, dense balls of matter at super-nuclear density. The behavior of matter at such high density is not well understood. X-ray observations of hot young neutron stars are one method of probing the physics of neutron stars. I will discuss X-ray observations of some young neutron stars, focusing on the youngest known neutron star, in Cassiopeia A. Our team proposed that its unusual X-ray properties are naturally explained by a carbon atmosphere, and discovered that its surface is measurably cooling with time. The observed rate of cooling, and its current temperature, require a rapid, short-lived transition in the neutron star's interior. Our proposed explanation is that the neutrons in the core are undergoing a transition to a superfluid state, radiating away their heat via neutrinos as the neutrons pair up.

February 24, 2012

Vector Inflation and the Baryon Asymmetry Problem
Stephon Alexander , Princeton University, Haverford College

Abstract: I present work in collaboration with David Spergel and Antonino Marciano on a new model of inflation that also resolves the baryon asymmetry problem. Inflation begins with a large plasma density of interacting gauge fields and fermions, which interact through gravity and the Chern-Simons term. The Chern-Simons term drives power from an initial white-noise spectrum of gauge fields into a narrow-band of superhorizon wave vectors. At the same time, the fermionic current and metric coupling amplifies the gauge field on superhorizon scales. This phase-correlation and amplification of the gauge field produces the correct conditions to maintain more than 60 e-folds of inflation. The Sakharov conditions for baryogenesis are self-consistently satisfied, and we calculate the net baryon asymmetry index in terms of the gauge configuration necessary for inflation, based on the chiral anomaly. Eventually the gauge field dissipates by producing the observed baryon assymetry nb/s ~ 10-10, through the chiral anomaly and inflation ends. I end with a few ideas for how this model can be probed with upcoming CMB experiments such as PLANCK.

February 17, 2012

Peeking into the crust of an accreting neutron star
Nathale Dagenaar , University of Michigan

Abstract: A profound reason to study neutron stars is that they provide a gateway to explore the behavior of matter under extreme physical conditions that are unattainable on Earth. When residing in X-ray binaries, neutron stars accrete matter from a companion star. The accretion of matter induces a series of nuclear reactions that deposit heat deep within the neutron star crust. Once the accretion switches off, the heated crust thermally relaxes. This crustal cooling can be observed by studying the thermal emission from quiescent neutron stars with sensitive X-ray satellites. Comparing these observations with theoretical simulations provides a unique opportunity to gain insight into the heat production in the crust and its thermal transport properties. I will present the latest results and challenges in this research field.

February 10, 2012

Non-Gaussian features of primordial magnetic fields
Leonardo Motta , Dartmouth College

Abstract: Little is known about the Lagrangian of inflation. Information about that epoch can be obtained from the non-gaussian corrections to cosmological correlations which have been explored extensively for the gravitational field. However, other cosmological observables may also be probed for this purpose. For instance, if new physics during inflation breaks the conformal invariance of electrodynamics, a relic cosmic magnetic field can be produced. We explored the non-gaussian correction that arises under this assumption in two models of inflation. In power-law inflation it is possible to produce a nG relic magnetic field at Mpc scales at the present time with a cross-correlation amplitude of 10-5 with matter fluctuations. This cross-correlation signal might be accessible by the SKA telescope, which would provide new information about the physics of the early universe.

December 16, 2011

Astrophysics with LIGO/Virgo, and the challenges of parameter estimation
Vivien Raymond , Northwestern University

Abstract: Gravitational waves are on the verge of opening a brand new window on the universe. However, gravitational wave astronomy comes with very unique challenges in data analysis and signal processing. LIGO/Virgo will soon enter their advanced phases and, among the anticipated detections, compact binary coalescences are of special interest because these events are one of the most promising for extracting astrophysical parameters of source systems. I will try to cover the challenges of gravitational wave observations, focusing on those sources. In particular, I will present a method using the Bayesian statistical framework and the Markov chain Monte Carlo algorithm for parameter estimation and model selection applied to spinning compact binary coalescences. In this method spinning parameters can be estimated, and in fact we find they have to be included in our model waveforms to avoid very significant biases in overall parameter estimation.

Please note this talk has been moved to 3:00 PM in light of finals week.

December 2, 2011

The Role of Gravitational Waves on the Evolution of Accreting Millisecond Pulsars
Alessandro Patruno , Astronomical Institute Anton Pannekoek, University of Amsterdam

Abstract: Accreting millisecond pulsars in low mass x-ray binaries are extreme objects that allow tests of fundamental physical processes like the production of gravitational waves. The spin frequency of the neutron star can be studied over long baselines and reveals details of the accretion torques acting upon the neutron star. One surprising finding of the last few years has been that several accreting millisecond pulsars do not show evidence of strong accretion torques that should in principle be responsible for the increase of their rotational velocity. Emission of gravitational radiation has been invoked in the past as a mechanism to balance accretion torques at some point of the evolution and limit the spin frequency reached by accreting pulsars. But are gravitational waves really sufficient to explain the most recent observations? I will discuss the latest findings on the topic and propose several scenarios that might help to shed light on this intriguing problem.

