Implications of the Gravitational Wave Event GW150914

TL;DR

The detection of GW150914 marked the start of gravitational-wave astronomy, providing new tests of gravity, insights into black hole origins, and constraints on alternative theories, while highlighting the importance of low-metallicity environments.

Contribution

This review discusses the implications of GW150914 for gravitational physics and astrophysics, emphasizing constraints on gravity modifications and black hole formation models.

Findings

Strong constraints on modifications to gravitational wave propagation.

Limited constraints on theories altering wave generation.

Black hole environments likely have low heavy element abundance.

Abstract

The era of gravitational-wave astronomy began on 14 September 2015, when the LIGO Scientific Collaboration detected the merger of two $\sim 30 M_\odot$ black holes at a distance of $\sim 400$ Mpc. This event has facilitated qualitatively new tests of gravitational theories, and has also produced exciting information about the astrophysical origin of black hole binaries. In this review we discuss the implications of this event for gravitational physics and astrophysics, as well as the expectations for future detections. In brief: (1) because the spins of the black holes could not be measured accurately and because mergers are not well calculated for modified theories of gravity, the current analysis of GW150914 does not place strong constraints on gravity variants that change only the generation of gravitational waves, but (2) it does strongly constrain alterations of the propagation of gravitational waves and alternatives to black holes. Finally, (3) many astrophysical models for the origin of heavy black hole binaries such as the GW150914 system are in play, but a reasonably robust conclusion that was reached even prior to the detection is that the environment of such systems needs to have a relatively low abundance of elements heavier than helium.