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Physics General Relativity and Quantum Cosmology

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General Relativity and Quantum Cosmology

Constraining the Progenitor Orbits of Sequential Black-Hole Mergers in Hierarchical Triples: Peters-Based Estimates

Felix M. Heinze and Zhoujian Cao

Two black-hole merger events may originate from sequential mergers in a pre-existing hierarchical black-hole triple if their spatial localizations overlap and the remnant mass of the first merger is compatible with a component mass of the second. In this scenario, the inner binary merges first, leaving a remnant that remains bound to the tertiary and subsequently undergoes a second merger. We present a method to constrain the progenitor triple's outer orbit by modeling the gravitational-wave-driven evolution of the remnant-tertiary binary using Peters' equations. Given an assumed upper bound on the recoil velocity of the first merger, the method requires only the black-hole masses, the time delay between the mergers, and the eccentricity at a reference frequency - quantities that can be inferred from the merger events themselves. We investigate the potential constraining power by constructing a constraint atlas spanning a range of system properties and identify the conditions under which the method places tight and reliable bounds on the progenitor triple's outer orbital parameters. By imposing an upper bound on the detector-band eccentricity, we also obtain meaningful constraints for candidate hierarchical mergers proposed in the literature, including GW190514-GW190521. We provide numerical tools for applying the method to user-specified systems and evaluating the reliability of the resulting Peters-based constraints. This framework offers a computationally inexpensive first step toward reconstructing hierarchical-merger progenitors and provides a foundation for extensions incorporating more sophisticated relativistic treatments.
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General Relativity and Quantum Cosmology · Cross-listed from hep-th

Post-Newtonian theory in de Sitter and anti-de Sitter spacetimes

R. Loganayagam and Avi Wadhwa

We develop the post-Newtonian framework for the near-zone dynamics of a binary in de Sitter and anti-de Sitter spacetime. We consider a binary interacting via a free scalar field in a fixed (A)dS background in $D+1$ spacetime dimensions, with $D\ge3$ and odd. Using multipole expansion and re-summation techniques, we solve for the field degrees of freedom to obtain an effective particle description in the form of a Fokker action. We derive the equations of motion for the binary up to second post-Newtonian (2PN) order and obtain the corresponding 2PN Hamiltonian and conserved charges in phase space. We show that the conformal algebra, corresponding to the isometry algebra of the (A)dS background, is realised in phase space as a curvature-dependent deformation of the Poincare algebra at each PN order. Our results provide a systematic extension of post-Newtonian effective dynamics to finite-curvature backgrounds. This could serve as a useful framework for studying bound-state dynamics in (A)dS as well as in the corresponding holographic CFT.
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General Relativity and Quantum Cosmology · Cross-listed from astro-ph.GA

Binary-lens amplification of starlight: from magnification flares to quasi-periodic tracking of orbital evolution

Hanxi Wang, Bence Kocsis and Miguel Zumalacárregui

Supermassive black hole binaries (SMBHB) are expected at the center of galaxies as a result of galaxy mergers. The dynamics and evolution of SMBHBs shed light on the evolution and formation of galaxies, the gravitational wave background and the nature of gravity. Many methods have been proposed to identify sub-parsec SMBHBs via the detection of gravitational waves or the modulation of the emission from the surrounding gaseous environment. In a previous study, we proposed a new method of identifying SMBHBs in non-active galactic nuclei via gravitational lensing. The caustics, i.e. the source positions with a divergent magnification, exhibit a complex extended geometry which rotates with the binary period. Each caustic-crossing produces an extreme magnification by up to a factor of $10^{6}$. In this work, we generalize the previous study and describe the rich phenomenology of binary-lens amplification of starlight (BLAST). We explore the electromagnetic transients produced when accounting for the relative proper motion of the source, either a single or a binary star, the eccentricity and precession of the SMBHB, and the relative inclination. Systems where a star rapidly drifts across the caustic produce a single- or double-peaked flare. In the opposite limit, a star with no relative proper motion produces quasi-periodic lensed starlight (QPLS), i.e., repeated peaks that track the system's orbital evolution. We determine the expected magnification and peak duration in different circumstances, and calculate the magnification curves and their Fourier transforms. This study may help construct search pipelines to identify SMBHBs in electromagnetic time-domain surveys.
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General Relativity and Quantum Cosmology

Cubic Quasinormal Modes in the Black-Hole Ringdown

Neev Khera and Huan Yang

We formulate and numerically solve the nonlinear Teukolsky equations through third order, obtaining a first-principles calculation of cubic quasinormal-mode amplitudes for Kerr black holes. Our approach combines lower-order metric reconstruction in split light-cone gauges with a hyperboloidal, compactified formulation solved on complex radial contours. Spin-dependent complex coupling coefficients fix the cubic amplitudes from products of their linear parent amplitudes. We apply these predictions to a nonprecessing numerical-relativity waveform, including cubic products of the $(2,2)$ fundamental mode, its first overtone, and their conjugates. The predicted cubic terms improve agreement over selected early-ringdown fitting windows and shift inferred linear-overtone amplitudes without adding fitting parameters. $3M$ after the peak of $|h_{22}|$, the summed cubic contribution reaches approximately $38\%$ of the total $h_{22}$ signal; $M$ is the initial total mass. These results identify an appreciable cubic response in early ringdown.
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General Relativity and Quantum Cosmology

Separation of variables for coupled radiative perturbations of Kerr-Newman-de Sitter black holes

Daisuke Yoshida

We extend Hintz's separation of variables for radiative gravitational and electromagnetic perturbations from Kerr-Newman to Kerr-Newman-de Sitter black holes. On subextremal backgrounds with nonzero rotation and charge, an explicit algebraic matrix transformation yields coupled two-component radial and angular ordinary differential systems linked by a common separation constant. The transformation is independent of the frequency and azimuthal number and applies to both signs of spin weight. For real frequencies, we establish a complete angular mode decomposition. We also translate regularity at the future black hole outer and cosmological horizons into ingoing and outgoing boundary conditions, respectively, for the separated radial systems. These results show that exact separation of the coupled radiative equations extends beyond asymptotically flat Kerr-Newman spacetime.
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General Relativity and Quantum Cosmology

An extension principle for Einstein-Maxwell-charged scalar field system with finite Doppler exponent

Xinliang An, Haoyang Chen and Taoran He

We prove a new extension principle for the spherically symmetric Einstein–Maxwell–charged scalar field system. Rather than imposing conditions for mass ratio $2m/r$ as in Christodoulou's small-mass-ratio criterion or as in An–Chen–Singh's proof [1] with $2m/r<3/8$, here we work through a proposal by An-Tan [2] with finite Doppler exponent (blueshift). By conducting two successive blow-up analyses, followed by a stationary comparison and a rigidity argument, we establish the uniform boundness of $\alpha'$ with $\alpha'=\partial_v\alpha/\partial_vr$ and $\alpha=\partial_v(r\phi)/\partial_vr$. This provides the desired extension principle in An-Tan [2] and completes the picture of weak cosmic censorship developed therein. As we were about to release our preprint, Cicortas communicated to us that together with his collaborators, they concomitantly used a different approach to obtain the corresponding extension principle with finite blueshift (Doppler exponent) for the uncharged Einstein-scalar field system. Hence, we decided to put the current paper as an appendix to An-Tan [2], thereby completing the proof of the weak cosmic censorship for the Einstein-Maxwell-charged scalar field system. At the same time, we are working with Cicortas and his collaborators to write a new preprint containing both approaches for the Einstein-scalar field system.
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General Relativity and Quantum Cosmology

