Ultra-relativistic journeys through compact astrophysical objects

Abstract

It has recently been proved that, for constant density stars, there is a critical value $\Lambda^{*}=1$ for the dimensionless density parameter $\Lambda\equiv 4\pi R^2\rho_{\text{max}}$ of the star above which the asymptotically measured travel time $T_{\text{s}}$ along a semi-circular trajectory that connects two antipodal points on the surface of the star is {\it shorter} than the travel time $T_{\text{c}}$ along the (shorter) straight-line trajectory that connects the two antipodal points through the center of the compact star [here $\{R,\rho_{\text{max}}\}$ are respectively the radius and the maximum density of the compact astrophysical object]. This intriguing observation provides a nice illustration of the general relativistic time dilation (redshift) effect in highly curved spacetimes. One expects that generic compact astrophysical objects whose dimensionless density parameters are smaller than some critical value $\Lambda^*$ would be characterized by the `normal' relation $T_{\text{c}}\leq T_{\text{s}}$ for the travel times between the two antipodal points. Motivated by this expectation, in the present paper we prove, using analytical techniques, that spherically symmetric compact astrophysical objects whose dimensionless density parameters are bounded from above by the model-independent relation $\Lambda\leq\Lambda^*={3\over2}[1-({{2}\over{\pi}})^{2/5}]$ are always (regardless of their inner density profiles) characterized by the normal dimensionless ratio $T_{\text{c}}/T_{\text{s}}\leq1$.