Multiparameter tests of general relativity using multiband gravitational-wave observations

In this Letter we show that multiband observations of stellar-mass binary black holes by the next generation of ground-based observatories (3G) and the space-based Laser Interferometer Space Antenna (LISA) would facilitate a comprehensive test of general relativity by simultaneously measuring all the post-Newtonian (PN) coefficients. Multiband observations would measure most of the known PN phasing coefficients to an accuracy below a few percent---two orders-of-magnitude better than the best bounds achievable from even `golden' binaries in the 3G or LISA bands. Such multiparameter bounds would play a pivotal role in constraining the parameter space of modified theories of gravity beyond general relativity.Comment: 7 pages, 4 figures. v3: version published in PR

Archival searches for stellar-mass binary black holes in LISA

Stellar-mass binary black holes will sweep through the frequency band of the Laser Interferometer Space Antenna (LISA) for months to years before appearing in the audio-band of ground-based gravitational-wave detectors. One can expect several tens of these events up to a distance of $500 \,\mathrm{Mpc}$ each year. The LISA signal-to-noise ratio for such sources even at these close distances will be too small for a blind search to confidently detect them. However, next generation ground-based gravitational-wave detectors, expected to be operational at the time of LISA, will observe them with signal-to-noise ratios of several thousands and measure their parameters very accurately. We show that such high fidelity observations of these sources by ground-based detectors help in archival searches to dig tens of signals out of LISA data each year.Comment: 12 pages, 7 figure

Comparison of post-Newtonian mode amplitudes with numerical relativity simulations of binary black holes

Gravitational waves from the coalescence of two black holes carry the signature of the strong field dynamics of binary black holes. In this work we have used numerical relativity simulations and post-Newtonian theory to investigate this dynamics. Post-Newtonian theory is a low-velocity expansion that assumes the companion bodies to be point-particles, while numerical relativity treats black holes as extended objects with horizons and fully captures their dynamics. There is a priori no reason for the waveforms computed using these disparate methods to agree with each other, especially at late times when the black holes move close to the speed of light. We find, remarkably, that the leading order amplitudes in post-Newtonian theory agree well with the full general relativity solution for a large set of spherical harmonic modes, even in the most dynamical part of the binary evolution, with only some modes showing distinctly different behavior than that found by numerical relativity simulations. In particular, modes with spherical harmonic indices l = m as well as l = 2, m = 1 are least modified from their dominant post-Newtonian behavior. Understanding the nature of these modes in terms of the post-Newtonian description will aid in formulating better models of the emitted waveforms in the strong field regime of the dynamics.Comment: 17 pages, 5 figure

The Accuracy of Neutron Star Radius Measurement with the Next Generation of Terrestrial Gravitational-Wave Observatories

In this paper, we explore the prospect for improving the measurement accuracy of masses and radii of neutron stars. We consider imminent and long-term upgrades of the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo, as well as next-generation observatories -- the Cosmic Explorer and Einstein Telescope. We find that neutron star radius with single events will be constrained to within roughly 500m with the current generation of detectors and their upgrades. This will improve to 200m, 100m and 50m with a network of observatories that contain one, two or three next-generation observatories, respectively. Combining events in bins of 0.05 solar masses we find that for stiffer (softer) equations-of-state like ALF2 (APR4), a network of three XG observatories will determine the radius to within 30m (100m) over the entire mass range of neutron stars from 1 to 2.0 solar masses (2.2 solar masses), allowed by the respective equations-of-state. Neutron star masses will be measured to within 0.5 percent with three XG observatories irrespective of the actual equation-of-state. Measurement accuracies will be a factor of 4 or 2 worse if the network contains only one or two XG observatories, respectively, and a factor of 10 worse in the case of networks consisting of Advanced LIGO, Virgo KAGRA and their upgrades. Tens to hundreds of high-fidelity events detected by future observatories will allow us to accurately measure the mass-radius curve and hence determine the dense matter equation-of-state to exquisite precision

Neutron star-black hole mergers in next generation gravitational-wave observatories

Observations by the current generation of gravitational-wave detectors have been pivotal in expanding our understanding of the universe. Although tens of exciting compact binary mergers have been observed, neutron star-black hole (NSBH) mergers remained elusive until they were first confidently detected in 2020. The number of NSBH detections is expected to increase with sensitivity improvements of the current detectors and the proposed construction of new observatories over the next decade. In this work, we explore the NSBH detection and measurement capabilities of these upgraded detectors and new observatories using the following metrics: network detection efficiency and detection rate as a function of redshift, distributions of the signal-to-noise ratios, the measurement accuracy of intrinsic and extrinsic parameters, the accuracy of sky position measurement, and the number of early-warning alerts that can be sent to facilitate the electromagnetic follow-up. Additionally, we evaluate the prospects of performing multi-messenger observations of NSBH systems by reporting the number of expected kilonova detections with the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope. We find that as many as $\mathcal{O}(10)$ kilonovae can be detected by these two telescopes every year, depending on the population of the NSBH systems and the equation of state of neutron stars.Comment: 30 pages, 15 figure

