The road to gravitational-wave astronomy

Gravitational-wave astronomy is an area of great promise, yet to be realized. While we are waiting for the first (undisputed!) direct detection of these elusive waves it is useful to take stock and consider the challenges that need to be met if we want this field to reach its full potential. This write-up provides a brief introduction to some of the key ideas and the current state-of-play, and lists a range of modelling questions that need to be considered in the future.     

Gamma ray astronomy and search for antimatter in the universe

Gamma ray astronomy provides a powerful tool for searching antimatter in the universe; it probably provides the only means to determine, if the universe has baryon symmetry. Presently existing gamma-ray observations can be interpreted without postulating the existence of antimatter. However, the measurements are not precise enough to definitely exclude the possibility of its existence. The search for antimatter belongs to one of the main scientific objectives of the Gamma Ray Observatory GRO of NASA, which will be launched in 1990 by the Space Shuttle.     

Measuring neutron-star radii with gravitational-wave detectors

Coalescing binary neutron stars (NS) are expected to be an important source of gravitational waves (GW) detectable by laser interferometers. We present here a simple method for determining the compactness ratio M/R of NS based on the observed deviation of the GW energy spectrum from point-mass behavior at the end of inspiral. Our method is based on the properties of quasiequilibrium binary NS sequences and does not require the computation of the full GW signal h(t). Combined with the measurement of the NS masses during inspiral, the determination of M/R will allow very strong constraints to be placed on the equation of state of dense nuclear matter.     

Matching of transducers to resonant gravitational-wave antennas

Summary We present here a detailed analysis of the signal and noise response of a resonant gravitational-wave antenna in two cases:a) with the transducer mounted directly on the bar,b) with the transducer mounted on an auxiliary resonant mechanical oscillator which, in turn, is connected to the bar. The sensitivity, minimum detectable spectralenergy density of the gravitational radiation, is given in terms of the ratio between an effective temperature and the antenna mass. The effective temperature depends on the temperature of the bar, on the noise temperature of the transducer, on the matching among bar, transducer and preamplifier, and on the data analysis algorithm. We derive an expression for the optimum matching conditions which holds for both casesa) andb). When the matching conditions are fulfilled, the effective temperature becomes equal to twice the noise temperature of the amplifier, provided the Wiener-Kolmogoroff algorithm is used in the data analysis.

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Riassunto Si presenta un'analisi dettagliata della risposta al segnale e al rumore di un'antenna gravitazionale risonante in due casi:a) col trasduttore montato direttamente sulla sparra,b) con il trasduttore montato su un'oscillatore meccanico risonante ausiliario, che a sua volta è collegato alla sbarra. La sensibilità, cioè la minima densità spettrale di energia rivelabile della radiazione gravitazionale, è espressa come rapporto tra una temperatura efficace e la massa dell'antenna. La temperatura efficace dipende dalla temperatura della sbarra, dalla temperatura di rumore del trasduttore, dall'adattamento tra sbarra, trasduttore e preamplificatore e dall'algoritmo di analisi dei dati. Si ricava un'espressione per le condizioni di adattamento ottimo che è valida sia nel casoa) che nel casob). Quando le condizioni di adattamento sono soddisfatte, la temperatura efficace è pari a due volte la temperatura di rumore dell'amplificatore, purchè nell'analisi dei dati si usi l'algoritmo di Wiener-Kolmogoroff.

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Резюме В этой работе предлагается подробный анализ резонансной антенны гравитационных волн в двух случаях:a) с датчиком, установленном непосредственно на стержне;b) с датчиком, установленном на вспомгательном резонансном механическом осцилляторе, который, в свою очередь, связан со стержнем. Приводятся чувствительность, минималная детектируемая спектральная плотность энергии гравитационного излучения в терминах отношения эффективной температуры и массы антенны. Эффективная температура зависит от температуры стержня, от температуры шума в датчике от согласования между стержнем, датчиком и предусилителем и от алгоритма анализа данных. Мы выводим выражение для оптимальных условий согласования, которые справедливы для обоих случаевa) иb). Когда условия согласования выполняются, эффективная температура становится равной удвоенной температуре шума предусилителя, при условии, что используется алгоритм Винера-колмогорова для анализа данных.

