It was 1916 when Albert Einstein first predicted the existence of gravitational waves, a ripple in space-time, in his general theory of relativity. Over a century later, scientists have for the first time detected one of the largest batches of these waves that passed through Earth.

The 35 new detections of the ripples in space-time take the total number of such observations to 90 since we first detected it in 2015. 32 of these ripples were detected from pairs of merging black holes, while three others likely come from the collision of a neutron star and a black hole.

The detection was done by an international team of scientists including Australian researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav). The detections were noted between November 2019 to March 2020. “Each new observing run brings new discoveries and surprises. The third observing run saw gravitational wave detection becoming an everyday thing, but I still think each detection is exciting!” said Dr Hannah Middleton, a post-doctoral researcher at OzGrav and co-author of the study.

WHAT ARE GRAVITATIONAL WAVES?

Gravitational waves are an invisible ripple in space that was first discovered by Albert Einstein over 100 years ago. Einstein predicted that something special happens when two bodies—such as planets or stars—orbit each other. He believed that this kind of movement could cause ripples in space. These ripples would spread out like the ripples in a pond when a stone is tossed in.

According to LIGO, the strongest gravitational waves are produced by cataclysmic events such as colliding black holes, supernovae (massive stars exploding at the end of their lifetimes), and colliding neutron stars. Other waves are predicted to be caused by the rotation of neutron stars that are not perfect spheres, and possibly even the remnants of gravitational radiation created by the Big Bang.

THE BIGGEST BATCH OF WAVES DETECTED

Scientists detected the current batch of waves emerging from mergers between possible neutron star – black hole pairs, a merger between a black hole and an object which could either be a light black hole or a heavy neutron star. While others were from a massive pair of black holes orbiting each other, with a combined mass 145 times heavier than the Sun, another was from a pair of black holes orbiting each other, in which at least one of the pair is spinning upright.

Some waves came from a pair of black holes orbiting each other which have a combined mass 112 times heavier than the Sun, which seems to be spinning upside-down and a ‘light’ pair of black holes that together weigh only 18 times the mass of the Sun. These waves show different properties of black holes and neutron stars which could be powering galaxies in some cases.

“It’s fascinating that there is such a wide range of properties within this growing collection of black holes and neutron star pairs. Properties like the masses and spins of these pairs can tell us how they’re forming, so seeing such a diverse mix raises interesting questions about where they came from,” study co-author and OzGrav PhD student Isobel Romero-Shaw said in a statement.

The observations provide a new understanding of black holes, which remains one of the most complex characters in the universe. Now, scientists can look at individual properties of these binary pairs and also study these cosmic events as a large collection – or population.

A GLOBAL COLLABORATION

Scientists were able to detect this major chunk of waves arriving at Earth through detectors at Laser Interferometer Gravitational-Wave Observatory (LIGO), Virog in Italy and Japan’s KAGRA observatories.

The LIGO and Virgo observatories are currently undergoing a further upgrade and will start the upcoming fourth observing period, in the second half of 2022, with even greater sensitivity, corresponding to a volume of the Universe almost 10 times larger than before and, therefore, a much greater probability of picking up gravitational signals.

“As we continue to observe more gravitational-wave signals, we will learn more and more about the objects that produce them, their properties as a population, and continue to put Einstein’s theory of General Relativity to the test,” says Dr Middleton.