In the present paper we investigate non-perturbatively and self-consistently the structure of neutron stars in $R$-squared gravity by simultaneously solving the interior and exterior problem. The mass-radius relations are obtained for several equations of state and for wide range of the $R$-squared gravity parameter $a$. Even though the deviation from general relativity for nonzero values of $a$ can be large, they are still comparable with the variations due to different modern realistic equations of state. That is why the current observations of the neutron star masses and radii alone can not put constraints on the value of the parameter $a$. We also compare our results with those obtained within the perturbative method and we discuss the differences between them.
There is a growing interest in investigating modified theories of gravity, primarily, with the aim of explaining the universe's accelerated expansion, which has been confirmed by several independent observations. Compact objects, like neutron stars, exhibit strong gravity effects and therefore are used to study modified gravity theories. We use the $f(R)=R+aR^2$ model, where R is the Ricci scalar and $a$ is a free parameter. This model has been studied both perturbatively and non-perturbatively. However, it was found that perturbative methods results in nonphysical solutions for the neutron st…
Deviations from the predictions of general relativity due to energy-momentum squared gravity (EMSG) are expected to become pronounced in the high density cores of neutron stars. We derive the hydrostatic equilibrium equations in EMSG and solve them numerically to obtain the neutron star mass-radius relations for four different realistic equations of state. We use the existing observational measurements of the masses and radii of neutron stars to constrain the free parameter, $α,$ that characterizes the coupling between matter and spacetime in EMSG. We show that $-10^{-38}\,\mathrm{cm^{3}/erg}<…
We derive a Tolman-Oppenheimer-Volkoff equation in neutron star systems within the modified $f(T, \mathcal{T})$-gravity class of models using a perturbative approach. In our approach $f(T, \mathcal{T})$-gravity is considered to be a static spherically symmetric space-time. In this instance the metric is built from a more fundamental tetrad vierbein which can be used to relate inertial and global coordinates. A linear function $f = T(r) + \mathcal{T}(r) + χh(T, \mathcal{T}) + \mathcal{O}(χ^{2})$ is taken as the Lagrangian density for the gravitational action. Finally we impose the polytropic eq…
We study the density distribution of the minimally-coupled scalar field dark energy inside a neutron star. The dark energy is considered in the hydrodynamical representation as a perfect fluid with three parameters (background density, equation of state, and effective sound speed). The neutron star matter is modeled with three unified equations of state, developed by the Brussels-Montreal group. With the calculated density distribution of the dark energy inside a neutron star (and its dependence on the dark energy parameters) we investigate how its presence impacts the macroscopic characterist…
We revisit the background solution for scalar matter coupled higher derivative gravity originally reported in arXiv: 1409.8019[hep-th]. In this letter, we choose a convenient ansatz for metric which determines the first order perturbative corrections to scalar as well as geometry.
