Binary Black Hole Mergers Research Articles

Binary black hole mergers from population III stars: uncertainties from star formation and binary star properties

ABSTRACT Population III (Pop. III) binary stars likely produced the first stellar-born binary black hole (BBH) mergers in the Universe. Here, we quantify the main sources of uncertainty for the merger rate density evolution and mass spectrum of Pop. III BBHs by considering four different formation histories and 11 models of the initial orbital properties of Pop. III binary stars. The uncertainty on the orbital properties affects the BBH merger rate density by up to two orders of magnitude, models with shorter orbital periods leading to higher BBH merger rates. The uncertainty on the star formation history has a substantial impact on both the shape and the normalization of the BBH merger rate density: the peak of the merger rate density shifts from z ∼ 8 up to z ∼ 16 depending on the assumed star formation rate, while the maximum BBH merger rate density for our fiducial binary population model spans from ∼2 to ∼30 Gpc−3 yr−1. The typical BBH masses are not affected by the star formation rate model and only mildly influenced by the binary population parameters. The primary black holes born from Pop. III stars tend to be rather massive (30–40 M⊙) with respect to those born from metal-rich stars (8–10 M⊙). We estimate that the Einstein Telescope will detect 10–104 Pop. III BBH mergers per year, depending on the star formation history and binary star properties.

Higgs Field-Induced Triboluminescence in Binary Black Hole Mergers

We conjecture that the Higgs potential can be significantly modified when it is in close proximity to the horizon of an astrophysical black hole, leading to the destabilization of the electroweak vacuum. In this situation, the black hole should be encompassed by a shell consisting of a “bowling substance” of the nucleating new-phase bubbles. In a binary black-hole merger, just before the coalescence, the nucleated bubbles can be prevented from falling under their seeding horizons, as they are simultaneously attracted by the gravitational potential of the companion. For a short time, the unstable vacuum will be “sandwiched” between two horizons of the binary black hole, and therefore the bubbles may collide and form micro-black holes, which are rapidly evaporated by thermal emission of Hawking radiation of all Standard Model species. This evaporation, being triggered by a gravitational wave signal from the binary black-hole merger, can manifest itself in observations of gamma rays and very-high-energy neutrinos, which makes it a perfect physics case for multi-messenger astronomical observations.

Understanding the progenitor formation galaxies of merging binary black holes

ABSTRACT With nearly a hundred gravitational wave detections, the origin of black hole mergers has become a key question. Here, we focus on understanding the typical galactic environment in which binary black hole (BBH) mergers arise. To this end, we synthesize progenitors of BBH mergers as a function of the redshift of progenitor formation, present-day formation galaxy mass, and progenitor stellar metallicity for 240 star formation and binary evolution models. We provide guidelines to infer the formation galaxy properties and time of formation, highlighting the interplay between the star formation rate and the efficiency of forming merging BBHs from binary stars, both of which strongly depend on metallicity. We find that across models, over 50 per cent of BBH mergers have a progenitor metallicity of a few tenths of Solar metallicity, however, inferring formation galaxy properties strongly depends on both the binary evolution model and global metallicity evolution. The numerous, low-mass black holes (≲ 15 M⊙) trace the bulk of the star formation in galaxies heavier than the Milky Way (MGal ≳ 1010.5 M⊙). In contrast, heavier BBH mergers typically stem from larger black holes forming in lower metallicity dwarf galaxies (MGal ≲ 109 M⊙). We find that the progenitors of detectable BBHs tend to arise from dwarf galaxies at a lower formation redshift (≲1). We also produce a posterior probability of the progenitor environment for any detected gravitational wave signal. For the massive GW150914 merger, we show that it likely came from a very low-metallicity (Z ≲ 0.025 Z⊙) environment.

