GW170817: Measurements of Neutron Star Radii and Equation of State

Accessing Universal Relations of Binary Neutron Star Waveforms in Massive Scalar-Tensor Theory

We investigate how the quasiuniversal relations connecting tidal deformability with gravitational waveform characteristics and/or properties of individual neutron stars that were proposed in the literature within general relativity would be influenced in the massive Damour-Esposito-Farese-type scalar-tensor gravity. For this purpose, we systematically perform numerical relativity simulations of ∼120 binary neutron-star mergers with varying scalar coupling constants. Although only three neutron-star equations of state are adopted, a clear breach of universality can be observed in the datasets. In addition to presenting difficulties in constructing quasiuniversal relations in alternative gravity theories, we also briefly compare the impacts of non-general-relativity physics on the waveform features and those due to the first order or cross-over quantum chromodynamical phase transition.

Real-time inference for binary neutron star mergers using machine learning

Mergers of binary neutron stars emit signals in both the gravitational-wave (GW) and electromagnetic spectra. Famously, the 2017 multi-messenger observation of GW170817 (refs. 1,2) led to scientific discoveries across cosmology3, nuclear physics4-6 and gravity7. Central to these results were the sky localization and distance obtained from the GW data, which, in the case of GW170817, helped to identify the associated electromagnetic transient, AT 2017gfo (ref. 8), 11 h after the GW signal. Fast analysis of GW data is critical for directing time-sensitive electromagnetic observations. However, owing to challenges arising from the length and complexity of signals, it is often necessary to make approximations that sacrifice accuracy. Here we present a machine-learning framework that performs complete binary neutron star inference in just 1 s without making any such approximations. Our approach enhances multi-messenger observations by providing: (1) accurate localization even before the merger; (2) improved localization precision by around 30% compared to approximate low-latency methods; and (3) detailed information on luminosity distance, inclination and masses, which can be used to prioritize expensive telescope time. Additionally, the flexibility and reduced cost of our method open new opportunities for equation-of-state studies. Finally, we demonstrate that our method scales to long signals, up to an hour in length, thus serving as a blueprint for data analysis for next-generation ground- and space-based detectors.

Minimum Neutron Star Mass in Neutrino-Driven Supernova Explosions

Supernova theory has struggled to explain the lightest known neutron star candidate with an accurate mass determination, the 1.174M_{⊙} companion in the eccentric compact binary system J0453+1559. To improve the theoretical lower limit for neutron star birth masses, we perform 3D supernova simulations for five stellar models close to the minimum mass for iron core collapse. We obtain a record-low neutron star mass of 1.192M_{⊙} and a substantial kick of ∼100  km s^{-1}. Given residual uncertainties in stellar evolution, a neutron star origin for the 1.174M_{⊙} object remains plausible.

Supernova Axions Convert to Gamma Rays in Magnetic Fields of Progenitor Stars

It has long been established that axions could have been produced within the nascent proto-neutron star formed following the type II supernova SN1987A, escaped the star due to their weak interactions, and then converted to gamma rays in the Galactic magnetic fields; the nonobservation of a gamma-ray flash coincident with the neutrino burst leads to strong constraints on the axion-photon coupling for axion masses m_{a}≲10^{-10}  eV. In this Letter, we use SN1987A to constrain higher mass axions, all the way to m_{a}∼10^{-3}  eV, by accounting for axion production from the Primakoff process, nucleon bremsstrahlung, and pion conversion along with axion-photon conversion on the still-intact magnetic fields of the progenitor star. Moreover, we show that gamma-ray observations of the next Galactic supernova, leveraging the magnetic fields of the progenitor star, could detect quantum chromodynamics axions for masses above roughly 50  μeV, depending on the supernova. We propose a new full-sky gamma-ray satellite constellation that we call the GALactic AXion Instrument for Supernova (GALAXIS) to search for such future signals along with related signals from extragalactic neutron star mergers.

Post-Minkowskian Theory Meets the Spinning Effective-One-Body Approach for Bound-Orbit Waveforms

Driven by advances in scattering amplitudes and worldline-based methods, recent years have seen significant progress in our ability to calculate gravitational two-body scattering observables. These observables effectively encapsulate the gravitational two-body problem in the weak-field and high-velocity regime [post-Minkowskian (PM)], with applications to the bound two-body problem and gravitational-wave modeling. We leverage PM data to construct a complete inspiral-merger-ringdown waveform model for nonprecessing spinning black holes within the effective-one-body (EOB) formalism SEOBNR-PM. This model is closely based on the highly successful SEOBNRv5 model, used by the LIGO-Virgo-KAGRA Collaboration, with its key new feature being an EOB Hamiltonian derived by matching the two-body scattering angle in a perturbative PM expansion. The model performs remarkably well, showing a median mismatch against 441 numerical-relativity (NR) simulations that is somewhat lower than a similarly calibrated version of SEOBNRv5. Comparisons of the binding energy with NR also demonstrate better agreement than SEOBNRv5, despite the latter containing additional calibration to NR simulations.

