致密物质状态方程:中子星与奇异星

中子星结构一直是核物理、粒子物理和天体物理共同关注的热点难题,双中子星并合事件GW170817的发现更是掀起这一研究的高潮。致密物质的状态方程是决定中子星结构的关键输入量,但是到目前为止,高密度的核物质状态方程行为依然很难确定。如今国内外已有许多运行或规划的大型核实验装置和天文观测设备,有望帮助我们很快解开致密物质状态方程的谜团。本文系统地阐述了基于微观多体理论和唯象模型对脉冲星类天体状态方程的研究现状,也讨论了奇异相变和奇异物质。结合理论计算和核物理实验及天文观测数据,致密物质状态方程的研究已取得相当多进展,但是也面临不少挑战,比如从实验和观测数据提取状态方程信息时的模型依赖,中子星各部分模型的不自洽以及各种依赖热密物质复杂动力学性质的实验和观测量。随着LIGO即将再运行而发现更多双中子星甚至中子星-黑洞等并合事件,多信使天文观测可望最终揭开中子星结构之谜。The matter state inside neutron stars (NSs) is an exciting problem in nuclear physics, particle physics and astrophysics. The equation of state (EOS) of NSs plays a crucial role in the present multimessenger astronomy, especially after the event of GW170817. Thanks to accruing studies with advanced telescopes and radioactive beam facilities, the unknown EOS of supranuclear matter could soon be understood. We review the current status of the EOS for pulsar-like compact objects, that have been studied with both microscopic many-body approaches and phenomenological models. The appearance of strange baryonic matter and strange quark matter are also discussed. We compare the theoretical predictions with different data coming from both nuclear physics experiments and astrophysical observations. Despite great progresses obtained in dense nuclear matter properties, there are various challenges ahead, such as the model dependence of the constraints extracted from either experimental or observational data, the lack of a consistent and rigorous many-body treatment of all parts of the star, the dependence of many observables on the turbulent dynamics of relevant hot dense system. As LIGO is about to run again and discover more NS merger events, multimessenger observations are expected to finally unravel the mystery of NS structure.     

Observational constraints on multimessenger sources of gravitational waves and high-energy neutrinos

Many astronomical sources of intense bursts of photons are also predicted to be strong emitters of gravitational waves (GWs) and high-energy neutrinos (HENs). Moreover some suspected classes, e.g., choked gamma-ray bursts, may only be identifiable via nonphoton messengers. Here we explore the reach of current and planned experiments to address this question. We derive constraints on the rate of GW and HEN bursts based on independent observations by the initial LIGO and Virgo GW detectors and the partially completed IceCube (40-string) HEN detector. We then estimate the reach of joint GW+HEN searches using advanced GW detectors and the completed km(3) IceCube detector to probe the joint parameter space. We show that searches undertaken by advanced detectors will be capable of detecting, constraining, or excluding, several existing models with 1 yr of observation.     

Gravitational-wave research with resonant antennas

Summary A brief historical survey of the GW experiments is presented. In particular, the experiments with resonant detectors are illustrated and the problems arising when comparing data of different detectors for the search of coincidences are discussed. The experimental results so far obtained are shown, among them the correlation during the SN1987A with various GW and neutrino detectors. Finally a warning for the long times needed to improve the sensitivity of the GW detectors is given and the improvements expected with spherical resonant detectors are mentioned.     

Quasi-normal modes and gravitational wave astronomy

We review the main results obtained in the literature on quasi-normal modes (QNM) of compact stars and black holes, in the light of recent exciting developments of gravitational wave (GW) detectors. QNMs are a fundamental feature of the gravitational signal emitted by compact objects in many astrophysical processes; we will show that their eigenfrequencies encode interesting information on the nature and on the inner structure of the emitting source and we will discuss whether we are ready for a GW asteroseismology.     

Gravitational wave bursts from cosmic strings

Cusps of cosmic strings emit strong beams of high-frequency gravitational waves (GW). As a consequence of these beams, the stochastic ensemble of gravitational waves generated by a cosmological network of oscillating loops is strongly non-Gaussian, and includes occasional sharp bursts that stand above the rms GW background. These bursts might be detectable by the planned GW detectors LIGO/VIRGO and LISA for string tensions as small as G&mgr; approximately 10(-13). The GW bursts discussed here might be accompanied by gamma ray bursts.     

