Einstein’s theory of General Relativity has withstood every test for more than a century.

The General Relativity picture of curved spacetime, where matter and energy determine how orbiting, inspiraling systems evolve over time, has made successful predictions that no other theory can match. Ripples in spacetime can be generated by fast orbiting stars (neutron stars, white dwarfs or black holes). Image credit: NASA.

From the bending of starlight to orbital decay, Einstein’s predictions for spacetime’s behavior have never failed.

As two neutron stars orbit each other, Einstein’s theory of general relativity predicts orbital decay, and the emission of gravitational radiation. In the final stages of a merger — never before observed in gravitational waves — the amplitude should spike so high that LIGO could, conceivably, detect them. Image credit: NASA (L), Max Planck Institute for Radio Astronomy / Michael Kramer.

Since 2015, the final stages of black hole and neutron star inspirals and mergers have been observed directly.

With numerous black hole-black hole mergers under its belt and even a neutron star-neutron star collision, gravitational wave astronomy has blossomed into a bona fide science over the past two years. Image credit: LIGO-Virgo/Frank Elavsky/Northwestern University.

The holy grail of black hole mergers, however, would be an inspiraling system that we could monitor consistently throughout the decay process, culminating in a merger.

While numerous black holes and even black hole pairs have been detected, we’d have to wait millions of years for any of them to merge. Image credit: NASA/Goddard Space Flight Center/S. Immler and H. Krimm.

Neutron stars are no better; the binaries we’ve found won’t result in a collision for some 80 million years.

Artist’s illustration of two merging neutron stars. Binary neutron star systems inspiral and merge as well, but the closest orbiting pair we’ve found won’t merge until nearly 100 million years have passed. Image credit: NSF / LIGO / Sonoma State University / A. Simonnet.

Over in Andromeda, the nearest large galaxy to the Milky Way, a number of unusual systems have been found.

Stars of all ages, types, and orbital configurations, including very tight binary stars, have been discovered via Hubble’s observations of Andromeda, the largest galaxy in the local group. Image credit: Full Hubble Field: NASA/ESA/J. Dalcanton, et al. & R. Gendler; Wide Optical Field: Robert Gendler.

One of them, J0045+41, was originally thought to be two stars orbiting one another with a period of just 80 days.

61 Cygni was the first star to have its parallax measured, but also is a difficult case due to its large proper motion. These two images, stacked in red and blue and taken almost exactly one year apart, show this binary star system’s fantastic speed. Astronomers originally thought that J0045+41 would be another such binary star system, but X-ray observations led to an even more peculiar conclusion. Image credit: Lorenzo2 of the forums at http://forum.astrofili.org/viewtopic.php?f=4&t=27548.

When additional observations were taken in the X-ray, they revealed a surprise: J0045+41 weren’t stars at all.

X-ray data ruled out the possibility of a binary system, while follow-up optical data demanded these be two black holes. Combined, they must be supermassive in origin, and at a great distance. Image credit: X-ray: NASA/CXC/Univ. of Washington/T.Dorn-Wallenstein et al.; Optical: NASA/ESA/J. Dalcanton, et al. & R. Gendler.

Instead, Trevor Dorn-Wallenstein’s team discovered a distant supermassive black hole pair, purely by coincidence.

The most massive black hole binary signal ever seen: OJ 287. This tight binary black hole system takes on the order of ~11–12 years to complete an orbit. The newly discovered system, J0045+41, orbits approximately 50 times as rapidly. Image credit: S. Zola & NASA/JPL.

Powerful relativistic effects will cause this orbit to decay, leading to a merger within 1,000 years.

An artistic representation of the configuration of the three LISA spacecraft, flying in formation, with two of the laser arms active. Given the masses and orbital parameters of any system, we can predict when a merger will occur. The supermassive black hole pair J0045+41, based on current data, may merge as soon as 350 years from now, and a space-based gravitational wave observatory will be uniquely poised to see it. Image credit: AEI/MM/exozet.

A long-period space-based gravitational wave detector would see the orbit, inspiral, and merger as it unfolds: a cosmic first.