Astrophysical Equation-of-State Constraints on the Color-Superconducting Gap

We demonstrate that astrophysical constraints on the dense-matter equation of state place an upper bound on the color-superconducting gap in dense matter above the transition from nuclear matter to quark matter. Pairing effects in the color-flavor locked quark matter phase increase the pressure at high density, and if this effect is sufficiently large then the requirements of causality and mechanical stability make it impossible to reach such a pressure in a way that is consistent with what is known at lower densities. The intermediate-density equation of state is inferred by considering extensions of chiral effective field theory to neutron star densities, and conditioning these using current astrophysical observations of neutron star radius, maximum mass, and tidal deformability (PSR J0348+0432, PSR J1624-2230, PSR J0740+6620, GW170817). At baryon number chemical potential μ=2.6  GeV we find a 95% upper limit on the color-flavor locked pairing gap Δ of 457 MeV using overly conservative assumptions and 216 MeV with more reasonable assumptions. This constraint may be strengthened by future astrophysical measurements as well as by future advances in high-density QCD calculations.

Neutrino Trapping and Out-of-Equilibrium Effects in Binary Neutron-Star Merger Remnants

We study out-of-thermodynamic-equilibrium effects in neutron-star mergers with 3D general-relativistic neutrino-radiation large-eddy simulations. During mergers, the cores of the neutron stars remain cold (T∼ a few MeV) and out of thermodynamic equilibrium with trapped neutrinos originating from the hot collisional interface between the stars. However, within ∼2 to 3 ms matter and neutrinos reach equilibrium everywhere in the remnant massive neutron star. Our results show that dissipative effects, such as bulk viscosity, if present, are only active for a short window of time after the merger.

Low-latency gravitational wave alert products and their performance at the time of the fourth LIGO-Virgo-KAGRA observing run

Multimessenger searches for binary neutron star (BNS) and neutron star-black hole (NSBH) mergers are currently one of the most exciting areas of astronomy. The search for joint electromagnetic and neutrino counterparts to gravitational wave (GW)s has resumed with ALIGO's, AdVirgo's and KAGRA's fourth observing run (O4). To support this effort, public semiautomated data products are sent in near real-time and include localization and source properties to guide complementary observations. In preparation for O4, we have conducted a study using a simulated population of compact binaries and a mock data challenge (MDC) in the form of a real-time replay to optimize and profile the software infrastructure and scientific deliverables. End-toend performance was tested, including data ingestion, running online search pipelines, performing annotations, and issuing alerts to the astrophysics community. We present an overview of the low-latency infrastructure and the performance of the data products that are now being released during O4 based on the MDC. We report the expected median latency for the preliminary alert of full bandwidth searches (29.5 s) and show consistency and accuracy of released data products using the MDC. We report the expected median latency for triggers from early warning searches (-3.1 s), which are new in O4 and target neutron star mergers during inspiral phase. This paper provides a performance overview for LIGO-Virgo-KAGRA (LVK) low-latency alert infrastructure and data products using theMDCand serves as a useful reference for the interpretation of O4 detections.

A High-Finesse Suspended Interferometric Sensor for Macroscopic Quantum Mechanics with Femtometre Sensitivity

We present an interferometric sensor for investigating macroscopic quantum mechanics on a table-top scale. The sensor consists of a pair of suspended optical cavities with finesse over 350,000 comprising 10 g fused silica mirrors. The interferometer is suspended by a four-stage, light, in-vacuum suspension with three common stages, which allows for us to suppress common-mode motion at low frequency. The seismic noise is further suppressed by an active isolation scheme, which reduces the input motion to the suspension point by up to an order of magnitude starting from 0.7 Hz. In the current room-temperature operation, we achieve a peak sensitivity of 0.5 fm/Hz in the acoustic frequency band, limited by a combination of readout noise and suspension thermal noise. Additional improvements of the readout electronics and suspension parameters will enable us to reach the quantum radiation pressure noise. Such a sensor can eventually be utilized for demonstrating macroscopic entanglement and for testing semi-classical and quantum gravity models.

