arXiv subjects

Simon Iteanu

Publications and source records attributed to Simon Iteanu.

6 recordsLinked to original sources

arXiv2026

Radial Perturbations of Black Holes in DHOST Theories

We study radial perturbations of static black holes with primary hair in a subfamily of degenerate higher-order scalar-tensor (DHOST) theories. We recast the equation of motion for the monopole degree of freedom into a flat radial wave equation and show that the associated operator can be extended, through appropriate boundary conditions, to a positive self-adjoint operator which ensures the stability of the radial mode. Remarkably, the coordinate choice that leads to the flat wave equation corresponds to the unitary gauge, in which the scalar field is uniform. As a result, the radial coordinate extends beyond the event horizon, into the black hole interior, in contrast with the tortoise coordinate in General Relativity. The same wave equation with the same coordinate choice applies to all solutions that are connected by disformal transformations. We also examine stealth black hole solutions, with either a constant or non constant kinetic term. In the former case, we find, to linear order, the absence of a propagating degree of freedom. In the latter case, we identify a stable radial degree of freedom, except for special values of the theory coupling constants.

gr-qc↗

arXiv2025

Axial perturbations of black holes with primary scalar hair

We study axial perturbations of static black holes with primary hair in a family of degenerate higher-order scalar-tensor (DHOST) theories. These solutions possess a scalar charge, fully independent of the mass, leading to a continuous one-parameter deformation of the standard Schwarzschild black hole. Starting from these solutions, we also construct new black holes, solutions of other DHOST theories, obtained via disformal transformations of the metric. In particular, we investigate two specific types of disformal transformations: the first leading to a theory where gravitational waves propagate at the speed of light, the second to a Horndeski theory, where the equations of motion remain second order. The dynamics of axial perturbations can be formally related to the general relativistic equations of motion of axial perturbations in an effective metric. The causal structure of the effective metric differs from that of the background metric, leading to distinct gravitational and luminous horizons. Using a WKB approximation, we compute the quasi-normal modes for the Schr\"odinger-like equation associated with the effective metric outside the gravitational horizon.

gr-qc↗

arXiv2024

Vanishing of Quadratic Love Numbers of Schwarzschild Black Holes

The induced conservative tidal response of self-gravitating objects in general relativity is parametrized in terms of a set of coefficients, which are commonly referred to as Love numbers. For asymptotically-flat black holes in four spacetime dimensions, the Love numbers are notoriously zero in the static regime. In this work, we show that this result continues to hold upon inclusion of nonlinearities in the theory for Schwarzschild black holes. We first solve the quadratic Einstein equations in the static limit to all orders in the multipolar expansion, including both even and odd perturbations. We show that the second-order solutions take simple analytic expressions, generically expressible in the form of finite polynomials. We then define the quadratic Love numbers at the level of the point-particle effective field theory. By performing the matching with the full solution in general relativity, we show that quadratic Love number coefficients are zero to all orders in the derivative expansion, like the linear ones.

gr-qc↗

arXiv2023

Large-scale turbulent driving regulates star formation in high-redshift gas-rich galaxies II: Influence of the magnetic field and the turbulent compressive fraction

The observed star formation rate (SFR) in galaxies is well below what it should be if gravitational collapse alone were at play. It has recently been shown that one candidate that might regulate star formation, the feedback from massive stars, is suitable only if the mean column density at the kiloparsec scale is lower than $\approx 20 M_\odot\cdot\mathrm{pc}^{-2}$. On the other hand, intense large-scale turbulent driving might slow down star formation in high-density environments to values that are compatible with observations. In this work, we explore the effect of the nature and strength of the turbulent driving, as well as the effect of the magnetic field. We performed a large series of feedback-regulated numerical simulations of the interstellar medium in which bidimensional large-scale turbulent driving was also applied. We determined the driving intensity needed to reproduce the Schmidt-Kennicutt relation for several gas column densities, magnetization, and driving compressibility. We confirm that in the absence of turbulent forcing and even with a substantial magnetic field, the SFR is too high, particularly at a high column density, compared to the Schmidt-Kennicutt relation. We find that the SFR outcome strongly depends on the initial magnetic field and on the compressibility of the turbulent driving. As a consequence, a higher magnetic field in high column density environment may lower the energy necessary to sustain a turbulence that is sufficiently intense to regulate star formation. Stellar feedback does not seem to be sufficient to regulate star formation in gas-rich galaxies where large-scale turbulent driving may be needed. The sources of this large-scale turbulence as well as its characteristics, such as its intensity, compressibility, and anisotropy, need to be understood and quantified.

astro-ph.GA↗

arXiv2022

Mass of cosmological perturbations in the hybrid and dressed metric formalisms of Loop Quantum Cosmology for the Starobinsky and exponential potentials

The hybrid and the dressed metric formalisms for the study of primordial perturbations in Loop Quantum Cosmology lead to dynamical equations for the modes of these perturbations that are of a generalized harmonic-oscillator type, with a mass that depends on the background but is the same for all modes. For quantum background states that are peaked on trajectories of the effective description of Loop Quantum Cosmology, the main difference between the two considered formalisms is found in the expression of this mass. The value of the mass at the bounce is especially important, since it is only in a short interval around this event that the quantum geometry effects on the perturbations are relevant. In a previous article, the properties of this mass were discussed for an inflaton potential of quadratic form, or with similar characteristics. In the present work, we extend this study to other interesting potentials in cosmology, namely the Starobinsky and the exponential potentials. We prove that there exists a finite interval of values of the potential (which includes the zero but typically goes beyond the sector of kinetically dominated inflaton energy density) for which the hybrid mass is positive at the bounce whereas the dressed metric mass is negative.

gr-qc↗

arXiv2022

Model of pulsar pair cascades in non uniform electric fields: growth rate, density profile and screening time

Time-dependent cascades of electron-positron pairs are thought to be the main source of plasma in pulsar magnetospheres and a primary ingredient to explain the nature of pulsar radio emission, a longstanding open problem in high-energy astrophysics. During these cascades - positive feedback loops of gamma-ray photon emission, via curvature radiation by TeV electrons and positrons, and pair production -, the plasma self-consistently develops inductive waves that couple to electromagnetic modes capable of escaping the pulsar dense plasma. In this work, we present an analytical description of pair cascades relevant in pulsars, including their onset, exponential growth and saturation stages. We study this problem in the case of a background linear electric field, relevant in pulsar polar caps, and using an heuristic model of the pair production process. The analytical results are confirmed with particle-in-cell simulations performed with OSIRIS including heuristic pair production.

astro-ph.HE↗