Detection Of Gravitational Radiation Research Articles

Quantum Measurement Theory in Gravitational-Wave Detectors.

The fast progress in improving the sensitivity of the gravitational-wave detectors, we all have witnessed in the recent years, has propelled the scientific community to the point at which quantum behavior of such immense measurement devices as kilometer-long interferometers starts to matter. The time when their sensitivity will be mainly limited by the quantum noise of light is around the corner, and finding ways to reduce it will become a necessity. Therefore, the primary goal we pursued in this review was to familiarize a broad spectrum of readers with the theory of quantum measurements in the very form it finds application in the area of gravitational-wave detection. We focus on how quantum noise arises in gravitational-wave interferometers and what limitations it imposes on the achievable sensitivity. We start from the very basic concepts and gradually advance to the general linear quantum measurement theory and its application to the calculation of quantum noise in the contemporary and planned interferometric detectors of gravitational radiation of the first and second generation. Special attention is paid to the concept of the Standard Quantum Limit and the methods of its surmounting.

COMPACT REMNANT MASS FUNCTION: DEPENDENCE ON THE EXPLOSION MECHANISM AND METALLICITY

The mass distribution of neutron stars and stellar-mass black holes provides vital clues into the nature of stellar core collapse and the physical engine responsible for supernova explosions. Using recent advances in our understanding of supernova engines, we derive mass distributions of stellar compact remnants. We provide analytical prescriptions for compact object masses for major population synthesis codes. In an accompanying paper, Belczynski et al., we demonstrate that these qualitatively new results for compact objects can explain the observed gap in the remnant mass distribution between ~2-5 solar masses and that they place strong constraints on the nature of the supernova engine. Here, we show that advanced gravitational radiation detectors (like LIGO/VIRGO or the Einstein Telescope) will be able to further test the supernova explosion engine models once double black hole inspirals are detected.

TIDAL PERTURBATIONS TO THE GRAVITATIONAL INSPIRAL OF J0651+2844

The recently discovered J0651+2844 is a detached, eclipsing white dwarf binary with an orbital period of 765 s. We investigate the prospects for the detection of gravitational radiation from this system and estimate the effect of the tidal deformation of the low-mass component on the period evolution of the system. Because of the high inclination of the system, the amplitude of the gravitational waves at Earth will be as much as a factor of two lower than that from an optimally oriented system. The dominant contribution of tidal corrections to the period evolution comes from the increase in rotational energy of the components as they spin up to remain tied to the orbital period. This contribution results in an advance of the timing of the eclipses by an additional 0.3 s after one year.

Interferometric gravitational wave detectors: state of the art and fundamental noise issues

Interferometric gravitational wave detectors, such as Virgo in Italy, LIGO in the US and GEO600 in Germany, have already completed scientific data runs. 2nd generation detectors, such as Advanced Virgo and Advanced LIGO will start operation within the next 5 years and preliminary design studies have already begun for 3rd generation detectors. It is hoped that we will soon be able to reach the holy grail of the first direct detection of gravitational radiation and start the exciting field of gravitational wave astronomy. The LIGO Scientific Collaboration (LSC) and the Virgo collaboration have published many observational results of great scientific relevance. For the future, the main challenge is to further increase the detector sensitivity by reducing the instrumental noise, in particular quantum noise, i.e. a fundamental noise term intrinsic to the measurement process itself. This can be achieved by the implementation of forefront quantum optics techniques.