Momentum Shell in Quarkyonic Matter from Explicit Duality: A Dual Model for Cold, Dense QCD

We present a model of cold QCD matter that bridges nuclear and quark matter through the duality relation between quarks and baryons. The baryon number and energy densities are expressed as functionals of either the baryon momentum distribution, f_{B}, or the quark distribution, f_{Q}, which are subject to the constraints on fermions, 0≤f_{B,Q}≤1. The theory is ideal in the sense that the confinement of quarks into baryons is reflected in the duality relation between f_{Q} and f_{B}, while other possible interactions among quarks and baryons are all neglected. The variational problem with the duality constraints is formulated and we explicitly construct analytic solutions, finding two distinct regimes: a nuclear matter regime at low density and a quarkyonic regime at high density. In the quarkyonic regime, baryons underoccupy states at low momenta but form a momentum shell with f_{B}=1 on top of a quark Fermi sea. Such a theory describes a rapid transition from a soft nuclear equation of state to a stiff quarkyonic equation of state. At this transition, there is a rapid increase in the pressure.

An updated nuclear-physics and multi-messenger astrophysics framework for binary neutron star mergers

The multi-messenger detection of the gravitational-wave signal GW170817, the corresponding kilonova AT2017gfo and the short gamma-ray burst GRB170817A, as well as the observed afterglow has delivered a scientific breakthrough. For an accurate interpretation of all these different messengers, one requires robust theoretical models that describe the emitted gravitational-wave, the electromagnetic emission, and dense matter reliably. In addition, one needs efficient and accurate computational tools to ensure a correct cross-correlation between the models and the observational data. For this purpose, we have developed the Nuclear-physics and Multi-Messenger Astrophysics framework NMMA. The code allows incorporation of nuclear-physics constraints at low densities as well as X-ray and radio observations of isolated neutron stars. In previous works, the NMMA code has allowed us to constrain the equation of state of supranuclear dense matter, to measure the Hubble constant, and to compare dense-matter physics probed in neutron-star mergers and in heavy-ion collisions, and to classify electromagnetic observations and perform model selection. Here, we show an extension of the NMMA code as a first attempt of analyzing the gravitational-wave signal, the kilonova, and the gamma-ray burst afterglow simultaneously. Incorporating all available information, we estimate the radius of a 1.4M⊙ neutron star to be [Formula: see text] km.

Strongly interacting matter exhibits deconfined behavior in massive neutron stars

Neutron-star cores contain matter at the highest densities in our Universe. This highly compressed matter may undergo a phase transition where nuclear matter melts into deconfined quark matter, liberating its constituent quarks and gluons. Quark matter exhibits an approximate conformal symmetry, predicting a specific form for its equation of state (EoS), but it is currently unknown whether the transition takes place inside at least some physical neutron stars. Here, we quantify this likelihood by combining information from astrophysical observations and theoretical calculations. Using Bayesian inference, we demonstrate that in the cores of maximally massive stars, the EoS is consistent with quark matter. We do this by establishing approximate conformal symmetry restoration with high credence at the highest densities probed and demonstrating that the number of active degrees of freedom is consistent with deconfined matter. The remaining likelihood is observed to correspond to EoSs exhibiting phase-transition-like behavior, treated as arbitrarily rapid crossovers in our framework.

Gravitational Waves from a Core g Mode in Supernovae as Probes of the High-Density Equation of State

Using relativistic supernova simulations of massive progenitor stars with a quark-hadron equation of state (EOS) and a purely hadronic EOS, we identify a distinctive feature in the gravitational-wave signal that originates from a buoyancy-driven mode (g mode) below the proto-neutron star convection zone. The mode frequency lies in the range 200≲f≲800  Hz and decreases with time. As the mode lives in the core of the proto-neutron star, its frequency and power are highly sensitive to the EOS, in particular the sound speed around twice saturation density.

Self-Consistent Picture of the Mass Ejection from a One Second Long Binary Neutron Star Merger Leaving a Short-Lived Remnant in a General-Relativistic Neutrino-Radiation Magnetohydrodynamic Simulation

We perform a general-relativistic neutrino-radiation magnetohydrodynamic simulation of a one second-long binary neutron star merger on the Japanese supercomputer Fugaku using about 85 million CPU hours with 20 736 CPUs. We consider an asymmetric binary neutron star merger with masses of 1.2M_{⊙} and 1.5M_{⊙} and a "soft" equation of state SFHo. It results in a short-lived remnant with the lifetime of ≈0.017  s, and subsequent massive torus formation with the mass of ≈0.05M_{⊙} after the remnant collapses to a black hole. For the first time, we find that after the dynamical mass ejection, which drives the fast tail and mildly relativistic components, the postmerger mass ejection from the massive torus takes place due to the magnetorotational instability-driven turbulent viscosity in a single simulation and the two ejecta components are seen in the distributions of the electron fraction and velocity with distinct features.

