117648248
Topics in Particle Physics
14856505
Weak-field gravity of circular cosmic strings.
A weak-field solution of Einstein's equations is constructed. It is generated by a circular cosmic string externally supported against collapse. The solution exhibits a conical singularity, and the corresponding deficit angle is the same as for a straight string for the same linear energy density. This confirms the deficit-angle assumption made in the Frolov-Israel-Unruh derivation of the metric describing a string loop at a moment of time symmetry.
14821200
RINGS AND BALLS
We examine the various linkings in space-time of “ball-like” and “ring-like” topological solitons in certain nonlinear sigma models in 2+1 and 3+1 dimensions. By going to theories where soliton overlaps are forbidden, these linkings become homotopically nontrivial and can be studied by investigating the topology of the corresponding configuration spaces. Our analysis reveals the existence of topological terms which give the linking number of the world tubes of distinct species of ball solitons in 2+1 dimensions, or which in 3+1 dimensions count the number of times a ball or ring soliton threads through the center of a ring of a different species. We explicitly construct these terms for our models, and generalize them to cases where soliton overlaps are no longer strictly forbidden so the terms are no longer purely topological. One of the (3+1)-dimensional theories we consider also has topological solitons which consist of two rings (of distinct species) linked in space.
18174177
Gravitational wave astronomy and cosmology
The first direct observation of gravitational waves' action upon matter has recently been reported by the BICEP2 experiment. Advanced ground-based gravitational-wave detectors are being installed. They will soon be commissioned, and then begin searches for high-frequency gravitational waves at a sensitivity level that is widely expected to reach events involving compact objects like stellar mass black holes and neutron stars. Pulsar timing arrays continue to improve the bounds on gravitational waves at nanohertz frequencies, and may detect a signal on roughly the same timescale as ground-based detectors. The science case for space-based interferometers targeting millihertz sources is very strong. The decade of gravitational-wave discovery is poised to begin. In this writeup of a talk given at the 2013 TAUP conference, we will briefly review the physics of gravitational waves and gravitational-wave detectors, and then discuss the promise of these measurements for making cosmological measurements in the near future.
6649820
Trust but Verify: The Case for Astrophysical Black Holes
This article is based on a pair of lectures given at the 2005 SLAC Summer Institute. Our goal is to motivate why most physicists and astrophysicists accept the hypothesis that the most massive, compact objects seen in many astrophysical systems are described by the black hole solutions of general relativity. We describe the nature of the most important black hole solutions, the Schwarzschild and the Kerr solutions. We discuss gravitational collapse and stability in order to motivate why such objects are the most likely outcome of realistic astrophysical collapse processes. Finally, we discuss some of the observations which — so far at least — are totally consistent with this viewpoint, and describe planned tests and observations which have the potential to falsify the black hole hypothesis, or sharpen still further the consistency of data with theory. 1. BACKGROUND Black holes are among the most fascinating and counterintuitive objects predicted by modern physical theory. Their counterintuitive nature comes not from obtuse features of gravitation physics, but rather from the extreme limit of well-understood and well-observed features — the bending of light and the redshifting of clocks by gravity. This limit is inarguably strange, and drives us to predictions that may reasonably be considered bothersome. Given the lack of direct observational or experimental data about gravity in the relevant regime, a certain skepticism about the black hole hypothesis is perhaps reasonable. Indeed, one can understand Eddington’s somewhat plaintive cry “I think there should be a law of Nature to prevent a star from behaving in this absurd way!” [1]. To the best of our knowledge, there is no such law of Nature. We expect that in any object which is sufficiently massive, gravity can overwhelm all opposing forces, causing that object to collapse into a black hole — a region of spacetime in which gravity is so intense that not even light can escape. Indeed, astronomical observations have now provided us with a large number of candidate black holes — dark objects which are so massive and compact that the hypothesis that they are the black holes of general relativity (GR) is quite plausible. These objects are now believed to be quite common — extremely massive black hole candidates (several 10 5 solar masses to ∼ 10 9 solar masses) reside in the cores of almost all galaxies, including our own; stellar mass candidates (several to several tens of solar masses) are seen in orbit with stars in our galaxy; and evidence suggests that “intermediate” mass black holes may fill the gap between these limits. Not only is there no law preventing stars from “behaving in this absurd way”, Nature appears to revel in such absurdities. It must be emphasized that, from the purely observational perspective, at present these objects are black hole candidates. We have not unequivocally established that any of these objects exhibits all of the properties that GR predicts for “real” black holes. GR is consistent with Newton’s theory of gravity in the appropriate limit. This means that, for example, orbits of black holes are totally consistent with Newtonian gravity except for strong field (small radius) orbits. Establishing whether these objects are the black holes of GR requires observations that probe close to the black hole itself. Because black holes are so compact, this is a challenging proposition. Consider the putative black hole at the core of our galaxy. If it is a black hole as described by GR, its radius should be about 10 7 kilometers. It is located roughly 7500 parsecs from the earth. The black hole thus subtends an angle δθ ∼ 10 7 km/(7.5×10 3 pc) ∼ 0.01 milliarcseconds on the sky! Resolving such small scale phenomena is extraordinarily difficult.
119440519
New physics and astronomy with the new gravitational wave observatories
Gravitational-wave detectors with sensitivities sufficient to measure the radiation from astrophysical sources are rapidly coming into existence. By the end of this decade, there will exist several ground-based instruments in North America, Europe, and Japan, and the joint American-European space-based antenna LISA should be either approaching orbit or in final commissioning in preparation for launch. The goal of these instruments will be to open the field of gravitational-wave astronomy: using gravitational radiation as an observational window on astrophysics and the universe. In this article, we summarize the current status of the various detectors currently being developed, as well as future plans. We also discuss the scientific reach of these instruments, outlining what gravitational-wave astronomy is likely to teach us about the universe.
5651041
Gravitational Wave Emission from Core Collapse of Massive Stars
We derive estimates for the characteristics of gravitational radiation from stellar collapse, using recent models of the core collapse of Chandrasekhar-massed white dwarfs (accretion-induced collapse), core-collapse supernovae and collapsars, and the collapse of very massive stars (300 M?). We study gravitational wave emission mechanisms using several estimation techniques, including two-dimensional numerical computation of quadrupole wave emission, estimates of bar-mode strength, estimates of r-mode emission, and estimates of waves from black hole ringing. We also review the rate at which the relevant collapses are believed to occur, which has a major impact on their relevance as astrophysical sources. Although the latest supernova progenitor simulations produce cores rotating much slower than those used in the past, we find that bar-mode and r-mode instabilities from core-collapse supernovae remain among the leading candidate sources for second-generation detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO II). Accretion-induced collapse (AIC) of a white dwarf could produce gravitational wave signals similar to those from core collapse. In the models that we examine, such collapses are not unstable to bar modes; we note that models recently examined by Liu & Lindblom, which have slightly more angular momentum, are certainly unstable to bar formation. Because AIC events are probably 1000 times less common than core-collapse supernovae, the typical AIC event will be much farther away, and thus the observed waves will be much weaker. In the most optimistic circumstances, we find that it may be possible to detect gravitational waves from the collapse of 300 M? Population III stars.
10713820
AIGO: a southern hemisphere detector for the worldwide array of ground-based interferometric gravitational wave detectors
This paper describes the proposed AIGO detector for the worldwide array of interferometric gravitational wave detectors. The first part of the paper summarizes the benefits that AIGO provides to the worldwide array of detectors. The second part gives a technical description of the detector, which will follow closely the Advanced LIGO design. Possible technical variations in the design are discussed.