On the formation of GW190521-like binary black hole merger systems

ABSTRACT GW190521 is the most massive merging binary black hole (BBH) system detected so far. At least one of the component BHs was measured to lie within the pair-instability supernova (PISN) mass gap (∼50–135 M⊙), making its formation a mystery. However, the transient observed signal allows alternative posterior distributions. There was suggestion that GW190521 could be an intermediate-mass ratio inspiral, with the component masses m1 ∼ 170 M⊙ and m2 ∼ 16 M⊙, happening to straddle the PISN mass gap. Under this framework, we perform binary population synthesis to explore the formation of GW190521-like systems via isolated binary evolution. We numerically calculate the binding energy parameter for massive stars at different metallicities, and employ them in our calculation for common envelope evolution. Our results prefer that the progenitor binaries formed in metal-poor environment with $\rm Z\le 0.0016$. The predicted merger rate density within redshift z = 1.1 is ${\sim} 4\times 10^{-5}{\text {--}}5\times 10^{-2} \, \rm Gpc^{-3}yr^{-1}$. We expect that such events are potentially observable by upcoming both space and ground-based gravitational wave detectors.

Exploring binary black hole mergers and host galaxies withsharkand COMPAS

ABSTRACTWe explore the connection between the gravitational wave (GW) merger rates of stellar-mass binary black holes (BBHs) and galaxy properties. We do this by generating populations of stars using the binary population synthesis code COMPAS and evolving them in galaxies from the semi-analytic galaxy formation model Shark, to determine the number of mergers occurring in each simulation time-step. We find that metal-rich and massive galaxies with star formation rate (SFR) greater than $1\, {\rm M}_{\odot }\, \rm yr^{-1}$ are 10 times more likely to have GW events compared to younger, less massive, and metal-poor galaxies. Our simulation with the default input parameters predicts a higher local merger rate density compared to the third GW transient catalogue (GWTC-3) prediction from LIGO, VIRGO, and KAGRA, due to short coalescence times, low metallicities, and a high SFR at low redshift in the simulation, which produces more BBHs that merge within the age of the Universe compared to observations. We identify alternate remnant mass models that more accurately reproduce the volumetric rate and provide updated fits to the merger rate as a function of redshift. We then investigate the relative fraction of GW events in our simulation that are in observable host galaxies from upcoming galaxy surveys, determining which of those are ideal for tracing host galaxies with high merger rates. The implications of this work can be utilized for constraining stellar evolution models, better informing follow-up programs, and placing informative priors on host galaxies when measuring cosmological parameters such as the Hubble constant.

Probing the delay time of supermassive black hole binary mergers with gravitational waves

ABSTRACT Merging supermassive black hole binaries are expected as a consequence of galaxy mergers, yet the detailed evolution path and underlying merging mechanisms of these binaries are still subject to large theoretical uncertainties. In this work, we propose to combine the (future) gravitational wave measurements of supermassive black hole binary merger events with the galaxy merger rate distributions from galaxy surveys/cosmological simulations, to infer the delay time of binary mergers, as a function of binary mass. The delay time encodes key information about binary evolution, which can be used to test the predictions of various evolution models. With a Mock data set of supermassive black hole binary merger events, we discuss how to infer the distribution of delay time with hierarchical Bayesian inference and test evolution models with the Bayesian model selection method. The astrophysical model uncertainties are also considered in the hierarchical Bayesian inference and Bayesian model selection.

Binary boson stars: Merger dynamics and formation of rotating remnant stars

Scalar boson stars have attracted attention as simple models for exploring the nonlinear dynamics of a large class of ultra compact and black hole mimicking objects. Here, we study the impact of interactions in the scalar matter making up these stars. In particular, we show the pivotal role the scalar phase and vortex structure play during the late inspiral, merger, and post-merger oscillations of a binary boson star, as well as their impact on the properties of the merger remnant. To that end, we construct constraint satisfying binary boson star initial data and numerically evolve the nonlinear set of Einstein-Klein-Gordon equations. We demonstrate that the scalar interactions can significantly affect the inspiral gravitational wave amplitude and phase, and the length of a potential hypermassive phase shortly after merger. If a black hole is formed after merger, we find its spin angular momentum to be consistent with similar binary black hole and binary neutron star merger remnants. Furthermore, we formulate a mapping that approximately predicts the remnant properties of any given binary boson star merger. Guided by this mapping, we use numerical evolutions to explicitly demonstrate, for the first time, that rotating boson stars can form as remnants from the merger of two non-spinning boson stars. We characterize this new formation mechanism and discuss its robustness. Finally, we comment on the implications for rotating Proca stars.

