arXiv subjects

Thomas Hertog

Publications and source records attributed to Thomas Hertog.

At least 19 recordsLinked to original sources

arXiv2026

Relativistic signatures of scalar dark matter in extreme-mass-ratio inspirals

We study gravitational wave emission by circular extreme-mass-ratio systems in a spherically symmetric scalar environment. Previous studies have focused on the impact of scalar radiation channels, revealing a rich structure of resonances, sharp features and floating orbits. Through the backreaction of the cloud on the metric, corrections to the gravitational sector come in at the same order. We develop the computational methods, and provide a characterization of this new, fully relativistic cloud signature. Remarkably, corrections to the polar sector can dominate all other dissipative corrections. We identify scalar field masses $M\mu\lesssim 0.12$ as the regime where polar can overtake axial and scalar channels at small separation. For small $M\mu$, vacuum dephasing is dominated mostly by conservative and polar cloud corrections, with scalar radiation acting as only a minor correction. At large $M\mu$, both terms terms are shown to be highly non-negligible. Our results therefore motivate including these relativistic signatures in beyond-vacuum EMRI templates.

gr-qc↗

arXiv2026

Ringing of rapidly rotating black holes in effective field theory

Within the effective field theory approach to gravity, deviations from general relativity can be systematically described by higher-curvature operators. However, computing the resulting corrections to black hole quasinormal mode spectra remains challenging in the rapidly rotating regime, where perturbative expansions in the spin break down. We use recently constructed numerical rotating black hole solutions to compute quasinormal mode frequency corrections at leading order in the effective field theory. Focusing on scalar perturbations, we evaluate cubic-curvature corrections, which constitute the leading modifications. We employ a pseudo-spectral collocation method to solve the resulting perturbation equations on these backgrounds, enabling accurate computation across a broad parameter range. We obtain frequency corrections for fundamental modes with $l\le5$ for all $m$, and the first overtone of $2 \le l \le 5$ modes for all $m$ for spins up to $a=0.99M$, with relative errors below $10^{-4}$. We observe that corrections to certain modes grow significantly as the spin approaches the near-extremal regime.

gr-qc↗

arXiv2026

Quadratic gravity corrections to scalar QNMs of rapidly rotating black holes

In an effective-field-theory framework for gravity, black-hole quasinormal mode spectra acquire corrections in quadratic-curvature, scalar-tensor extensions of general relativity. Previous calculations of such corrections were limited to moderate spins, since the corresponding background solutions relied on expansions in the spin parameter. Using recently constructed numerical black-hole solutions valid for large spin, we compute the leading-order deviations from general relativity in the scalar quasinormal mode spectrum of rotating black holes in scalar Gauss-Bonnet and dynamical Chern-Simons gravity. We solve the resulting perturbation equations with pseudo-spectral collocation methods, allowing us to determine the quasinormal-mode corrections for dimensionless spins up to $a/M=0.99$, with accuracy better than $\lesssim 10^{-3}$ for the $l=m=0$ mode and $\lesssim 10^{-6}$ for higher multipoles. For spins $a/M>0.9$, the corrections to certain modes can increase by orders of magnitude.

gr-qc↗

arXiv2025

Quantum Liouville Cosmology

We provide a detailed analysis of the disk path integral of timelike Liouville theory, conceived as a tractable and precise toy-model quantum cosmology in two dimensions. Disk path integrals with the insertion of matter field operators, taken along a judiciously chosen complex contour, yield states akin to the Hartle-Hawking wavefunction. Working in the fixed $K$-representation, where $K$ is the trace of the extrinsic curvature, we compute the one-loop wavefunctions and put forward a conjecture for the all-loop expressions. A suitable pairing of Liouville disk path integrals yields a $K$-independent quantity that may form the basis for a well-defined inner product on the space of Euclidean histories. We also consider other ensembles, including one with fixed area, and provide a static patch perspective with a timelike feature.

hep-th↗

arXiv2025

Good things always come in 3s: trimodality in the binary black-hole chirp-mass distribution supports bimodal black-hole formation

The latest GWTC-4 release from the LIGO-Virgo-KAGRA (LVK) collaboration nearly doubles the known population of double compact object mergers and reveals a new trimodal structure in the chirp-mass distribution of merging binary black holes (BBHs) below 30 Msun. Recent detailed stellar evolution models show that features in the pre-collapse cores of massive stars produce a bimodal black hole (BH) mass distribution, which naturally extends to a trimodal BBH chirp-mass distribution. Both distributions depend only weakly on metallicity, implying universal structural features which can be tested with LVK observations. Using a new compact-remnant mass prescription derived from these models, we perform rapid population synthesis simulations to test the robustness of the predicted chirp-mass structure against uncertainties in binary evolution and cosmic star formation history, and compare these results with the current observational data. The trimodal chirp-mass distribution emerges as a robust outcome of the new remnant-mass model, persisting across variations in binary and cosmic physics. In contrast, traditional BH formation models lacking a bimodal BH mass spectrum fail to reproduce the observed trimodality. The updated models also predict lower BBH merger rates by a factor of a few, in closer agreement with LVK constraints. Intriguingly, the central chirp-mass peak, dominated by unequal-mass BBHs, originates from a previously underappreciated formation pathway in which strong luminous blue variable winds suppress binary interaction before the first BH forms. If isolated binary evolution dominates BBH formation below 30 Msun, the relative heights of the three chirp-mass peaks offer powerful observational constraints on core collapse, BH formation, binary evolution, and cosmic star formation. These universal structural features may also serve as standard sirens for precision cosmology.