A Scale-Critical $\mu<\frac38$ Extension Principle for Spherically Symmetric Einstein-Scalar Field System

Xinliang An, Haoyang Chen and Jaydeep Singh

We present a new quantitative, scale-critical $C^1$ extension principle for the spherically symmetric Einstein–scalar field system. We derive estimates for solutions in the causal past of a possibly singular endpoint $\mathcal{O}$ of the center. Assume that there is no trapped surface in this region and assume that the Doppler exponent (blue shift) along each incoming null cone is uniformly bounded. We then prove that the solution can extend regularly across $\mathcal{O}$ provided the mass ratio $\mu=2m/r$ satisfies $\sup\mu<\frac38$. This improves the small-mass-ratio condition in Christodoulou's extension principle by an explicit threshold of order one. Our proof has three new ingredients. First, we derive several new uniform scale-critical pointwise estimates for the scalar field and its derivatives. Second, we obtain refined estimates in an exterior self-similar domain within the triangular causal past region. Third, we establish a family of higher-order Morawetz-type identities and identify the ranges of $\mu$, with which there are coercive spacetime estimates. The unweighted identity yields the threshold $\mu\leq\frac14$, while employing extra self-similar weight raises the threshold to $\mu<\frac38$ and provides a weighted control, which implies that the solution's BV norm along an outgoing null cone is small, so that Christodoulou's theory of bounded variation solutions gives the extension.
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General Relativity and Quantum Cosmology

Analytical Time-Domain Waveforms for Black-Hole Scattering in the Post-Minkowskian Expansion

Stefano De Angelis, Vincent F. He and Julio Parra-Martinez

We present the first analytic computation of the time-domain multipoles of the gravitational waveform from hyperbolic black-hole scattering at leading order in the post-Minkowskian expansion, to all orders in velocity. We perform the partial-wave projection as a cut loop integral, which enables the use of dimensional regularization and associated integration techniques. As opposed to the frequency domain, the time-domain multipoles are logarithmic and do not contain Bessel-like functions. Our results pave the way for the incorporation of post-Minkowskian fluxes in waveform models for binary mergers.
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General Relativity and Quantum Cosmology

Analytical Time-Domain Waveforms for Black-Hole Scattering in the Self-Force Expansion

Chris Kavanagh, Julio Parra-Martinez and Davide Usseglio

We obtain a systematic understanding of the time-domain gravitational waveform of hyperbolic black-hole scattering at first order in the self-force expansion and all orders in the post-Newtonian/post-Minkowskian (PN/PM) expansion. The continuous frequency spectrum of unbound orbits makes the inverse Fourier transform to the time domain diverge order by order; we introduce a dimensional-regularization-inspired prescription that cures this uniformly. We show that the full waveform then follows, at any PM and PN order, from a single family of integrals, generated by the Lauricella function $F_D^{(3)}$, which we calculate using strategies familiar from the study of Feynman integrals. As a proof of principle, we use this understanding to compute the quadrupolar 1SF waveform through relative 10PM-20PN, that is to order ${\cal O}(G^{10} v^{42})$, the full 1SF waveform to 10PM-10PN, as well as the asymptotic fluxes and the dissipative scattering angle at the corresponding accuracy.
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General Relativity and Quantum Cosmology · Cross-listed from cond-mat.quant-gas

Analog Cosmology in a Quenched Bose-Einstein Condensate Bubble

Ruotong Zhai, Brendan Rhyno, Zain Mehdi et al. (+3 more)

Recent interest and realization of ultracold bosons confined in shell-shaped, or bubble, traps have opened up several directions in the study of Bose-Einstein condensates (BECs) in intrinsically curved geometries. Here, we explore the prospect of using thin spherical shell-shaped BECs as analog gravity simulators of an expanding or contracting curved universe. We study an interacting BEC bubble subject to a temporal quench of the interaction strength. Our theoretical approach, which uses a Bogoliubov treatment following a dimensional reduction of a spherical BEC to the low-energy excitations of a shell-shaped structure, demonstrates how this system serves as a laboratory analog for a positively curved universe having a time-varying scale factor. We investigate spontaneous pair production triggered by quenches associated with exponentially decaying or growing interaction strengths, focusing on the emergence of oscillations in the structure factor—analogous to Sakharov oscillations in the Cosmic Microwave Background angular power spectrum. We quantify the entanglement of the produced pairs via logarithmic negativity, and show how this measure may be extracted from the power spectrum. These findings establish a bridge between quantum information science and early-universe cosmology in a laboratory setting.
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All authors Ruotong Zhai, Brendan Rhyno, Zain Mehdi, Naceur Gaaloul, Smitha Vishveshwara, Daniel E. Sheehy

General Relativity and Quantum Cosmology · Cross-listed from astro-ph.CO

Origin of the red spectral tilt in Inflation without an Inflaton

Raul Jimenez, Daniele Bertacca, Mariam Abdelaziz et al. (+3 more)

In Inflation without an Inflaton (IWI), a de Sitter (dS) background drives inflation, and the graviton bilinear generates scalar perturbations at second order, with no inflaton or additional fundamental scalar field. We investigate how this scenario can account for the observed scalar spectral tilt. The tensor-mode dependence of the scalar power spectrum gives the consistency relation $n_s-1=2n_T$, since it contains four tensor mode functions. The scalar spectral tilt $n_s\simeq0.9649$ at CMB scales therefore gives $n_T\simeq-0.0175$, independent of the tensor-to-scalar ratio $r$, hence different from the standard single-field slow-roll prediction $n_T=-r/8$. A full time-dependent calculation of $\phi_2$ in exact dS leads to an almost scale-invariant scalar power spectrum, with a small blue infrared residual on the order of $n_s-1\simeq+4\times10^{-4}$. In order to account for the observed red tilt, we introduce a slow drift in the Hubble rate, parametrized by $\varepsilon=-\dot H/H^2$. The tensor tilt is then given by $n_T=-2\varepsilon$, implying $n_s-1\simeq-4\varepsilon$, at leading order. The observed tilt requires $\varepsilon_\ast\simeq0.0088$, far exceeding the drift that can be sustained by the classical graviton backreaction, for which we find $|\varepsilon_{\rm self}|\lesssim10^{-11}$. Therefore, we investigate the quantum break-time of dS as the origin of the background evolution. In this picture, the scalar tilt becomes $n_s-1=-4C_qN_{sp}(H/m_{pl})^2$, where $C_q$ is an order-one depletion coefficient, and the tilt is directly connected to the particle content of dS, $N_{sp}$. The evolution of the drift itself further predicts a positive running of $\alpha_s=+\tfrac12(1-n_s)^2\simeq+6\times10^{-4}$. These results conclude the distinctive observational predictions of the IWI scenario, establishing it as a falsifiable alternative to standard inflaton-driven models.
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All authors Raul Jimenez, Daniele Bertacca, Mariam Abdelaziz, Sabino Matarrese, Angelo Ricciardone, Marisol Traforetti

General Relativity and Quantum Cosmology

Search for Compact Binary Coalescences with Sub-solar Mass Components in Data from the Second Part of LIGO-Virgo-KAGRA's Fourth Observing Run

The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration: A. G. Abac et al. (+1815 more)