November 18, 2011

A Variety of Stellar Deaths
Atish Kamble , UW-Milwaukee

Abstract: Transients in the Universe come in several varieties and some of the most energetic ones, which are also plausible sources of gravitational waves, are produced in the process of stellar death. Although quite a few are discovered traditionally at optical or at higher frequencies, radio waves from these sources reveal crucial information about the progenitors and processes that shine them. The past few years have seen a surge in the number of transient discoveries and in our understanding of them. This is expected only to get better in the near future with the systematic investigation of the transient sky coming under multi-frequency coverage thanks to the advent of several dedicated facilities such as PTF, ASKAP and others. In that context, I will discuss a few different transients exemplifying the power of radio investigation.

November 11, 2011

Everything you ever wanted to know about the gravitational-wave memory
Marc Favata , UW-Milwaukee

Abstract: The gravitational-wave memory refers to a time-varying but non-oscillatory contribution to the gravitational-wave amplitude. This memory comes in linear and nonlinear varieties. The linear memory typically arises from sources with unbound components, like supernova explosions or gravitational two-body scattering. The nonlinear memory arises from the gravitational waves produced by gravitational-waves. The largest memory signals are produced by binary black hole mergers and provide insight into the highly nonlinear sector of general relativity. I will discuss the memory effect in detail, focusing on post-Newtonian and numerical relativity calculations of the nonlinear memory and the prospects for its detection.

November 9, 2011 (Wednesday)

Neutron star masses from the flushed face of irradiated pulsar companions
Rene Breton , University of Toronto

Abstract: Typical neutron star densities are beyond the reach of Earth laboratory experiments and the study of their equation of state can provide important knowledge about the behaviour of ultra-dense matter. While the neutron star equation of state remains elusive due to observational challenges (e.g. namely the lack of reliable simultaneous mass and radius measurements), the most massive neutron stars constrain it to increasingly stiff models. The most promising candidates to search for massive neutron stars are the binary millisecond pulsars, which are old, once-slowly rotating pulsars that have been spun-up by accreting mass from a close companion star. Empirically, the so-called black-widow systems seem particularly promising: for the prototype system, PSR B1957+20, we recently inferred a mass of 2.4 solar masses. If confirmed by further study, this would make it the heaviest know neutron star. In this talk, I will describe how the light curve and spectrum of the strongly irradiated companion was used to determine the black-widow pulsar mass. I will also discuss perspectives of several new mass measurements in similar systems detected with the help of the Fermi gamma-ray observatory.

Please note this talk will begin at 2PM.

November 4, 2011

The Formation and Evolution of Massive Black Holes in Cosmological Simulations
Jillian Bellovary , University of Michigan

Abstract: Massive black holes (MBHs) are inextricably connected to the formation of massive galaxies, but their formation, evolution, and specific effects on their hosts are not clearly understood. Cosmological simulations of galaxy formation, including prescriptions for MBH formation, mergers, accretion, and feedback, are a unique way to shed light on this issue. I adopt a novel approach to forming seed black holes in galaxy halos which is dictated directly by the physics of primordial, zero-metallicity gas and motivated by physical models of massive black hole formation. Our simulations explain why massive black holes are found in some bulgeless and dwarf galaxies, but we also predict that their occurrence becomes rarer and rarer in low-mass systems. I also predict a population of wandering MBHs in the halos of massive galaxies, which are the remnant cores of tidally stripped satellite galaxies. These objects may be observed as off-nuclear ultraluminous X-ray sources if the cores retain a gas reservoir and are perturbed in some way, or if a nearby star is tidally disrupted.

October 28, 2011

Midwest Relativity Meeting Practice Talks
Various , UW-Milwaukee

October 14, 2011

Charting the Dynamic Sky: An Industrial Approach (The Palomar Transient Factory)
S. R. Kulkarni , Director, Caltech Optical Observatories, California Institute of Technology

Abstract: Cosmic explosions were first noted nearly two thousand years ago. However, secure recognition and study began only a hundred years ago. What was once termed as Stella Nova (new stars) are now divided into two major families, novae and supernovae (with real distinct classes in each). Equally the variable stars have a rich phenomenology. Together, supernovae and variable stars have contributed richly to key problems in modern astrophysics: distances to galaxies, cosmography and build up of elements in the Universe. The Palomar Transient Factory (PTF), an innovative 2-telescope system, was designed to explicitly to chart the transient sky with a particular focus on events which lie in the nova-supernova gap. PTF is now finding an extragalactic transient every 20 minutes and a Galactic (strong) variable every 10 minutes. Extensive spectroscopy and photometry allowed us: to identify an emerging class of ultra-luminous supernovae, discover luminous red novae, undertake UV spectroscopy of Ia supernovae, discover low budget supernovae, clarify sub-classes of core collapse and thermo-nuclear explosions, map the systematics of core collapse supernovae, identify a trove of eclipsing binaries and the curious AM CVns.

