Star tracks in the ghost condensate
S. L. Dubovsky
TL;DR
This work analyzes an infrared modification of gravity via ghost condensate, showing that naive expectations of sizable static gravity modifications at ~1000 km are thwarted by strong retardation and the motion of objects through the ghost frame. The key insight is that moving massive bodies leave narrow, exponentially amplified "star tracks" along their trajectories, rather than altering nearby fields instantaneously. The authors derive a general expression for the gravitational potential in this theory and study the cases of moving point masses and finite-size sources, revealing track geometry and wave-zone behavior. They discuss observational prospects—mad stars, microlensing, and gravitational waves—as potential signatures, while highlighting parameter regimes where tracks could be detectable and stressing the need for UV completion to fully understand non-linear effects. Overall, the paper identifies star tracks as a distinctive, testable consequence of ghost-condensate gravity with implications for constraining the theory’s characteristic scales.
Abstract
We consider the infrared modification of gravity by ghost condensate. Naively, in this scenario one expects sizeable modification of gravity at distances of order 1000 km, provided that the characteristic time scale of the theory is of the order of the Hubble time. However, we argue that this is not the case. The main physical reason for the conspiracy is a simple fact that the Earth (and any other object in the Universe) has velocity of at least of order 10^{-3}c with respect to the rest frame of ghost condensate. Combined with strong retardation effects present in the ghost sector, this fact implies that no observable modification of the gravitational field of nearby objects occurs. Instead, the physical manifestation of ghost condensate is the presence of ``star tracks'' -- narrow regions of space with growing gravitational and ghost fields inside -- along the trajectory of any massive object. We briefly discuss the possibilities to observe these tracks.