Initial Laser Interferometer Gravitational-Wave Observatory Research Articles

LIGO analogy lab—A set of undergraduate lab experiments to demonstrate some principles of gravitational wave detection

The first direct detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in September 2015 proved their existence, as predicted by Einstein's General Theory of Relativity, and ushered in the era of gravitational-wave interferometry. In this article, we present a set of lab course experiments at different levels of advancement, which give students insight into the basic LIGO operating principle and advanced detection techniques. Starting with methods for folding an optical cavity, we advance to analogy experiments with sound waves that can be detected with a Michelson interferometer with an optical cavity arm. In that experiment, students also learn how the sensitivity of the device can be tuned. In a last step, we show how optical heterodyne detection (the mixing of a signal with a reference oscillator) was used in Initial LIGO. We hope these experiments not only give students an understanding of some LIGO techniques but also awaken a fascination for how unimaginably tiny signals, created by powerful cosmic events a billion years ago or earlier, can be detected today here on Earth.

The PyCBC search for gravitational waves from compact binary coalescence

We describe the PyCBC search for gravitational waves from compact-object binary coalescences in advanced gravitational-wave detector data. The search was used in the first Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) observing run and unambiguously identified two black hole binary mergers, GW150914 and GW151226. At its core, the PyCBC search performs a matched-filter search for binary merger signals using a bank of gravitational-wave template waveforms. We provide a complete description of the search pipeline including the steps used to mitigate the effects of noise transients in the data, identify candidate events and measure their statistical significance. The analysis is able to measure false-alarm rates as low as one per million years, required for confident detection of signals. Using data from initial LIGO's sixth science run, we show that the new analysis reduces the background noise in the search, giving a increase in sensitive volume for binary neutron star systems over previous searches.

Gravitational wave detector with cosmological reach

Twenty years ago, construction began on the Laser Interferometer Gravitational-wave Observatory (LIGO). Advanced LIGO, with a factor of ten better design sensitivity than Initial LIGO, will begin taking data this year, and should soon make detections a monthly occurrence. While Advanced LIGO promises to make first detections of gravitational waves from the nearby universe, an additional factor of ten increase in sensitivity would put exciting science targets within reach by providing observations of binary black hole inspirals throughout most of the history of star formation, and high signal to noise observations of nearby events. Design studies for future detectors to date rely on significant technological advances that are futuristic and risky. In this paper we propose a different direction. We resurrect the idea of a using longer arm lengths coupled with largely proven technologies. Since the major noise sources that limit gravitational wave detectors do not scale trivially with the length of the detector, we study their impact and find that 40~km arm lengths are nearly optimal, and can incorporate currently available technologies to detect gravitational wave sources at cosmological distances $(z \gtrsim 7)$.

Searching for Gravitational Waves from the Coalescence of High-mass Black Hole Binaries

We search for gravitational waves from the coalescence (inspiral, merger and ringdown) of binary black holes with data from the Laser Interferometer Gravitational-Wave Observatory (LIGO). Provided with well-described waveform models from General Relativity, matched filtering is employed in the GSTLAL analysis pipeline as the optimal detection technique for weak signals in Gaussian noise. The GSTLAL analysis pipeline filters data with waveform template banks, identifies triggers with SNR greater than 4, forms coincident triggers between multiple detectors in the LSC-Virgo Collaboration, and attempts to optimally separate signal from detector background noise fluctuations using a Chisquared test. We analyze high-statistics simulations of binary merger waveforms injected into LIGO recolored S6 data to evaluate the pipeline search sensitivity and to test the readiness of the pipeline for Advanced LIGO. With Advanced LIGO fully in operation by 2015 and the upgraded analysis pipelines, the expected detection rate is increased to as much as 100 events/year or more as compared to 0.01–1 events/year in Initial LIGO. Our work will make it possible to detect gravitational waves from binary black hole coalescence in Advanced LIGO data with high confidence. KEYWORDS: LIGO, Gravitational Waves, General Relativity, Coalescence, Black Hole Binaries, Noise Fluctuations, Matched Filtering, Chi-squared Test, Simulations, GSTLAL Analysis Pipeline