Generation of a stable low-frequency squeezed vacuum field with periodically poled KTiOPO4 at 1064 nm

We report the generation of a stable continuous-wave low-frequency squeezed vacuum field with a squeezing level of 7.4+/-0.1 dB at 1064 nm, the wavelength at which laser-interferometric gravitational wave (GW) detectors operate, using periodically poled KTiOPO4 (PPKTP) in a subthreshold optical parametric oscillator. The squeezing was observed in a broad band of frequencies above 700 Hz where the sensitivity of the currently operational GW detectors is limited by shot noise. PPKTP has the advantages of higher nonlinearity, smaller pump-induced seed absorption, and wider temperature tuning range than alternative nonlinear materials such as MgO-doped or periodically poled LiNbO3, and is, therefore, an excellent material for generation of squeezed vacuum fields for application to laser interferometers for GW detection.     

Effects of relativistic parameter sets on tidal deformabilities and f-mode oscillations of neutron stars

The implications of relativistic parameter sets established at saturation density on the tidal deformabilities and f-mode oscillations of neutron stars (NSs) are examined using constraints from the gravitational wave (GW) event GW170817 and NICER. According to our findings, the isovector saturation parameters have a greater impact on the radii and tidal deformabilities of NSs than the isoscalar saturation parameters. Our analysis also examines the impact of saturation properties on f-mode frequencies and finds that f-mode frequencies with 1.4 M_⊙ (solar mass) are roughly between 1.95 and 2.15 kHz. These findings could be confirmed by future advanced GW detectors. A good linear parameter-independent correlation between f-mode frequencies inferred from saturation parameters in the entire region is also observed, and we attempt to fit an updated version of this universal relationship. Furthermore, we used chiral effective theory (χEFT) together with the multi-messenger astronomy constraints to further reinforce the rationality of the conclusions we have reached.     

A brief history of gravitational wave research

For the benefit of the readers of this journal, the editors requested that we prepare a brief review of the history of the development of the theory, the experimental attempts to detect them, and the recent direct observations of gravitational waves (GWs). The theoretical ideas and disputes beginning with Einstein in 1916 regarding the existence and nature of gravitational waves and the extent to which one can rely on the electromagnetic analogy, especially the controversies regarding the quadrupole formula and whether gravitational waves carry energy, are discussed. The theoretical conclusions eventually received strong observational support from the binary pulsar. This provided compelling, although indirect, evidence for gravitational waves carrying away energy—as predicted by the quadrupole formula. On the direct detection experimental side, Joseph Weber started more than fifty years ago. In 1966, his bar for GW detection reached a strain sensitivity of a few times ${10}^{?16}$. His announcement of coincident signals (now considered spurious), stimulated many experimental efforts from room temperature resonant masses to cryogenic detectors and laser-interferometers. Now there are km-sized interferometric detectors (LIGO Hanford, LIGO Livingston, Virgo and KAGRA). Advanced LIGO first reached a strain sensitivity of the order of ${10}^{?22}$. During their first 130 days of observation (O1 run), with the aid of templates generated by numerical relativity, they did make the first detections: two 5-σ GW events and one likely event. Besides earth-based GW detectors, the drag-free sensitivity of the LISA Pathfinder has already reached to the LISA goal level, paving the road for space GW detectors. Over the whole GW spectrum (from aHz to THz) there are efforts for detection, notably the very-low-frequency band (pulsar timing array [PTA], 300 pHz – 100 nHz) and the extremely-low (Hubble)-frequency (cosmic microwave background [CMB] experiment, 1 aHz – 10 fHz).     

Impacts of gravitational-wave standard siren observations from Einstein Telescope and Cosmic Explorer on weighing neutrinos in interacting dark energy models

Multi-messenger gravitational wave (GW) observation for binary neutron star merger events could provide a rather useful tool to explore the evolution of the Universe. In particular, for the third-generation GW detectors, i.e. the Einstein Telescope (ET) and the Cosmic Explorer (CE), proposed to be built in Europe and the U.S., respectively, lots of GW standard sirens with known redshifts could be obtained, which would exert great impacts on the cosmological parameter estimation. The total neutrino mass could be measured by cosmological observations, but such a measurement is model-dependent and currently only gives an upper limit. In this work, we wish to investigate whether the GW standard sirens observed by ET and CE could help improve the constraint on the neutrino mass, in particular in the interacting dark energy (IDE) models. We find that the GW standard siren observations from ET and CE can only slightly improve the constraint on the neutrino mass in the IDE models, compared to the current limit. The improvements in the IDE models are weaker than those in the standard cosmological model. Although the limit on neutrino mass can only be slightly updated, the constraints on other cosmological parameters can be significantly improved by using the GW observations.     