Binary black holes population and cosmology in new lights: signature of PISN mass and formation channel in GWTC-3

ABSTRACT The mass, spin, and merger rate distribution of the binary black holes (BBHs) across cosmic redshifts provide a unique way to shed light on their formation channel. Along with the redshift dependence of the BBH merger rate, the mass distribution of BBHs can also exhibit redshift dependence due to different formation channels and dependence on the metallicity of the parent stars. We explore the redshift dependence of the BBH mass distribution jointly with the merger rate evolution from the third gravitational wave (GW) catalogue GWTC-3 of the LIGO–Virgo–KAGRA collaboration. We study possible connections between peak-like features in the mass spectrum of BBHs and processes related to supernovae physics and time delay distributions. We obtain a preference for short-time delays between star formation and BBH mergers. Using a power-law form for the time delay distribution ($(t^{\rm min}_d)^{d}$), we find d < −0.7 credible at 90 per cent interval. The mass distribution of the BBHs could be fitted with a power-law form with a redshift-dependent peak feature that can be linked to the pair instability supernovae (PISN) mass-scale MPISN(Z*) at a stellar metallicity Z*. For a fiducial value of the stellar metallicity Z* = 10−4, we find the $\rm M_{\rm PISN}(Z_*)=44.4^{+7.9}_{-6.3}$$\rm M_\odot$. This is in accordance with the theoretical prediction of the lower edge of the PISN mass-scale and differs from previous analyses. Although we find a strong dependence of the PISN value on metallicity, the model that we explored is not strongly favoured over those that do not account for metallicity as the Bayes factors are inconclusive. In the future with more data, evidence towards metallicity dependence of the PISN will have a significant impact on our understanding of stellar physics.

An autoencoder neural network integrated into gravitational-wave burst searches to improve the rejection of noise transients

The gravitational-wave (GW) detector data are affected by short-lived instrumental or terrestrial transients, called ‘glitches’, which can simulate GW signals. Mitigation of glitches is particularly difficult for algorithms which target generic sources of short-duration GW transients (GWT), and do not rely on GW waveform models to distinguish astrophysical signals from noise, such as coherent WaveBurst (cWB). This work is part of the long-term effort to mitigate transient noises in cWB, which led to the introduction of specific estimators, and a machine-learning based signal-noise classification algorithm. Here, we propose an autoencoder neural network, integrated into cWB, that learns transient noises morphologies from GW time-series. We test its performance on the glitch family known as ‘blip’. The resulting sensitivity to generic GWT and binary black hole mergers significantly improves when tested on LIGO detectors data from the last observation period (O3b). At false alarm rate of one event per 50 years the sensitivity volume increases up to 30% for signal morphologies similar to blip glitches. In perspective, this tool can adapt to classify different transient noise classes that may affect future observing runs, enhancing GWT searches.

Compact object mergers: exploring uncertainties from stellar and binary evolution with sevn

ABSTRACT Population-synthesis codes are an unique tool to explore the parameter space of massive binary star evolution and binary compact object (BCO) formation. Most population-synthesis codes are based on the same stellar evolution model, limiting our ability to explore the main uncertainties. Here, we present the new version of the code sevn, which overcomes this issue by interpolating the main stellar properties from a set of pre-computed evolutionary tracks. We describe the new interpolation and adaptive time-step algorithms of sevn, and the main upgrades on single and binary evolution. With sevn, we evolved 1.2 × 109 binaries in the metallicity range 0.0001 ≤ Z ≤ 0.03, exploring a number of models for electron-capture, core-collapse and pair-instability supernovae, different assumptions for common envelope, stability of mass transfer, quasi-homogeneous evolution, and stellar tides. We find that stellar evolution has a dramatic impact on the formation of single and BCOs. Just by slightly changing the overshooting parameter (λov = 0.4, 0.5) and the pair-instability model, the maximum mass of a black hole can vary from ≈60 to ≈100 M⊙. Furthermore, the formation channels of BCOs and the merger efficiency we obtain with sevn show significant differences with respect to the results of other population-synthesis codes, even when the same binary-evolution parameters are used. For example, the main traditional formation channel of BCOs is strongly suppressed in our models: at high metallicity (Z ≳ 0.01) only <20 per cent of the merging binary black holes and binary neutron stars form via this channel, while other authors found fractions >70 per cent.