Dark sirens to resolve the Hubble-Lemaître tension

The planned sensitivity upgrades to the LIGO and Virgo facilities could uniquely identify host galaxies of dark sirens—compact binary coalescences without any electromagnetic counterparts—within a redshift of z = 0.1. This is aided by the higher-order spherical harmonic modes present in the gravitational-wave signal, which also improve distance estimation. In conjunction, sensitivity upgrades and higher modes will facilitate an accurate, independent measurement of the host galaxy's redshift in addition to the luminosity distance from the gravitational-wave observation to infer the Hubble–Lemaître constant H 0 to better than a few percent in 5 yr. A possible Voyager upgrade or third-generation facilities would further solidify the role of dark sirens for precision cosmology in the future

Measuring properties of primordial black hole mergers at cosmological distances: effect of higher order modes in gravitational waves

Primordial black holes (PBHs) may form from the collapse of matter overdensities shortly after the Big Bang. One may identify their existence by observing gravitational wave (GW) emissions from merging PBH binaries at high redshifts $z\gtrsim 30$, where astrophysical binary black holes (BBHs) are unlikely to merge. The next-generation ground-based GW detectors, Cosmic Explorer and Einstein Telescope, will be able to observe BBHs with total masses of $\mathcal{O}(10-100)~M_{\odot}$ at such redshifts. This paper serves as a companion paper of arXiv:2108.07276, focusing on the effect of higher-order modes (HoMs) in the waveform modeling, which may be detectable for these high redshift BBHs, on the estimation of source parameters. We perform Bayesian parameter estimation to obtain the measurement uncertainties with and without HoM modeling in the waveform for sources with different total masses, mass ratios, orbital inclinations and redshifts observed by a network of next-generation GW detectors. We show that including HoMs in the waveform model reduces the uncertainties of redshifts and masses by up to a factor of two, depending on the exact source parameters. We then discuss the implications for identifying PBHs with the improved single-event measurements, and expand the investigation of the model dependence of the relative abundance between the BBH mergers originating from the first stars and the primordial BBH mergers as shown in arXiv:2108.07276.Comment: 11 pages, 11 figure

On the single-event-based identification of primordial black hole mergers at cosmological distances

The existence of primordial black holes (PBHs), which may form from the collapse of matter overdensities shortly after the Big Bang, is still under debate. Among the potential signatures of PBHs are gravitational waves (GWs) emitted from binary black hole (BBH) mergers at redshifts z ≳ 30, where the formation of astrophysical black holes is unlikely. Future ground-based GW detectors, the Cosmic Explorer and Einstein Telescope, will be able to observe equal-mass BBH mergers with total mass of (10–100)M⊙ at such distances. In this work, we investigate whether the redshift measurement of a single BBH source can be precise enough to establish its primordial origin. We simulate BBHs of different masses, mass ratios and orbital orientations. We show that for BBHs with total masses between 20 M ⊙ and 40 M ⊙ merging at z ≥ 40, one can infer z > 30 at up to 97% credibility, with a network of one Einstein Telescope, one 40 km Cosmic Explorer in the US, and one 20 km Cosmic Explorer in Australia. This number reduces to 94% with a smaller network made of one Einstein Telescope and one 40 km Cosmic Explorer in the US. We also analyze how the measurement depends on the Bayesian priors used in the analysis and verify that priors that strongly favor the wrong model yield smaller Bayesian evidences

A Horizon Study for Cosmic Explorer: Science, Observatories, and Community

Gravitational-wave astronomy has revolutionized humanity's view of the universe. Investment in the field has rewarded the scientific community with the first direct detection of a binary black hole merger and the multimessenger observation of a neutron-star merger. Each of these was a watershed moment in astronomy, made possible because gravitational waves reveal the cosmos in a way that no other probe can. Since the first detection of gravitational waves in 2015, the National Science Foundation's LIGO and its partner observatory, the European Union's Virgo, have detected over fifty binary black hole mergers and a second neutron star merger -- a rate of discovery that has amazed even the most optimistic scientists.This Horizon Study describes a next-generation ground-based gravitational-wave observatory: Cosmic Explorer. With ten times the sensitivity of Advanced LIGO, Cosmic Explorer will push the gravitational-wave astronomy towards the edge of the observable universe ($z \sim 100$). This Horizon Study presents the science objective for Cosmic Explorer, and describes and evaluates its design concepts for. Cosmic Explorer will continue the United States' leadership in gravitational-wave astronomy in the international effort to build a "Third-Generation" (3G) observatory network that will make discoveries transformative across astronomy, physics, and cosmology