Reduction of thermal fluctuations in a cryogenic laser interferometric gravitational wave detector

The thermal fluctuation of mirror surfaces is the fundamental limitation for interferometric gravitational wave (GW) detectors. Here, we experimentally demonstrate for the first time a reduction in a mirror's thermal fluctuation in a GW detector with sapphire mirrors from the Cryogenic Laser Interferometer Observatory at 17 and 18 K. The detector sensitivity, which was limited by the mirror's thermal fluctuation at room temperature, was improved in the frequency range of 90 to 240 Hz by cooling the mirrors. The improved sensitivity reached a maximum of 2.2×10(-19) m/√Hz at 165 Hz.     

Generic gravitational-wave signals from the collapse of rotating stellar cores

We perform general relativistic (GR) simulations of stellar core collapse to a protoneutron star, using a microphysical equation of state (EOS) and an approximation of deleptonization. We show that for a wide range of rotation rates and profiles the gravitational-wave (GW) burst signals from the core bounce are generic, known as type I. In our systematic study, using both GR and Newtonian gravity, we identify and quantify the influence of rotation, the EOS, and deleptonization on this result. Such a generic type of signal templates will facilitate a more efficient search in current and future GW detectors of both interferometric and resonant type.     

Identify real gravitational wave events in the LIGO-Virgo catalog GWTC-1 and GWTC-2 with convolutional neural network

In recent years, machine learning models have been introduced into the field of gravitational wave (GW) data processing. In this paper, we apply the convolutional neural network (CNN) to LIGO O1, O2, O3a data analysis to search the released 41 GW events which are emitted from binary black hole (BBH) mergers (here we exclude the events from binary neutron star (BNS) mergers, and the events that are not detected simultaneously by Hanford (H) and Livingston (L) detectors), and use time sliding method to reduce the false alarm rate (FAR). According to the results, the 41 confirmed GW events of BBH mergers can be classified successfully by our CNN model. Furthermore, through restricting the number of consecutive prewarning from sequential samples intercepted continuously in LIGO O2 real time-series and vetoing the coincidences of noise from H and L, the FAR is limited to be less than once in 2 months. It is helpful to promote LIGO real time data processing.     

Higher-order Laguerre-Gauss mode generation and interferometry for gravitational wave detectors

We report on the first experimental demonstration of higher-order Laguerre-Gauss (LG(p)(?)) mode generation and interferometry using a method scalable to the requirements of gravitational wave (GW) detection. GW detectors which use higher-order LG(p)(?) modes will be less susceptible to mirror thermal noise, which is expected to limit the sensitivity of all currently planned terrestrial detectors. We used a diffractive optic and a mode-cleaner cavity to convert a fundamental LG(0)(0) Gaussian beam into an LG(3)(3) mode with a purity of 98%. The ratio between the power of the LG(0)(0) mode of our laser and the power of the LG(3)(3) transmitted by the cavity was 36%. By measuring the transmission of our setup using the LG(0)(0), we inferred that the conversion efficiency specific to the LG(3)(3) mode was 49%. We illuminated a Michelson interferometer with the LG(3)(3) beam and achieved a visibility of 97%.     

Displacement-noise-free gravitational-wave detection with a single Fabry-Perot cavity: A toy model

We propose a detuned Fabry-Perot cavity, pumped through both the mirrors, as a toy model of the gravitational-wave (GW) detector partially free from displacement noise of the test masses. It is demonstrated that the noise of cavity mirrors can be eliminated, but the one of lasers and detectors cannot. The isolation of the GW signal from displacement noise of the mirrors is achieved in a proper linear combination of the cavity output signals. The construction of such a linear combination is possible due to the difference between the reflected and transmitted output signals of detuned cavity. We demonstrate that in low-frequency region the obtained displacement-noise-free response signal is much stronger than the -limited sensitivity of displacement-noise-free interferometers recently proposed by S. Kawamura and Y. Chen. However, the loss of the resonant gain in the noise cancelation procedure results is the sensitivity limitation of our toy model by displacement noise of lasers and detectors.     