We report on a gravitational wave search for compact binary coalescences with secondary mass between 0.2 M and 1 M and primary mass between 0.2 M and 10 M, restricting the component mass ratio to lie between 0.1 and 1. The analysis uses data collected by the LIGO and Virgo detectors between 2024 April 10 15:00:00 and 2025 January 28 17:00:00, the second part of the fourth observing run of the LIGO-Virgo-KAGRA Collaboration. Results from two out of three search pipelines show outlier candidates, but not at a sufficient level of significance to provide evidence for detection. We report the combined sensitivity of the current searches over the first and second parts of the observing run to the target sub-solar mass (SSM) black hole (BH) population, as well as to neutron star (NS) binaries with one or more sub-solar component. We place upper limits on the merger rate of SSM BHs, ranging from $50 {\rm Gpc}^{-3} {\rm yr}^{-1}$ to~$5000 {\rm Gpc}^{-3} {\rm yr}^{-1}$ (90% confidence) for different component mass combinations. We use the merger rate limits to constrain two illustrative dark matter (DM) scenarios that can form SSM compact objects: primordial BHs, and dark BHs forming in a dissipative DM model. Our search limits the fraction of DM in primordial BHs, assuming a monochromatic mass distribution, to be $\leq$ (0.018-0.032) at 1 M, and $\leq$ (0.10-0.18) at 0.35 M for early-forming binaries; or to be less than 1 at masses above (0.62-0.74) M for late-forming binaries. For the dissipative model, we constrain the fraction of DM for a range of minimum dark black hole masses below $1\,M$ to be as low as (1.0-1.1) x $10^{-5}$; we also exclude minimum masses above (0.62-0.74) M, if dark BHs constitute all of the DM. For binary NSs that include sub-solar mass components, assuming a simple, fixed population model we find a merger rate upper limit of $249 {\rm Gpc}^{-3} {\rm yr}^{-1}$.
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All authors The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration: A. G. Abac, A. Abe, I. Abouelfettouh, F. Acernese, K. Ackley, A. Adam, S. Adhicary, D. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I.-L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu, L. Albers, W. Ali, S. Al-Kershi, C. Allan, C. Allene, A. Allocca, S. Al-Shammari, J. A. Alvarez, S. Alvarez-Lopez, W. Amar, O. Amarasinghe, A. Amato, F. Amicucci, C. Amra, A. B. Anand, C. Anand, A. Ananyeva, S. B. Anderson, W. G. Anderson, M. Andia, M. Ando, F. Andrade-Oliveira, M. Andrés-Carcasona, J. L. Andrey, T. Andrić, J. Anglin, J. Anna, J. M. Antelis, S. Antier, T. Aoki, M. Aoumi, E. Z. Appavuravther, E. A. Appelt, S. Appert, S. K. Apple, K. Arai, A. Araya, M. C. Araya, M. Arca Sedda, F. Arciprete, J. S. Areeda, N. Aritomi, F. Armato, S. Armstrong, N. Arnaud, M. Arogeti, S. M. Aronson, G. Ashton, Y. Aso, L. Asprea, M. Assiduo, S. Assis de Souza Melo, S. M. Aston, P. Astone, P. S. Aswathi, F. Attadio, F. Aubin, K. AultONeal, G. Avallone, N. Avdeev, E. A. Avila, S. Babak, C. Badger, S. Bae, S. Bagnasco, S. Baimukhametova, L. Baiotti, T. Baka, K. A. Baker, T. Baker, G. Balbi, G. Baldi, N. Baldicchi, M. Ball, G. Ballardin, M. Ballelli, S. W. Ballmer, S. Banagiri, B. Banerjee, D. Bankar, T. M. Baptiste, P. Baral, M. Baratti, J. C. Barayoga, K. Baric, B. C. Barish, D. Barker, D. Barman, N. Barman, F. Barone, B. Barr, M. Barrios, L. Barsotti, M. Barsuglia, D. Barta, M. A. Barton, I. Bartos, A. Basalaev, R. Bassiri, A. Basti, M. Bawaj, J. C. Bayley, A. C. Baylor, P. A. Baynard II, M. Bazzan, V. M. Bedakihale, F. Beirnaert, M. Bejger, A. S. Bell, C. Bellani, D. S. Bellie, D. Beltran-Martinez, E. Benedetti, W. Benoit, I. Bentara, M. Ben Yaala, S. Bera, F. Bergamin, B. K. Berger, M. Beroiz, C. P. L. Berry, I. Berry, D. Bersanetti, T. Bertheas, A. Bertolini, J. Betzwieser, D. Beveridge, N. Bevins, J. Bezerra-Sobrinho, R. Bhandare, R. Bhatt, A. Bhattacharjee, D. Bhattacharjee, S. Bhattacharyya, S. Bhaumik, V. Biancalana, F. Bianchi, I. A. Bilenko, M. Bilicki, G. Billingsley, A. Binetti, S. Bini, S. Biot, O. Birnholtz, S. Biscoveanu, A. Bisht, M. Bitossi, M.-A. Bizouard, S. Blaber, J. Black, J. K. Blackburn, L. A. Blagg, C. D. Blair, D. G. Blair, M. Bloch, N. Bode, N. Boettner, P. Bogdan, G. Boileau, M. Boldrini, G. N. Bolingbroke, L. D. Bonavena, V. A. Bonhomme, E. Bonilla, M. S. Bonilla, A. Bonino, R. Bonnand, A. Borchers, N. Borghi, V. Boschi, S. Bose, V. Bossilkov, Y. Bothra, A. Boudon, T. D. Boybeyi, M. Boyle, A. Bozzi, C. Bradaschia, M. J. Brady, P. R. Brady, A. Branch, M. Branchesi, T. Briant, A. Brillet, M. Brinkmann, P. Brockill, E. Brockmueller, A. F. Brooks, D. D. Brown, M. L. Brozzetti, S. Brunett, G. Bruno, R. Bruntz, J. Bryant, Y. Bu, F. Bucci, A. Buchicchio, A. Buggiani, O. Bulashenko, T. Bulik, H. J. Bulten, A. Buonanno, K. Burtnyk, R. Buscicchio, N. Busdon, D. Buskulic, R. L. Byer, R. Cabrita, V. A. Cáceres-Barbosa, L. Cadonati, G. Cagnoli, C. Cahillane, A. Calafat, J. Calderón Bustillo, J. D. Callaghan, T. A. Callister, E. Calloni, S. R. Callos, K. Cannon, V. Cantory, H. Cao, L. A. Capistran, E. Capocasa, G. Capoccia, E. Capote, C. Capuano, G. Capurri, F. Carbognani, K. J. Cardona-Martínez, M. Carlassara, M. Carpinelli, G. Carrillo, G. Carullo, A. Casallas-Lagos, J. Casanueva Diaz, C. Casentini, S. Caudill, M. Cavaglià, R. Cavalieri, A. Ceja, G. Cella, P. Cerdá-Durán, N. Chabbra, W. Chaibi, A. Chakraborty, P. Chakraborty, S. Chakraborty, S. Chalathadka Subrahmanya, C. Chan, J. C. L. Chan, M. Chan, C.-Y. Chang, K. Chang, S. Chao, P. Charlton, E. Chassande-Mottin, C. Chatterjee, Debarati Chatterjee, Deep Chatterjee, M. Chaturvedi, S. Chaty, A. Chen, A. H.-Y. Chen, D. Chen, H. Chen, H. Y. Chen, S. Chen, Yanbei Chen, Yiwen Chen, G. Cheng, H. P. Cheng, P. Chessa, T. Cheunchitra, H. T. Cheung, S. Y. Cheung, F. Chiadini, G. Chiarini, A. Chiba, A. Chincarini, D. Chintala, A. Chiummo, A. Chopra, C. Chou, S. Choudhary, N. Christensen, Y. K. Chu, S. S. Y. Chua, G. Ciani, P. Ciecielag, M. Cieślar, M. Cifaldi, B. Cirok, F. Clara, J. A. Clark, T. A. Clarke, A. Claveus, M. R. Claypool, S. Clesse, F. Cleva, S. M. Clyne, E. Coccia, E. Codazzo, P.