Cosmography Using Strongly Lensed Gravitational Waves from Binary Black Holes

Third generation gravitational wave (GW) detectors are expected to detect millions of binary black hole (BBH) mergers during their operation period. A small fraction of them (∼1%) will be strongly lensed by intervening galaxies and clusters, producing multiple observable copies of the GW signals. The expected number of lensed events and the distribution of the time delay between lensed images depend on the cosmology. We develop a Bayesian analysis method for estimating cosmological parameters from the detected number of lensed events and their time delay distribution. The expected constraints are comparable to that obtained from other cosmological measurements, but probing a different redshift regime (z∼10) that is not explored by other probes.

Degeneracy in the Inference of Phase Transitions in the Neutron Star Equation of State from Gravitational Wave Data

Gravitational wave (GW) detections of binary neutron star inspirals will be crucial for constraining the dense matter equation of state (EOS). We demonstrate a new degeneracy in the mapping from tidal deformability data to the EOS, which occurs for models with strong phase transitions. We find that there exists a new family of EOS with phase transitions that set in at different densities and that predict neutron star radii that differ by up to ∼500  m but that produce nearly identical tidal deformabilities for all neutron star masses. Next-generation GW detectors and advances in nuclear theory may be needed to resolve this degeneracy.

Neural Importance Sampling for Rapid and Reliable Gravitational-Wave Inference

We combine amortized neural posterior estimation with importance sampling for fast and accurate gravitational-wave inference. We first generate a rapid proposal for the Bayesian posterior using neural networks, and then attach importance weights based on the underlying likelihood and prior. This provides (1) a corrected posterior free from network inaccuracies, (2) a performance diagnostic (the sample efficiency) for assessing the proposal and identifying failure cases, and (3) an unbiased estimate of the Bayesian evidence. By establishing this independent verification and correction mechanism we address some of the most frequent criticisms against deep learning for scientific inference. We carry out a large study analyzing 42 binary black hole mergers observed by LIGO and Virgo with the SEOBNRv4PHM and IMRPhenomXPHM waveform models. This shows a median sample efficiency of ≈10% (2 orders of magnitude better than standard samplers) as well as a tenfold reduction in the statistical uncertainty in the log evidence. Given these advantages, we expect a significant impact on gravitational-wave inference, and for this approach to serve as a paradigm for harnessing deep learning methods in scientific applications.

Plausible presence of new state in neutron stars with masses above 0.98MTOV

We investigate the neutron star (NS) equation of state (EOS) by incorporating multi-messenger data of GW170817, PSR J0030 + 0451, PSR J0740 + 6620, and state-of-the-art theoretical progresses, including the information from chiral effective field theory (χEFT) and perturbative quantum chromodynamics (pQCD) calculation. Taking advantage of the various structures sampling by a single-layer feed-forward neural network model embedded in the Bayesian nonparametric inference, the structure of NS matter's sound speed c_s is explored in a model-agnostic way. It is found that a peak structure is common in the c_s² posterior, locating at 2.4-4.8ρ_sat (nuclear saturation density) and c_s² exceeds c²/3 at 90% credibility. The non-monotonic behavior suggests evidence of the state deviating from the hadronic matter inside the very massive NSs. Assuming the new/exotic state is featured as it is softer than typical hadronic models or even with hyperons, we find that a sizable (⩾10^-3M_⊙) exotic core, likely made of quark matter, is plausible for the NS with a gravitational mass above about 0.98M_TOV, where M_TOV represents the maximum gravitational mass of a non-rotating cold NS. The inferred M_TOV=2.18_-0.13^+0.27M_⊙ (90% credibility) is well consistent with the value of 2.17_-0.12^+0.15M_⊙ estimated independently with GW170817/GRB 170817A/AT2017gfo assuming a temporary supramassive NS remnant formed after the merger. PSR J0740 + 6620, the most massive NS detected so far, may host a sizable exotic core with a probability of ≈0.36.