October 7, 2011

The Physics and Cosmology of TeV Blazars
Phil Chang , University of Wisconsin-Milwaukee

Abstract: The universe is teeming with very high energy gamma ray sources (> 100 GeV), but it is generally thought that while the phenomenology is interesting their impact on the rest of the universe is minor at best. On energetic grounds, this assumption seems well-founded because the energy density in TeV photons is 0.2% of that of ionizing photons from quasars. However, as I hope to show in this talk, this is not the case. Rather, the greater efficiency by which TeV photons can be converted to heating in the intergalactic medium (IGM) allows TeV blazars to punch well above their energetic weight. I will discuss the nature of this conversion via plasma instabilities and its plasma physics. I will then discuss how the resultant heating from these TeV sources makes dramatic differences in the formation of structure in the universe. In particular, I will discuss how it gives rise to the inverted temperature-density profile of the IGM, the bimodality of galaxy clusters, and the paucity of dwarf galaxies in galactic halos and voids.

September 23, 2011

21cm Cosmology
Peter Timbie , University of Wisconsin-Madison

Abstract: Future surveys of the redshifted 21 cm line from neutral hydrogen gas promise to revolutionize the field of cosmology. These surveys may ultimately encompass much larger volumes than galaxy surveys can study. They will measure the power spectrum of matter with high precision, leading to tight constraints on dark energy, neutrino mass, and other cosmological parameters. I will describe some current and planned surveys and summarize the challenges that must be overcome.

September 21, 2011

Black Hole Rainbows: Testing General Relativity with Quasi-Normal Modes
Sarah Gossan , Cardiff University, 2:00 PM

Abstract: Please note this talk will be given at 2 PM in Physics 481. The quasi-normal modes that characterise the ringdown emission from an excited black hole, immediately after the merger of a binary system, are described by general relativity models. In these models, the frequencies and decay times of the modes are dependent only on the mass and spin of the black hole, by the no-hair theorem, and the relative amplitudes of the modes have dependence only on the mass ratio of the binary components prior to merger. It is thus possible by measuring the frequencies, decay times and relative amplitudes of each mode to determine not only the mass and spin of the black hole, but additionally the mass ratio of the binary components that formed the black hole. A test of general relativity can be conducted by measuring these parameters for multiple modes and confirming the predicted parameter consistency. We developed a Bayesian method to measure the individual modes and extract the system parameters, allowing the consistency of modes predicted by general relativity to be tested, as well as estimating the deviations of these parameters away from the values predicted by general relativity. Preliminary results will be presented displaying both the accuracy to which system parameters can be extracted and deviations from general relativity that can be determined.

September 2, 2011

Gravitational waves from black hole-neutron star binaries: dependence on the black hole spins and on neutron star equations of state
Koutarou Kyoutoku , Yukawa Institute for Theoretical Physics, Kyoto University, Japan

Abstract: Black hole-neutron star (BH-NS) binaries are ones of the most promising sources of gravitational waves for ground-based laser-interferometric gravitational-wave detectors, and are also possible candidates of the short-hard gamma-ray bursts. Because tidal disruption of the NS is strongly affected by the NS radius and therefore the equation of state (EOS) at high density, we can extract the information about EOSs from the gravitational-wave observation if the NS tidal disruption occurs outside the innermost stable circular orbit of the BH. Furthermore, in such cases, massive accretion disks are formed around the BH, which is required to generate gamma-ray bursts. We present recent results obtained by our numerical-relativity simulations of the BH-NS binary mergers, focusing on the dependence of gravitational waves on BH spins and on NS EOSs.

August 26, 2011

Black Holes in String Theory
Satoshi Nawata , Tata Institute of Fundamental Research

Abstract: The pioneering work of Bekenstein and Hawking in the 70's produced a universal area law for black hole entropy S=A/4G valid in the infinite size limit. One of the important successes of string theory is that one can obtain a statistical understanding of the thermodynamic entropy of certain supersymmetric black holes in terms of microscopic counting. This talk will describe some of this progress in our understanding of the quantum structure of black holes. The plan of this talk is as follows.

  1. Black holes in string theory which have been extensively studied are supersymmetric states and are stable under quantum corrections. To begin with, I will describe the simplest toy model, the supersymmetric version of quantum mechanics (i.e. the (0+1)-dimensional nonlinear σ-model). The Witten index Tr(-1)F, where F is the fermion number operator, counts the number of supersymmetric states up to sign, and this turns out to be the Euler characteristic of a target manifold.
  2. Next, I will explain the first success by Strominger and Vafa to explain the Beckenstein-Hawking entropy in terms of microscopic counting. We consider black holes in the zero temperature limit which can be realized as supersymmetric extremal black holes M=Q. In string theory, the bound states of D-branes become these extremal black holes. Given charges Qi of a black hole, we enumerate microstates dQi and compute the statistical entropy given by Sstat=log(d). In the infinite limit Qi → ∞ of charges, it was shown that this statistical entropy provides the area of the black hole.
  3. If time permits, I would like to explain subleading corrections to the black hole entropy with finite charges Qi. The leading term, which of course is just the ordinary Bekenstein-Hawking entropy S=A/4G, is too universal since it follows from the Einstein-Hilbert action which is the leading low energy effective action in all phases. Finite size effects are controlled by the higher derivative terms of metric in the effective action. Wald proposed a way to compute the black hole entropy from the Einstein-Hilbert action with higher derivative terms. Recently, it has been shown that for certain extremal black holes the counting of microstates reproduces this Wald entropy.
Note: A less technical version of this same talk will also be given at 3:00 PM in Physics 135.

