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The final state of a binary black hole merger is predicted with high precision by numerical relativity, but could there be a simple thermodynamic principle within general relativity that governs the selection of the remnant? Using post-Newtonian relations between the mass 𝑀 (including the binding energy) and angular momentum 𝐽 of quasicircular, nonspinning binaries, we uncover a puzzling result: When the binary’s instantaneous 𝑀 and 𝐽 are mapped to those of a hypothetical Kerr black hole, the corresponding entropy exhibits a maximum during the evolution. This maximum occurs at values of 𝑀 and 𝐽, strikingly close to those of the final remnant predicted by numerical relativity. Consistent behavior is observed when using the relation between 𝑀 and 𝐽 obtained from numerical relativity evolution. Although this procedure is somewhat ad hoc, the agreement between the masses and spins of the final state obtained from numerical relativity and the results of this maximum entropy procedure is remarkable, with agreement to within a few percent when using either post-Newtonian or numerical relativity results for 𝑀 and 𝐽. These findings allow us to propose an entropy maximization conjecture for binary black hole mergers, hinting that thermodynamic principles may govern the selection of the final black hole state. 

Abstract We implement frequency-dependent antenna responses and develop likelihood classes (standard likelihood, multibanded likelihood, and the relative binning (RB) likelihood) capable of handling the same within the framework ofBilby. We validate the approximate likelihoods by comparing them with the exact likelihood for a GW170817-like signal (luminosity distance scaled such that signal-to-noise ratio ∼1000) containing higher-order modes of radiation. We use the RB likelihood to perform parameter estimation (PE) for a GW170817-like signal, including Earth-rotation effects, detector-size effects, and higher-order modes. To the best of our knowledge, we are the first to perform proof-of-concept Bayesian PE, including all the above-mentioned effects. We study the system in several detector networks consisting of a single 40 km Cosmic Explorer, a 20 km CE, and a present-generation detector at A+ sensitivity. The PE runs with RB take around a day to complete on a typical cluster. We find that higher-order modes significantly improve sky localization in scenarios where only a single next-generation detector is operating. This advantage diminishes when a second detector is added to the network; however, in such cases, we observe improved recovery of the polarization angle. While ignoring higher-order modes leads to a reduction in precision, it does not introduce bias in parameter recovery. In contrast, neglecting the effects of Earth’s rotation and the finite size of long-arm detectors can result in biased parameter estimates. 

Abstract We examine the role of LIGO-India in facilitating multimessenger astronomy in the era of next-generation observatories. A network with two L-shaped Cosmic Explorer (CE) detectors and one triangular Einstein Telescope (ET) would precisely localize nearly the entire annual binary neutron star (NS) merger population up to a redshift of 0.5—over 10,000 events would be localized within 10 deg², including approximately 150 events within 0.1 deg². Luminosity distance would be measured to within 10% for over 9000 events and within 1% for ∼100 events. Surprisingly, replacing the 20 km CE detector with LIGO-India operating at A^♯sensitivity (I^♯) yields a nearly identical performance. The factor-of-5 shorter arms are offset by a fourfold increase in baseline relative to a second CE in the US, preserving localization accuracy, with over 9000 events within 10 deg²and ∼90 events within 0.1 deg². This configuration detects ∼6000 events with luminosity distance uncertainties under 10%, including ∼50 with under 1%. Both networks provide $\mathcal{O}\left(100\right)$ early-warning detections up to 10 minutes before merger, with localization areas ≤10 deg². WhileI^♯enables excellent localization and early warnings, its shorter arms and narrower sensitivity band would limit its reach for other science goals, such as detecting Population III binary black hole mergers atz≳ 10, NS mergers atz∼ 2, or constraining cosmological parameters. 

Several theoretical proposals describe horizonless compact objects that can mimic black holes in their gravitational wave signatures; however, their spin-induced quadrupole moments (SIQMs) may reveal their distinct nature. Using the tight bounds on the SIQM of GW241011, we place constraints on the nature of its primary. Across exotic compact object models considered in this Letter, we find that rotating boson stars with quartic self-interactions cannot explain the nature of the primary, whereas models of sufficiently large compactness, $C\gtrsim 0.24$ , may still be viable contenders. 

Abstract Gravitational waves (GWs) from compact binary coalescences arestandard sirensthat provide a direct measure of the source’s luminosity distance, enabling an independent measurement of the Hubble constant (H₀). While abright siren—a GW event with an identified electromagnetic (EM) counterpart—provided the first such constraint, most detections, currently dominated by black hole mergers, lack EM signatures. A measurement ofH₀is still possible with thesedark sirensby statistically associating GW events with galaxies in existing catalogs based on the sky localization. In this work, we explore the potential of two subsets of dark sirens categorized by their localization precision: ’golden’ dark sirens, defined by a sky area localization ≤0.1deg², and ’silver’ dark sirens, which are more common but less precisely localized (≤1deg²). Using thefifth internal data releaseof the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), we assess the suitability of the Visible Integral-field Replicable Unit Spectrograph (VIRUS) for spectroscopic follow-up of dark sirens. VIRUS exposures of the standard HETDEX depth provide precise redshifts and exquisite completeness withinz= 0.2. After a single year of observations with the upgraded LIGO-A^#network, the combined sample of golden and silver dark sirens withz≤ 0.2 atH₀ = 70 km s⁻¹Mpc⁻¹and follow-up VIRUS observations can potentially yield a few-percent constraint onH₀. Our predictions suggest that spectroscopic redshift surveys such as HETDEX can play a key role in realizing high-precision cosmology with dark sirens in the near future. 

Abstract The precise measurement of neutron star (NS) spins can provide important insight into the formation and evolution of compact binaries containing NSs. While traditional methods of NS spin measurement rely on pulsar observations, gravitational-wave detections offer a complementary avenue. However, determining component spins with gravitational waves is hindered by the small dimensionless spins of the NSs and the degeneracy in the mass and spin parameters. This degeneracy can be addressed by the inclusion of higher-order modes in the waveform, which are important for systems with unequal masses. This study shows the suitability of NS–black hole mergers, which are naturally mass-asymmetric, for precise NS spin measurements. We explore the effects of the black hole masses and spins, higher-mode content, inclination angles, and detector sensitivity on the measurement of NS spin. We find that networks with next-generation observatories like the Cosmic Explorer and the Einstein Telescope can distinguish NS dimensionless spin of 0.04 (0.1) from zero at 1σconfidence for events within ∼350 (∼1000) Mpc. Networks with A+ and A^♯detectors achieve similar distinction within ∼30 (∼70) Mpc and ∼50 (∼110) Mpc, respectively.