Two black holes collided 1.4 billion years ago radiating out ripples as gravitational waves, the second of its kind ever detected, the American Astronomical Society announced on Wednesday. On February 11, the Laser Interferometer Gravitational-wave Observatory scientists had established the existence of gravitational waves with their first detection, a ground-breaking one.

The black holes that were involved in the new collision that was detected were 14 and 8 times the mass of the sun, while the final one that was created from the synthesis started out with a mass 21 times that of our sun, but ended at a mass about as much as the star at the centre of our solar system, reported The Atlantic.

The signal this time is believed to be weaker than what the researchers found the first time around, reported Discover Magazine. “The first detected signal was very loud, so loud that it was possible to see the waveform in the data stream by naked eye. In this second detection the signal is buried in the noise,” said Marco Cavaglià, LIGO spokesperson and an astronomer at the University of Mississippi.

During the experiment, the scientists came across a third signal, but it was too weak for them to officially record it. However, the third signal is being read to help understand the frequency of collisions that cause gravitational waves. “The best guess we have is that binary black holes merge in our universe at the rate of a few per hour,” says LIGO scientist Jolien Creighton, who works at University of Wisconsin-Milwaukee.

Considering their success in detecting gravitational waves in very recent times, LIGO is planning to put together their first black hole census soon. “It does imply that we should have tens of detections over the next few years, and hundreds through the end of the decade. That’s enough to do some pretty significant astronomy. That’s a big population,” believes LIGO scientist Chad Hanna. He is associated with Pennsylvania State University.

Gravitational waves, first predicted by Albert Einstein in 1916, are ripples in the curvature of space-time, which is the very fabric of the universe. These waves are actual physical ripples that move away from each other and closer together, thus stretching and squeezing the space they exist in.

The strongest gravitational waves, which scientists had hoped to detect, occur when objects with tremendous gravity undergo a huge amount of acceleration. For instance, this could happen when two black holes merge to form another, or when massive stars explode. The discovery announced in February was the result of two black holes colliding 1.3 billion light years away. Scientists said it corresponded well with Einstein's theory and that there was no ambiguity in their discovery.

LIGO has a system of two detectors, one in Louisiana and another in Washington, to detect miniscule vibrations passing between gravitational waves. In 1974, the indirect detection of gravitational waves won scientists the Nobel Physics Prize.