-F. Cohadon, D. E. Cohen, E. Colangeli, O. Cole, M. Colleoni, C. G. Collette, J. Collins, S. Colloms, A. Colombo, G. Compère, C. M. Compton, G. Connolly, L. Conti, T. R. Corbitt, I. Cordero-Carrión, S. Corezzi, N. J. Cornish, A. Corsi, S. Cortese, L. A. Corubolo, L. Cotnoir, R. Cottingham, J. A. Cotturone, M. W. Coughlin, P. Couvares, R. Coyne, A. Cozzumbo, J. D. E. Creighton, T. D. Creighton, S. Crook, R. Crouch, J. Csizmazia, K. Csukás, T. J. Cullen, A. Cumming, E. Cuoco, M. Cusinato, R. R. Cuzinatto, L. V. da Conceição, T. Dal Canton, S. Dall'Osso, S. Dal Pra, G. Dálya, Y. Dang, B. D'Angelo, S. Danilishin, O. Danner, S. D'Antonio, K. Danzmann, K. E. Darroch, L. P. Dartez, R. Das, S. Das, A. Dasgupta, V. Dattilo, A. Daumas, I. Dave, A. Davenport, T. F. Davies, D. Davis, M. C. Davis, P. Davis, E. J. Daw, M. Dax, J. De Bolle, E. deBruin, M. Deenadayalan, J. Degallaix, U. Deka, M. De Laurentis, C. J. Delgado Mendez, F. De Lillo, S. Della Torre, W. Del Pozzo, O. M. del Rio, A. Demagny, F. De Marco, G. Demasi, F. De Matteis, C. de Melo, N. Demos, T. Dent, A. Depasse, N. DePergola, R. De Pietri, R. De Rosa, C. De Rossi, E. K. Derrick, M. Desai, D. DeSantis, S. Deshmukh, V. Deshmukh, R. De Simone, S. Determan, S. Dhage, A. Dhani, R. Dhatri, R. Dhurkunde, R. Diab, C. Diaz, M. C. Díaz, F. Diaz Guerra, M. Di Cesare, M. A. Dicorato, T. Dietrich, C. Di Fronzo, M. Di Giovanni, D. Diksha, J. Ding, S. Di Pace, I. Di Palma, D. Di Piero, F. Di Renzo, Divyajyoti, A. Dmitriev, J. P. Docherty, Z. Doctor, N. Doerksen, E. Dohmen, A. Doke, A. Domiciano De Souza, L. D'Onofrio, F. Donovan, K. L. Dooley, S. Doravari, O. Dorosh, F. Dosopoulou, M. Drago, J. C. Driggers, M. Dubois, R. S. Dumbreck, U. Dupletsa, D. D'Urso, P. Dutta Roy, H. Duval, S. Dwivedi, S. E. Dwyer, C. Eassa, M. Eberhardt, M. Ebersold, M. Ebiri, G. Eddolls, A. Effler, J. Eichholz, H. Einsle, M. Eisenmann, M. Emma, K. Endo, R. Enficiaud, V. Ernst, L. Errico, R. Espinosa, M. Esposito, R. C. Essick, H. Estellés, T. Etzel, M. Evans, T. Evstafyeva, J. M. Ezquiaga, F. Fabrizi, V. Fafone, S. Fairhurst, X. Fan, A. M. Farah, B. Farr, W. M. Farr, M. Favata, M. Fays, M. Fazio, J. Feicht, M. M. Fejer, J.-N. Feldhusen, E. Fenyvesi, A. Feo, J. Fernandes, T. Fernandes, G. Fernández Rodríguez, D. Fernando, S. Ferraiuolo, T. A. Ferreira, M. Ferrer-Martinez, F. Fidecaro, P. Figura, I. Fiori, M. Fishbach, R. P. Fisher, S. K. Fitzgerald, V. Fiumara, R. Flaminio, B. Flanagan, S. M. Fleischer, L. S. Fleming, F. Flocco, E. Floden, H. Fong, J. A. Font, F. Fontinele-Nunes, C. Foo, B. Fornal, P. W. F. Forsyth, A. Fragkos, N. Franchini, A. Franco-Ordovas, F. Frappez, F. Frasconi, J. P. Freed, Z. Frei, A. Freise, O. Freitas, R. Frey, W. Frischhertz, P. Fritschel, V. V. Frolov, M. Fuentes-Garcia, R. Fujii, T. Fujimori, Y. Fujiwara, P. Fulda, M. Fyffe, B. U. Gadre, J. R. Gair, S. Galaudage, V. Galdi, M. Galimberti, A. Gamboa, S. Gamoji, A. Ganguly, B. Garaventa, P. García Abia, J. García-Bellido, C. García-Quirós, J. W. Gardner, S. Garg, J. Gargiulo, X. Garrido, A. Garron, F. Garufi, P. A. Garver, C. Gasbarra, F. Gautier, V. Gayathri, T. Gayer, G. Gemme, A. Gennai, V. Gennari, J. George, R. George, O. Gerberding, L. Gergely, A. Ghinassi, Archisman Ghosh, Sayantan Ghosh, Shaon Ghosh, Shrobana Ghosh, Suprovo Ghosh, Tathagata Ghosh, J. A. Giaime, K. D. Giardina, D. R. Gibson, C. Gier, F. Gittins, J. Glanzer, F. Glotin, E. Glowacki, J. Godfrey, R. V. Godley, O. Godwin, A. S. Goettel, E. Goetz, J. Golomb, S. Gomez Lopez, G. González, P. Goodarzi, S. R. Goode, A. Goodwin-Jones, M. Gosselin, S. M. Goss-Grubbs, C. Gostiaux, R. Gouaty, D. W. Gould, D. Goupilliere, K. Govorkova, A. Grado, V. Graham, A. E. Granados, M. 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Trozzo, R. J. Trudeau, T. Tsang, S. Tsuchida, K. Tsuji, L. Tsukada, A. Tuci, M. Turconi, C. Turski, H. Ubach, A. S. Ubhi, N. Uchikata, T. Uchiyama, R. P. Udall, T. Uehara, V. Undheim, V. Upadhyaya, L. E. Uronen, T. Ushiba, M. Vacatello, H. Vahlbruch, G. Vajente, J. Valencia, M. Valentini, E. Vallejo-Pagès, S. A. Vallejo-Peña, S. Vallero, M. van Dael, E. Van den Bossche, J. F. J. van den Brand, C. Van Den Broeck, M. van der Kolk, M. van der Sluys, A. Van de Walle, J. van Dongen, K. Vandra, M. VanDyke, H. van Haevermaet, J. V. van Heijningen, P. Van Hove, J. Vanier, J. Vanosky, N. van Remortel, M. Vardaro, A. F. Vargas, V. Varma, A. Vecchio, G. Vedovato, J. Veitch, P. J. Veitch, S. Venikoudis, P. Verdier, M. Vereecken, D. Verkindt, B. Verma, S. Verma, Y. Verma, S. M. Vermeulen, F. Vetrano, A. Veutro, A. Viceré, S. Vidyant, A. D. Viets, A. Vijaykumar, A. Vilkha, N. Villanueva Espinosa, E. T. Vincent, J.-Y. Vinet, S. Viret, S. Vitale, A. Vives, L. Vizmeg, B. Vizzone, H. Vocca, D. Voigt, E. R. G. von Reis, J. S. A. von Wrangel, W. E. Vossius, L. Vujeva, S. P. Vyatchanin, J. Wack, L. E. Wade, M. Wade, K. J. Wagner, L. Wallace, R.-Z. Wan, H. Wang, P. Wang, W. H. Wang, Y. F. Wang, Z. Wang, R. L. Ward, J. Warner, M. Was, T. Washimi, N. Y. Washington, D. Watarai, B. Weaver, S. A. Webster, N. L. Weickhardt, M. Weinert, A. J. Weinstein, R. Weiss, L. Wen, K. Wette, C. Wheeler, J. T. Whelan, B. F. Whiting, E. G. Wickens, D. Wilken, B. M. Williams, D. Williams, M. J. Williams, N. S. Williams, J. L. Willis, B. Willke, M. Wils, L. Wimmer, C. W. Winborn, A. Wingfield, J. Winterflood, C. C. Wipf, G. Woan, N. E. Wolfe, H. T. Wong, I. C. F. Wong, T. Wouters, J. L. Wright, M. Wright, B. Wu, C. Wu, D. S. Wu, H. Wu, K. Wu, T. Wu, Z. Wu, E. Wuchner, D. M. Wysocki, V. A. Xu, Y. Xu, N. Yadav, H. Yamamoto, K. Yamamoto, T. S. Yamamoto, T. Yamamoto, R. Yamazaki, T. Yan, H. Yang, K. Z. Yang, Y. Yang, Z. Yarbrough, J. Yébana Carrilero, A. B. Yelikar, X. Yin, J. Yokoyama, T. Yokozawa, M. Yoshihara, S. Yuan, H. Yuzurihara, M. Zanatta, M. Zanolin, M. Zeeshan, T. Zelenova, J.-P. Zendri, M. Zeoli, M. Zerrad, M. Zevin, H. Zhang, J. Zhang, L. Zhang, N. Zhang, R. Zhang, T. Zhang, C. Zhao, J. Zhao, Yue Zhao, Yuhang Zhao, L.-M. Zheng, Y. Zheng, L. Zhizhong, H. Zhong, H. Zhou, H. O. Zhu, X.-J. Zhu, Z.-H. Zhu, Z. Zhu, D. Z. Zieba, A. B. Zimmerman, L. Zimmermann, M. E. Zucker