Constraining the Nuclear Symmetry Energy with Multimessenger Resonant Shattering Flares

Much effort is devoted to measuring the nuclear symmetry energy through neutron star (NS) and nuclear observables. Since matter in the NS core may be nonhadronic, observables like radii and tidal deformability may not provide reliable constraints on properties of nucleonic matter. By performing the first consistent inference using ensembles of core and crust equations of state from astrophysical and nuclear data, we demonstrate that coincident timing of a resonant shattering flare (RSF) and gravitational wave signal during binary NS inspiral probes the crust-core transition region and provides constraints on the symmetry energy comparable to terrestrial nuclear experiments. We show that nuclear masses, RSFs, and measurements of NS radii and tidal deformabilities constrain different density ranges of the equation of state, providing complementary probes.

Gravitational Wave Signal for Quark Matter with Realistic Phase Transition

The cores of neutron stars (NSs) near the maximum mass can realize a transitional change to quark matter (QM). Gravitational waves from binary NS mergers are expected to convey information about the equation of state (EOS) sensitive to the QM transition. Here, we present the first results of gravitational wave simulation with the realistic EOS that is consistent with ab initio approaches of χEFT and pQCD and is assumed to go through smooth crossover. We compare them to results obtained with another EOS with a first-order hadron-quark phase transition. Our results suggest that early collapse to a black hole in the post-merger stage after NS merger robustly signifies softening of the EOS associated with the QM onset in the crossover scenario. The nature of the hadron-quark phase transition can be further constrained by the condition that electromagnetic counterparts should be energized by the material left outside the remnant black hole.

Neutrino transport in general relativistic neutron star merger simulations

Numerical simulations of neutron star-neutron star and neutron star-black hole binaries play an important role in our ability to model gravitational-wave and electromagnetic signals powered by these systems. These simulations have to take into account a wide range of physical processes including general relativity, magnetohydrodynamics, and neutrino radiation transport. The latter is particularly important in order to understand the properties of the matter ejected by many mergers, the optical/infrared signals powered by nuclear reactions in the ejecta, and the contribution of that ejecta to astrophysical nucleosynthesis. However, accurate evolutions of the neutrino transport equations that include all relevant physical processes remain beyond our current reach. In this review, I will discuss the current state of neutrino modeling in general relativistic simulations of neutron star mergers and of their post-merger remnants. I will focus on the three main types of algorithms used in simulations so far: leakage, moments, and Monte-Carlo scheme. I will review the advantages and limitations of each scheme, as well as the various neutrino-matter interactions that should be included in simulations. We will see that the quality of the treatment of neutrinos in merger simulations has greatly increased over the last decade, but also that many potentially important interactions remain difficult to take into account in simulations (pair annihilation, oscillations, inelastic scattering).

A physics-based neural network reconstruction of the dense matter equation of state from neutron star observables

We introduce a novel technique that utilizes a physics-driven deep learning method to reconstruct the dense matter equation of state from neutron star observables, particularly the masses and radii. The proposed framework involves two neural networks: one to optimize the EoS using Automatic Differentiation in the unsupervised learning scheme; and a pre-trained network to solve the Tolman–Oppenheimer–Volkoff (TOV) equations. The gradient-based optimization process incorporates a Bayesian picture into the proposed framework. The reconstructed EoS is proven to be consistent with the results from conventional methods. Furthermore, the resulting tidal deformation is in agreement with the limits obtained from the gravitational wave event, GW170817.

Gravitational Waves from Accretion-Induced Descalarization in Massive Scalar-Tensor Theory

Many classes of extended scalar-tensor theories predict that dynamical instabilities can take place at high energies, leading to the formation of scalarized neutron stars. Depending on the theory parameters, stars in a scalarized state can form a solution-space branch that shares a lot of similarities with the so-called mass twins in general relativity appearing for equations of state containing first-order phase transitions. Members of this scalarized branch have a lower maximum mass and central energy density compared to Einstein ones. In such cases, a scalarized star could potentially overaccrete beyond the critical mass limit, thus triggering a gravitational phase transition where the star sheds its scalar hair and migrates over to its nonscalarized counterpart. Such an event resembles, but is distinct from, a nuclear or thermodynamic phase transition. We dynamically track a gravitational transition by first constructing hydrostatic, scalarized equilibria for realistic equations of state, and then allowing additional material to fall onto the stellar surface. The resulting bursts of monopolar radiation are dispersively stretched to form a quasicontinuous signal that persists for decades, carrying strains of order ≳10^{-22}  (kpc/L)^{3/2} Hz^{-1/2} at frequencies of ≲300  Hz, detectable with the existing interferometer network out to distances of L≲10  kpc, and out to a few hundred kpc with the inclusion of the Einstein Telescope. Electromagnetic signatures of such events, involving gamma-ray and neutrino bursts, are also considered.

