The standard seminar time and place is Wednesday, 1:30pm, Room 4102. Fall 2013 Seminars coming soon.

Non-standard times and/or places are indicated in red type.

For more information about GRT group seminars contact Sam Gralla or Alexandre Le Tiec. For information concerning the Elementary Particle Theory group

seminars see the EPT seminar page, the Theoretical Quarks, Hadrons, Nuclei group seminars see the TQHN seminar page, and scheduled

seminars in the UMD Physics Department see the Department calendar. The webpages of the UMD-NASA Goddard Joint Space-Science Institute

and the Maryland Center for Fundamental Physics also hold regular events of interest to the group.

Seminars from previous semesters can be found here: spring 2002, fall 2002, spring 2003, fall 2003, spring 2004, fall 2004, spring 2005,

fall 2005, spring 2006, fall 2006, spring 2007, fall 2007, spring 2008, fall 2008, spring 2009, fall 2009, 2010, spring 2011, fall 2011, fall 2012.

===============================================

ABSTRACTS

===============================================

Jonathan McKinney, UMD

Black Hole Event Horizon Phenomena: Observations Confront Theory

The black holes in M87 and SgrA* have event horizons with the largest

angular size on the sky among all black holes in the Universe. Such

event horizon scales are now accessible to Earth-wide radio arrays,

so these systems potentially offer the best chance to probe and even

test Einstein's general relativity theory. I discuss the latest observations,

rough interpretations, theories, and simulations that attempt to make

sense of what goes on near black hole event horizons.

===============================================

Donghui Jeong, Johns Hopkins

New Ways of Searching for the Primordial Gravitational Wave from

Large Scale Structure

Primordial gravitational wave (PGW) is a valuable probe of the physics

of the early universe as the amplitude of it is directly related to the energy

scale of inflation. Most popular method of detecting PGW is using a parity

odd (B-mode) polarization pattern of the cosmic microwave background

radiation. Then, do we have any signature from the large scale structure?

In this talk, we shall discuss two possible ways that PGW affect the large

scale structure: 1) through light deflection and 2) through intrinsic

correlation. We shall show that one can probe these two effects from the

B-mode of weak gravitational lensing and the off-diagonal correlation of

galaxies. For both cases, the intrinsic correlation dominates over the light

deflection in standard cosmologies as PGW amplitude decays once the

mode comes inside of the horizon.

reference:

Jeong & Kamionkowski [arXiv:1203.0302]

Schmidt & Jeong [arXiv:1204.3625]

Jeong & Schmidt [arXiv:1205.1512]

Schmidt & Jeong [arXiv:1205.1514]

===============================================

Jeremy Schnittman, NASA Goddard

Finding Stellar-Mass Black Holes with Gravitational Lensing

Over the past two decades, ground-based optical surveys of millions of

stars in the galactic bulge have produced thousands of microlensing

events. Due to a fundamental mass-distance-velocity degeneracy in

the microlensing light curve, it has been very difficult to determine

accurate lens masses for most of these events. Here we discuss two

promising techniques for breaking this degeneracy: high-precision

astrometry, and additional timing features present in binary

systems. We show that the current observational data is of sufficient

quality to detect the first known binary black holes. With these

detections, we will be able to learn important information about the

evolution of massive stars and binary systems.

===============================================

Francois Foucart, CITA

The Outcome of Black Hole-Neutron Star Mergers: Predictions from

Numerical Relativity

The gravitational wave signal emitted by the merger of a black hole and

a neutron star will be observable by the next generation of gravitational

wave detectors. Such mergers can also be accompanied by detectable

electromagnetic signals: short gamma-ray bursts and their afterglow,

optical transients from the radioactive decay of neutron-rich material

ejected during merger, and radio emission as that ejecta decelerates in

the interstellar medium. The existence of these counterparts and the

characteristics of the gravitational wave signal are however sensitive

to the parameters of the binary (masses, spins, eccentricity), and to the

unknown equation of state of the neutron star material. In this talk, I will

discuss what numerical simulations have told us about the outcome of

black hole-neutron star mergers, and about their gravitational wave

emission. I will focus in particular on the few general relativistic

simulations performed in the range of black hole masses currently favored

by both population synthesis models and the observation of stellar mass

black holes, and on their strong differences with respect to previous

results obtained for lower black hole masses.

