Detecting Gravitational Waves

"We think Cosmic Explorer could detect hundreds of thousands of black hole binaries and up to a million neutron star mergers per year."

Astronomers may have detected the most massive collision of two black holes ever discovered, a chaotic merger that occurred some 7 billion years ago, the signs of which have only just reached us. The cataclysmic event offered researchers a front-row seat to the birth of one of the Universe’s most elusive objects. The distant show included two major players: one black hole roughly 66 times the mass of our Sun, and another black hole roughly 85 times the mass of our Sun. The two came close together, rapidly spinning around one another several times per second before eventually crashing together in a violent burst of energy that sent shockwaves throughout the Universe. The result of their merger? One single black hole roughly 142 times the mass of our Sun. “For every event like this one, there will be roughly 500 mergers of smaller black holes, so it’s very rare,” [Salvatore Vitale, an assistant professor at the LIGO Lab of MIT studying gravitational waves, tells The Verge.]

“This quantum noise is like a popcorn crackle in the background that creeps into our interferometer, and is very difficult to measure,” adds Nergis Mavalvala, the Marble Professor of Astrophysics and associate head of the Department of Physics at MIT. With the new squeezer technology, LIGO has shaved down this confounding quantum crackle, extending the detectors’ range by 15 percent. Combined with an increase in LIGO’s laser power, this means the detectors can pick out a gravitational wave generated by a source in the universe out to about 140 megaparsecs, or more than 400 million light years away. This extended range has enabled LIGO to detect gravitational waves on an almost weekly basis.

Now, physicists from MIT and elsewhere have studied the ringing of an infant black hole, and found that the pattern of this ringing does, in fact, predict the black hole’s mass and spin — more evidence that Einstein was right all along. “We all expect general relativity to be correct, but this is the first time we have confirmed it in this way,” says the study’s lead author, Maximiliano Isi, a NASA Einstein Fellow in MIT’s Kavli Institute for Astrophysics and Space Research.

Scientists from MIT and Harvard University have proposed a more accurate and independent way to measure the Hubble constant, a unit of measurement that describes the rate at which the universe is expanding. Using gravitational waves emitted by a relatively rare system: a black hole-neutron star binary, a hugely energetic pairing of a spiraling black hole and a neutron star, should yield the most accurate value yet for the Hubble constant.

Since LIGO’s first detection of gravitational waves, we’ve gained unexpected insight into the cosmos. Theorists had predicted that what follows the initial fireball of a neutron star merger is a “kilonova” — a phenomenon by which leftover material from a collision glows with light. Using gravitational waves, scientists could pinpoint and then record new light-based observations indicating that heavy elements, such as lead and gold, are created in these kilonova and subsequently distributed throughout the universe — opening the window of a long-awaited “multi-messenger” astronomy.

Gravitational waves emanating from the collision of two black holes holes was detected for the first time by LIGO. This computer simulation shows two black holes, each roughly 30 times the mass of the sun, about to merge together 1.3 billion years ago.