Charged anisotropic matter with linear or nonlinear equation of state

TL;DR

This paper develops analytical solutions for charged, anisotropic self-gravitating fluids with linear or nonlinear equations of state, exploring their properties and potential astrophysical applications like strange quark stars and dark matter models.

Contribution

It extends the Krori and Barua method to include pressure anisotropy and complex equations of state, providing new solutions to Einstein-Maxwell equations for charged anisotropic spheres.

Findings

Solutions satisfy general relativity energy conditions.

Large electric fields and charges are possible even with zero net charge.

Pressure anisotropy allows for more massive and highly charged configurations.

Abstract

Ivanov pointed out substantial analytical difficulties associated with self-gravitating, static, isotropic fluid spheres when pressure explicitly depends on matter density. Simplification achieved with the introduction of electric charge were noticed as well. We deal with self-gravitating, charged, anisotropic fluids and get even more flexibility in solving the Einstein-Maxwell equations. In order to discuss analytical solutions we extend Krori and Barua's method to include pressure anisotropy and linear or non-linear equations of state. The field equations are reduced to a system of three algebraic equations for the anisotropic pressures as well as matter and electrostatic energy densities. Attention is paid to compact sources characterized by positive matter density and positive radial pressure. Arising solutions satisfy the energy conditions of general relativity. Spheres with vanishing net charge contain fluid elements with unbounded proper charge density located at the fluid-vacuum interface. Notably the electric force acting on these fluid elements is finite, although the acting electric field is zero. Net charges can be huge ($10^{19}\,C$) and maximum electric field intensities are very large ($10^{23}-10^{24}\,statvolt/cm$) even in the case of zero net charge. Inward-directed fluid forces caused by pressure anisotropy may allow equilibrium configurations with larger net charges and electric field intensities than those found in studies of charged isotropic fluids. Links of these results with charged strange quark stars as well as models of dark matter including massive charged particles are highlighted. The van der Waals equation of state leading to matter densities constrained by cubic polynomial equations is briefly considered. The fundamental question of stability is left open.