August 22, 2011

Special Seminar: Undergraduate Summer Research Presentations
Christopher Biwer, David Day, Jesse DePinto, Robin Karr, Matthew Rohr and Josh Speagle , University of Wisconsin-Milwaukee

Abstract: A series of talks from UWM students (hosted by professor David Kaplan) will be presented in the following order:

  1. Real-Time Decision Making for Gravitational Wave Detection, Christopher Biwer
  2. A Search for Low Redshift Analogs to the Earliest Galaxies, David Day
  3. Detecting Arhived Radio Transients, Jesse DePinto
  4. The Development and Use of a New Small Radio Telescope, Robin Karr
  5. Pulsar J0645+51: An Addition to the International Pulsar Timing Array, Matthew Rohr
  6. The X-Ray Counterpart of the High-B Pulsar J0726-26: Implications for the Evolution of Neutron Stars, Josh Speagle
Please note that this special presentation is scheduled to run from 2 - 3:30 PM.

August 19, 2011

The Impact of QFT Renormalization and the Initial State on the Observable Consequences of Inflation
Jose Navarro Salas , Universidad de Valencia (Spain)

Abstract: We analyze the generation of primordial perturbations in a (single-field) slow-roll inflationary universe. In momentum space, these (Gaussian) perturbations are characterized by a zero mean and a non-zero variance (mean square fluctuation). However, in position space the variance diverges in the ultraviolet. The requirement of a finite variance in position space forces one to regularize it. This can be achieved by proper renormalization in an expanding universe in a unique way. This affects the predicted scalar and tensorial power spectra for wavelengths that today are at observable scales. As a consequence, the imprint of slow-roll inflation on the CMB anisotropies is significantly altered. We will also report briefly on the impact of a non-Gaussian initial state on the scalar trispectrum.

August 5, 2011

The Large Observatory for X-ray Timing (LOFT)
Luigi Stella , INAF Astronomical Observatory of Rome - Italy

Abstract: High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultra-dense matter and to black hole masses and spins. A 10 m2-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme "Matter under extreme conditions," namely: does matter orbiting close to the event horizon follow the prediction of general relativity? What is the equation of state of matter in neutron stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of the four Cosmic Vision M3 candidate missions to undergo an assessment phase, will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. Thanks to an innovative design and the development of large-area monolithic Silicon Drift Detectors, the Large Area Detector (LAD) on board LOFT will achieve an effective area of ~12 m2 (more than an order of magnitude larger than any spaceborne predecessor) in the 2-30 keV range (up to 50 keV in expanded mode), yet still fits a conventional platform and small/medium-class launcher. With this large area and a spectral resolution of < 260 eV, LOFT will yield unprecedented information on strongly curved spacetimes and matter under extreme conditions of pressure and magnetic field strength.

July 29, 2011

Gravitational Waves from Cosmological Phase Transitions
Tom Giblin , Perimeter Institute

Abstract: Cosmological phase transitions occurred. I will talk about recent advances in modeling possible phase transitions when these transitions are mediated by scalar fields. I will discuss first- and second-order transitions, at various scales, and show how we can compute the background of stochastic gravitational waves produced during (and after) these transitions.

May 13, 2011

Gravitational self-force in a radiation gauge a for particle in circular orbit around Kerr black hole and filling in the missing pieces in a radiation gauge self-force calculation
Abhay Shah & John Friedman , University of Wisconsin-Milwaukee

May 6, 2011

A Reduced Basis representation of gravitational waveforms
Evan Ochsner , University of Wisconsin-Milwaukee

Abstract: The reduced basis approach provides a compact representation of a family of gravitational waveforms in some parameter range. This set of waveforms can be embedded into a finite-dimensional subspace of the infinite-dimensional Hilbert space of all functions with finite norm. A set of basis functions are found which span this subspace and which can reconstruct physical waveforms to arbitrarily high precision. This talk will begin with a description of the mathematical foundations of the approach and the algorithm to construct a reduced basis. Next we will present results of the method applied to non-spinning inspiral waveforms, spin-aligned inspiral waveforms, and ringdown waveforms. Last, we will discuss some of the advantages of the approach, issues that need to be addressed to take full advantage of the approach, and directions of future work.

April 22, 2011

The Generation & Evolution of Cosmic Magnetic Fields
Ellen Zweibel , University of Wisconsin-Madison

Abstract: Despite recent progress in cosmology, the origin of magnetic fields is still uncertain. I will review what we know about magnetic fields in galaxies, galaxy clusters, and the intergalactic medium, discuss dynamo theories, and talk about how observations of magnetic fields in the interstellar medium can be used to constrain dynamo processes.

