Detection of primordial gravitational-wave backgrounds generated during the early Universe phase transitions is a key science goal for future ground-based detectors. The rate of compact binary ...mergers is so large that their cosmological population produces a confusion background that could masquerade the detection of potential primordial stochastic backgrounds. In this paper, we study the ability of current and future detectors to resolve the confusion background to reveal interesting primordial backgrounds. The current detector network of LIGO and Virgo and the upcoming KAGRA and LIGO-India will not be able to resolve the cosmological compact binary source population, and its sensitivity to stochastic background will be limited by the confusion background of these sources. We find that a network of three (and five) third generation (3G) detectors of Cosmic Explorer and Einstein Telescope will resolve the confusion background produced by binary black holes leaving only about 1.3% (respectively, 0.075%) unresolved; in contrast, as many as 25% (respectively, 7.7%) of binary neutron star sources remain unresolved. Consequently, the binary black hole population will likely not limit observation of primordial backgrounds, but the binary neutron star population will limit the sensitivity of 3G detectors to ΩGW ∼ 10−11 at 10 Hz (respectively, ΩGW ∼ 3 × 10−12).
We show how gravitational-wave observations with advanced detectors of tens to several tens of neutron-star binaries can measure the neutron-star radius with an accuracy of several to a few percent, ...for mass and spatial distributions that are realistic, and with none of the sources located within 100 Mpc. We achieve such an accuracy by combining measurements of the total mass from the inspiral phase with those of the compactness from the postmerger oscillation frequencies. For estimating the measurement errors of these frequencies, we utilize analytical fits to postmerger numerical relativity waveforms in the time domain, obtained here for the first time, for four nuclear-physics equations of state and a couple of values for the mass. We further exploit quasiuniversal relations to derive errors in compactness from those frequencies. Measuring the average radius to well within 10% is possible for a sample of 100 binaries distributed uniformly in volume between 100 and 300 Mpc, so long as the equation of state is not too soft or the binaries are not too heavy. We also give error estimates for the Einstein Telescope.
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 coefficients. Multiband observations would measure most of the known post-Newtonian phasing coefficients to an accuracy below a few percent-2 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.
We propose a novel method to test the consistency of the multipole moments of compact binary systems with the predictions of general relativity (GR). The multipole moments of a compact binary system, ...known in terms of symmetric and trace-free tensors, are used to calculate the gravitational waveforms from compact binaries within the post-Newtonian (PN) formalism. For nonspinning compact binaries, we derive the gravitational wave phasing formula, in the frequency domain, parametrizing each PN order term in terms of the multipole moments which contribute to that order. Using GW observations, this parametrized multipolar phasing would allow us to derive the bounds on possible departures from the multipole structure of GR and hence constrain the parameter space of alternative theories of gravity. We compute the projected accuracies with which the second-generation ground-based detectors, such as the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), the third-generation detectors such as the Einstein Telescope and Cosmic Explorer, as well as the space-based detector Laser Interferometer Space Antenna (LISA) will be able to measure these multipole parameters. We find that while Advanced LIGO can measure the first two or three multipole coefficients with good accuracy, Cosmic Explorer and the Einstein Telescope may be able to measure the first four multipole coefficients which enter the phasing formula. Intermediate-mass-ratio inspirals, with mass ratios of several tens, in the frequency band of the planned space-based LISA mission should be able to measure all seven multipole coefficients which appear in the 3.5PN phasing formula. Our finding highlights the importance of this class of sources for probing the strong-field gravity regime. The proposed test will facilitate the first probe of the multipolar structure of Einstein’s general relativity.
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 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.