===============================================

Rob Owen, Oberlin

Spacetime Vorticity, Tendicity, and the Near-Field Structure of

Dynamical Black Holes

Numerical relativity is an invaluable tool for exploring the strongly

nonlinear dynamics of curved spacetime. However one of the first

steps in numerical simulation -- fixing a coordinate system in which

to express the partial differential equations -- breaks the fundamental

physical symmetry of the true problem. A good intuitive understanding

of black hole collisions would require tools that reduce, or at least

clarify, the effects of their own inherent coordinate ambiguity. I will

report one such effort that I've been pursuing along with a number of

colleagues, involving the stretching and twisting of fleets of inertial

observers, and will also comment on the relationship between this

formalism and some (more mainstream) constructions that define

quasilocal quantities such as black-hole spin and multipole moments.

===============================================

James Lattimer, Stonybrook

A Convergence on an Understanding of the Dense Matter Equation

of State

Determining the equation of state of neutron star matter has been a

long-sought goal. Recently, there has been a remarkable convergence

in our understanding of dense matter from several directions: multiple

nuclear experiments, theoretical neutron matter studies, pulsar mass

determinations, and estimates of neutron star masses and radii from

X-ray sources. The key parameters are related to the symmetry energy

of matter near the nuclear saturation density, which is closely related

to the neutron star mass-radius relation. Observations indicate that

the maximum neutron star mass is in excess of 2 solar masses, and,

together with nuclear experimental and theoretical studies, restrict

the radii of neutron stars with approximately 1.4 - 1.5 solar masses to

lie in the range 11 to 12.5 km. In addition, the rapid cooling recently

found for the neutron star in the Cassiopeia supernova remnant indicates

that both neutron superfluidity and proton superconductivity exist in its

interior, and tightly constrain their respective critical temperatures.

===============================================

Lam Hui, Columbia

Testing Gravity with Pulsars, Black Holes and the CMB

We will discuss three different topics all connected with the nature

of gravity: 1. a way to measure gravitational waves using scattering

with binary systems; 2. a way to test generic scalar-tensor theories by

looking for off-centered supermassive black holes; 3. a way to probe

fluctuations on superhorizon scales using the microwave background.

===============================================

Scott Field, UMD

Fast Evaluation of Asymptotic Waveforms from Gravitational

Perturbations

In the context of blackhole perturbation theory, I describe evaluation

of an asymptotic waveform from data (Regge-Wheeler-Zerilli master

functions given as a time-series) recorded at a fixed radial location.

The asymptotic waveform is represented as a convolution of the data

with a time-domain kernel comprised of a few damped exponentials.

In turn, each exponentials' strength and damping rate arises from a

sum-of-poles approximation of a Laplace frequency domain kernel.

I will motivate the origin of the frequency domain kernel as well as

the numerical techniques needed for its sum-of-poles approximation.

The method is used to study late-time decay tails at null-infinity,

"teleportation" of a signal between two finite radial values, and

luminosities from extreme-mass-ratio binaries. Through numerical

simulations with data recorded as close as r = 30M, I compute

asymptotic waveforms with late-time -4 decay (for l = 2 perturbations),

and also luminosities from circular and eccentric orbits that match

frequency domain results to relative errors of better than 10^{-9}.

These results are achieved without a compactification scheme,

extrapolation procedure or solving a PDE.