General Relativity and Quantum Cosmology

Biases in Ringdown Tests of General Relativity from Gravitational-Wave Microlensing

Sreelakshmi Mundakkal Dasan, Anuj Mishra and Apratim Ganguly

Gravitational-wave (GW) ringdown signals provide a powerful probe of the strong-field regime of general relativity (GR) through black hole spectroscopy and tests of the Kerr hypothesis. However, unmodeled propagation effects can bias the inference of remnant properties and the interpretation of ringdown features. We investigate the impact of gravitational lensing on ringdown inference and tests of GR using GW150914-like simulated signals spanning the long-wavelength, wave-optics, and geometric-optics regimes. We perform complementary time-domain post-merger and frequency-domain full inspiral–merger–ringdown analyses using unlensed waveform models. We find that lensing in the wave-optics regime can produce substantial biases in the inferred remnant mass and spin and can strongly affect the inferred modal content of the ringdown. In particular, it can spuriously favor the presence of the first overtone, with $\log_{10}\mathcal{B}^{221}_{220}\sim40$, even though the injected signal has no support for it. Parameterized ringdown tests can likewise yield strong apparent support for deviations from the Kerr quasinormal mode (QNM) spectrum, with $\log_{10}\mathcal{B}^{\rm modGR}_{\rm GR}\sim8$. In contrast, the evidence for higher-order modes remains largely unchanged for the simulated signals considered here. The parameterized analysis using the entire signal further shows that microlensing can bias the inferred frequency of the dominant $(2,2,0)$ QNM. These results demonstrate that unmodeled microlensing can produce systematic biases and spurious signatures in ringdown analyses, potentially affecting precision tests of the Kerr hypothesis.
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General Relativity and Quantum Cosmology

Scalar Laplace-Beltrami harmonics and curvature-induced radial mode mixing in Friedmann-like cosmologies

Eduardo Bittencourt, Peter K. S. Dunsby, Bob Osano et al. (+1 more)

We develop a harmonic framework for spherically symmetric Friedmann-like geometries with constant scalar curvature but non-zero trace-free Ricci curvature. This preserves Friedmann-type isotropic expansion while introducing a radially varying tidal field and supporting anisotropic stress. We formulate the scalar Laplace-Beltrami operator as a Sturm-Liouville problem and establish its self-adjoint domains and regularity at minimal spheres. Robertson-Walker harmonics are recovered in the maximally symmetric limit. Trace-free curvature terms are expressed as spectral projection kernels. Rotational symmetry preserves angular labels and parity, while the tidal profile couples different radial eigenmodes. A finite-shell calculation displays this off-diagonal structure, verifies consistency between radial representations, and confirms the expected weak-field scaling. A minimally coupled scalar field remains diagonal in the exact basis, indicating operator-dependent mixing rather than mixing solely due to inhomogeneity. We formulate transverse-traceless kernels, whose dynamical application requires a specified model for the supporting anisotropic matter. This converts curvature-induced mode mixing into a spectral problem for perturbations beyond maximally symmetric cosmologies.
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All authors Eduardo Bittencourt, Peter K. S. Dunsby, Bob Osano, Muzikayise E. Sikhonde

General Relativity and Quantum Cosmology · Cross-listed from hep-ph

Correlation between gravitational wave frequencies as a signature of new black hole physics

Ngo Phuc Duc Loc

We point out that for black holes (BH) of the same mass, despite of arising from two entirely different physical arguments, the peak frequency of the coarse-grained gravitational wave (GW) spectrum of gravitons $f_\text{peak}^\text{graviton}$ is very close to the peak frequency of the GW spectrum produced from binary BH mergers $f_\text{peak}^\text{merger}$. In particular, we find that $f_\text{peak}^\text{graviton}/f_\text{peak}^\text{merger}\approx 2.3$ independent from BH mass or population. If BHs are still evaporating, the peak frequencies scale as $\propto M_i^{-1}$ with $M_i$ is the initial BH mass. If BHs evaporated in the radiation (matter) dominated era, the peak frequencies scale as $\propto M_i^{1/2}$ ($\propto M_i$) respectively after taking into account redshift effect. Any deviation from the aforementioned magical ratio would indicate new BH physics. The sweetest frequency ballpark to look for this correlation is around $10^{14}\ \rm Hz-10^{20}\ \rm Hz$ where both GW spectra are allowed to have maximum amplitude. As a demonstration of what the new physics could be, we consider a phenomenological self-similar regular BH model of Hayward featuring a single hair parameter. The observed ratio of peak frequencies can be used to directly infer the hair parameter, along with crucial properties of the novel BH solution such as Hawking temperature, horizon size, and innermost stable circular orbit. We also comment on the pros and cons of correlating frequencies among other GW sources such as formation, population fluctuation, and sudden transition of adiabatic perturbation.
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General Relativity and Quantum Cosmology

All Quiet on the Vector Front: Scalar Growth and Perturbative Control in Horndeski–Weyl-squared Gravity

