Astrophysical Sources of Stochastic Gravitational-Wave Background
T. Regimbau, V. Mandic
备注：
12 pages, 3 figures,accepted for publication in CQG, GWDAW12 conference proceedings version corrected in comparison published version where we found an error in equation (4)
We review the spectral properties of stochastic backgrounds of astrophysical origin and discuss how they may differ from the primordial contribution by their statistical properties. We show that stochastic searches with the next generation of terrestrial interferometers could put interesting constrains on the physical properties of astrophysical populations, such as the ellipticity and magnetic field of magnetars, or the coalescence rate of compact binaries.
Gravitational waves transport very detailed information on the structure and evolution of astrophysical sources. For instance a binary system in the early stages of its evolution emits a wavetrain at specific frequencies that depend on the characteristics of the obital motion; as the orbit shrinks and circularize, due to radiation reaction effects, the orbiting bodies get closer and tidally interact. This interaction may result in the excitation of the proper modes of oscillation of the stars, and in the emission of gravitational signals that carry information on the mode frequencies, and cons…
Stars and black holes are sources of gravitational radiation in many phases of their life, and the signals they emit exhibit features that are characteristic of the generating process. Emitted since the beginning of star formation, these signals also contribute to create a stochastic background of gravitational waves. We shall show how the spectral properties of this background can be estimated in terms of the energy spectrum of each single event and of the star formation rate history, which is now deducible from astronomical observations. We shall further discuss the process of scattering of …
We explore the effects of a positive cosmological constant on astrophysical and cosmological configurations described by a polytropic equation of state. We derive the conditions for equilibrium and stability of such configurations and consider some astrophysical examples where our analysis may be relevant. We show that in the presence of the cosmological constant the isothermal sphere is not a viable astrophysical model since the density in this model does not go asymptotically to zero. The cosmological constant implies that, for polytropic index smaller than five, the central density has to e…
Key targets for gravitational wave (GW) observatories, such as LIGO and the next generation interferometric detector, Advanced LIGO, include core-collapse of massive stars and the final stage of coalescence of compact stellar remnants. The combined GW signal from such events occurring throughout the Universe will produce an astrophysical GW background (AGB), one that is fundamentally different from the GW background by very early Universe processes. One can classify contributions to the AGB for different classes of sources based on the strength of the GW emissions from the individual sources, …
We use the StarTrack stellar population synthesis code to analyze properties of double compact object binaries as sources of gravitational waves. Since the distribution of lifetimes of these objects extends up to the Hubble time we conclude that a proper calculation of the expected rate must include a full cosmological model. We present such model, calculate the expected coalescence rates, and analyze the intrinsic sensitivity of these rates to the model assumptions. We find that the rate alone is a very poor estimator of the underlying stellar evolution model. However we show that the distrib…
Every astrophysical object (dark or not) is a gravitational lens, as well as a receiver/observer of the light from sources lensed by other objects in its neighborhood. For a given pair of source and lens, there is a thin on-axis tubelike volume behind the lens in which the radiation flux from the source is greatly increased due to gravitational lensing. Any objects which pass through such a thin tube or beam will experience strong bursts of radiation, i.e., Extreme Gravitational Lensing Events (EGLEs). We have studied the physics and statistics of EGLEs. EGLEs may have interesting astrophysica…
We study the phenomenological consequences of the recently proposed idea of a running gravitational coupling on macroscopic scales. When applied to the rotation curves of galaxies, we find that their flatness requires the presence of baryonic dark matter. Bounds on the variation of the gravitational coupling from primordial nucleosynthesis and the change of the period of binary pulsars are analysed. We also study constraints on the variations of $G$ with scale from gravitational lensing and the cosmic virial theorem, as well as briefly discuss the implications of such a scenario for structure …
Relativistic spin-orbit and spin-spin couplings has been shown to modify the gravitational waveforms expected from inspiraling binaries with a black hole and a neutron star. As a result inspiral signals may be missed due to significant losses in signal-to-noise ratio, if precession effects are ignored in gravitational-wave searches. We examine the sensitivity of the anticipated loss of signal-to-noise ratio on two factors: the accuracy of the precessing waveforms adopted as the true signals and the expected distributions of spin-orbit tilt angles, given the current understanding of their physi…
Astrophysical implications of gravitational microlensing of gravitational waves emitted by rotating neutron stars (NSs) are investigated. In particular, attention is focused on the following situations: i) NSs in the galactic bulge lensed by a central black hole of $2.6\times 10^6 M_{\odot}$ or by stars and MACHOs distributed in the galactic bulge, disk and halo between Earth and the sources; ii) NSs in globular clusters lensed by a central black hole of $\sim 10^3 M_{\odot}$ or by stars and MACHOs distributed throughout the Galaxy. The detection of such kind of microlensing events will give a…
Astrophysical sources of gravitational radiation are likely to have been formed since the beginning of star formation. Realistic source rates of formation throughout the Universe have been estimated from an observation-based determination of the star formation rate density evolution. Both the radiation emitted during the collapse to black holes and the spin-down radiation, induced by the r-mode instability, emitted by hot, young rapidly rotating neutron stars have been considered. We have investigated the overall signal produced by the ensemble of sources exploring the parameter space and disc…
