"These mysterious cosmic structures at the heart of nearly every galaxy consume light and matter and are impossible to glimpse with traditional telescopes. But now, for the first time, astrophysicists have gathered knowledge directly from these titans, in the form of gravitational waves that ripple through space and time. What they learned suggests that the population of massive black hole pairs that are merging numbers in the hundreds of thousands -- perhaps even millions.

The gravitational waves from these mergers are all contributing to an underlying background hum of the universe that researchers can detect from Earth. The findings, from a collaboration of more than 100 scientists, help confirm what will one day happen to the supermassive black hole at our galaxy's center, known as Sagittarius A, as it crashes into the black hole at the heart of the Andromeda galaxy. "The Milky Way galaxy is on a collision course with the Andromeda galaxy, and in about 4.5 billion years, the two galaxies are set to merge," said Joseph Simon, a University of Colorado, Boulder, astrophysicist and a member of the North American Nanohertz Observatory for Gravitational Waves, or Nanograv, which helped lead the new work with support from the National Science Foundation.

That merger, he said, will eventually result in the black hole at the center of Andromeda and Sagittarius A sinking into the center of the newly combined galaxy and forming what is known as a binary system. The results were announced in a series of papers published Wednesday in the Astrophysical Journal Letters. "Before now, we didn't even know if supermassive black holes merged, and now we have evidence that hundreds of thousands of them are merging," said Chiara Mingarelli, a Yale University astrophysicist and a member of Nanograv. The new work could answer questions such as how these black holes grow, and how often their host galaxies merge, the researchers said. "