Antonio De Felice, Guillem Domènech and Jan Tränkle

Higher-order curvature corrections to general relativity arise naturally in many fundamental theories and provide phenomenologically interesting low-energy effective field theories. However, higher-derivative terms can introduce ghost modes, which call the validity of such models into question. Here, we thoroughly investigate the classical linear stability of the most general second-order scalar-tensor theory, namely the Horndeski theory of gravity, augmented by a Weyl-squared term in the action. We consider shift-symmetric de Sitter and power-law FLRW backgrounds and study the evolution of linear perturbations. Vector perturbations are free of gradient and tachyonic instabilities for a positive Weyl-induced mass squared. Tensor perturbations necessarily contain a ghost; their superhorizon branches are bounded, while a frozen-background calculation identifies a separate finite-momentum stability condition. The scalar sector instead contains a growing long-wavelength branch for all expanding backgrounds we consider, which is inherited by at least one of the gauge-invariant Bardeen potentials. We show that this growth enters the Bardeen potentials through an inverse-gradient factor $a^2/k^2$: the dynamical scalar fields remain bounded on the inflationary backgrounds, the direct contribution of the growing shear to the clock-matched tidal field cancels, and the electric part of the Weyl tensor decays. We discuss the implications of our results for perturbative control and for the phenomenological viability of theories including quadratic curvature corrections.
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General Relativity and Quantum Cosmology

Directional effects of the Unruh effect on steering harvesting

Shu-Min Wu, Rui-Yang Xu and Zhihong Liu

We investigate directional quantum steering harvesting by two uniformly accelerated non-identical Unruh-DeWitt detectors coupled to a massless scalar field in the Minkowski vacuum. We show that the Unruh effect does not simply degrade harvested steering, but can reshape its directionality in a strongly parameter-dependent manner. At low detector energy scales, acceleration predominantly suppresses steering, whereas at higher energy scales, moderate acceleration can enhance steering and even activate an initially absent steering direction. Most strikingly, within a finite acceleration window, the Unruh effect enhances steering in one direction while simultaneously suppressing it in the opposite direction, revealing an intrinsically directional influence of relativistic acceleration. Detector non-identicality provides an additional control mechanism: a moderate energy-gap mismatch can enhance or activate one steering direction while suppressing the reverse one, whereas sufficiently large energy-gap mismatch suppresses the harvested steering. Moreover, the maximal steering asymmetry approximately marks the transition from two-way to one-way steering. Our results establish relativistic acceleration and detector non-identicality as means of controlling the strength and directionality of vacuum-harvested quantum correlations.
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General Relativity and Quantum Cosmology

Testing Charged Black Holes Embedded in Anisotropic Matter and Surrounded by Plasma through Gravitational Lensing: Bounds from the Sloan Lens ACS Survey

Ritesh Pandey, Shubham Kala, Amare Abebe et al. (+2 more)

We investigate weak gravitational lensing by a charged black hole surrounded by anisotropic matter, described by the two-component Kiselev metric, and immersed in a homogeneous plasma. We first examine the black hole geometry and horizon structure and then derive the weak-deflection angle from the Hamiltonian description of light propagation in plasma, using an expansion with a fixed turning point that we verify against direct numerical integration. Using the gravitational lens equation, we obtain the Einstein-ring angle, the image positions, the total magnification and the fractional magnifications of the primary and secondary images. We then confront the model with the Einstein radii of two galaxy-scale lenses of the Sloan Lens ACS Survey, J0008$-$0004 and J0029$-$0055. Because the Einstein angle cannot be smaller than the reduced deficit deflection, the observed radii give the mass-independent bounds $K<6.5\times10^{-6}$ and $K<4.2\times10^{-6}$, respectively. The charge correction to the Einstein radius is suppressed by $r_s/R_E\approx 5\times10^{-6}$ and is unobservable, while the plasma parameter at the optical wavelengths of the SLACS imaging is of order $10^{-22}$ and cannot be constrained. Our results show that galaxy-scale strong lensing data place stringent limits on anisotropic matter surrounding the lens, whereas plasma effects can be probed only at low radio frequencies.
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All authors Ritesh Pandey, Shubham Kala, Amare Abebe, Hemwati Nandan, G. G. L. Nashed

General Relativity and Quantum Cosmology

Quantum noise and torque correlations in a torsion-bar Sagnac speed meter

Yuki Inoue, Mario Onglao, Daiki Tanabe et al. (+3 more)

Quantum fluctuations entering through weakly transmitting mirrors can limit a torsion-bar Sagnac speed meter for low-frequency gravitational-wave detection, even when its optical layout suppresses radiation-pressure back action. We show that, in the Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter (CHRONOS), vacuum fields entering from behind the end and middle test masses can dominate the low-frequency quantum noise despite cancellation of the torques on opposite ends of each bar. For a reference configuration with mirror power transmissions of only $10^{-7}$, these fields contribute $89.7\%$ of the total quantum-noise power spectral density at 0.1 Hz. Omitting them would underestimate the amplitude spectral density by a factor of 3.11. Our input-output model connects the correlations produced by successive optical interactions to the forces on each bar and the measured signal. This connection shows why strong torque cancellation does not, by itself, ensure a corresponding improvement in detector sensitivity. Replacing mirror transmission with absorption at fixed reflectivity leaves the excess quantum noise essentially unchanged for the two operating points studied. The predicted signal response and quantum noise agree with independent FINESSE 3 calculations as recycling detuning and homodyne angle are varied. These results establish rear vacuum fields as a limiting noise source in this torsion-bar speed meter and provide quantitative requirements for its mirror and readout design.
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All authors Yuki Inoue, Mario Onglao, Daiki Tanabe, Masashi Hazumi, Vivek Kumar, Apimook Watcharangkool

General Relativity and Quantum Cosmology

A catalog of long, eccentric and precessing binary black hole simulations from ICTS-TIFR

Vaishak Prasad, Prayush Kumar, Michael Boyle et al. (+3 more)

We present a catalogue of 31 binary black hole simulations performed with the Spectral Einstein Code (SpEC) on the Sonic cluster at ICTS-TIFR, Bengaluru between August 2023 and December 2024, targeting the sparsely covered corner of parameter space in which orbital eccentricity and spin-orbit precession are present simultaneously: 17 configurations are both eccentric and precessing, spanning mass ratios $1 \le q \le 11$ and initial eccentricities up to $\hat{e}_0 = 0.40$. The longest, EccPrecDiff002, inspirals for 80 orbits (159 gravitational-wave cycles), to our knowledge the longest simultaneously eccentric and precessing binary black hole evolution to date. Compile-time optimisation of SpEC and its third-party library stack for the AMD Zen-2 architecture delivered a 90% speedup of the initial-data solver and a 40% speedup of the evolution, roughly half of the latter traceable to a single defect in how pre-2.34 glibc reported cache topology on AMD processors. The campaign consumed $1.18 \times 10^{6}$ CPU-hours. We document the post-processing pipeline and validate the frame transformation and waveforms. Eccentricity is measured from the waveforms and compared against the initial data. The first law of binary black hole mechanics, a test of approximate helical symmetry, is satisfied to better than 0.54% for every configuration with $e_{\omega} < 0.2$, and we show that even precessing systems can be regarded as possessing approximate helical symmetry in the co-precessing frame. Using the aligned-spin simulations to calibrate the numerical floor, the co-precessing approximation's mirror-symmetry violation lies three to four ordersof magnitude above it, growing from 0.7% to 5.3%with opening angle. EccPrecDiff002 spans the band above 5 Hz without hybridisationfor $M \le 62\,M_\odot$, giving a single-interferometer signal-to-noise ratio of 961 in Cosmic Explorer at 1 Gpc.
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All authors Vaishak Prasad, Prayush Kumar, Michael Boyle, Lawrence E. Kidder, Harald P. Pfeiffer, Mark A. Scheel

