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1. Study of the intermediate mass ratio black hole binary merger up to 1000:1 with numerical relativity

   https://doi.org/10.1088/1361-6382/acc7ef  

   Lousto, Carlos O; Healy, James (April 2023, Classical and Quantum Gravity)

   Abstract We explicitly demonstrate that current numerical relativity techniques are able to accurately evolve black hole binaries with mass ratios of the order of 1000:1. This proof of principle is relevant for future third generation gravitational wave detectors and space mission LISA, as by purely numerical methods we would be able to accurately compute gravitational waves from the last stages of black hole mergers, as directly predicted by general relativity. We perform a sequence of simulations in the intermediate to small mass ratio regime, m 1 p / m 2 p = 1 / 7 , 1 / 16 , 1 / 32 , 1 / 64 , 1 / 128 , 1 / 256 , 1 / 512 , 1 / 1024 , with the small hole starting from rest at a proper distance D ≈ 13 M . We compare these headon full numerical evolutions with the corresponding semianalytic point particle perturbative results finding an impressive agreement for the total gravitational radiated energy and linear momentum as well as for the waveform spectra. We display numerical convergence of the results and identify the minimal numerical resolutions required to accurately solve for these very low amplitude gravitational waves. This work represents a first step towards the considerable challenge of applying numerical-relativity waveforms to interpreting gravitational-wave observations by LISA and next-generation ground-based gravitational-wave detectors. 

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2. Gravitational Waves from Black Hole Emission

   https://doi.org/10.3390/universe11010021  

   Islam, Tousif; Khanna, Gaurav; Liebling, Steven L (January 2025, Universe)

   Using adiabatic point-particle black hole perturbation theory, we simulate plausible gravitational wave (GW) signatures in two exotic scenarios (i) where a small black hole is emitted by a larger one (‘black hole emission’) and (ii) where a small black hole is emitted by a larger one and subsequently absorbed back (‘black hole absorption’). While such scenarios are forbidden in general relativity (GR), alternative theories (such as certain quantum gravity scenarios obeying the weak gravity conjecture, white holes, and Hawking radiation) may allow them. By leveraging the phenomenology of black hole emission and absorption signals, we introduce straightforward modifications to existing gravitational waveform models to mimic gravitational radiation associated with these exotic events. We anticipate that these (incomplete but) initial simulations, coupled with the adjusted waveform models, will aid in the development of null tests for GR using GWs. 

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3. Effect of ignoring eccentricity in testing general relativity with gravitational waves

   https://doi.org/10.1103/PhysRevD.108.064003  

   Narayan, Purnima; Johnson-McDaniel, Nathan K.; Gupta, Anuradha (September 2023, Physical Review D)

   Detections of gravitational waves emitted from binary black hole coalescences allow us to probe the strong-field dynamics of general relativity (GR). One can compare the observed gravitational-wave signals with theoretical waveform models to constrain possible deviations from GR. Any physics that is not included in these waveform models might show up as apparent GR deviations. The waveform models used in current tests of GR describe binaries on quasicircular orbits, since most of the binaries detected by ground-based gravitational-wave detectors are expected to have negligible eccentricities. Thus, a signal from an eccentric binary in GR is likely to show up as a deviation from GR in the current implementation of these tests. We study the response of four standard tests of GR to eccentric binary black hole signals with the forecast O4 sensitivity of the LIGO-Virgo network. Specifically, we consider two parametrized tests (TIGER and FTI), the modified dispersion relation test, and the inspiral-merger-ringdown consistency test. To model eccentric signals, we use nonspinning numerical relativity simulations from the SXS catalog with three mass ratios (1, 2, 3), which we scale to a redshifted total mass of 80M⊙ and luminosity distance of 400 Mpc. For each of these mass ratios, we consider signals with eccentricities of ∼0.05 and ∼0.1 at 17 Hz. We find that signals with larger eccentricity lead to very significant false GR deviations in most tests while signals having smaller eccentricity lead to significant deviations in some tests. For the larger eccentricity cases, one would even get a deviation from GR with TIGER at ∼90% credibility at a distance of ≳1.5 Gpc. Thus, it will be necessary to exclude the possibility of an eccentric binary in order to make any claim about detecting a deviation from GR. 

