[Paper Review] Dynamical Chameleon Neutron Stars: stability, radial oscillations and scalar radiation in spherical symmetry

Alexandru Dima, Miguel Bezares+1 more|arXiv (Cornell University)|Jul 9, 2021

Pulsars and Gravitational Waves Research140 references21 citations

TL;DR

This paper presents the first fully nonlinear numerical simulations of neutron stars in chameleon scalar-tensor gravity, confirming their stability under radial perturbations and revealing deviations in radial oscillation spectra from general relativity. It further computes scalar radiation during collapse, showing detectable fluxes that could be probed by future gravitational-wave detectors.

ABSTRACT

Scalar-tensor theories whose phenomenology differs significantly from general relativity on large (e.g. cosmological) scales do not typically pass local experimental tests (e.g. in the solar system) unless they present a suitable "screening mechanism". An example is provided by chameleon screening, whereby the local general relativistic behavior is recovered in high density environments, at least in weak-field and quasi-static configurations. Here, we test the validity of chameleon screening in strong-field and highly relativistic/dynamical conditions, by performing fully non-linear simulations of neutron stars subjected to initial perturbations that cause them to oscillate or even collapse to a black hole. We confirm that screened chameleon stars are stable to sufficiently small radial oscillations, but that the frequency spectrum of the latter shows deviations from the general relativistic predictions. We also calculate the scalar fluxes produced during collapse to a black hole, and comment on their detectability with future gravitational-wave interferometers.

Motivation & Objective

• To test the robustness of chameleon screening in strong-field, dynamical conditions beyond the static or weak-field approximations.
• To investigate the stability of neutron stars coupled to a chameleon scalar field under radial perturbations.
• To characterize the spectrum of radial oscillations in chameleon neutron stars and compare them to general relativity predictions.
• To compute scalar wave fluxes during dynamical collapse to black holes and assess their detectability with future gravitational-wave observatories.

Proposed method

• Performing fully non-linear numerical relativity simulations in spherical symmetry using a generalized Z4 formulation with adaptive mesh refinement.
• Constructing initial data for chameleon neutron stars by solving the coupled Einstein-scalar field equations with a chameleon potential and conformal coupling.
• Implementing a scalar field evolution equation with a density-dependent effective mass to model chameleon screening.
• Using spectral methods to extract scalar waveforms and compute fluxes during radial oscillations and gravitational collapse.
• Applying a modified harmonic gauge condition to maintain numerical stability in strong-field regimes.
• Validating results against known general relativistic limits and comparing scalar charge and oscillation frequencies with GR predictions.

Experimental results

Research questions

• RQ1 Can chameleon screening remain effective in highly relativistic, dynamical neutron star configurations?
• RQ2 Do chameleon neutron stars exhibit stable radial oscillations, and how do their frequencies deviate from general relativity?
• RQ3 What is the amplitude and spectral content of scalar radiation emitted during radial oscillations and collapse to black holes?
• RQ4 Can future gravitational-wave detectors distinguish scalar radiation from chameleon neutron stars from standard general relativistic signals?

Key findings

• Chameleon neutron stars are nonlinearly stable under small radial perturbations, confirming the robustness of chameleon screening in strong-field, dynamical regimes.
• The radial oscillation spectrum of chameleon neutron stars shows measurable deviations from general relativity, with shifts in mode frequencies due to the scalar field's contribution.
• During collapse to a black hole, chameleon neutron stars emit significant scalar radiation, with flux amplitudes that could be detectable by third-generation gravitational-wave detectors like the Einstein Telescope.
• The scalar flux is enhanced in regions of partial descreening within pressure-dominated cores, where the scalar field is less suppressed.
• The monopole scalar radiation during collapse is dominated by the time-varying scalar charge, which is largest during the most dynamical phases of collapse.
• The results suggest that chameleon theories with screening mechanisms are not ruled out by neutron star dynamics and could be constrained by future gravitational-wave observations.