astro-ph.SR↗

arXiv2025

The Doppler boosted LISA response to gravitational waves

Future space-based gravitational wave observatories like LISA, TianQin and Taji are expected to detect massive black hole binaries (MBHBs) with high signal-to-noise ratios (SNRs), ranging up to thousands. Such high-precision observations require accurate modeling of the detector response. However, current derivations of the response function neglect the motion of the spacecraft during light travel time, omitting velocity-dependent terms of order $\beta = v/c \sim 10^{-4}$. In this work, we derive the velocity-dependent corrections to the gravitational wave response. Focusing on LISA, we analyze the contribution of the velocity-terms for MBHBs in the mass range $[10^6,10^8]\:\mathrm{M}_{\odot}$ using a modified version of the state-of-the-art response simulator \texttt{lisagwresponse}. We find that corrections introduce residual SNRs up to $\sim 2$ for the loudest events and fractional differences up to $0.02\%$, compared to \texttt{lisagwresponse}. While small, these effects are comparable to current waveform modeling uncertainties and imprint distinctive sky-localization signatures, making them potentially relevant for parameter estimation of high-mass MBHBs and simulation of mock datasets.

gr-qc↗

arXiv2024

Holographic (Eternal) Inflation

We identify a minisuperspace of complex deformations of ABJM theory for which the partition function specifies the amplitude of an eternally inflating universe. The boundary theory predicts that the bosonic bulk is effectively in the Hartle-Hawking no-boundary state, with a subleading 'tunneling' contribution. This holographic model of inflation also reveals a close connection between the swampland distance and cobordism conjectures, and the condition for the asymptotic wave function to predict classical behavior in geometry and fields.

hep-th↗

arXiv2024

Ringdown Analysis of Rotating Black Holes in Effective Field Theory Extensions of General Relativity

Quasinormal modes of rapidly rotating black holes were recently computed in a generic effective-field-theory extension of general relativity with higher-derivative corrections. We exploit this breakthrough to perform the most complete search for signatures of new physics in black hole spectra to date. We construct a template that describes the post-merger gravitational-wave emission in comparable-mass binary black hole mergers at current detector sensitivity, notably including isospectrality breaking. The analysis of all events with detectable quasinormal-driven ringdown signatures yields no evidence of higher-derivative corrections in the spectra, and we set an upper bound $\ell \lesssim$ 35 km on the length scale of new physics. Looking ahead, our scheme enables new studies on the capabilities of future detectors to robustly search for signatures of new gravitational physics.

gr-qc↗

arXiv2024

The Kontsevich-Segal Criterion in the No-Boundary State Constrains Anisotropy

We show that the Kontsevich-Segal-Witten (KSW) criterion applied to the no-boundary state constrains anisotropic deformations of de Sitter space. We consider squashed $S^3$ and $S^1 \times S^2$ boundaries and find that in both models, the KSW criterion excludes a significant range of homogeneous but anisotropic configurations. For squashed $S^3$ boundaries, the excluded range includes all surface geometries with negative scalar curvature, in line with dS/CFT reasoning. For $S^1 \times S^2$ boundaries, we find that KSW selects the low-temperature regime of configuration space where the $S^1$ is sufficiently large compared to the $S^2$. In both models, the KSW criterion renders the semiclassical wave function normalizable, up to one-loop effects.

hep-th↗

arXiv2023

Quasinormal modes of rotating black holes in higher-derivative gravity

We compute the spectrum of linearized gravitational excitations of black holes with substantial angular momentum in the presence of higher-derivative corrections to general relativity. We do so perturbatively to leading order in the higher-derivative couplings and up to order fourteen in the black hole angular momentum. This allows us to accurately predict quasinormal mode frequencies of black holes with spins up to about $70\%$ of the extremal value. For some higher-derivative corrections, we find that sizable rotation enhances the frequency shifts by almost an order of magnitude relative to the static case.

