https://phys.org/news/2019-07-pair-supermassive-black-holes-collision.html Astronomers have spotted a distant pair of titanic black holes headed for a collision. Each black hole's mass is more than 800 million times that of our sun. As the two gradually draw closer together in a death spiral, they will begin sending gravitational waves rippling through space-time. Those cosmic ripples will join the as-yet-undetected background noise of gravitational waves from other supermassive black holes. Even before the destined collision, the gravitational waves emanating from the supermassive black hole pair will dwarf those previously detected from the mergers of much smaller black holes and neutron stars. "Supermassive black hole binaries produce the loudest gravitational waves in the universe," says co-discoverer Chiara Mingarelli, an associate research scientist at the Flatiron Institute's Center for Computational Astrophysics in New York City. Gravitational waves from supermassive black hole pairs "are a million times louder than those detected by LIGO." more at link...... extract: "The gravitational waves generated by supermassive black hole pairs are outside the frequencies currently observable by experiments such as LIGO and Virgo. Instead, gravitational wave hunters rely on arrays of special stars called pulsars that act like metronomes. The rapidly spinning stars send out radio waves in a steady rhythm. If a passing gravitational wave stretches or compresses the space between Earth and the pulsar, the rhythm is slightly thrown off". the paper: https://iopscience.iop.org/article/10.3847/2041-8213/ab2a14 Discovery of a Close-separation Binary Quasar at the Heart of a z ~ 0.2 Merging Galaxy and Its Implications for Low-frequency Gravitational Waves Abstract Supermassive black hole (SMBH) binaries with masses of ~108–109 M ⊙ are expected to dominate the contribution to the as-yet undetected gravitational wave background (GWB) signal at the nanohertz frequencies accessible to pulsar timing arrays. We currently lack firm empirical constraints on the amplitude of the GWB due to the dearth of confirmed SMBH binaries in the required mass range. Using Hubble Space Telescope/Wide Field Camera 3 images, we have discovered a z ~ 0.2 quasar hosted in a merger remnant with two closely separated (0Please Register or Log in to view the hidden image!13 or ~430 pc) continuum cores at the heart of the galaxy SDSS J1010+1413. The two cores are spatially coincident with two powerful [O iii]-emitting point sources with quasar-like luminosities (L AGN ~ 5 × 1046 erg s−1), suggesting the presence of a bound SMBH system, each with M BH > 4 × 108 M ⊙. We place an upper limit on the merging timescale of the SMBH pair of 2.5 billion years, roughly the universe lookback time at z ~ 0.2. There is likely a population of quasar binaries similar to SDSS J1010+1413 that contribute to a stochastic GWB that should be detected in the next several years. If the GWB is not detected this could indicate that SMBHs merge only over extremely long timescales, remaining as close separation binaries for many Hubble times, the so-called "final-parsec problem."