Searching for Gravitational Waves from Sub-Solar Mass Black Holes

We are searching for gravitational-wave signals from sub-solar mass black hole binary systems in initial Laser Interferometer Gravitational-wave Observatory (LIGO) data. The most likely candidates for such systems are primordial black holes that have formed from the collapse of quantum fluctuations in the early universe. Primordial black holes have not yet been ruled out by microlensing experiments, but the allowable masses have been restricted. The gravitational- wave strain from such an inspiralling binary system is well modeled with the post- Newtonian formalism. Therefore, a modeled search for gravitational-wave signals is employed. The search technique is known as matched filtering and is implemented using a codebase that is well-suited for fast searches with long signals. One of the biggest challenges in performing this search is dealing with the heavy computational burden. The gravitational-wave signals from such low-mass binary systems are long (about 10 minutes) and require a large number of models, or templates, spread across the parameter space. A large effort has been focused on speeding up the search while using a reasonable amount of computational resources.

Improved estimate of the detectability of gravitational radiation from a magnetically confined mountain on an accreting neutron star

We give an improved estimate of the detectability of gravitational waves from magnetically confined mountains on accreting neutron stars. The improved estimate includes the following effects for the first time: three-dimensional hydromagnetic ('fast') relaxation, three-dimensional resistive ('slow') relaxation, realistic accreted masses M a ≤ 2 × 10 ―3 M ⊙ (where the mountain is grown ab initio by injection) and verification of the curvature rescaling transformation employed in previous work. Typically, a mountain does not relax appreciably over the lifetime of a low-mass X-ray binary. The ellipticity reaches ∈ ≈ 2 × 10- 5 for M a = 2 x 10- 3 M ⊙ . The gravitational wave spectrum for triaxial equilibria contains an additional line, which, although weak, provides valuable information about the mountain shape. We evaluate the detectability of magnetic mountains with initial and advanced Laser Interferometer Gravitational Wave Observatory (LIGO). For a standard, coherent matched filter search, we find a signal-to-noise ratio of d = 28(M a /10 ―4 M ⊙ ) (1 + 5.5M a /10 ―4 M ⊙ ) ―1 (D/10kpc) ―1 (T 0 /14 d) 1/2 for initial LIGO, where D is the distance and To is the observation time. From the non-detection of gravitational waves from low-mass X-ray binaries to date, and the wave strain limits implied by the spin frequency distribution of these objects (due to gravitational wave braking), we conclude that there are other, as yet unmodelled, physical effects that further reduce the quadrupole moment of a magnetic mountain, most notably sinking into the crust.

Gravitational waves from inspiralling compact binaries: Hexagonal template placement and its efficiency in detecting physical signals

Matched filtering is used to search for gravitational waves emitted by inspiralling compact binaries in data from the ground-based interferometers. One of the key aspects of the detection process is the design of a template bank that covers the astrophysically pertinent parameter space. In an earlier paper, we described a template bank that is based on a square lattice. Although robust, we showed that the square placement is overefficient, with the implication that it is computationally more demanding than required. In this paper, we present a template bank based on an hexagonal lattice, which size is reduced by 40% with respect to the proposed square placement. We describe the practical aspects of the hexagonal template bank implementation, its size, and computational cost. We have also performed exhaustive simulations to characterize its efficiency and safeness. We show that the bank is adequate to search for a wide variety of binary systems (primordial black holes, neutron stars, and stellar-mass black holes) and in data from both current detectors (initial LIGO, Virgo and GEO600) as well as future detectors (advanced LIGO and EGO). Remarkably, although our template bank placement uses a metric arising from a particular template family, namely, stationary phase approximation, we show that it can be used successfully with other template families (e.g., Pad\'e resummation and effective one-body approximation). This quality of being effective for different template families makes the proposed bank suitable for a search that would use several of them in parallel (e.g., in a binary black hole search). The hexagonal template bank described in this paper is currently used to search for nonspinning inspiralling compact binaries in data from the Laser Interferometer Gravitational-Wave Observatory (LIGO).

