Tolman IV fluid sphere in bigravity

Abstract

We present Tolman IV spacetime representing compact fluid sphere in bigravity. Here, we have explored the effect of scale parameter k in the local matter distribution of compact stars. We have modelled for three well-known compact stars, and it shows that lower values of k lead to stiffer EoS. This claim is also supported by the graphical analysis. It can be observed that the sound speed and the adiabatic index are more for lower values of k. It is also seen that all the solutions of Einstein’s field equations are still satisfying the field equations in the presence of a background metric $\gamma _{\mu \nu }$. However, the density and pressure are modified by extra term from the constant curvature background, thus affecting the EoS. One can also think that the parameter $\alpha \equiv 1/k^2$ as coupling constant between the $g_{\mu \nu }$ and $\gamma _{\mu \nu }$ and consequently more the coupling stiffer is the EoS. As $k\rightarrow \infty$, the background de Sitter spacetime reduces to Minkowski’s spacetime and the coupling vanishes. The solution satisfies the causality condition, all the energy conditions and equilibrium under gravity and hydrostatic forces. The stability of the local stellar structure is enhanced by reducing the scalar curvature of the background spacetime.