Gravitational wave background from a sample of Cataclysmic Variables

In this paper we analyze the class of Cataclysmic Variables (CVs) as sources of Gravitational Radiation, basing our analysis only on known objects (168 CVs taken from the recent Ritter Catalog), as we have already done in two previous papers for two other classes of astrophysical objects, eccentric binaries and pulsars. We try to evaluate the Gravitational Wave background, outlining all the potentially interesting sources, in two different ways, showing the substantial agreement of the results obtained. Although not completely new, such results are based on real samples of data, plus some statistical analysis, and therefore constitute a solid basis for planning the construction of GW detectors (especially space-borne GW antennas). Moreover, they provide the possibility of experimentally proving the effectiveness of the mechanism of Gravitational Radiation on CV evolution. Furthermore, for the sake of completeness, the GW emission from the known low-mass X-ray binaries and CV related objects has been evaluated.     

Heterodyne detection enhanced by quantum correlation

Heterodyne detectors as phase-insensitive(PI) devices have found important applications in precision measurements such as space-based gravitational-wave(GW) observation. However, the output signal of a PI heterodyne detector is supposed to suffer from signal-to-noise ratio(SNR) degradation due to image band vacuum and imperfect quantum efficiency. Here, we show that the SNR degradation can be overcome when the image band vacuum is quantum correlated with the input signal.We calculate the noise figure of the detector and prove the feasibility of heterodyne detection with enhanced noise performance through quantum correlation. This work should be of great interest to ongoing space-borne GW signal searching experiments.     

Increase of the Number of Detectable Gravitational Waves Signals Due to Gravitational Lensing

This article deals with the gravitational lensing (GL) of gravitational waves (GW). We compute the increase in the number of detected GW events due to GL. First, we check that geometrical optics is valid for the GW frequency range on which Earth-based detectors are sensitive, and that this is also partially true for what concerns the future space-based interferometer LISA. To infer this result, both the diffraction parameter and a cut-off frequency are computed. Then, the variation in the number of GW signals is estimated in the general case, and applied to some lens models: point mass lens and singular isothermal sphere (SIS profile). An estimation of the magnification factor has also been done for the softened isothermal sphere and for the King profile. The results appear to be strongly model-dependent, but in all cases the increase in the number of detected GW signals is negligible. The use of time delays among images is also investigated.     

Improving the sensitivity of future GW observatories in the 1�C10?Hz band: Newtonian and seismic noise

The next generation gravitational wave interferometric detectors will likely be underground detectors to extend the GW detection frequency band to frequencies below the Newtonian noise limit. Newtonian noise originates from the continuous motion of the Earth??s crust driven by human activity, tidal stresses and seismic motion, and from mass density fluctuations in the atmosphere. It is calculated that on Earth??s surface, on a typical day, it will exceed the expected GW signals at frequencies below 10 Hz. The noise will decrease underground by an unknown amount. It is important to investigate and to quantify this expected reduction and its effect on the sensitivity of future detectors, to plan for further improvement strategies. We report about some of these aspects. Analytical models can be used in the simplest scenarios to get a better qualitative and semi-quantitative understanding. As more complete modeling can be done numerically, we will discuss also some results obtained with a finite-element-based modeling tool. The method is verified by comparing its results with the results of analytic calculations for surface detectors. A key point about noise models is their initial parameters and conditions, which require detailed information about seismic motion in a real scenario. We will describe an effort to characterize the seismic activity at the Homestake mine which is currently in progress. This activity is specifically aimed to provide informations and to explore the site as a possible candidate for an underground observatory. Although the only compelling reason to put the interferometer underground is to reduce the Newtonian noise, we expect that the more stable underground environment will have a more general positive impact on the sensitivity. We will end this report with some considerations about seismic and suspension noise.     

Chemical Evolution of the Universe and its Consequences for Gravitational-Wave Astrophysics

Gravitational waves (GW) emitted by merging black holes (BH) and neutron stars are now routinely detected. Those are the afterlives of massive stars that formed all across the Universe—at different cosmic times and with different metallicities. Birth metallicity plays an important role in the evolution of massive stars. Consequently, the population properties of mergers are sensitive to the metallicity dependent cosmic star formation history (f_SFR(Z,z)). In particular, within the isolated formation scenarios (the focus of this paper), a strong low metallicity preference of the formation of BH mergers is found. The origin of this dependence and its consequences are discussed. Most importantly, uncertainty in the f_SFR(Z,z) (substantial even at low redshifts) cannot be ignored in the models. This poses a challenge for the interpretation of the observed GW source population properties. Possible improvements and the role of future GW detectors are considered. Recent efforts to determine f_SFR(Z,z) and the factors that dominate its uncertainty are summarized. Many of those factors stem from the uncertain properties of faint and distant galaxies. The fact that they leave imprint on the redshift-dependent properties of mergers makes GW a promising (and complementary to electromagnetic observations) tool to study galaxy chemical evolution.