Bimodal Black Hole Mass Distribution and Chirp Masses of Binary Black Hole Mergers

In binary black hole mergers from isolated binary-star evolution, both black holes are from progenitor stars that have lost their hydrogen-rich envelopes by binary mass transfer. Envelope stripping is known to affect the pre-supernova core structures of such binary-stripped stars and thereby their final fates and compact remnant masses. In this paper, we show that binary-stripped stars give rise to a bimodal black hole mass spectrum with characteristic black hole masses of about 9 M ⊙ and 16 M ⊙ across a large range of metallicities. The bimodality is linked to carbon and neon burning becoming neutrino dominated, which results in interior structures that are difficult to explode and likely lead to black hole formation. The characteristic black hole masses from binary-stripped stars have corresponding features in the chirp-mass distribution of binary black hole mergers: peaks at about 8 and 14 M ⊙ and a dearth in between these masses. Current gravitational-wave observations of binary black hole mergers show evidence for a gap at 10–12 M ⊙ and peaks at 8 and 14 M ⊙ in the chirp-mass distribution. These features are in agreement with our models of binary-stripped stars. In the future, they may be used to constrain the physics of late stellar evolution and supernova explosions and may even help measure the cosmological expansion of the universe.

Accelerating tests of general relativity with gravitational-wave signals using hybrid sampling

The Advanced LIGO/Virgo interferometers have observed $\ensuremath{\sim}100$ gravitational-wave transients enabling new questions to be answered about relativity, astrophysics, and cosmology. However, many of our current procedures for computing these constraints will not scale well with the increased size of future transient catalogs. We introduce a novel hybrid sampling method in order to more efficiently perform parameterized tests of general relativity with gravitational-wave signals. Applying our method to the binary black hole merger GW150914 and simulated signals we find that our method is approximately an order of magnitude more efficient than the current method with conservative settings for our hybrid analysis. While we have focused on the specific problem of measuring potential deviations from relativity, our method is of much wider applicability to any problem that can be decomposed into a simple and more complex model(s).

Late-forming primordial black holes: Beyond the CMB era

The intermediate mass black hole range, ${10}^{2}\ensuremath{\lesssim}{M}_{\mathrm{BH}}/{M}_{\ensuremath{\bigodot}}\ensuremath{\lesssim}{10}^{5}$, has long offered enticing possibilities for primordial black holes (PBHs), with populations in this range postulated to be responsible for some of the black hole binary merger detected events as well as the existence of supermassive black holes embedded at galactic centers. However, a prominent bound derived from PBH accretion during recombination severely restricts the mass fraction of intermediate mass PBHs. We address this problem by proposing a formation scenario in which ``primordial'' black holes form late in our cosmological history, beyond the cosmic microwave background era, and bypassing this bound. During this crucial epoch, our population of compact objects exist as thermal balls supported by thermal pressure, which eventually cool to Fermi balls supported by degeneracy pressure and finally collapse to PBHs. Our mechanism is a viable production method for both the mass gap LIGO-VIRGO-KAGRA detections and the James Webb Space Telescope observation of an early time $z>10$ supermassive black hole. Furthermore, we present the remarkable possibility of PBH formation after the present era, which we term future PBH. Such a population would evade most, if not all, bounds on the PBH mass spectrum in the literature and open up previously unthought-of possibilities. Light future PBHs could form below the Hawking evaporation threshold and convert the bulk of the matter in the Universe into radiation.

