We derive dynamical and gravitational lensing properties of local sources in the Hassan-Rosen bimetric gravity theory. Observations of elliptical galaxies rule out values of the effective length-scale of the theory, in units of the Hubble radius, in the interval 10 −6 λ g /r H 10 −3 , unless the proportionality constant between the metrics at the background level is far from unity, in which case general relativity is effectively restored for local sources. In order to have background solutions resembling the concordance cosmological model, without fine-tuning of the parameters of the model, we are restricted to the upper interval, or λ g /r H ∼ 1, for which the Vainshtein mechanism is expected to restore general relativity for local sources. Except for a limited range of parameter values, the Hassan-Rosen theory is thus consistent with the observed lensing and dynamical properties of elliptical galaxies.

Regularity of the horizon radius rg of a collapsing body constrains a limiting form of a spherically symmetric energy-momentum tensor near it. Its nonzero limit belongs to one of four classes that are distinguished only by two signs. As a result, close to rg the geometry can always be described by either an ingoing or outgoing Vaidya metric with increasing or decreasing mass. If according to a distant outside observer the trapped regions form in finite time, then the Einstein equations imply violation of the null energy condition. In this case the horizon radius and its rate of change determine the metric in its vicinity, and the hypersurface r = rg(t) is timelike during both the expansion and contraction of the trapped region. We present the implications of these results for the firewall paradox and discuss arguments that the required violation of the null energy condition is incompatible with the standard analysis of black hole evaporation.

We investigate the possibility that quantum effects responsible for black hole radiation do not allow for horizon crossing of gravitationally collapsing matter in a finite time as seen by distant observers. We consider this in the context of the collapse of evaporating massive thin dust shells using two families of metrics to describe the exterior geometry: the outgoing Vaidya metric and the retarded Schwarzschild metric. We describe how this hypothesis results in a modified equation of motion for the shell. In each case the collapse is accelerated due to evaporation, but the Schwarzschild radius is not crossed. Instead the shell is always at a certain sub-Planckian distance from this would-be horizon that depends only on the mass and evaporation rate, while a comoving observer encounters firewall-like energy density and flux with a natural cutoff.

We consider the effective stress-energy tensors for the foreground and background sectors in ghost-free bimetric gravity. By considering the symmetries of the theory, we show that the foreground and background null energy conditions (NECs) are strongly anti-correlated. In particular, the NECs can only be simultaneously fulfilled when they saturate, corresponding to foreground and background cosmological constants. In all other situations, either the foreground or the background is subject to a NEC-violating contribution to the total stress-energy.Comment: v1: 16 pages; v2: 2 references adde