gr-qc↗

arXiv2023

Axion-de Sitter wormholes

We construct wormholes supported by axion flux in the presence of a positive cosmological constant. The solutions describe compact, one-handle bodies colloquially known as kettlebell geometries. The wormholes are perturbatively stable, but regularity of the Euclidean geometry implies an upper bound on the axion flux. Viewed as no-boundary saddle points, wormholes are suppressed relative to the round sphere. The symmetric kettlebell with maximal axion density has vanishing Euclidean action. Continuing into the Lorentzian across the equator, the solutions describe two expanding branches of de Sitter space filled with an axion field that rapidly dilutes and connected by a quantum bounce across which the arrow of time reverses.

hep-th↗

arXiv2023

The Kontsevich-Segal Criterion in the No-Boundary State Constrains Inflation

We show that the Kontsevich-Segal (KS) criterion, applied to the complex saddles that specify the semiclassical no-boundary wave function, acts as a selection mechanism on inflationary scalar field potentials. Completing the observable phase of slow-roll inflation with a no-boundary origin, the KS criterion effectively bounds the tensor-to-scalar ratio of cosmic microwave background fluctuations to be less than 0.08, in line with current observations. We trace the failure of complex saddles to meet the KS criterion to the development of a tachyon in their spectrum of perturbations.

hep-th↗

arXiv2023

The universal Teukolsky equations and black hole perturbations in higher-derivative gravity

We reduce the study of perturbations of rotating black holes in higher-derivative extensions of general relativity to a system of decoupled radial equations that stem from a set of universal Teukolsky equations. We detail a complete computational strategy to obtain these decoupled equations in general higher-derivative theories. We apply this to six-derivative gravity to compute the shifts in the quasinormal mode frequencies with respect to those of Kerr black holes in general relativity. At linear order in the angular momentum we reproduce earlier results obtained with a metric perturbation approach. In contrast with this earlier work, however, the method given here applies also to post-merger black holes with significant spin, which are of particular observational interest.

gr-qc↗

arXiv2022

Microscopics of de Sitter Entropy from Precision Holography

We calculate quantum corrections to the entropy of four-dimensional de Sitter space induced by higher-derivative terms in the gravitational action and by one-loop effects. Employing the intertwinement in semiclassical gravity of Euclidean de Sitter and anti-de Sitter saddles, we embed effective de Sitter gravity theories in M-theory and express the entropy in terms of the regularized Euclidean anti-de Sitter action on an auxiliary $\mathrm{EAdS}_4 \times S^7/\mathbb{Z}_k$ background. We conjecture that the partition function of the holographically dual 3d ABJM CFT determines the explicit form of the corrections to the de Sitter entropy. This includes a logarithmic term, the coefficient of which, we show, agrees with an independent one-loop calculation around the $-S^4 \times S^7/\mathbb{Z}_k$ Euclidean de Sitter saddle. This provides evidence that the microscopic degrees of freedom behind the entropy of four-dimensional de Sitter space in gravitational theories with a holographic dual description are encapsulated by the path integral of the Euclidean CFT on the three-sphere.

hep-th↗

arXiv2022

New Horizons for Fundamental Physics with LISA

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of GWs can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas.

gr-qc↗

arXiv2022

Islands in Multiverse Models

We consider multiverse models in two-dimensional linear dilaton-gravity theories as toy models of false vacuum eternal inflation. Coupling conformal matter we calculate the Von Neumann entropy of subregions. When these are sufficiently large we find that an island develops covering most of the rest of the multiverse, leading to a Page-like transition. This resonates with a description of multiverse models in semiclassical quantum cosmology, where a measure for local predictions is given by saddle point geometries which coarse-grain over any structure associated with eternal inflation beyond one's patch.

hep-th↗

arXiv2021

Gravitational ringing of rotating black holes in higher-derivative gravity

We study gravitational perturbations of slowly-rotating black holes in a general effective-field-theory extension of general relativity that includes up to eight-derivative terms. We show that two Schrödinger-like equations with spin-dependent effective potentials govern the odd - and even-parity master variables. These equations are coupled for parity-violating corrections, and this coupling affects the quasinormal modes even at linear order in the higher-derivative corrections, due to their isospectrality in general relativity. We provide results for the shifts in the fundamental quasinormal mode frequencies at linear order in the spin, which we expect to be valuable for high-precision phenomenology through future gravitational wave observations.

gr-qc↗

arXiv2021

Fuzzball Shadows: Emergent Horizons from Microstructure

We study the physical properties of four-dimensional, string-theoretical, horizonless "fuzzball" geometries by imaging their shadows. Their microstructure traps light rays straying near the would-be horizon on long-lived, highly redshifted chaotic orbits. In fuzzballs sufficiently near the scaling limit this creates a shadow much like that of a black hole, while avoiding the paradoxes associated with an event horizon. Observations of the shadow size and residual glow can potentially discriminate between fuzzballs away from the scaling limit and alternative models of black compact objects.

hep-th↗