Exotic Compact Objects and the Fate of the Light-Ring Instability

Abstract

Ultracompact objects with light-rings (LRs) but without an event horizon could mimic black holes (BHs) in their strong gravity phenomenology. But are such objects dynamically viable? Stationary and axisymmetric ultracompact objects that can form from smooth, quasi-Minkowski initial data must have at least one stable LR, which has been argued to trigger a spacetime instability; but its development and fate have been unknown. Using fully non-linear numerical evolutions of ultracompact bosonic stars free of any other known instabilities and introducing a novel adiabatic effective potential technique, we confirm the LRs triggered instability, identifying two possible fates: migration to non-ultracompact configurations or collapse to BHs. In concrete examples we show that typical migration/collapse time scales are not larger than ∼ 10 3 light-crossing times, unless the stable LR potential well is very shallow. Our results show that the LR instability is effective in destroying horizonless ultracompact objects that could be plausible BH imitators.

Shadows of boson and Proca stars with thin accretion disks

“…Model (29) does not apply here given the presence of an event horizon. these assumptions is dropped, since in such a case one could find solutions for ultracompact bosonic stars with light rings, which have recently been shown to be potentially affected by instabilities [56]. In any case, for the present solutions without light rings only the direct emission is present, the location of the effective maximum of emission in the luminosity models employed here, after accounting for the gravitational redshift, determines the FIG.…”

Dynamical gravastars may evade no-go results for exotic compact objects

“…The second objection that has been raised against an exotic compact object mimicking Sg A* or M87 concerns the existence of a second, interior light sphere and possible associated nonlinear instabilities. The argument, developed in the papers [8]- [10], [11] and [12], [13], shows that on topological grounds one in general expects an even number of light spheres around a spherically symmetric exotic compact object. The outer one is the usual one, which is unstable in the sense that light near this sphere moves away from it, either inwards or outwards.…”

“…Stability of the inner light ring raises the possibility of a pileup of null geodesics at that radius, leading to a possible dynamical instability that, on a sufficiently long time scale, could blow up or collapse an exotic compact object [15]- [17], [10]. Simulations for two models of bosonic compact objects in [10] suggests that for these models this instability occurs on physically accessible time scales, ruling out these compact objects as candidates for black hole mimickers. However, for gravastars the situation may be very different, and this objection may be evaded.…”

“…This means that for parameters giving physically realistic gravastars, the time scale for instability development at the inner light sphere may be very large on a cosmological time scale. Again, detailed gravastar simulations will be needed to assess whether the objections raised in [10] are relevant.…”

Impact of the wavelike nature of Proca stars on their gravitational-wave emission

“…As the latter is approached, the PS solutions become ultracompact; i.e., they develop a light ring pair [29] for ω=μ ≲ 0.711. This creates a spacetime instability [30] which motivates us to avoid this region of the parameter space. The compactness is defined as…”