A little more than three months after detecting gravitational waves from the merger of two massive black holes, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors recorded on December 26, 2015, gravitational waves from the merger of two smaller black holes nearly 1.4 billion years ago. This has confirmed that the merger of binary black holes recorded on September 14, 2015 was not a chance discovery, and opened a new category of objects to be observed in the universe. While the September event was from the merger of black holes 36 and 29 times the mass of the Sun, the December event was from the merger of smaller black holes that had 14 and eight times its mass. As a result of their lighter masses, the signal from the last 27 orbits of the black holes before they merged lasted more than one second in LIGO’s frequency band. Unlike the September event, when three times the mass of the Sun was radiated as gravitational waves, such waves from the December event came from one mass of the Sun. Hence, the signals from the December event were a lot weaker compared with the first one (which was like a short-duration burst), and distributed over a longer stretch of time, thus getting buried in noise. Yet, scientists were able to tease out the signal thanks to the seminal work of Indian scientists in adapting a special technique for gravitational wave data analysis and theoretical modelling of the expected signals.
The two observations were made by two LIGO detectors located in the U.S. — Livingston in Louisiana and Hanford in Washington — during a four-month run from September 2015. The next observation run beginning September 2016 will have an improved sensitivity of 25 to 75 per cent. As a result, the volume of the universe that can be studied will increase by 1.5 to two times, and the detectors will be in operation for a longer duration of six months. Hence, three times the number of events will be witnessed. The Virgo detector, a third interferometer located near Pisa, Italy, which has a design that is close to LIGO but is not quite identical, is expected to become operational during the latter half of LIGO’s upcoming observation run. Simultaneous operation of the three detectors and the 26 millisecond difference in the arrival time of incoming gravitational wave signals between LIGO and VIRGO will improve the ability to locate the source of each new event. The precision of source location will further improve when the arrival time difference increases to 39 milliseconds as LIGO-India, the fourth detector, begins operations by January 2023. It’s a promising time ahead for science.