April 15, 2011

Black hole-neutron star close encounters: An update
Branson Stephens , UWM

April 8, 2011

Astrometric Effects of a Stochastic Gravitational Wave Background
Eanna Flanagan , Cornell

Abstract: A stochastic gravitational wave background causes the apparent positions of distant sources to fluctuate, with angular deflections of order the characteristic strain amplitude of the gravitational waves. These fluctuations may be detectable with high precision astrometry, as first suggested by Braginsky et al. in 1990. Several researchers have made order of magnitude estimates of the upper limits obtainable on the gravitational wave spectrum \Omega_gw(f), at frequencies of order f ~ 1 yr-1, both for the future space-based optical interferometry missions GAIA and SIM, and for VLBI interferometry in radio wavelengths with the SKA. For GAIA, tracking N ~ 106 quasars over a time of T ~ 1 yr with an angular accuracy of \Delta \theta ~ 10 \mu as would yield a sensitivity level of \Omega_gw ~ (\Delta \theta)2/(N T2 H_02) ~ 10-6, which would be roughly comparable with pulsar timing. We take a first step toward firming up these estimates by computing in detail the statistical properties of the angular deflections caused by a stochastic background. We compute analytically the two point correlation function of the deflections on the sphere, and the spectrum as a function of frequency and angular scale. The fluctuations are concentrated at low frequencies (for a scale invariant stochastic background), and at large angular scales, starting with the quadrupole. The magnetic-type and electric-type pieces of the fluctuations have equal amounts of power.

April 1, 2011

Gravitational Wave Detection through Pulsar Timing
Joris Verbiest , Max-Planck-Institut, Bonn

Abstract: Radio timing of millisecond pulsars (MSPs) has allowed some of the most precise tests in astrophysics to date, including the first indirect detection of gravitational waves, in 1982. Almost 30 years later, the direct detection of gravitational waves - and the birth of gravitational wave science - is predicted to be achieved by the same means. Specifically, simulations of hierarchical galaxy formation models predict a population of supermassive black-hole binaries in the nearby (z<2) Universe. These binaries would emit gravitational waves that would affect pulsar timing at the 10 to 100 ns level, over timescales of years to decades. In this talk I present results from the first sizeable long-term MSP timing campaign, confirming that the timing precision required for gravitational wave detection is achievable within the next decade. I further comment on how this gravitational wave sensitivity can be improved through ongoing pulsar surveys, international collaboration, system upgrades, studies of the ISM and the next generation of radio telescopes.

March 11, 2011

Magnetic Field-Decay-Induced Electron Captures: a Strong Heat Source in Magnetar Crusts
David Kaplan , UWM

Abstract: We propose a new heating mechanism in magnetar crusts. Magnetars' crustal magnetic fields are much stronger than their surface fields; therefore, magnetic pressure partially supports the crust against gravity. The crust loses magnetic pressure support as the field decays and must compensate by increasing the electron degeneracy pressure; the accompanying increase in the electron Fermi energy induces nonequilibrium, exothermic electron captures. The total heat released via field-decay electron captures is comparable to the total magnetic energy in the crust. Thus, field-decay electron captures are an important, if not the primary, mechanism powering magnetars' soft X-ray emission.

March 11, 2011

Optimal strategies for non-Einsteinian gravitational wave detection using a pulsar timing array
Sydney Chamberlin , UWM

Abstract: Alternative theories of gravity hold the potential to answer fundamental questions in physics, from the origin of cosmic expansion and dark matter to quantum gravity. In the next few years gravitational wave astronomy could deliver experimental results which support or rule out modified theories of gravity. We investigate the detection of non-Einsteinian gravitational waves using a pulsar timing array. We analyze the optimal statistical technique for the detection of a low frequency, stochastic background of gravitational waves, including non-Einsteinian polarizations.

March 4, 2011

Non-gaussianities from single-field inflation
Leonard Parker , UWM

Abstract: I discuss constraints on non-gaussianities of the primordial inflaton perturbations resulting from inflation. Measurement of non-gaussianities of the CMB anisotropies and of the large- scale-structure of the universe can reveal information about these primordial non-gaussianities. In work with Ivan Agullo, soon to appear in Physical Review D, we show how the primordial perturbations and their non-gaussianities are related to the quantum state at very early times during inflation.

February 25, 2011

The truth is out there - Galaxy evolution and more from photometric redshifts
Ralf Kotulla , UWM

Abstract: Semi-analytical models (SAMs) summarize much of our current knowledge about galaxy evolution and apply it to cosmological simulations of structure formation. As such, they are calibrated to match the observational signatures in the local universe. However, detailed comparisons at higher redshifts are necessary to check how well these models perform at predicting the evolution of galaxies. I present the results of such a study, based on photometric redshifts of a range of deep-fields, showing that at least at the high-mass end SAMs are still missing some of the ingredients to fully reproduce observations, and also give an outlook on what else we can infer from the data at hand.

February 11, 2011

The Instability of 5-Dimensional Black Strings
Frans Pretorius , Princeton University

Abstract: I will describe recent numerical results of the evolution of 5-dimensional black strings, subject to the Gregory-Laflamme instability. We find that the horizon qualitatively behaves very much like a thin stream of low viscosity fluid subject to the Rayleigh-Plateau instability, where ever smaller spherical "beads", connected by ever thinning string segments, form. This cascade to small length scales unfolds in a self-similar manner, and thus though with finite computational resources we cannot evolve the spacetime to the end-state, we can extrapolate the observed trends. Doing so suggests that the string segments will reach zero radius in finite asymptotic time, and hence arbitrarily large spacetime curvatures will be revealed to external observers. Thus, black strings are an example of a spacetime that exhibits generic violation of cosmic censor in 5-dimensional Einstein gravity.