General Relativity and Quantum Cosmology

Hierarchical Parameter Estimation for Intrinsic Parameters of Extreme-mass-ratio Inspirals Using Deep Learning

Ping Shen, Wen-Biao Han and Minghui Du

Extreme-mass-ratio inspirals (EMRIs) stand as prime targets for future space-borne gravitational wave (GW) detectors, yet their precise parameter estimation is severely hindered by waveform complexity and high-dimensional parameter spaces. In this work, we propose a deep learning framework capable of recovering the seven intrinsic EMRI parameters without relying on detailed priors for extrinsic parameters. Our approach first employs a U-Net to extract robust signal features from noisy time-frequency data. These features are then fed into a multi-channel convolutional neural network (CNN) equipped with a dual attention mechanism, which decouples the time and frequency dimensions to jointly model long-range temporal evolution and persistent spectral structures, thereby enhancing sensitivity to EMRI time-frequency ridges in noisy data. Remarkably, this method reduces the search ranges for the parameters $\{\log M, a, \mu, e_0, p_0 \}$ and $\{t_p, {x_I}_0 \}$ by factors of $\sim 10^{2}$ and $\sim 10^{1}$, respectively, effectively shrinking the total volume of the seven-dimensional parameter space by $\sim 10^{8}$. Such a dramatic reduction enables a rapid pre-screening of the intrinsic parameter space, substantially saving computational cost for subsequent full parameter estimation. Although our framework is developed with Taiji mission specifications in mind, the methodology is applicable to data from other space-borne GW detectors.
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General Relativity and Quantum Cosmology

Tidal deformability of white dwarfs in Eddington-inspired Born-Infeld gravity

Santanu Pradhan and Tapobrata Sarkar

We compute the static quadrupole Love number $k_2$ and the dimensionless tidal deformability $\Lambda$ of cold white dwarfs in Eddington-inspired Born-Infeld (EiBI) gravity. Matter couples to the physical metric while perturbation equations are solved in an auxiliary metric, so the physical tidal field near the surface depends on derivatives of the equation of state. This dependence is regular for a surface polytropic index $1<\Gamma_{\rm env}<3/2$, but not for the $\Gamma = 5/3$ Chandrasekhar equation of state. The mechanism, an algebraic map between the metric that carries perturbations and the metric that matter feels, is common to Palatini-type theories, so the same type of surface condition should apply to tidal calculations in this wider class. We use thermodynamically consistent envelopes that exactly reach zero enthalpy. At five masses from \(0.6\) to \(1.3\,\Msun\) and five sampled Born-Infeld couplings \(\kappa\), with $|\kappa|\le5\times10^6\,\mathrm{m^2}$, the EiBI-GR differences are approximately linear in $\kappa$. The largest absolute fractional shifts are \(3.89\times 10^{-5}\) in the stellar radius $R$, \(4.09\times 10^{-5}\) in $k_2$, and \(2.36\times 10^{-4}\) in $\Lambda$. Across nine envelope models, half the spread of each quantity relative to the baseline envelope is at most \(0.0186\) in $R$ and \(0.0857\) in \(k_2\), but only \(3.02\times 10^{-5}\) in \(\Lambda\) as the changes in \(k_2\) and \(R^5\) largely cancel. A high-precision calculation shows that the envelope dependence of the EiBI-GR difference in \(\Lambda\) is small. Half the spread of this difference ranges from \(1.067\times 10^{-12}\) to \(6.208\times 10^{-11}\) over the 20 nonzero mass-coupling points.
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General Relativity and Quantum Cosmology

A quantum probabilistic approach to the emergence of an inflationary Starobinsky Universe

Eftychios Kaimakkamis

We propose that the necessary conditions for the existence of inflation could be explained by a quantum probabilistic approach. We interpret physical states $\psi$ obtained via the path integral formulation as probability density amplitudes for the Universe to be measured at a particular geometric configuration. First, we examine the problem of ordering ambiguities in the Wheeler-DeWitt (WDW) equation. We develop a formalism to handle ambiguities in the canonical quantization of a $1d$ minisuperspace by defining ambiguity functions within the WDW equation. We define a Hilbert space inner product for calculating physical observables, and using an argument from the hermiticity of the Hamiltonian, we obtain an expression for its measure $\mu$ in addition to constraints on the ambiguity functions. By defining a new quantum state $\Psi =\sqrt{\mu} \psi$ and using the path integral formalism, we are able to resolve ambiguities and prove the theory is universal. We then generalize for any $M$-dimensional minisuperspace system, for which we can prove universality in the semi-classical approximation for a large class of models. We also show that this universality extends to all orders of quantum theory if the target space manifold of the theory is flat. We calculate probability amplitudes for various cosmological scenarios involving Starobinsky inflation. We summarize the Hartle-Hawking no-boundary proposal for GR and we comment on the distinct possibilities of defining a contour of integration: the sign choice of Hartle-Hawking vs that of Linde-Vilenkin. We make an analogous calculation for $R^{2}$ gravity and find that for the Linde- Vilenkin choice it is statistically more likely for the Universe to emerge in its inflationary stage. Finally, we calculate transition amplitudes when the kinetic energy of the inflaton field $\phi$ takes non-zero values.
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General Relativity and Quantum Cosmology · Cross-listed from hep-th

Nambu's Dynamics, Strings and Gravitized Quantum Theory

Tristan Hübsch and Djordje Minic

In this article we present a new view on string theory that is rooted in quantum foundations extended to the domain of quantum gravity in the form of gravitized quantum theory, in which the canonical probability measure captured by the Born rule of quantum theory, turns out to be dynamical and contextual. We claim that this gravitization of quantum theory is the essence of string theory. String theory, according to this view, originates in foundations of quantum theory and Nambu's dynamics, which features in the non-perturbative definition of string theory in terms of a T-duality covariant and non-commutative formulation, called the metastring. Nambu's dynamics and the associated cubic and hypercubic objects also appear as the natural language for exploring the new observables and phenomena implied by a dynamical Born rule, including triple and higher order quantum interference, that can be investigated in experiments involving massive quantum probes, and that are also needed for direct empirical tests of the Born rule, a cornerstone of quantum theory.
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General Relativity and Quantum Cosmology

Detectability of Gravitational Waves from Extreme-Mass-Ratio Inspirals on Periodic Orbits around Bardeen Black Holes

Li Huang and Xing He

We investigate gravitational wave (GW) signatures from extreme-mass-ratio inspirals (EMRIs) around a Bardeen black hole, modeling the small object as a test particle, and provide a framework for probing strong-field spacetime geometry. We first present the equations of motion for test particles around a Bardeen black hole and characterize circular orbits in the parameter space of energy and angular momentum. Using Levin's classification, we adopt the extended classification scheme to classify periodic orbits around a Bardeen black hole. We then compute GW waveforms in both the time and frequency domains, as well as the corresponding characteristic strains, for these periodic orbits and compare them with the sensitivity curves of the Laser Interferometer Space Antenna (LISA), the evolved Laser Interferometer Space Antenna (eLISA), the TianQin mission, the Big Bang Observer (BBO), and the Decihertz Interferometer Gravitational wave Observatory (DECIGO). We find that detectability is controlled primarily by the luminosity distance. At a luminosity distance of 200 Mpc, the signals can exceed the sensitivity curves of BBO and DECIGO and are comparable to the LISA sensitivity curve, whereas at 2 Gpc, they remain below the sensitivity curves of all detectors considered. These results suggest that EMRIs on periodic orbits around Bardeen black holes offer a promising avenue to probe strong-field gravity and the properties of Bardeen black holes.
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General Relativity and Quantum Cosmology · Cross-listed from hep-ph

