The Collapsar's Gravitational Boomerang: Cracking the GW190814 Anomaly ✦

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LIGO and Virgo detectors catch ripples in spacetime, but some signals force us to rewrite the scripts of stellar death. GW190814—the merger of a black hole of 23 solar masses with a compact object of 2.6 solar masses—is a discordant chord in the symphony of standard models. The mass ratio of 0.11 is too extreme for isolated binaries, and the companion's mass falls into the 'desert' between neutron stars and black holes. Traditional channels—binary evolution, globular clusters, nuclei of galaxies—are powerless to explain this duo. But if we look into the heart of a collapsar, where a disk of matter, as thin as a fraction of the radius, cools via neutrino emission, an elegant mechanism emerges.

The collapse of a rapidly rotating star turns its core into a central black hole, while the remains contract into an accretion disk. This disk, like an overheated pancake on a cosmic griddle, loses energy via neutrinos and becomes gravitationally unstable—the Jeans criterion kicks in. Neutrino cooling acts as an invisible sculptor, allowing the disk to thin to the limit where gravity takes over. The disk breaks into clumps, forming seeds of neutron stars or light black holes. But instead of chaotic absorption, a dance begins: gravitational interactions fling one fragment outward, like a boomerang, onto an elongated orbit. Braking via the emission of gravitational waves, this fragment spirals back after weeks or months to merge with the central hole—this time on a circular orbit, within the detectors' sensitivity band. Numerical simulations using the AR-CHAIN code, incorporating post-Newtonian corrections up to 3.5 PN, essentially run the film of this drama in reverse: from ejection to the final merger chord, where the orbit circularizes and the mass ratio naturally falls into the observed range: q ~ H³, with H the disk thickness.

This scenario doesn't just explain the anomaly. It turns GW190814 into a bright standard siren: if tied to the supernova SN2019npv, the redshift from optical observations and the distance from gravitational waves yield an independent measurement of the Hubble constant—70.5 km/s/Mpc. Each such union becomes a cosmic beacon, whose light—gravitational waves—travels billions of years to tell us the universe's expansion rate, without needing an electromagnetic counterpart. Thus we gain a tool to measure the universe's expansion that could resolve the Hubble tension. Moreover, the proposed channel predicts a new class of kilonovae—embedded in the supernova shell, opening a hunt for unique transients with next-generation telescopes. From the collapsar disk to cosmological distances, physics weaves a tapestry where stellar death becomes a key to the nature of objects in the mass gap, first hinted at by Chandrasekhar. Future observational campaigns may witness the gravitational boomerang in action—and then we will read another chapter in the history of cosmic violence, begun with the insights of Einstein and honed by the detectors of Weiss.