A simple generalization to Einstein's general relativity (GR) was recently proposed which allows a correction term $T_{αβ}T^{αβ}$ in the action functional of the theory. This theory is called Energy-Momentum Squared Gravity (EMSG) and introduces a new coupling parameter $η$. EMSG resolves the big bang singularity and has a viable sequence of cosmological epochs in its thermal history. {Interestingly, in the vacuum EMSG is equivalent to GR, and its effects appear only inside the matter-energy distribution. More specifically, its consequences appear in high curvature regime. Therefore it is natu…
All Quiet on the Vector Front: Scalar Growth and Perturbative Control in Horndeski--Weyl-squared Gravity
Yukawa Institute for Theoretical Physics, Kyoto University（京都大学汤川理论物理研究所）；Institute for Theoretical Physics, Leibniz University Hannover（汉诺威莱布尼茨大学理论物理研究所）；Max Planck Institute for Gravitational Physics, Albert Einstein Institute（马克斯·普朗克引力物理研究所爱因斯坦研究所）；Dipartimento di Fisica e Astronomia “G. Galilei”, Università degli Studi di Padova（帕多瓦大学伽利略物理与天文系）；INFN, Sezione di Padova（意大利国家核物理研究所帕多瓦分部）
Magnetized neutron stars: perturbative versus fully-numerical approaches
Inter-University Centre for Astronomy and Astrophysics（大学间天体物理中心）；Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris（巴黎天文台，巴黎文理研究大学，索邦大学，法国国家科学研究中心，UMR 7095，巴黎天体物理研究所）；Observatoire astronomique de Strasbourg, CNRS, Université de Strasbourg（斯特拉斯堡天文台，法国国家科学研究中心，斯特拉斯堡大学）；Institut für Kernphysik, Theoriezentrum, Technische Universität Darmstadt（达姆施塔特工业大学核物理研究所，理论中心）
We present a novel universal relation for binary neutron star mergers with long-lived neutron star remnants: inspired by recent work based on numerical relativity simulations, we propose a novel approach using perturbative calculations that allow us to relate the pre-merger neutron star binary tidal deformability to the effective compactness of the post-merger remnant. Our results allow for the prediction of the stellar parameters of a long-lived remnant neutron star from the study of gravitational waves emitted during the pre-merger phase.
Rotating Fermion-Boson Stars in $R$-squared Gravity
Universitat de València（瓦伦西亚大学）；Universidad Nacional Autónoma de México（墨西哥国立自治大学）；Sofia University “St. Kliment Ohridski”（索非亚大学（圣克利门特·奥赫里斯基））；Bulgarian Academy of Sciences（保加利亚科学院）
In this paper, we study the Jeans analysis in the context of energy-momentum-squared gravity (EMSG). More specifically we find the new Jeans mass for non-rotating infinite mediums as the smallest mass scale for local perturbations that can be stable against its own gravity. Furthermore, for rotating mediums, specifically for rotating thin disks in the context of EMSG, we find a new Toomre-like criterion for the local gravitational stability. Finally, the results are applied to a hyper-massive neutron star, as an astrophysical system. Using a simplified toy model we have shown that, for a posit…
In scalar-tensor theories, spontaneous scalarization is a phase transition that can occur in ultra-dense environments such as neutron stars. The scalar field develops a non-trivial configuration once the stars exceeds a compactness threshold. We recently pointed out that, if the scalar exhibits some additional coupling to matter, it could give rise to significantly different microphysics in these environments. In this work we study, at the non-perturbative level, a toy model in which the photon is given a large mass when spontaneous scalarization occurs. Our results demonstrate clearly the eff…
Future observations of continuous gravitational waves from single neutron stars, apart from their monumental astrophysical significance, could also shed light on fundamental physics and exotic particle states. One such avenue is based on the fact that magnetic fields cause deformations of a neutron star, which results in a magnetic-field-induced quadrupole ellipticity. If the magnetic and rotation axes are different, this quadrupole ellipticity may generate continuous gravitational waves which may last decades, and may be observable in current or future detectors. Light, milli-magnetic monopol…
Gaussian processes provide a promising framework by which to extrapolate the equation of state (EoS) of cold, catalyzed matter beyond $1-2$ times nuclear saturation density. Here we discuss how to extend Gaussian processes to include nontrivial features in the speed of sound, such as bumps, kinks, and plateaus, which are predicted by nuclear models with exotic degrees of freedom. Using a fully Bayesian analysis incorporating measurements from X-ray sources, gravitational wave observations, and perturbative QCD results, we show that these features are compatible with current constraints and rep…
The structure of an interior dipole magnetic field of neutron stars in $f(R)$ gravity is considered. For this purpose, the perturbative approaches are used when both the deviations from general relativity and the deformations of spherically symmetric configurations associated with the presence of the magnetic field are assumed to be small. Solutions are constructed which describe relativistic, spherically symmetric configurations consisting of a gravitating magnetized perfect fluid modeled by a realistic equation of state. Comparing configurations from general relativity and modified gravity, …