The Novello-DeLorenci-Luciane (NDL) field theory of gravitation predicts that gravitational waves (GWs) follow geodesics of a modified (effective) geometry with a speed lower than the velocity of light. The theory also demonstrates that GWs exhibit the phenomenon of birefringence, formerly believed to be exclusive of electromagnetic waves. Here prospective astrophysical tests of these predictions are proposed. I point out that future measurements of gravitational waves in coincidence with a non-gravitational process such as {\it a neutrino burst} (and likely a burst of gamma-rays) may prove us…
Stochastic gravitational waves (GW) associated with unresolved astrophysical sources at frequency bands of the ongoing GW interferometers LIGO/VIRGO and LISA are studied. We show that GW noise from rotating galactic neutron stars with low magnetic fields may reach the advanced LIGO sensitivity level at frequency $f\sim 100$ Hz. Within LISA frequency band (10^{-4}-10^{-1} Hz), the GW background from galactic binary stars is shown to mainly contribute up to a frequency of 3.10^{-2} Hz, depending on the galactic rate of binary white dwarf mergers. To be detectable by LISA, relic GW backgrounds sh…
Albert Einstein postulated the equivalence of energy and mass, developed the theory of special relativity, explained the photoelectric effect, and described Brownian motion in five papers, all published in 1905, 100 years ago. With these papers, Einstein provided the framework for understanding modern astrophysical phenomena. Conversely, astrophysical observations provide one of the most effective means for testing Einstein's theories. Here, I review astrophysical advances precipitated by Einstein's insights, including gravitational redshifts, gravitational lensing, gravitational waves, the Le…
We study the impact of astrophysical processes on the gamma-ray background produced by the annihilation of dark matter particles in cosmological halos, with particular attention to the consequences of the formation of supermassive black holes. In scenarios where these objects form adiabatically from the accretion of matter on small seeds, dark matter is first compressed into very dense ``spikes'', then its density progressively decreases due to annihilations and scattering off of stellar cusps. With respect to previous analyses, based on non-evolving halos, the predicted annihilation signal is…
The origin of magnetic fields in astrophysical objects is a challenging problem in astrophysics. Throughout the years, many scientists have suggested that non-minimal gravitational-electromagnetic coupling (NMGEC) could be the origin of the ubiquitous astrophysical magnetic fields. We investigate the possible origin of intense magnetic fields $\sim 10^{15}-10^{16}$ by NMGEC near rotating neutron stars and black holes, connected with magnetars, quasars, and gamma-ray bursts. Whereas these intense magnetic fields are difficult to explain astrophysically, we find that they are easily explained by…
The {\em Van Cittert Zernike} theorem consists of a simple relation between the measured coherence of the radiation and the characteristics of the emitting stochastic source. In the present work, an extension of the theorem to partially correlated sources in curved spacetime is derived. The generalized theorem is used to evaluate the changes in the spectrum of light travelling through a gravitational lens. Some possible astrophysical applications are discussed, accompanied by a numerical estimation of the spectrum modification in the process of propagation through a gravitational lens.
Gravitational microlensing has proved to be a versatile astrophysical tool. Recently, the question of whether higher order relativistic corrections can influence the observable properties of microlensing has been addressed. This letter extends earlier studies, investigating higher order relativistic terms in astrometric shifts during microlensing events, revealing that they produce no effects. The magnitude of the relativistic corrections is examined in an astrophysical context and is found to be negligible.
Angular fluctuations of stochastic gravitational wave backgrounds (GWB) produced by extragalactic astrophysical sources are calculated. The angular properties of such backgrounds are determined by the large scale structure of Universe (galaxy clustering). The evolution of star formation rate with redshift is taken into account. Fluctuations of the metric strain amplitude associated with such noises at angular scales of about one degree are found to be of order 5-20% slowly growing toward smaller angular scales. This feature can be potentially used to separate astrophysical GWB from cosmologica…
Cosmological consequences of a string-motivated dark energy scenario featuring a scalar field coupled to the Gauss-Bonnet invariant are investigated. We study the evolution of the universe in such a model, identifying its key properties. The evolution of the homogeneous background and cosmological perturbations, both at large and small scales, are calculated. The impact of the coupling on galaxy distributions and the cosmic microwave background is examined. We find the coupling provides a mechanism to viably onset the late acceleration, to alleviate the coincidence problem, and furthermore to …
We use astrophysical data to shed light on fundamental physics by constraining parametrized theoretical cosmological and gravitational models. Gravitational parameters are those constants that parametrize possible departures from Einstein's general theory of relativity. We develop a general framework to describe torsion in the spacetime around the Earth, and show that certain observables of the Gravity Probe B experiment can be computed in this framework. We also search for viable theories of gravity where the Ricci scalar R in the Lagrangian is replaced by an arbitrary function f(R). Making u…
The origin of the cosmic gamma-ray background (CGB) is a longstanding mystery in high-energy astrophysics. Possible candidates include ordinary astrophysical objects such as unresolved blazars, as well as more exotic processes such as dark matter annihilation. While it would be difficult to distinguish them from the mean intensity data alone, one can use anisotropy data instead. We investigate the CGB anisotropy both from unresolved blazars and dark matter annihilation (including contributions from dark matter substructures), and find that the angular power spectra from these sources are very …