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Search for monochromatic sources with the GEOGRAV gravitational-wave antenna

Summary The data of 46 months of the GEOGRAV gravitational-wave antenna are analysed to search for monochromatic Doppler-shifted signals from rotating neutron stars, in the frequency band (856.2÷859.2) Hz. Two different methods were used. With the first, the gravitational radiation from the whole sky is investigated and a limit ofh≤1·10⁻²¹ is found. With the second method only the radiation from a single source is searched. The Galactic Centre and the Large Magellanic Cloud are considered as sources and the limit ofh≤6·10⁻²² is found.     

Multimessenger astronomy with the Einstein Telescope

Gravitational waves (GWs) are expected to play a crucial role in the development of multimessenger astrophysics. The combination of GW observations with other astrophysical triggers, such as from gamma-ray and X-ray satellites, optical/radio telescopes, and neutrino detectors allows us to decipher science that would otherwise be inaccessible. In this paper, we provide a broad review from the multimessenger perspective of the science reach offered by the third generation interferometric GW detectors and by the Einstein Telescope (ET) in particular. We focus on cosmic transients, and base our estimates on the results obtained by ET’s predecessors GEO, LIGO, and Virgo.     

Displacement-noise-free gravitational-wave detection with two Fabry-Perot cavities

We propose two detuned Fabry-Perot cavities, each pumped through both the mirrors, positioned in line as a toy model of the gravitational-wave (GW) detector free from displacement noise of the test masses. It is demonstrated that the noise of cavity mirrors can be completely excluded in a proper linear combination of the cavities output signals. This model is illustrated by a simplified round trip model (without Fabry-Perot cavities). We show that in low-frequency region the obtained displacement-noise-free response signal is stronger than the one of the interferometer recently proposed by S. Kawamura and Y. Chen.     

Polarization Sagnac interferometer with postmodulation for gravitational-wave detection

We describe a polarization Sagnac interferometer with an in-loop half-wave plate that allows signal detection at the reciprocal port of the beam splitter while maintaining the ability to detect the signal at a dark fringe. Postmodulation and balanced heterodyne detection are used to recover the signal. This topology is simple to control because of its common-path characteristics and its collinear signal and local oscillator. The robustness of this scheme to amplitude and frequency fluctuations of the laser is demonstrated. Intraloop birefringence in this interferometer acts as a loss, reducing the power on the detector. The magnitude of this loss is discussed and experimentally verified.     

Multiple physical elements to determine the gravitational-wave signatures of core-collapse supernovae

We review recent progress in the theoretical predictions of gravitational waves (GWs) of core-collapse supernovae. Following a brief summary of the methods in the numerical modeling, we summarize multiple physical elements that determine the GW signatures which have been considered to be important in extracting the information of the long-veiled explosion mechanism from the observation of the GWs. We conclude with a summary of the most urgent tasks to make the dream come true.     

A perspective view to hard-X-ray astronomy

Summary The present observational and instrumental status of hard-X-ray astronomy ((10÷200) keV) is discussed. The relevance of observations in this energy range is stressed and a few examples of unsolved observational problems in galactic and in extragalactic astronomy are discussed. In these examples we focus on the possibility to solve the problems with observations using detectors of the current generation. In this framework, the performances of the most sensitive hard-X-ray detectors are discussed with particular emphasis on the control of systematic errors. Quite simple but unavoidable considerations on limits of the present generation of hard-X-ray detectors (supported by results of simple simulations) lead to the conclusion that a decisive breakthrough can be achieved only using optics with a sufficiently good concentration power. In particular we discuss the feasibility of hard-X-ray telescopes (with a concentration power ≫1), using either grazing incidence or Bragg diffraction. The use of concentrators in this energy band can also make feasible polarimetric measures of a substantial sample of X-ray sources, up to now severely limited by the very low detection efficiency of the devices used in polarimetry. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Squeezing in the audio gravitational-wave detection band

We demonstrate the generation of broadband continuous-wave optical squeezing from 280 Hz-100 kHz using a below-threshold optical parametric oscillator (OPO). The squeezed state phase was controlled using a noise locking technique. We show that low frequency noise sources, such as seed noise, pump noise, and detuning fluctuations, present in optical parametric amplifiers, have negligible effect on squeezing produced by a below-threshold OPO. This low frequency squeezing is ideal for improving the sensitivity of audio frequency measuring devices such as gravitational-wave detectors.     

