GRAVITY THEORY SEMINARS 2004
Abstracts:
Jan. 28, Wed., 12:15pm, Room 1305
Emil Mottola, LANL
``Vacuum Energy and Condensate Stars: A Quantum Alternative to Black Holes''
A new solution for the endpoint of gravitational collapse is proposed. By extending the concept of Bose-Einstein condensation to gravitational systems, a cold, compact object with an interior de Sitter condensate phase and an exterior Schwarzschild geometry of arbitrary total mass M is constructed. These are separated by a phase boundary with a small but finite thickness boundary layer with eq. of state p=+\rho, replacing both the Schwarzschild and de Sitter classical horizons. The new solution has no singularities, no event horizons, and a global time. Its entropy ismaximized under small fluctuations and is given by the standard hydrodynamic entropy ofthe thin shell, which is of order M^{3/2} instead of the Bekenstein-Hawking entropy, S = 4\pi k_B G M^2/\hbar c. Unlike black holes, a collapsed star of this kind is thermodynamically stable and has no information paradox.
Jan. 29, Thu., 2:00pm, Room 1305
Emil Mottola, LANL
``Dark Energy, Conformal Invariance, and the CMBR''
The discovery of dark energy comprising some 70% of the energy density of the universe has brought the long-standing cosmological constant problem to the attention of observational cosmologists. The consistent treatment of quantum vacuum energy in gravity and cosmology very likely requires some revision in classical general relativity. The minimal revision required is the inclusion of the trace anomaly of massless fields, whose fluctuations do not decouple from the metric even at the very largest space and time scales. The trace anomaly predicts a conformally invariant phase of gravity, which has consequences for the spectrum and statistics of the CMBR. Conformal invariance leads in general also to deviations from naive classical scaling. The spectral index of the two-point function of density fluctuations is given in terms of the quantum trace anomaly and is greater than one, leading to less power at large distance scales than a strict Harrison-Zel'dovich spectrum, a hint of which appears in the most recent WMAP data. Conformal invariance also implies non-gaussian statistics for the higher point correlations and determines the large angular dependence of the three-point correlations of the CMBR.
Feb. 9, Mon., 2:30pm, Room 1201
Victor Flambaum, University of South Wales, Australia, and IAS
``Effects of variation of fundamental constants from Big Bang to atomic clocks''
Theories unifying gravity with other
interactions suggest temporal and spatial variation of the fundamental
"constants" in expanding Universe. I discuss effects of variation of
the fine structure constant \alpha=e^2/\hbar c, strong interaction,
quark mass and gravitational constant. The measurements of these
variations cover lifespan of the Universe from few minutes after Big
Bang to the present time and give controversial results. There are
some hints for the variation in Big Bang nucleosynthesis, quasar
absorption spectra and Oklo natural nuclear reactor data.
A very promising method to search for the variation
of the fundamental constants consists in comparison of
different atomic clocks. A billion times enhancement of
the variation effects happens in transition between accidentally
degenerate atomic energy levels.
Apr. 16, Fri., 2:00pm, Room 4208
Éanna Flanagan, Cornell University
``The accelerating Universe and gravitation theories obtained from the Palatini variational principle''
We consider theories of gravity where the Lagrangian is a nonlinear function of the Ricci scalar, in the Palatini variational formalism where the connection and metric are independent. We show that such theories are equivalent to scalar-tensor theories in which the scalar field kinetic energy term is absent from the action. By integrating out the scalar field, the theory can be recast as general relativity coupled to a modified matter action. We explain why this rules out some models that have been proposed for the Universe's recent acceleration. We also note that this class of theories is finely tuned from the point of view of loop corrections. A more general class of theories, stable under loop corrections, is given by taking the Lagrangian to be some function of (i) the Ricci scalar computed from the metric, and (ii) a second Ricci scalar computed from the connection. We show that such theories can be recast as tensor-biscalar theories. A subclass of these theories is compatible with solar system experiments. [Based on astro-ph/0308111, gr-qc/0309015 and gr-qc/0403063]
Apr. 23, Fri., 2:00pm, Room 4208
Cole Miller, University
of Maryland
``Constructing Templates for the Detection of Gravitational Waves from Binaries''
Binaries are the best established of all proposed sources of gravitational radiation, and there is a significant worldwide effort to produce optimal strategies for their detection by ground-based or space-based interferometers. I will give a status report on current issues from an astrophysicist's perspective, including post-Newtonian expansions, statistical issues, and the construction of low-dimensional templates for matched filtering. I will also discuss briefly a type of source that, if it exists, will allow direct observational resolution of many current issues in strong-gravity gravitational radiation.