A template bank to search for gravitational waves from inspiralling compact binaries: I. Physical models

Gravitational waves from coalescing compact binaries are searched for using the matched filtering technique. As the model waveform depends on a number of parameters, it is necessary to filter the data through a template bank covering the astrophysically interesting region of the parameter space. The choice of templates is defined by the maximum allowed drop in signal-to-noise ratio due to the discreteness of the template bank. In this paper we describe the template-bank algorithm that was used in the analysis of data from the Laser Interferometer Gravitational Wave Observatory (LIGO) and GEO 600 detectors to search for signals from binaries consisting of non-spinning compact objects. Using Monte Carlo simulations, we study the efficiency of the bank and show that its performance is satisfactory for the design sensitivity curves of ground-based interferometric gravitational wave detectors GEO 600, initial LIGO, advanced LIGO and Virgo. The bank is efficient in searching for various compact binaries such as binary primordial black holes, binary neutron stars, binary black holes, as well as a mixed binary consisting of a non-spinning black hole and a neutron star.

The gravitational wave 'probability event horizon' for double neutron star mergers

Gravitational waves generated by the final merger of double neutron star (DNS) binary systems are a key target for the gravitational wave (GW) interferometric detectors, such as LIGO (Laser Interferometer Gravitational-wave Observatory), and the next generation detectors, Advanced LIGO. The cumulative GW signal from DNS mergers in interferometric data will manifest as ‘geometrical noise’: a non-continuous stochastic background with a unique statistical signature dominated by the spatial and temporal distribution of the sources. Because geometrical noise is highly non-Gaussian, it could potentially be used to identify the presence of a stochastic GW background from DNS mergers. We demonstrate this by fitting to a simulated distribution of transients using a model for the DNS merger rate and idealized Gaussian detector noise. Using the cosmological ‘probability event horizon’ concept and recent bounds for the Galactic DNS merger rate, we calculate the evolution of the detectability of DNS mergers with observation time. For Advanced LIGO sensitivities and a detection threshold assuming optimal filtering, there is a 95 per cent probability that a minimum of one DNS merger signal will be detectable from the ensemble of events comprising the stochastic background during 12–211 d of observation. For initial LIGO sensitivities, we identify an interesting regime where there is a 95 per cent probability that at least one DNS merger with signal-to-noise ratio greater than unity will occur during 4–68 d of observation. We propose that there exists an intermediate detection regime with pre-filtered signal-to-noise ratio less than unity, where the DNS merger rate is high enough that the geometrical signature could be identified in interferometer data.

The Cosmic Coalescence Rates for Double Neutron Star Binaries

We report on the newly increased event rates due to the recent discovery of the highly relativistic binary pulsar J07373039. Using a rigorous statistical method, we present the calculations reported by Burgay et al., which produce a coalescence rate for Galactic double neutron star (DNS) systems that is higher by a factor of 6–7 compared to estimates made prior to the new discovery. Our method takes into account known pulsar survey selection effects and biases due to small-number statistics. This rate increase has dramatic implications for gravitational wave detectors. For the initial Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, the most probable detection rates for DNS in-spirals are one event per 5–250 yr; at 95% confidence, we obtain rates up to one per 1.5 yr. For the advanced LIGO detectors, the most probable rates are 20–1000 events per year. These predictions, for the first time, bring the expectations for DNS detections by the initial LIGO detectors to the astrophysically relevant regime. We also use our models to predict that the large-scale Parkes Multibeam pulsar survey with acceleration searches could detect an average of three to four binary pulsars similar to those known at present. Subject headings: binaries: close — gravitational waves — methods: statistical — stars: neutron

Multistage active vibration isolation system

The major obstacle to the detection of low-frequency gravitational waves with an earth-based interferometer is seismic noise. The current design of the initial Laser Interferometer Gravitational-Wave Observatory (LIGO) receiver, now under construction, projects that ground noise will limit the operating band to frequencies above 40 Hz. In this article, we describe recent progress on the JILA active vibration isolation system. This device is being constructed to demonstrate the technology needed for useful reduction of low-frequency seismic noise in a gravitational wave interferometer. It consists of three spring-mounted stages, each of which provides both active and passive isolation. To date, all of the control loops on the first two of the three stages have been closed. Together they can reduce large vibrations by at least 70 dB in both vertical and horizontal directions at 1.5 Hz and above.