Collapsing neutron stars could betray scalar-tensor gravity through radiation

A new 3D numerical code built on the Einstein Toolkit evolves rapidly rotating neutron stars in massive scalar-tensor theories through gravitational collapse to a black hole. The tensor gravitational-wave signal alone cannot distinguish these modified-gravity collapses from general relativity. However, the accompanying scalar radiation carries roughly 10⁻³ solar masses of energy — four orders of magnitude more than the quadrupolar tensor emission — and fast rotation amplifies the effect by sustaining larger scalar field amplitudes. Detecting this scalar burst would serve as a direct observational probe of scalar-tensor gravity.