Short Gamma rays bursts from binary black holes merger

In this paper, we introduced a model related to the astronomical events having the co-detection of GW associated with Short Gamma Ray Bursts (SGRBs). Study shows that the existence of magnetized accretion disks is responsible for creating the events of GWs associated with electromagnetic (EM) counterparts from binary BHs mergers. Our model leads from space-time instability to the emission of EM radiations as its counterparts. Starting from Maxwell's stress tensor, we found the condition for the growth of instability that is proportional to the angular velocity and is independent of the magnetic field strength. Considering the event GW150914 with the final product as a Kerr BHs in an equatorial plan, we discussed the motion of particles under an effective potential on circular orbits around it and determined its frequency, redshift factor, and epicyclic frequency. Next, considering the evolution of mass and angular momentum, we calculated the remnant mass, rotational energy, and the magnetic field strength acting along the axis of rotation. The accretion rate and its luminosity are determined by the restoring and shear forces of the magnetic field. The energy extraction efficiency of the flow is determined to be very low. Results show the presence of a weak transient caused by magneto-rotational instability with a strong poloidal magnetic field that causes turbulence in the accretion disk onto black holes.

Cosmological magnetic fields from primordial Kerr-Newman black holes

The origin of our universe's cosmological magnetic fields remains a mystery. In this study, we consider whether these magnetic fields could have been generated in the early universe by a population of charged, spinning primordial black holes. To this end, we calculate the strength and correlation length of the magnetic fields generated by this population, and describe their evolution up to the current epoch. We find that near-extremal black holes in the mass range $ M \sim 10^{28} -10^{36} \, {\rm g}$ could potentially generate magnetic fields with present day values as large as $B \sim 10^{-20} - 10^{-15} \, {\rm G}$; those with $M \gtrsim10^{38} \, \rm{g}$ could have produced even larger fields $B \gtrsim 10^{-14} \, {\rm G}$. To motivate this scenario, we briefly discuss how new physics may have induced a chemical potential which could have briefly maintained the black holes in an electrically charged state in the early universe. Finally, we comment on a correlation between the parameters of the cosmological magnetic field and the stochastic gravitational wave background coming from the merger of primordial black hole binaries as the primary observable signature of this scenario.

Globular cluster formation histories, masses, and radii inferred from gravitational waves

ABSTRACT Globular clusters (GCs) are found in all types of galaxies and harbour some of the most extreme stellar systems, including black holes that may dynamically assemble into merging binary black holes (BBHs). Uncertain GC properties, including when they formed, their initial masses and sizes, affect their production rate of BBH mergers. Using the gravitational-wave transient catalogue (GWTC-3), we measure that dynamically assembled BBHs – those that are consistent with isotropic spin directions – make up ${61^{+29}_{-44}\%}$ of the total merger rate, with a local merger rate of ${10.9^{+16.8}_{-9.3}}$ Gpc−3 yr−1 rising to ${58.9^{+149.4}_{-46.0}}$ Gpc−3 yr−1 at z = 1. We assume that this inferred rate describes the contribution from GCs and compare it against the Cluster Monte Carlo (cmc) simulation catalogue to directly fit for the GC initial mass function, virial radius distribution, and formation history. We find that GC initial masses are consistent with a Schechter function with slope ${\beta _m = -1.9^{+0.8}_{-0.8}}$ . Assuming a mass function slope of βm = −2 and a mass range between 104–$10^8\, \mathrm{ M}_\odot$ , we infer a GC formation rate at z = 2 of ${5.0^{+9.4}_{-4.0}}$ Gpc−3 yr−1, or ${2.1^{+3.9}_{-1.7}}\times 10^6\, \mathrm{ M}_\odot$ Gpc−3 yr−1 in terms of mass density. We find that the GC formation rate probably rises more steeply than the global star formation rate between z = 0 and z = 3 (82 per cent credibility) and implies a local number density that is ${f_\mathrm{ev} = 22.6^{+29.9}_{-16.2}}$ times higher than the observed density of survived GCs. This is consistent with expectations for cluster evaporation, but may suggest that other environments contribute to the rate of BBH mergers with significantly tilted spins.

