# Gravitational Waves with Orbital Angular Momentum

## TL;DR

This paper proposes that gravitational waves can carry orbital angular momentum in beam-like forms, which could lead to new detectable strains and improve understanding of post-merger black hole spins.

## Contribution

It introduces the concept of gravitational wave beams with orbital angular momentum, challenging the plane wave approximation and suggesting new detection possibilities.

## Key findings

- Gravitational wave beams may carry orbital angular momentum.
- Such beams could produce additional strain signals in detectors.
- Current constraints on post-merger spins can be refined with this approach.

## Abstract

Compact orbiting binaries like the black hole binary system observed in GW150914 carry large amount of orbital angular momentum. The post-ringdown compact object formed after merger of such a binary configuration has only spin angular momentum, and this results in a large orbital angular momentum excess. One significant possibility is that the gravitational waves generated by the system carry away this excess orbital angular momentum. An estimate of this excess is made. Arguing that plane gravitational waves cannot possibly carry any orbital angular momentum, a case is made in this paper for gravitational wave beams carrying orbital angular momentum, akin to optical beams. Restricting to certain specific beam-configurations, we predict that such beams may produce a new type of strain, in addition to the longitudinal strains measured at aLIGO for GW150914 and GW170817. Current constraints on post-ringdown spins, derived within the planewave approximation of gravitational waves, therefore stand to improve. The minimal modification that might be needed on a laser-interferometer detector (like aLIGO or VIRGO) to detect such additional strains is also briefly discussed.