The Einstein Telescope is a conceived third-generation gravitational-wave detector that is envisioned to be an order of magnitude more sensitive than advanced LIGO, Virgo, and Kagra, which would be ...able to detect gravitational-wave signals from the coalescence of compact objects with waveforms starting as low as 1 Hz. With this level of sensitivity, we expect to detect sources at cosmological distances. In this paper we introduce an improved method for the generation of mock data and analyze it with a new low-latency compact binary search pipeline called gstlal. We present the results from this analysis with a focus on low-frequency analysis of binary neutron stars. Despite compact binary coalescence signals lasting hours in the Einstein Telescope sensitivity band when starting at 5 Hz, we show that we are able to discern various overlapping signals from one another. We also determine the detection efficiency for each of the analysis runs conducted and show a proof of concept method for estimating the number signals as a function of redshift. Finally, we show that our ability to recover the signal parameters has improved by an order of magnitude when compared to the results of the first mock data and science challenge. For binary neutron stars we are able to recover the total mass and chirp mass to within 0.5% and 0.05%, respectively.
Observation of gravitational waves (GWs) in two different frequency bands is referred to as multiband GW astronomy. With the planned Laser Interferometric Space Antenna (LISA) operating in the ...10−4–0.1 Hz range, and third-generation (3G) ground-based detectors such as the Cosmic Explorer (CE) and Einstein Telescope (ET) operating in the 1–104 Hz range, multiband GW astronomy could be a reality in the coming decades. In this paper, we present the potential of multiband observations of intermediate-mass binary black holes (IMBBHs) of component masses ∼102–103 M⊙ to test general relativity (GR). We show that mutiband observations of IMBBHs would permit multiparameter tests of GR-tests where more than one post-Newtonian (PN) coefficient is simultaneously measured-yielding more rigorous constraints on possible modifications to GR. We also find that the improvement due to multibanding can often be much larger than the best of the bounds from either of the two observatories. The origin of this result, as we shall demonstrate, can be traced to the lifting of degeneracies among the various parameters when the information from LISA and 3G is taken together. A binary of redshifted total mass of 200 M⊙ gives the best bounds. Such a system at 1 Gpc and mass ratio m1/m2=2 would bound the deviations on all the PN coefficients to below 10% when they are measured individually, and additionally place simultaneous bounds on the first seven PN coefficients to below 50%.
Binary neutron-star mergers will predominantly produce black-hole remnants of mass ∼ 3 – 4 M ⊙ , thus populating the putative low-mass gap between neutron stars and stellar-mass black holes. If these ...low-mass black holes are in dense astrophysical environments, mass segregation could lead to "second-generation" compact binaries merging within a Hubble time. In this paper, we investigate possible signatures of such low-mass compact binary mergers in gravitational-wave observations. We show that this unique population of objects, if present, will be uncovered by the third-generation gravitational-wave detectors, such as Cosmic Explorer and Einstein Telescope. Future joint measurements of chirp mass M and effective spin χeff could clarify the formation scenario of compact objects in the low-mass gap. As a case study, we show that the recent detection of GW190425 (along with GW170817) favors a double Gaussian mass model for neutron stars, under the assumption that the primary in GW190425 is a black hole formed from a previous binary neutron-star merger.
Gravitational-wave (GW) observations of binary black holes offer the best probes of the relativistic, strong-field regime of gravity. Gravitational radiation in the leading order is quadrupolar. ...However, nonquadrupole (higher order) modes make appreciable contribution to the radiation from binary black holes with large mass ratios and misaligned spins. The multipolar structure of the radiation is fully determined by the intrinsic parameters (masses and spin angular momenta of the companion black holes) of a binary in quasicircular orbit. Following our previous work S. Dhanpal, A. Ghosh, A. K. Mehta, P. Ajith, and B. S. Sathyaprakash, Phys. Rev. D 99, 104056 (2019)., we develop multiple ways of testing the consistency of the observed GW signal with the expected multipolar structure of radiation from binary black holes in general relativity. We call this a no-hair test of binary black holes as this is similar to testing the no-hair theorem for isolated black holes through mutual consistency of the quasinormal mode spectrum. We use Bayesian inference on simulated GW signals that are consistent/inconsistent with binary black holes in general relativity to demonstrate the power of the proposed tests. We also make estimate systematic errors arising as a result of neglecting companion spins.
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