Apr. 29, Thu., 12:30pm, Room 1126
Albert Roura,
University of Maryland
``Non-relativistic particles propagating on linearized gravitational waves and atom interferometry''
We consider non-relativistic particles propagating on the spacetime corresponding to a linearized gravitational wave. It is explicitly shown that, in contrast to previous claims, the quantum mechanical description in terms of geodesic coordinates and that based on the geodesic deviation equation are equivalent: they just correspond to the use of different coordinate systems and are related by a time-dependent unitary transformation. This is then illustrated by computing the phase shifts for an atom interferometer due to the passage of a gravitational wave. We point out and explain a discrepancy with previous results. We also discuss the fact that, once the correct result is employed, the MIGO scheme for the detection of gravitational waves is no better than the existing ones based on laser interferometry (e.g., LIGO and LISA).
May 7, Fri., 2:00pm, Room 4208
Richard Woodard,
University of Florida
``Stochastic Inflation''
Explicit quantum field theoretic computations in a locally de Sitter background have revealed important effects driven by logarithms of the scale factor. These infrared logarithms arise from two sources: (1) The propagators of gravitons and massless, minimally coupled scalars; and (2) Integrating interaction vertices over the past light-cone of an observation point. These IR logarithms pose an interesting conundrum: because the scale factor grows without bound, they must eventually become so large that perturbation theory breaks down. In this talk I develop a leading logarithm approximation in which the quantum field operator becomes stochastic: random but definite. Because the approximate field operator ceases to obey the uncertainty principle, one can use the classical equations of motion to gain analytic control over the regime in which the IR logarithms become nonperturbatively large.
May 14, Fri., 2:00pm, Room 4208
Ho Jung Paik,
University of Maryland
``Gravitational Wave Detection on the Moon''
The Moon is extremely quiet seismically due to its lack of plate tectonics and its spin locked to its orbital motion. The Moon is thus an ideal gravitational wave detector by itself, if instrumented properly. An interesting approach to instrumentation is locating six sensitive displacement sensors on the Moon in an icosahedral configuration. A highly sensitive horizontal displacement sensor can be constructed by combining a magnetically levitated test mass with a superconducting inductive transducer. The displacement sensors will detect all the quadrupole modes of the Moon over a large bandwidth (1 mHz - 1 Hz) or can be operated as a wideband detector below its lowest quadrupole mode frequency (< 1 mHz). A detector formed with six tangential displacement sensors in an icosahedral configuration preserves all the advantages of a detector with six radial sensors; it has a uniform cross section for all sky, can determine the source direction and wave polarization, and discriminate against seismic noise. Although the detector will cover the LISA band, the most interesting frequency range is 0.1-1 Hz, which wll be missed by both LISA and ground detectors. A displacement sensor with a 100-kg mass at 2 K tuned to 0.3 Hz has an intrinsic noise level of 10-16 mHz-1/2. This yields hmin ~ 10-21 Hz-1/2, which is within three orders of magnitude from the goal of the Big Bang Observer (BBO). The lunar detector will detect many interesting astrophysical sources in clusters of galaxies in a new window and serve as a pathfinder for BBO.
Sept. 8, Wed., 2:00pm, Room 1304
Albert Roura,
University of Maryland
``MIGO is no better than LIGO''
It will be shown that a recent claim that matter wave interferometers (MIGO) have a much higher sensitivity than laser interferometers (LIGO) for a comparable physical setup is unfounded, and the mistake made in the earlier analysis will be pointed out. Furthermore, the additional claim that only a description based on the geodesic deviation equation in the rigid frame can produce the correct physical result will also be disproved. The equations for the quantum dynamics of non-relativistic massive particles in a linearly perturbed spacetime derived here can be useful for treating a wider class of related physical problems. Finally, a general discussion on the use of atom interferometers for the detection of gravitational waves will be provided.
Sept. 22, Wed., 2:00pm, Room 4102
Neil Lambert, King's College London
``Brane Decay''
I will discuss non-supersymmetric D-branes and their decay into closed string radiation.