===============================================

B. Sathyaprakash, Cardiff

Shedding Light on Black Holes

Observation of black holes in the gravitational window will impact

many areas in astrophysics, cosmology and fundamental physics: Black

hole mass functions and their spins could tell us about astrophysics

of their formation and evolution; their redshift distribution and

demographics might reveal the origin of galaxies and if seed black

holes were small or large; accurate measurement of distances to black

hole coalescences could be useful for cosmography; accurate phasing of

the signals could help test general relativity and explore black hole

spacetimes. In this talk I will discuss the prospects for deploying

black holes for observational astronomy and cosmology as well as what

we could learn about black holes from gravitational wave observations.

===============================================

Sam Gralla, UMD

Thermodynamics of a Black Hole with Moon

Much of black hole thermodynamics is limited to systems with a high

degree of symmetry. In this talk I will discuss a non-stationary, non-

axisymmetric black hole spacetime that nevertheless admits a standard

thermodynamics: a black hole corotating with an orbiting moon. More

precisely, we consider a Kerr black hole perturbed by a particle on the

circular orbit whose frequency matches that of the event horizon. The

key point is that the spacetime has a "helical" Killing vector that generates

the event horizon, allowing the surface gravity to be defined in the

standard way. The surface gravity is uniform on the horizon and should

correspond to the Hawking temperature of the black hole. We calculate

the change in surface gravity/temperature, finding it negative: the moon

has a cooling effect on the black hole. We also calculate the area/entropy

of the perturbed black hole, finding no change from the background Kerr

value. Generalizations to alternative theories, higher dimensions, and

alternative asymptotics (e.g., ads) should be possible, allowing one to

probe the behavior of an interacting black hole in a variety of settings.

This work is in collaboration with Alexandre Le Tiec.

===============================================

Dam Son, Chicago

Newton-Cartan Geometry and the Effective Field Theory of the

Quantum Hall States

A nonrelativistic system in an external metric exhibits a gauge version

of Galilean invariance. This symmetry put constraints on the effective

field theory of quantum Hall states. We describe how the effective field

theory satisfying these constraints can be constructed using the formalism

of the Newton-Cartan geometry. Physical consequences for electromagnetic

response at nonzero wavenumbers are derived.

===============================================

Claudia de Rham, Case Western

Pulsar Tests of Modified Gravity

Modifications of Gravity usually come hand in hand with new polarizations

that can be probed on astrophysical and cosmological scales. In specific

models such as Massive Gravity, the force mediated by these extra

polarizations are screened via the Vainshtein mechanism. After reviewing

the theoretical framework behind this mechanism, I will show how it

affects the gravitational radiation emitted by binary pulsar systems.

In the simplest model, the mechanism successfully screens the effect

from scalar fields conformally coupled to matter, although gravitational

radiation is less suppressed relative to its general relativity predictions

than static fifth forces effects within pulsar systems. I will then discuss

extension of the mechanism to more general Galileon theories.

===============================================

Ira Rothstein, Carnegie Mellon

Generating Solutions to Einsteins Equations from the Yang-Mills Action

In this talk I will show how to generate solutions to Einsteins equations

without any reference to the Einstein-Hilbert action starting from the

Yang-Mills action by utilizing on-shell unitarity techniques in conjunction

with BCFW (Britto, Cachazo, Feng, Witten) recursion relations.

===============================================

Frans Pretorius, Princeton

Eccentric Compact Object Mergers

Binary compact object mergers are among the primary gravitational wave

sources expected to be observed by the next generation of ground-based

gravitational wave detectors. Mergers where one or both compact objects

are neutron stars will further produce electromagnetic emission, and

coincident observation of this together with gravitational wave emission

could teach us much about the progenitor systems, test general relativity

in the dynamical strong field regime, and help elucidate the nature of

matter at nuclear density. I will discuss some ongoing work modeling

such mergers within the context of general relativity coupled to ideal

hydrodynamics, focusing on black hole-neutron star and binary neutron

systems merging with sizeable eccentricity. Large eccentricity is expected

for mergers that occur following dynamical capture in dense cluster

environments, and though they may be rarer than the traditional quasi-

circular inspiral, they could exhibit strikingly different behavior, including

zoom-whirl orbital dynamics and large amounts of unbound material for

cases where the neutron star is tidally disrupted.