February 4, 2011

Convex Optimization as a Foundation for Time Domain Window Design and Novel Methods of NMR Spectral Analysis
Jeff Kline , University of Wisconsin-Milwaukee

Abstract: A fundamental technique of protein NMR spectroscopy is time domain windowing of the acquired signal, called the free induction decay or FID. The FID is typically modeled in the time domain as a linear combination of modulated and decaying ``atoms'' in the presence of additive noise. Typical analysis of the FID involves the identification of ``peaks'' living in the spectrum of the FID. The ability to accurately distinguish true peaks from peaks that arise from a conspiracy of noise is therefore central to NMR spectroscopy. I will discuss a novel and nonlinear representation of the FID that outperforms the best-possible linear representations, as measured by peak-maximum to noise ratio. The starting point of this representation is the construction of a sequence of mutually orthonormal windows, each window is required to have minimal coherence with the model atom of the FID. Due to the mutual orthogonality of the windows and since each of the windows is coherent with the atom, the ``true peaks'' of the FID spectrum can be discerned from the ``false peaks'' not due to atoms. Although the application discussed in my talk is NMR spectroscopy, the techniques I describe should generalize to more generic situations.

January 13, 2011

Heavy Vortices and Dark Energy
Philip Chang , University of Toronto

Abstract: I will discuss two problems in the fields of planet formation and cosmology. The first problem concerns the formation of planetesimals. The formation of planetesimals starts with coagulation of dust particles up to pebbles on cm scales and ends with gravitational accretion of km-size bodies. How these pebbles grow from cm scales to km scales remains a major unsolved problem in astrophysics. Vortices in protoplanetary disks may serve as nurseries for these planetesimals by concentrating dust in there cores, where these pebbles can directly form these km-sized bodies by gravitational collapse. I will show that this is not the case. Rather, the concentration of these pebbles in the cores of these vortices leads to a new instability, the heavy-core instability. I will discuss the general physics of this instability and argue that this instabilty precludes vortices from being the nurseries of planetesimal formation. The second problem I will discuss is using stellar evolution to place constraints on the nature of dark energy. One recent suggestion is the dark energy arises from the presence of a scalar field, i.e., a fifth force, that couples to matter like gravity. Locally (in our high density patch of the universe) this scalar field is not observed, but it operates in the low density regions of the universe. I will show that the effect of this scalar field will modified stellar structure and evolution in these parts of the universe and how careful observations of the colors of red giants can put constraints on these theories.

December 10, 2010

Modeling the dynamics and gravitational-wave emission of compact binaries
Alessandra Buonanno , University of Maryland

Abstract: Recent developments at the interface of analytical and numerical relativity have deepened our understanding of the two-body problem in general relativity, revealing an intriguing simplicity. I will review those advances within the effective-one-body approach, focusing on the most dynamic and non-linear phase of the evolution. I will discuss the implications of the advances in the search for gravitational waves from comparable mass and extreme mass-ratio binary black holes. Finally, I will briefly review recent results of modeling binary neutron stars within the same approach.

December 3, 2010

Precessing compact binaries: Gravitational wave astrophysics
Richard O'Shaughnessy , UWM

Abstract: Second-generation ground-based gravitational wave observatories will measure the signature of at least several tens of merging compact binaries, including the amplitude and phase modulations spin-orbit precession introduces in their waveforms. For any given event, these modulations complicate parameter estimation and even detection, introducing intrinsic (e.g., change in orbit duration) and practical (e.g., mismatch with nonspinning templates) selection biases. In this talk, we quantify some of these biases for low (<15 Msun) and high (>100 Msun) mass binaries. We review scenarios for low and high-mass compact binary formation. For high-mass equal-mass binaries binaries, we argue the at- times-enormous biases will nonetheless average out for astrophysically plausible populations. For low-mass binaries, we demonstrate that (a) spin-orbit misalignment in low-mass binaries is a quantitative measure of the formation mechanism; that (b) that practical selection biases introduced by current searches should filter out strongly misaligned BH-NS binaries, noticeably biasing the detected population towards aligned spin; and that (c) assuming these biases aren't eliminated (e.g., by targeting ill-recovered spin configurations with special templates), the birthrate inferred from a detected population of BH-NS binaries will therefore depend up to a factor of 2 on proposed spin-orbit misalignment distributions.

November 19, 2010

A Non-radial Oscillation Model for Radio Pulsars
Rachel Rosen , NRAO

Abstract: We present a non-radial oscillation model that can successfully reproduce many properties observed in pulsars. By demonstrating the presence of oscillation modes in pulsars, we hope to pave the way for asteroseismology of neutron stars. This model is an alternative to the drifting spark model of Ruderman and Sutherland (1975). A non-radial oscillation model can explain behavior in the average pulse profile and in individual pulses, including drifting subpulses, quasi-stationary behavior, subpulse phase jumps, orthogonal polarization modes, and changes in the spin down rate. We quantitatively fit our data to both PSRs B0943+10 and B0809+74 and find that our model is able to reproduce the morphology seen in these stars

November 12, 2010

LHC Phenomenology of Regge Excitations in the Randall-Sundrum Model
Xing Huang , UWM

Abstract: We consider string realizations of the Randall-Sundrum effective theory and explore the search for the lowest massive Regge excitation of the gluon and of the extra (color singlet) gauge boson inherent of D-brane constructions. In these curved backgrounds, the higher-spin Regge recurrences of Standard Model fields localized near the IR brane are warped down to close to the TeV range and hence can be produced at collider experiments. Assuming that the theory is weakly coupled, we make use of four gauge boson amplitudes evaluated near the first Regge pole to determine the discovery potential of LHC.