Impact of Dynamical Tides on the Reconstruction of the Neutron Star Equation of State

Gravitational waves (GWs) from inspiraling neutron stars afford us a unique opportunity to infer the as-of-yet unknown equation of state of cold hadronic matter at supranuclear densities. During the inspiral, the dominant matter effects arise due to the star's response to their companion's tidal field, leaving a characteristic imprint in the emitted GW signal. This unique signature allows us to constrain the cold neutron star equation of state. At GW frequencies above ≳800  Hz, however, subdominant tidal effects known as dynamical tides become important. In this Letter, we demonstrate that neglecting dynamical tidal effects associated with the fundamental (f) mode leads to large systematic biases in the measured tidal deformability of the stars and hence in the inferred neutron star equation of state. Importantly, we find that f-mode dynamical tides will already be relevant for Advanced LIGO's and Virgo's fifth observing run (∼2025)-neglecting dynamical tides can lead to errors on the neutron radius of O(1  km), with dramatic implications for the measurement of the equation of state. Our results demonstrate that the accurate modeling of subdominant tidal effects beyond the adiabatic limit will be crucial to perform accurate measurements of the neutron star equation of state in upcoming GW observations.

Probing the Incompressibility of Nuclear Matter at Ultrahigh Density through the Prompt Collapse of Asymmetric Neutron Star Binaries

Using 250 neutron star merger simulations with microphysics, we explore for the first time the role of nuclear incompressibility in the prompt collapse threshold for binaries with different mass ratios. We demonstrate that observations of prompt collapse thresholds, either from binaries with two different mass ratios or with one mass ratio but combined with the knowledge of the maximum neutron star mass or compactness, will constrain the incompressibility at the maximum neutron star density K_{max} to within tens of percent. This otherwise inaccessible measure of K_{max} can potentially reveal the presence of hyperons or quarks inside neutron stars.

Temperature Effects on Core g-Modes of Neutron Stars

Neutron stars provide a unique physical laboratory in which to study the properties of matter at high density and temperature. We study a diagnostic of the composition of high-density matter, namely, g-mode oscillations, which are driven by buoyancy forces. These oscillations can be excited by tidal forces and couple to gravitational waves. We extend prior results for the g-mode spectrum of cold neutron star matter to high temperatures that are expected to be achieved in neutron star mergers using a parameterization for finite-temperature effects on equations of state recently proposed by Raithel, Özel and Psaltis. We find that the g-modes of canonical mass neutron stars (≈1.4M⊙) are suppressed at high temperatures, and core g-modes are supported only in the most massive (≥2M⊙) of hot neutron stars.

Testing Quantum Gravity in the Multi-Messenger Astronomy Era

Quantum gravity (QG) remains elusive despite almost century-long efforts to combine general relativity and quantum mechanics. All the approaches triggered and powered by purely theoretical considerations eventually failed with a prevailing feeling of a complete lack of guidance from the experimental side. Currently, however, this circumstance is beginning to change considerably. We have entered the era of multi-messenger astronomy. The electromagnetic window to the universe—so far the only one—has been tremendously enlarged in the energy range beyond gamma rays up to ultra-high-energy photons and has been complemented by other messengers: high-energy cosmic rays, cosmic neutrinos, and gravitational waves (GWs). This has created a unique environment in which to observationally constrain various phenomenological QG effects. In this paper, we focus on the LIV phenomenology manifested as energy-dependent time-of-flight delays and strong lensing time delays. We review results regarding time-of-flight delays obtained with GRBs. We also recall the idea of energy-dependent lensing time delays, which allow one to constrain LIV models independently of the intrinsic time delay. Lastly, we show how strongly a gravitationally lensed GW signal would place interesting constraints on the LIV.

Constraining neutron-star matter with microscopic and macroscopic collisions

Interpreting high-energy, astrophysical phenomena, such as supernova explosions or neutron-star collisions, requires a robust understanding of matter at supranuclear densities. However, our knowledge about dense matter explored in the cores of neutron stars remains limited. Fortunately, dense matter is not probed only in astrophysical observations, but also in terrestrial heavy-ion collision experiments. Here we use Bayesian inference to combine data from astrophysical multi-messenger observations of neutron stars1-9 and from heavy-ion collisions of gold nuclei at relativistic energies10,11 with microscopic nuclear theory calculations12-17 to improve our understanding of dense matter. We find that the inclusion of heavy-ion collision data indicates an increase in the pressure in dense matter relative to previous analyses, shifting neutron-star radii towards larger values, consistent with recent observations by the Neutron Star Interior Composition Explorer mission5-8,18. Our findings show that constraints from heavy-ion collision experiments show a remarkable consistency with multi-messenger observations and provide complementary information on nuclear matter at intermediate densities. This work combines nuclear theory, nuclear experiment and astrophysical observations, and shows how joint analyses can shed light on the properties of neutron-rich supranuclear matter over the density range probed in neutron stars.