Superhorizon enhancement of the hydrodynamic GW tail in Standard Model embedded warm inflation

Simone Biondini and M. Laine

Hydrodynamic fluctuations in the primordial plasma generate a gravitational-wave (GW) background that grows like $f^3_\mathrm{\scriptscriptstyle 0}$ in terms of the current frequency. However, within the ($10^{-4}_{ } ... 10^4_{ })\,$Hz band of upcoming interferometers, the modes likely re-entered the Hubble horizon when the hydrodynamic noise was no longer at its maximal value. To address this situation, we define a model-independent transfer function for GWs generated by hydrodynamic (or other white-noise) fluctuations at any time. Employing appropriate variables, we show how this transfer function, together with the simpler vacuum one, can be determined numerically for a general expansion history. The results are illustrated for Standard Model embedded warm inflation scenarios, including both the vacuum and hydrodynamic parts. Following previous estimates, we find that a pre-exit-generated hydrodynamic tail becomes almost flat below $(10^5_{ } ... 10^7_{ })\;$Hz. Its small-$f^{ }_\mathrm{\scriptscriptstyle 0}$ intercept remains finite, and can then moderately influence observational constraints through the tensor-to-scalar ratio ($r$).
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General Relativity and Quantum Cosmology · Cross-listed from hep-th

Linear stability matrix and Lyapunov exponents for stationary spacetime

Xu Ye and Shao-Wen Wei

Lyapunov exponents characterize the average rate at which nearby trajectories in phase space converge or diverge and therefore provide a useful criterion for dynamical stability. In this work, we develop a general framework for analyzing Lyapunov exponents in stationary spacetimes and extend previous studies beyond circular orbits. For a generic stationary spacetime described by $g_{\mu\nu}(r,\theta)$, we show that the phase space relevant to linear perturbations is spanned by $(r,p_r,\theta,p_\theta)$ and the corresponding linear stability matrix has rank four. When the spacetime admits a rank-2 Killing tensor, the linear stability matrix can be reduced to a block-diagonal form, allowing the Lyapunov exponents to be obtained analytically. By imposing the appropriate constraint conditions, the formalism further yields the linear stability matrices and Lyapunov exponents for circular and spherical orbits. The validity of the method is illustrated by several representative examples, including the (inverted) harmonic oscillator, planar Kepler motion, and particle motion in Schwarzschild and Kerr spacetimes. These results provide a unified framework for investigating the stability properties of particle orbits in both classical and relativistic dynamical systems.
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General Relativity and Quantum Cosmology · Cross-listed from astro-ph.HE

Search for Sub-Solar Mass Binaries in the First and Second Part of LIGO's Fourth Observing Run O4a-O4b

Keisi Kacanja, Kanchan Soni and Alexander H. Nitz

The observation of sub-solar mass neutron stars and black holes would signify a new class of astrophysical objects since these systems are not expected to form via standard stellar evolution. We present the first sub-solar mass search for binaries consisting of neutron star or black holes that uses the data from both the second part (O4b) and first part (O4a) of the fourth observing run of the Advanced LIGO detectors. The search covers primary masses 0.1 to 2 $M_\odot$ and secondary masses 0.1 to 1 $M_\odot$. Tidal effects up to $7\times10^5$ are accounted for in extremely low-mass neutron stars. No statistically significant candidates are identified. We place a $90\%$ confidence upper limit on the merger rate $\mathcal{R}_{90}$ for sub-solar mass black holes and neutron stars to be $< 5.2\times10^{2}\,\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$ for a chirp mass of 0.5 $M_\odot$, a factor $2.2 \times$ lower compared to previous O1-O4a searches. We constrain the effective local dark matter fraction to be $\tilde{f}_\textrm{PBH}<0.6\%$ for $M_{\textrm{PBH}}=0.8 \,M_\odot$, approximately $1.5$ factor improvement to previous O1-O4a constraints. Given our model assumptions, our local dark matter fraction constraints are $2-7$ times lower than the OGLE microlensing survey for $M_{\textrm{PBH}}\ge{0.6}~M_\odot$.
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Tuesday 13 October · still arriving

General Relativity and Quantum Cosmology

Gravity from generalized entropy

Hussain Gohar

Gravity from entropy (GfE) identifies its Lagrangian with the geometric quantum relative entropy (GQRE) between the spacetime metric and a metric induced by matter and curvature, treating the metric itself as a quantum operator. We extend this construction to the Tsallis, Rényi, and Sharma-Mittal families, which we call gravity from generalized entropy (GfGE), and determine their common Einstein limit, weak-field spectra, auxiliary-field formulation, and horizon entropy. The entropy choice controls the leading higher-curvature terms and additional modes. We confirm that the linearized GfE spectrum contains a healthy scalar and a Planck-scale tachyonic spin-2 ghost. We identify the conditions that remove this ghost: curvature must enter through its scalar trace, or the entropy generator must be flat to quadratic order at the identity. The latter gives exact general relativity for Tsallis at $q=0$ and ghost-free $f(R,L_m)$ theories for Rényi and Sharma-Mittal. On shell, the auxiliary G-field is proportional to the tensor in Wald's entropy formula. For analytic entropy functions, horizons with a single curvature scale have an area term followed by constant and inverse-area corrections. We also extend the volume-constraint cosmological constant to the full family; its leading entropy dependence is set by the scalar mass. A finite set of entropy-function derivatives controls the leading weak-curvature departures from general relativity.
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General Relativity and Quantum Cosmology

The WDM time-frequency transform in gravitational-wave data analysis II: Evolutionary spectrum and noise covariance

Aaron D. Johnson, Katerina Chatziioannou and Robert Rosati

Noise cannot be treated as stationary over the timescales required for upcoming gravitational-wave detectors, minutes for ground-based and months for space-based detectors. Nonstationarity leads to a non-diagonal frequency-domain noise covariance matrix, thus motivating a shift to the time-frequency domain. We introduce evolutionary power spectra for describing nonstationary noise in the Wilson-Daubechies-Meyer time-frequency basis. Under the framework of oscillatory processes, we show that, to leading order in the ratio of the Meyer filter and the nonstationarity timescales, the evolutionary spectrum is a power-frequency distribution that can be estimated from a single noise realization. We derive the corresponding noise covariance matrix and show that it is band tri-diagonal, like its stationary counterpart. The exact covariance to all orders is positive semi-definite by construction and dense depending on the noise. We further derive the optimal tiling of the time-frequency plane that minimizes the error of a diagonal covariance approximation and show that coarse-graining the plane suppresses correlations quadratically. Using simulated data with a LISA Galactic foreground and a drifting powerlaw spectrum, we show that the stationarity approximation leads to signal-to-noise ratio loss and severely biases inference. For realistic LISA data, approximating the covariance as diagonal in the Wilson-Daubechies-Meyer domain is accurate for the narrow-band analysis of a single resolved Galactic binary, but insufficient for a broad-band analysis. Finally, we provide mitigation strategies for the wraparound correlations induced across the data time edges whenever the spectrum differs between the start and the end of the data.
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