Estimate of the background of a gravitational-wave detector due to cosmic rays

Summary The authors examine once more the effect of cosmic rays on a resonating gravitational-wave antenna in view of the very high sensitivities that are required for detecting the supernovae of the Virgo Cluster. They show that, at sea-level, the secondaries generated in the bar by the electromagnetic interaction of high-energy muons produce signals with rates much larger than that expected from supernovae. This inconvenience is eliminated in an underground laboratory.

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Riassunto Gli autori esaminano nuovamente l'effetto dei raggi cosmici su di una antenna gravitazionale risonante, in considerazione della elevata sensibilità che è necessario raggiungere per rivelare le supernovae del Virgo Cluster. Essi mostrano che al livello del mare i secondari generati nella sbarra dalla interazione electtromagnetica dei muoni di alta energia producono segnali con frequenza statistica maggiore di quella prevista per le supernovae. Questo inconveniente è eliminato in un laboratorio sotterraneo.

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Резюме В работе исследуется влияние космических лучей на резонансную антенну гравитационнын волн, в виду высокой чувствиельности, необходимой для детектирования сверхновых в созвездии Девы. Доказывается, что на уровне моря вторичные частицы, образованные в результате электромагнитного взаимодействия высокоэнергетических мюонов произодят сигналы с интенсивностями, много большими, чем ожидаемые интенсивности сигналов от сверхновых. Это противоречие устраняется в подземной лабоатории.

     

Dynamics of the universe with global rotation

We analyze the dynamics of the FRW models with global rotation in terms of dynamical system methods. We reduce the dynamics of these models to the FRW models with some fictitious fluid which scales like radiation matter. This fluid mimics dynamical effects of global rotation. The significance of the global rotation of the Universe for the resolution of the acceleration and horizon problems in cosmology is investigated. It is found that the dynamics of the Universe can be reduced to the two-dimensional Hamiltonian dynamical system. Then the construction of the Hamiltonian allows for full classification of evolution paths. On the phase portraits we find the domains of cosmic acceleration for the globally rotating universe as well as the trajectories for which the horizon problem is solved. We show that the FRW models with global rotation are structurally stable. This proves that the universe acceleration is due to the global rotation. It is also shown how global rotation gives a natural explanation of the empirical relation between angular momentum for clusters and superclusters of galaxies. The relation J ~ M² is obtained as a consequence of self similarity invariance of the dynamics of the FRW model with global rotation. In derivation of this relation we use the Lie group of symmetry analysis of differential equation.     

Oscillating universe with quintom matter

In this Letter, we study the possibility of building a model of the oscillating universe with quintom matter in the framework of 4-dimensional Friedmann–Robertson–Walker background. Taking the two-scalar-field quintom model as an example, we find in the model parameter space there are five different types of solutions which correspond to: (I) a cyclic universe with the minimal and maximal values of the scale factor remaining the same in every cycle, (II) an oscillating universe with its minimal and maximal values of the scale factor increasing cycle by cycle, (III) an oscillating universe with its scale factor always increasing, (IV) an oscillating universe with its minimal and maximal values of the scale factor decreasing cycle by cycle, and (V) an oscillating universe with its scale factor always decreasing.     

A further visit to the steady state universe

It is shown that within the framework of stochastic electrodynamics observations of the apparent magnitudes and redshifts of galaxies as well as number counts cannot be considered as arguments against the steady state cosmology. A bright (a black hole viewed from inside) universe cosmology is briefly considered.