October 22, 2010

Non-axisymmetry configurations in General Relativity: A PN Approximation of Dedekind ellipsoids
Norman Gurlebeck , Charles University

Abstract: A changing quadrupole moment leads to gravitational radiation in General Relativity. Does this imply that stationary but non-axisymmetric, isolated systems cannot exist? To learn something about the answer to this question, a PN approximation of the Newtonian triaxial and homogeneous Dedekind ellipsoids is investigated. I shall discuss a generalization of the ansatz used by Chandrasekhar and Elbert (1978), in particular its axisymmetric limit. Contrary to Chandrasekhar & Elbert's ansatz this generalization permits an axially symmetric and rigidly rotating limit (PN Maclaurin spheroids). The additional freedom in the generalized solution can also be used to remove a singularity which occurs in their work.

October 12, 2010

Gravitational waves from first order phase transitions
Chiara Caprini , Institut de Physique Théorique, CEA Saclay

Abstract: First order phase transitions in the early universe give rise to a stochastic background of gravitational waves. Three main processes lead to the production of the gravitational wave signal: the collision of the broken phase bubbles, the turbulence in the primordial plasma stirred by the bubble collision, and the magnetic fields amplified by the turbulence. In this seminar I will show how the main features of the gravitational wave spectrum can be predicted by simple, general arguments based on the sources properties, such as their time evolution and space structure. I will discuss detection prospects, in particular for the electroweak and the QCD phase transitions, which can generate a GW signal potentially detectable respectively by the space interferometer LISA and by pulsar timing arrays.

October 8, 2010

Searching for transient gravitational waves; Data analysis
Stefanos Giampanis , UWM

Abstract: Brief introduction to the methods used in looking for known and unknown signals in noisy data streams. Stefanos Giampanis will discuss the Frequentist and Bayesian statistical methods and interpretation and elaborate on a toy model for transient gravitational waves from known neutron stars. The talk will focus mainly on the latter, emphasizing the "matched filtering" approach and the Bayesian detection statistic.

October 1, 2010

Neutron star equation of state using gravitational wave observations of coalescing neutron star binaries
Jocelyn Read , AEI

September 17, 2010

Binary mergers and some initial attempts to calculate electromagnetic counterparts
David Neilsen , Brigham Young University and Perimeter Institute

Abstract: The detection of electromagnetic radiation from binary merger events can aid searches for gravitational waves. The possibility of precursor radiation could alert us to a future merger, and the detection of an afterglow would help to isolate the source for a detected signal. I will talk about some of our recent work with black hole and neutron star binaries, and some of our efforts to calculate electromagnetic radiation from these mergers.

September 10, 2010

Black hole-neutron star binary simulations: a progress report
Branson Stephens , UWM

Abstract: Recent work to develop a code for compact object binary simulations in full general relativity will be described, as well as preliminary simulations of black hole-neutron star binary mergers. This work is motivated by the possible association of short-duration gamma-ray bursts (GRBs) with mergers of black holes (BHs) and neutron stars (NSs). Gravitational waves from BH-NS and NS-NS mergers may eventually be detected by LIGO coincidently with short GRBs. It is still not known how (or whether) compact binary mergers can produce sufficiently massive accretion disks to power short GRBs, or how these disks could produce the observed late time X-ray flares following the prompt gamma-ray spike. We are working toward address such questions using numerical simulations.

July 9, 2010

Low-latency searches for unmodeled gravitational waves in LIGO and Virgo
Jamie Rollins , Columbia University

July 8, 2010

Searching for the X-ray counterparts of LIGO/VIRGO gravitational wave
Luigi Stella , INAF, Astronomical Observatory of Rome

June 25, 2010

Cosmological Perturbations from Staggered Inflation
Evan Pease , Kenyon College

Jun 25, 2010

Supernovae and Cosmic Anisotropy
Cary Watts , Kenyon College

May 14, 2010

Life Before the LVC: GRBs, Standard Sirens, Bayesian Statistics and Cosmology
Fiona Speirits , University of Glasgow

Abstract: Despite Gamma Ray Bursts (GRBs) being discovered in the 1960s, surprisingly little is understood about the most energetic events in the Universe since the Big Bang. However, these unprecedented energy levels allow GRBs to be detected to very high redshift - an attractive attribute that Cosmologists are keen to exploit. I will discuss methods developed by the community to attempt to standardise GRBs as cosmological distance indicators, highlighting potential pitfalls to conventional analysis methods. Perhaps of even greater interest to the future of Cosmological model selection will be the discovery of gravitational wave standard sirens. With the potential for exquisite accuracy, these compact object binary mergers could provide the most accurate constraints on Cosmological parameters yet achieved. I will present examples of the best case scenario potential of standard sirens, while also highlighting and quantifying the main source of error in the form of weak gravitational lensing and discussing novel attempts to reduce this signal degradation.