Oct. 6, Wed., 2:00pm, Room 4102
Markus Luty, University
of Maryland
``IR Modification of GR''
I will give an informal blackboard talk about IR modification of gravity in general, and "ghost condensation" or the simplest Higgs phase of gravity in particular. The talk will be on the paper hep-th/0312099 and more recent work to be published.
Oct. 13, Wed., 2:00pm, Room 4102
Cole Miller, University
of Maryland
``High Amplitude Extreme Mass Ratio Inspirals''
Recent observations and N-body simulations suggest that thousand solar mass black holes can form in compact massive young star clusters. Any such clusters in the bulge of their host galaxy will spiral to the center within a few hundred million years, where their intermediate-mass black holes are likely to merge eventually with the galaxy's supermassive black hole. If such mergers are common, then future space-based gravitational wave detectors such as the Laser Interferometer Space Antenna will detect them with such a high signal to noise ratio that towards the end of the inspiral the orbits will be clearly visible in a simple power density spectrum, without the need for matched filtering. We discuss the astrophysics of the inspiral of clusters in the nuclear region of a galaxy and the subsequent merger of intermediate-mass with supermassive black holes. We also examine the prospects for understanding the spacetime geometry of rotating black holes, based on phase connection of the strong signals visible near the end of these extreme mass ratio inspirals.
Oct. 15, Fri., 2:00pm, Room 4102
Matthew Kleban, IAS
``Poincaré Recurrences and Topological Diversity''
Finite entropy thermal systems undergo Poincaré recurrences if their time evolution is unitary. According to the AdS/CFT correspondence, AdS black holes are dual to such a system. However, correlators computed in the BH geometry damp exponentially. This constitutes a very precise formulation of the information paradox. We attempt to reproduce the expected behavior with a sum over bulk geometries.
Nov. 1, Mon., 2:30pm, Room 1201
David Reitze,
University of Florida
``The Laser Interferometer Gravitational Wave Observatory: Lasers at the Frontiers of Astrophysics''
In 2004, the Laser Interferometer Gravitational Wave Observatory (LIGO) Science Collaboration reported the first searches for gravitational waves from the universe. LIGO, one of the largest projects ever undertaken by the National Science Foundation, has as its goal the detection and study of gravitational waves from large-scale astrophysical sources. Gravitational waves were predicted by Einstein almost 90 years ago but never been observed directly despite a number of experiments over the last 40 years. While strong indirect evidence comes from long-term precision astronomical measurements of the periastron shift of a pulsating binary neutron star system, it is only with the construction of large-scale interferometers that direct detection of gravitational waves is possible. Gravitational waves are tiny dynamical strains applied to space-time by motion of massive astrophysical systems such as binary black holes and neutron star systems. LIGO uses ultrahigh precision interferometry to detect the changes in positions of mirrors ("test masses") induced by the passage of gravitational wave. Direct observation of gravitational waves presents a formidable challenge, because the magnitude of the dynamic strain is expected to be less than 10-22 near 100 Hz. In this talk, I will give an overview of LIGO, discussing the astrophysical goals and the challenges faced in reaching them with an emphasis on the starring role that lasers and optics play in the search for gravitational waves.
Nov. 30, Tue., 4:00pm, Room 1410
Edward Seidel,
Lousiana State University
``Using Supercomputers to Collapse Gravitational Waves, Collide Black Holes (and study other Cataclysms)''
Einstein's equations of general relativity govern such exotic phenomena as black holes, neutron stars, and gravitational waves. Unfortunately they are among the most complex in physics, and require very large scale computational power --which we are just on the verge of achieving-- to solve in the general case. I will motivate and describe the structure of these equations, and the worldwide effort to develop advanced computational tools to solve them in their full generality for the first time since they were written down nearly a century ago. I will focus on applications of these tools to extract new physics of relativistic systems. In particular, I will summarize recent progress in the study of black hole collisions, considered to be promising sources of observable gravitational waves that may soon be seen for the first time by the worldwide network of gravitational wave detectors (LIGO, VIRGO, GEO, and others) currently under construction.
Dec. 10, Fri., 10:30pm, Building 60 auditorium
at the Naval Research Laboratory
Seth Lloyd, MIT
``Quantum Limits to Measuring Spacetime Geometry''
This talk investigates the accuracy to which systems such as GPS, gravitational interferometers, etc., can measure the geometry of spacetime. Simple concepts from the physics of computation are used to put absolute limits to the total number of ticks of clocks and clicks of detectors that can take place within a volume of space and time.
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