Pulsar timing sensitivities to gravitational waves from relativistic metric theories of gravity

Pulsar timing experiments aimed at the detection of gravitational radiation have been performed for decades now. With the forthcoming construction of large arrays capable of tracking multiple millisecond pulsars, it is very likely we will be able to make the first detection of gravitational radiation in the nano-Hertz band, and test Einstein's theory of relativity by measuring the polarization components of the detected signals. Since a gravitational wave predicted by the most general relativistic metric theory of gravity accounts for {\it six} polarization modes (the usual two Einstein's tensor polarizations as well as two vector and two scalar wave components), we have estimated the single-antenna sensitivities to these six polarizations. We find pulsar timing experiments to be significantly more sensitive, over their entire observational frequency band ($\approx 10^{-9} - 10^{-6}$ Hz), to scalar-longitudinal and vector waves than to scalar-transverse and tensor waves. At $10^{-7}$ Hz and with pulsars at a distance of $1$ kpc, for instance, we estimate an average sensitivity to scalar-longitudinal waves that is more than two orders of magnitude better than the sensitivity to tensor waves. Our results imply that a direct detection of gravitational radiation by pulsar timing will result into a test of the theory of general relativity that is more stringent than that based on monitoring the decay of the orbital period of a binary system.

Gravitational wave detection with single-laser atom interferometers

We present a new general design approach of a broad-band detector of gravitational radiation that relies on two atom interferometers separated by a distance L. In this scheme, only one arm and one laser will be used for operating the two atom interferometers. We consider atoms in the atom interferometers not only as perfect inertial reference sensors, but also as highly stable clocks. Atomic coherence is intrinsically stable and can be many orders of magnitude more stable than a laser. The unique one-laser configuration allows us to then apply time-delay interferometry to the responses of the two atom interferometers, thereby canceling the laser phase fluctuations while preserving the gravitational wave signal in the resulting data set. Our approach appears very promising. We plan to investigate further its practicality and detailed sensitivity analysis.

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.

Bayesian model comparison in gravitational wave data analysis

We estimate the probability of detecting a gravitational wave signal from coalescing compact binaries in simulated data from a ground-based interferometer detector of gravitational radiation using Bayesian model selection. The simulated waveform of the chirp signal is assumed to be a spin-less Post-Newtonian (PN) waveform of a given expansion order, while the searching template is taken to be either of the same PN family as the simulated signal or one level below its PN expansion order. Within the Bayesian framework we estimate the detection probabilities and the statistical uncertainties due to noise as a function of the signal-to-noise ratio (SNR), and the posterior distributions of the parameters characterizing the chirp signal. Our analysis indicates that the detection probabilities are not compromised when simplified models are used, while the accuracies in the determination of the parameters can be significantly worsened.

The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays

Massive black hole binary systems, with masses in the range ~10^4-10^10 \msun, are among the primary sources of gravitational waves in the frequency window ~10^-9 Hz - 0.1 Hz. Pulsar Timing Arrays (PTAs) and the Laser Interferometer Space Antenna (LISA) are the observational means by which we will be able to observe gravitational radiation from these systems. We carry out a systematic study of the generation of the stochastic gravitational-wave background from the cosmic population of massive black hole binaries. We consider a wide variety of assembly scenarios and we estimate the range of signal strength in the frequency band accessible to PTAs. We show that, taking into account the uncertainties surrounding the actual key model parameters, the amplitude lies in the interval h_c(f = 10^-8 Hz)~5x10^-16 - 8x10^-15. The most optimistic predictions place the signal level at a factor of ~3 below the current sensitivity of Pulsar Timing Arrays, but within the detection range of the complete Parkes PTA for a wide variety of models, and of the future Square-Kilometer-Array PTA for all the models considered here. We also show that at frequencies >10^-8 Hz the frequency dependency of the generated background follows a power-law significantly steeper than f^-2/3, that has been considered so far. Finally we show that LISA observations of individual resolvable massive black hole binaries are complementary and orthogonal to PTA observations of a stochastic background from the whole population in the Universe. In fact, the detection of gravitational radiation in both frequency windows will enable us to fully characterise the cosmic history of massive black holes.