April 2, 2010

The North American Nanohertz Observatory of Gravitational Waves
Andrea Lommen , Franklin and Marshall College

Abstract: NANOGrav is a consortium of radio astronomers and gravitational wave physicists whose goal is to detect gravitational waves using an array of millisecond pulsars as clocks. Whereas interferometric gravitational wave experiments use lasers to create the long arms of the detector, NANOGrav uses earth-pulsar pairs. The limits that pulsar timing places on the energy density of gravitational waves in the universe are on the brink of limiting models of galaxy formation and have already placed limits on the tension of cosmic strings. Pulsar timing has traditionally focused on stochastic sources, but most recently I have been investigating the idea of detecting individual gravitational wave bursts wherein there are some interesting advantages. I will also demonstrate how the array can be used to reconstruct the waveform and obtain its direction

March 5, 2010

Solving the Poisson equation with multidomain spectral methods
Ben Lackey , UWM

Abstract: Poisson-like equations are used in both Newtonian gravity and general relativity for constructing stellar models. Spectral methods provide a way to rapidly solve these equations with a small number of gridpoints and with accuracy approaching double precision. However, discontinuous functions produce Gibbs phenomena that must be treated carefully. I will give a brief overview of spectral methods and then describe a method for calculating the structure of Newtonian stars with density discontinuities

February 26, 2010

Gravitational wave radiation from the center of GRB
Hyun Kyu Lee , Hanyang University

February 19, 2010

The double pulsar: tomography of pulsar magnetosphere and a new GR test
Maxim Lyutikov , Purdue University

Abstract: The long awaited discovery of the binary radio pulsar system, PSR J0737-3039A/B, surpassed most expectations, both theoretical and observational, as a tool to probe general relativity, stellar evolution and pulsar theories. I will describe rich observational properties of the system, like eclipses, orbital variations in magnetospheric activity, evidence for reconnection between the rapidly changing magnetic field in the wind with the magnetospheric fields. An eclipse model also provides a quantitative measurement of relativistic spin precession and offers a new test of theories of gravity. A number of methods used in studying interaction of the Solar wind with planetary magnetospheres can be directly applied to this amazing system.

February 5, 2010

Geometry and dynamics of a tidally deformed black hole
Eric Poisson , University of Guelph

Abstract: I describe a program of research that aims to produce an improved understanding of the tidal deformation of black holes. In this work the tidal environment is described in the most general terms compatible with the assumption that the tidal interaction is both weak and slow. The tidal deformation is obtained by integrating the Einstein field equations in the local neighbourhood of the black hole; this is done in the context of (nonlinear) perturbation theory. An outcome of this investigation is the description of how the black hole grows in area as a result of the tidal interaction, a phenomenon known as tidal heating. I conclude by revealing an analogy between the tidal dynamics of black holes and the tidal dynamics of viscous bodies in Newtonian gravity; the analogy is surprisingly close and holds in near-quantitative detail.

29 January 2010

Clarifying Some Remaining Questions in the Anomaly Puzzle in N=1 Supersymmetric Yang-Mills Theory
Xing Huang , UWM

Abstract: The anomaly puzzle in N = 1 supersymmetric gauge theories is well known. The problem is that there is no consistent way to write an anomaly equation in the operator form. A real superfield, $\CJ_{\mu}$, called the supercurrent can be constructed. The lowest component of this superfield is the R-current (a chiral $U(1)$ current). The other components of $\CJ_{\mu}$ are related to the supersymmetry current $J_{\xa\mu}$ and the stress tensor $\vartheta_{\mu\nu}$ through linear transformations. This leads to a contradiction. On the one hand, the anomaly of R-symmetry should be only of one-loop according to the Adler-Bardeen theorem. On the other hand, the trace of the stress tensor, which is another component of the super anomaly equation should be proportional to the $\xb$-function. The two components should be proportional to the same factor and this seems to imply the $\xb$-function is exhausted at one loop. However, explicit perturbative calculations show higher order corrections to the $\xb$-function. There are various solutions to this puzzle. Our work is mainly about clarifying some the remaining problems. By doing so, we try to integrate various proposed solutions into one consistent framework. In this talk, I will try to present a complete and consistent picture of how this anomaly puzzle is resolved.

22 January 2010

Searching for a stochastic background of gravitational waves with LIGO's co-located Hanford detectors: trials and tribulations
Nickolas Fotopoulos , UWM

Abstract: A search for a stochastic gravitational-wave background with H1 and H2, LIGO's Hanford detectors, promises a factor of 10 improvement over existing searches, but H1 and H2 operate within the same vacuum system, so they are immersed in a common cacophony. The correlated noise has proven fatal to stochastic background searches with this pair so far. I'll review methods to identify all narrow-band and all environmental correlations between the instruments and how far they have brought us. Finally, I'll discuss the newest efforts, which attempt to use Bayesian techniques to identify the posterior odds with which we should believe that there is significant environmental correlation or not. One theme in this talk will be why we believe any measurement at all.


UWM Center for Gravitation and Cosmology | http://www.gravity.phys.uwm.edu/ | contact@gravity.phys.uwm.edu