Reducing the impact of weak-lensing errors on gravitational-wave standard sirens

ABSTRACT The mergers of supermassive black hole binaries can serve as standard sirens: the gravitational-wave (GW) analogue of standard candles. The upcoming space-borne GW detectors will be able to discover such systems and estimate their luminosity distances precisely. Unfortunately, weak gravitational lensing can induce significant errors in the measured distance of these standard sirens at high redshift, severely limiting their usefulness as precise distance probes. The uncertainty due to weak lensing can be reduced if the lensing magnification of the siren can be estimated independently, a procedure called ‘delensing’. With the help of up-to-date numerical simulations, here we investigate how much the weak-lensing errors can be reduced using convergence maps reconstructed from shear measurements. We also evaluate the impact of delensing on cosmological parameter estimation with bright standard sirens. We find that the weak-lensing errors for sirens at zs = 2.9 can be reduced by about a factor of two on average, but to achieve this would require expensive ultra-deep-field observations for every siren. Such an approach is likely to be practical in only limited cases, and the reduction in the weak-lensing error is therefore likely to be insufficient to significantly improve the cosmological parameter estimation. We conclude that performing delensing corrections is unlikely to be worthwhile, in contrast to the more positive expectations presented in previous studies. For delensing to become more practicable and useful in the future will require significant improvements in the resolution/depth of weak-lensing surveys and/or the methods to reconstruct convergence maps from these surveys.

Primordial black holes from D-parity breaking in SO(10) grand unified theory

The growing evidence of gravitational waves from binary black hole mergers has renewed the interest in study of primordial black holes (PBH). Here we study a mechanism for the formation of PBH from collapse of pseudo-topological domain walls which form out of equilibrium during inflation and then collapse post inflation. We apply the study to domain wall formation due to D-parity embedded in a supersymmetric grand unified theory (GUT) based on SO(10) and compare the abundance of resulting PBH with the existing constraints. Thus the macroscopic relics can then be used to constrain or rule out a GUT, or demand a refinement of the theory of PBH formation in this class of GUTs.

The Locations of Features in the Mass Distribution of Merging Binary Black Holes Are Robust against Uncertainties in the Metallicity-dependent Cosmic Star Formation History

New observational facilities are probing astrophysical transients such as stellar explosions and gravitational-wave sources at ever-increasing redshifts, while also revealing new features in source property distributions. To interpret these observations, we need to compare them to predictions from stellar population models. Such models require the metallicity-dependent cosmic star formation history () as an input. Large uncertainties remain in the shape and evolution of this function. In this work, we propose a simple analytical function for . Variations of this function can be easily interpreted because the parameters link to its shape in an intuitive way. We fit our analytical function to the star-forming gas of the cosmological TNG100 simulation and find that it is able to capture the main behavior well. As an example application, we investigate the effect of systematic variations in the parameters on the predicted mass distribution of locally merging binary black holes. Our main findings are that (i) the locations of features are remarkably robust against variations in the metallicity-dependent cosmic star formation history, and (ii) the low-mass end is least affected by these variations. This is promising as it increases our chances of constraining the physics that govern the formation of these objects (https://github.com/LiekeVanSon/SFRD_fit/tree/7348a1ad0d2ed6b78c70d5100fb3cd2515493f02/).

Gravitational waves in Einstein–Cartan theory: On the effects of dark matter spin tensor

This article studies the effects of an arbitrary dark matter spin tensor on the propagation of gravitational wave amplitude in the context of Einstein–Cartan theory. We choose to work with an arbitrary spin tensor because, given our ignorance of the nature of dark matter, it is sensible not to make further hypotheses on its spin and not to assume any particular model for its spin tensor (or its vanishing). The analysis focuses on a “weak-torsion regime”, such that gravitational wave emission, at leading and subleading orders, does not deviate from standard General Relativity. We show that, in principle, the background torsion induced by an eventual dark matter spin component could lead to an anomalous dampening or amplification of the gravitational wave amplitude, after going across a long cosmological distance. We assess the importance of this torsion-induced anomalous amplitude propagation for binary black hole mergers in a way as model-free as possible in Einstein–Cartan gravity. It is possible to prove that at its best, for realistic late-universe cosmological scenarios, the effect is tiny and falls below detection thresholds, even for near-future interferometers such as LISA. Therefore, detecting this effect may not be impossible, but it is still beyond our technological capabilities. As a model-independent result in the Einstein–Cartan context, it also implies that mergers are robust standard sirens without considering any potential dark-matter-induced torsional effects.