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4. Perturbations of spinning black holes in dynamical Chern-Simons gravity: Slow rotation equations

   https://doi.org/10.1103/PhysRevD.109.104029  

   Wagle, Pratik; Li, Dongjun; Chen, Yanbei; Yunes, Nicolás (May 2024, Physical Review D)

   The detection of gravitational waves resulting from the coalescence of binary black holes by the LIGO-Virgo-Kagra Collaboration has inaugurated a new era in gravitational physics. These gravitational waves provide a unique opportunity to test Einstein’s general relativity and its modifications in the regime of extreme gravity. A significant aspect of such tests involves the study of the ringdown phase of gravitational waves from binary black hole coalescence, which can be decomposed into a superposition of various quasinormal modes. In general relativity, the spectra of quasinormal modes depend on the mass, spin, and charge of the final black hole, but they can also be influenced by additional properties of the black hole spacetime, as well as corrections to the general theory of relativity. In this work, we focus on a specific modified theory known as dynamical Chern-Simons gravity. We employ the modified Teukolsky formalism developed in a previous study and lay down the foundations to investigate perturbations of slowly rotating black holes admitted by the theory. Specifically, we derive the master equations for the ${\mathrm{\Psi }}_0$and ${\mathrm{\Psi }}_4$Weyl scalar perturbations that characterize the radiative part of gravitational perturbations, as well as the master equation for the scalar field perturbations. We employ metric reconstruction techniques to obtain explicit expressions for all relevant quantities. Finally, by leveraging the properties of spin-weighted spheroidal harmonics to eliminate the angular dependence from the evolution equations, we derive two, radial, second-order, ordinary differential equations for ${\mathrm{\Psi }}_0$and ${\mathrm{\Psi }}_4$, respectively. These two equations are coupled to another radial, second-order, ordinary differential equation for the scalar field perturbations. This work is the first attempt to derive a master equation for black holes in dynamical Chern-Simons gravity using curvature perturbations. The master equations we obtain can then be numerically integrated to obtain the quasinormal mode spectrum of slowly rotating black holes in this theory, making progress in the study of ringdown in dynamical Chern-Simons gravity. Published by the American Physical Society2024 

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5. The missing link in gravitational-wave astronomy: A summary of discoveries waiting in the decihertz range

   https://doi.org/10.1007/s10686-021-09713-z  

   Sedda, Manuel Arca; Berry, Christopher P.; Jani, Karan; Amaro-Seoane, Pau; Auclair, Pierre; Baird, Jonathon; Baker, Tessa; Berti, Emanuele; Breivik, Katelyn; Caprini, Chiara; et al (June 2021, Experimental Astronomy)

   Abstract Since 2015 the gravitational-wave observations of LIGO and Virgo have transformed our understanding of compact-object binaries. In the years to come, ground-based gravitational-wave observatories such as LIGO, Virgo, and their successors will increase in sensitivity, discovering thousands of stellar-mass binaries. In the 2030s, the space-based LISA will provide gravitational-wave observations of massive black holes binaries. Between the $$\sim 10$$ ∼ 10 –10 3 Hz band of ground-based observatories and the $$\sim 10^{-4}$$ ∼ 1 0 − 4 –10 − 1 Hz band of LISA lies the uncharted decihertz gravitational-wave band. We propose a Decihertz Observatory to study this frequency range, and to complement observations made by other detectors. Decihertz observatories are well suited to observation of intermediate-mass ( $$\sim 10^{2}$$ ∼ 1 0 2 –10 4 M ⊙ ) black holes; they will be able to detect stellar-mass binaries days to years before they merge, providing early warning of nearby binary neutron star mergers and measurements of the eccentricity of binary black holes, and they will enable new tests of general relativity and the Standard Model of particle physics. Here we summarise how a Decihertz Observatory could provide unique insights into how black holes form and evolve across cosmic time, improve prospects for both multimessenger astronomy and multiband gravitational-wave astronomy, and enable new probes of gravity, particle physics and cosmology. 

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