Bayesian comparison of post-Newtonian approximations of gravitational wave chirp signals

We estimate the probability of detecting a gravitational wave signal from coalescing compact binaries in simulated data from a ground-based interferometer detector of gravitational radiation using Bayesian model selection. The simulated wave form of the chirp signal is assumed to be a spinless post-Newtonian (PN) wave form of a given expansion order, while the searching template is assumed to be either of the same post-Newtonian family as the simulated signal or one level below its post-Newtonian expansion order. Within the Bayesian framework, and by applying a reversible jump Markov Chain Monte Carlo simulation algorithm, we compare PN1.5 vs PN2.0 and PN3.0 vs PN3.5 wave forms by deriving the detection probabilities, the statistical uncertainties due to noise as a function of the signal-to-noise ratio, and the posterior distributions of the parameters. Our analysis indicates that the detection probabilities are not compromised when simplified models are used for the comparison, while the accuracies in the determination of the parameters characterizing these signals can be significantly worsened, no matter what the considered post-Newtonian order expansion comparison is.

The Search for Gravitational Waves

The search for gravitational waves has reached a new era as currently a network of interferometric gravitational wave detectors is in operation around the globe at its initial target sensitivity, in parallel with existing acoustic bar-type detectors, and searches are underway aimed at the first direct detection of gravitational radiation from astrophysical sources. This paper briefly summarizes the status of operating gravitational wave facilities, plans for future detector upgrades and the status of the space-based gravitational wave detector LISA. It also describes some of the most promising sources of gravitational waves for ground-based detectors like LIGO and VIRGO and LISA.

Current and future status of gravitational wave astronomy - gravitational wave facilities

AbstractCurrently a network of interferometric gravitational wave detectors is in operation around the globe, in parallel with existing acoustic bar-type detectors. Searches are underway aimed at the first direct detection of gravitational radiation from astrophysical sources. This paper briefly summarizes the current status of operating gravitational wave facilities, plans for future detector upgrades, and the status of the planned space-based gravitational wave detector LISA.

Gravitational radiation of slowly rotating neutron stars

The gravitational rotation of slowly rotating neutron stars with rough surfaces is examined. The source of the gravitational waves is assumed to be the energy transferred to the crust of the star during irregular changes in its angular rotation velocity. It is shown that individual pulsars whose angular velocity regularly undergoes glitches will radiate a periodic gravitational signal that can be distinguished from noise by the latest generation of detectors. Simultaneous recording of a gravitational signal and of a glitch in the angular velocity of a pulsar will ensure reliable detection of gravitational radiation.

Gauge invariance and the detection of gravitational radiation

The detection of gravitational radiation raises some subtle issues concerning the coordinate invariance of general relativity. This paper explains these issues and their resolution by using an analogy with the Aharonov-Bohm effect of quantum mechanics.

Extreme mass ratio inspirals: LISA's unique probe of black hole gravity

In this paper, I attempt to summarize past and present ongoing work on the problem of the inspiral of a small body in the gravitational field of a much more massive Kerr black hole. Such extreme mass ratio systems, expected to occur in galactic nuclei, will constitute prime sources of gravitational radiation for the future LISA gravitational radiation detector. The paper's main goal is to provide a survey of basic celestial mechanics in Kerr spacetime and calculations of gravitational waveforms and backreaction on the small body's orbital motion, based on the traditional ‘flux-balance’ method and the Teukolsky black hole perturbation formalism.

Time-Delay Interferometry.

Equal-arm interferometric detectors of gravitational radiation allow phase measurements many orders of magnitude below the intrinsic phase stability of the laser injecting light into their arms. This is because the noise in the laser light is common to both arms, experiencing exactly the same delay, and thus cancels when it is differenced at the photo detector. In this situation, much lower level secondary noises then set the overall performance. If, however, the two arms have different lengths (as will necessarily be the case with space-borne interferometers), the laser noise experiences different delays in the two arms and will hence not directly cancel at the detector. In order to solve this problem, a technique involving heterodyne interferometry with unequal arm lengths and independent phase-difference readouts has been proposed. It relies on properly time-shifting and linearly combining independent Doppler measurements, and for this reason it has been called Time-Delay Interferometry (TDI). This article provides an overview of the theory and mathematical foundations of TDI as it will be implemented by the forthcoming space-based interferometers such as the Laser Interferometer Space Antenna (LISA) mission. We have purposely left out from this first version of our “Living Review” article on TDI all the results of more practical and experimental nature, as well as all the aspects of TDI that the data analysts will need to account for when analyzing the LISA TDI data combinations. Our forthcoming “second edition” of this review paper will include these topics.