Status and Perspectives of Continuous Gravitational Wave Searches

The birth of gravitational wave astronomy was triggered by the first detection of a signal produced by the merger of two compact objects (also known as a compact binary coalescence event). The following detections made by the Earth-based network of advanced interferometers had a significant impact in many fields of science: astrophysics, cosmology, nuclear physics and fundamental physics. However, compact binary coalescence signals are not the only type of gravitational waves potentially detectable by LIGO, Virgo, and KAGRA. An interesting family of still undetected signals, and the ones that are considered in this review, are the so-called continuous waves, paradigmatically exemplified by the gravitational radiation emitted by galactic, fast-spinning isolated neutron stars with a certain degree of asymmetry in their mass distribution. In this work, I will review the status and the latest results from the analyses of advanced detector data.

Research Facilities for Europe’s Next Generation Gravitational-Wave Detector Einstein Telescope

The Einstein Telescope is Europe’s next generation gravitational-wave detector. To develop all necessary technology, four research facilities have emerged across Europe: The Amaldi Research Center (ARC) in Rome (Italy), ETpathfinder in Maastricht (The Netherlands), SarGrav in the Sos Enattos mines on Sardinia (Italy) and E-TEST in Liége (Belgium) and its surroundings. The ARC pursues the investigation of a large cryostat, equipped with dedicated low-vibration cooling lines, to test full-scale cryogenic payloads. The installation will be gradual and interlaced with the payload development. ETpathfinder aims to provide a low-noise facility that allows the testing of full interferometer configurations and the interplay of their subsystems in an ET-like environment. ETpathfinder will focus amongst others on cryogenic technologies, silicon mirrors, lasers and optics at 1550 and 2090 nm and advanced quantum noise reduction schemes. The SarGrav laboratory has a surface lab and an underground operation. On the surface, the Archimedes experiment investigates the interaction of vacuum fluctuations with gravity and is developing (tilt) sensor technology for the Einstein Telescope. In an underground laboratory, seismic characterisation campaigns are undertaken for the Sardinian site characterisation. Lastly, the Einstein Telecope Euregio meuse-rhine Site & Technology (E-TEST) is a single cryogenic suspension of an ET-sized silicon mirror. Additionally, E-TEST investigates the Belgian–Dutch–German border region that is the other candidate site for Einstein Telescope using boreholes and seismic arrays and hydrogeological characterisation. In this article, we describe the Einstein Telescope, the low-frequency part of its science case and the four research facilities.

Constraints on the Maximum Densities of Neutron Stars from Postmerger Gravitational Waves with Third-Generation Observations

Using data from 289 numerical relativity simulations of binary neutron star mergers, we identify, for the first time, a robust quasiuniversal relation connecting the postmerger peak gravitational-wave frequency and the value of the density at the center of the maximum mass nonrotating neutron star. This relation offers a new possibility for precision equation-of-state constraints with next-generation ground-based gravitational-wave interferometers. Mock Einstein Telescope observations of fiducial events indicate that Bayesian inferences can constrain the maximum density to ∼15% (90% credibility level) for a single signal at the minimum sensitivity threshold for a detection. If the postmerger signal is included in a full-spectrum (inspiral-merger-postmerger) analysis of such a signal, the pressure-density function can be tightly constrained up to the maximum density, and the maximum neutron star mass can be measured with an accuracy better than 12% (90% credibility level).

Finding Structure in the Speed of Sound of Supranuclear Matter from Binary Love Relations

Analyses that connect observations of neutron stars with nuclear-matter properties can rely on equation-of-state insensitive relations. We show that the slope of the binary Love relations (between the tidal deformabilities of binary neutron stars) encodes the baryon density at which the speed of sound rapidly changes. Twin stars lead to relations that present a signature "hill," "drop," and "swoosh" due to the second (mass-radius) stable branch, requiring a new description of the binary Love relations. Together, these features can reveal new properties and phases of nuclear matter.

Determination of the ^{27}Al Neutron Distribution Radius from a Parity-Violating Electron Scattering Measurement

We report the first measurement of the parity-violating elastic electron scattering asymmetry on ^{27}Al. The ^{27}Al elastic asymmetry is A_{PV}=2.16±0.11(stat)±0.16(syst)  ppm, and was measured at ⟨Q^{2}⟩=0.02357±0.00010  GeV^{2}, ⟨θ_{lab}⟩=7.61°±0.02°, and ⟨E_{lab}⟩=1.157  GeV with the Q_{weak} apparatus at Jefferson Lab. Predictions using a simple Born approximation as well as more sophisticated distorted-wave calculations are in good agreement with this result. From this asymmetry the ^{27}Al neutron radius R_{n}=2.89±0.12  fm was determined using a many-models correlation technique. The corresponding neutron skin thickness R_{n}-R_{p}=-0.04±0.12  fm is small, as expected for a light nucleus with a neutron excess of only 1. This result thus serves as a successful benchmark for electroweak determinations of neutron radii on heavier nuclei. A tree-level approach was used to extract the ^{27}Al weak radius R_{w}=3.00±0.15  fm, and the weak skin thickness R_{wk}-R_{ch}=-0.04±0.15  fm. The weak form factor at this Q^{2} is F_{wk}=0.39±0.04.

Ranking Love Numbers for the Neutron Star Equation of State: The Need for Third-Generation Detectors

Gravitational-wave measurements of the tidal deformability in neutron-star binary coalescences can be used to infer the still unknown equation of state (EOS) of dense matter above the nuclear saturation density. By employing a Bayesian-ranking test, we quantify the ability of current and future gravitational-wave observations to discriminate among families of nuclear-physics based EOS which differ in particle content and ab initio microscopic calculations. While the constraining power of GW170817 is limited, we show that even twenty coalescences detected by LIGO-Virgo at design sensitivity are not enough to discriminate between EOS with similar softness but distinct microphysics. However, just a single detection with a third-generation detector such as the Einstein Telescope or Cosmic Explorer will rule out several families of EOS with very strong statistical significance and can discriminate among models which feature similar softness, hence, constraining the properties of nuclear matter to unprecedented levels.

Anisotropic Compact Stars in D → 4 Limit of Gauss–Bonnet Gravity

In the frame of Gauss–Bonnet gravity and in the limit of D→4, based on the fact that spherically symmetric solution derived using any of regularization schemes will be the same form as the original theory, we derive a new interior spherically symmetric solution assuming specific forms of the metric potentials that have two constants. Using the junction condition we determine these two constants. By using the data of the star EXO 1785-248, whose mass is M=1.3±0.2M⊙ and radius l=8.849±0.4 km, we calculate the numerical values of these constants, in terms of the dimensionful coupling parameter of the Gauss–Bonnet term, and eventually, we get real values for these constants. In this regard, we show that the components of the energy–momentum tensor have a finite value at the center of the star as well as a smaller value to the surface of the star. Moreover, we show that the equations of the state behave in a non-linear way due to the impact of the Gauss–Bonnet term. Using the Tolman–Oppenheimer–Volkoff equation, the adiabatic index, and stability in the static state we show that the model under consideration is always stable. Finally, the solution of this study is matched with observational data of other pulsars showing satisfactory results.

Toward Calibration of the Global Network of Gravitational Wave Detectors with Sub-Percent Absolute and Relative Accuracy

The detection of gravitational-wave signals by the LIGO and Virgo observatories during the past few years has ushered us into the era of gravitational-wave astronomy, shifting our focus from detection to source parameter estimation. This has imposed stringent requirements on calibration in order to maximize the astrophysical information extracted from these detected signals. Current detectors rely on photon radiation pressure from auxiliary lasers to achieve required calibration accuracy. These photon calibrators have made significant improvements over the last few years, realizing fiducials displacements with sub-percent accuracy. This achieved accuracy is directly dependent on the laser power calibration. For the next observing campaign, scheduled to begin at the end of 2022, a new scheme is being implemented to achieve improved laser power calibration accuracy for all of the GW detectors in the global network. It is expected to significantly improve absolute and relative calibration accuracy for the entire network.

Can we distinguish quark stars from neutron stars with measurements of global properties?

The phase state of the dense stellar matter is an exciting topic in the area of nuclear astrophysics. It may be probed by observed properties of neutron stars from, for example, the currently operating satellites (NICER, Neutron star Interior Composition Explorer) and the gravitational-wave laser interferometers (Advanced LIGO, Virgo, and KAGRA). Based on our recent constrained parameter spaces of the equation of states of neutron stars and quark stars from LIGO/Virgo and NICER, we discuss the important role of an even-accurate determination of the stellar radius for distinguishing possible quark stars from neutron stars and our understanding of the QCD phase transition at finite density.