Event rate estimates for LISA extreme mass ratio capture sources

One of the most exciting prospects for the LISA gravitational wave observatory is the detection of gravitational radiation from the inspiral of a compact object into a supermassive black hole. The large inspiral parameter space and low amplitude of the signal make detection of these sources computationally challenging. We outline here a first-cut data analysis scheme that assumes realistic computational resources. In the context of this scheme, we estimate the signal-to-noise ratio that a source requires to pass our thresholds and be detected. Combining this with an estimate of the population of sources in the universe, we estimate the number of inspiral events that LISA could detect. The preliminary results are very encouraging—with the baseline design, LISA can see inspirals out to a redshift z = 1 and should detect over a thousand events during the mission lifetime.

General relativity experiments in space

Space technology has led to increased accuracy in the measurements of the gravitational redshift and the Shapiro time delay, has permitted the first measurements of the Nordtvedt effect and geodetic (or de Sitter) effect to be made, and has further constrained the parameters in the Parameterized Post Newtonian formalism. These important made, and has further constrained the parameters in the Parameterized Post Newtonian formalism. These important experimental tests of the general of relativity would not have been possible without the capability of launching instruments into space, accurately tracking spacecraft with dual frequency transponders, landing spacecraft on Mars, placing retroreflectors on the surface of the moon, and making precise range measurements to those retroreflectors. The reasons why this technology has been essential are examined. In light of this experience, a number of planned and proposed experimental tests of general relativity are discussed. Increased accuracy in measurements of the geodetic effect and tests of the weak equivalence principle, the first measurements of the Lense-Thrrring effect, and detection of gravitational radiation in space are expected in the next decade.

On the limit of Brans–Dicke theory when ω → ∞

We discuss the problem of the limit of the Brans–Dicke theory (BDT) in the limit ω ⟶ ∞, when the trace of the stress tensor is not zero. It is shown that a BDT solution, with T ≠ 0, known in the framework of anisotropic cosmology, fails to converge to the general relativity theory (GRT) corresponding solution. Considering the spherically symmetric field problem, it is shown that the argument leading to the convergence in the static case is lost when the non-static case is considered. These remarks suggest that the non-convergence of BDT to GRT could be the general behaviour, even if T ≠ 0, except for some (most?) of the highly symmetric solutions, including most of the known ones. The impact on gravitational radiation detection is emphasized.

Acoustic quality factor of aluminium and selected aluminium alloys from 50 mK to 300 K

The amplitude-independent quality factors ( Qs) of polycrystalline aluminium of 99.999 wt% purity, and the commercial aluminium alloys 2024, 6061 and 7075 have been measured in the temperature range 50 mK–300 K. Data were obtained at about 1 kHz from a torsional mode of machined monolithic mechanical resonators. The as-machined alloys exhibited maximum Qs of 1.5 to 9.5 million. The annealed alloys exhibited maximum Qs from 4.3 to 7.4 million. After several weeks at room temperature, substantially higher Qs were observed in all of the annealed materials, as compared to values obtained within a few days subsequent to the thermal treatment. Nevertheless, the maximum Qs were appreciably lower than those which have been measured in aluminium–magnesium alloys. Three loss peaks were observed in the aluminium, and a single peak in two of the alloys, even after annealing. The temperature dependence of the shear moduli was also obtained. This study is part of the continuing effort in this laboratory to evaluate the acoustic loss of high- Q materials at low temperatures, of particular interest for the construction of sensitive detectors of gravitational radiation.