The black hole collision rocking the Earth after 7 billion years

The collision involved two black holes so massive that their very existence challenges preexisting scientific theories

An illustration depicting two black holes of about 66 and 85 solar masses spiralling into each other to form the huge black hole. AP

Astronomers have witnessed the birth of a giant black hole in a cosmic collision so violent it rocked our planet despite happening half-way across the universe.

Described as "mindboggling" by experts, the collision involved two black holes so massive that their very existence challenges theories about how these bizarre objects are formed. The analysis comes from the international LIGO-VIRGO collaboration - two observatories which operate three wave-detection systems in America and Europe. Here is what you need to know:

How was the collision detected?

The discovery was made by astronomers at two observatories, LIGO in the United States and VIRGO in Italy, using instruments that detect gravitational waves – ripples in the fabric of space and time predicted to exist by Albert Einstein more than a century ago. First detected in 2015, gravitational waves cause changes in the paths of laser beams which can reveal a wealth of information about the most violent events in the universe - such as the collision of black holes.

When did the collision take place?

Astronomers first detected tiny changes in the laser beams at LIGO and VIRGO in May last year. A detailed analysis of the signals has now revealed the collision actually took place around seven billion years ago – two billion years before the creation of the Earth. This makes the collision the most distant ever recorded. Even travelling at light-speed, it has taken around half the age of the universe for the gravitational waves to finally reach the Earth.

So what exactly happened?

Two black holes – objects whose gravity is so strong not even light can escape their clutches – spiralled into each other and created a brand new black hole. Dozens of such events have been detected before. What makes this event so special is the sheer size of the black holes involved - 66 and 85 times the mass of the sun – and of their  “offspring”.

Why the excitement over the new discovery?

Until now, astronomers have believed such black holes were typically formed when giant stars run out of nuclear fuel and collapse under their own gravity. But calculations pointed to limits on the mass of such black holes. Stars around 65 to 135 times heavier than the Sun were thought to explode before they could collapse into black holes. The discovery of a collision involving at least one black hole in this “impossible” mass range suggests astronomers will have to re-think their ideas of black hole formation.

What could explain the “impossible” black hole?

A big clue comes from the aftermath of last May’s collision. According to the data, the two black holes merged to create an even bigger black hole around 142 times more massive than the Sun. This suggests that at least one of its “parents” was itself the result of a previous black hole merger, rather than the collapse of a star. In other words, astronomers may have witnessed the birth of a “grandchild” black hole.

But wait – those numbers don’t add up correctly…

In everyday life, combining two masses of, say, 66 and 85 kg would create an object weighing 151 kg, not 142 kg. But in cosmic events like black hole collisions, normal rules give way to Einstein’s theory of relativity – and his most famous formula relating mass and energy: E = Mc2. The difference between the combined masses of the two original black holes and their “offspring” – around 9 solar masses – was released as energy in the form of the gravitational waves. Calculations based on Einstein’s formula show that at its peak the energy release exceeded the output of all the stars in the universe.

So why was nothing seen?

The clue is in the name: like black holes themselves, their offspring have gravity fields so strong light can’t escape their clutches. So astronomers didn’t expect to see any signs of the merger using conventional telescopes. But a chance observation by astronomers in California suggests the newly-formed black hole may have briefly signalled its birth before becoming invisible.

During a routine scan of the night sky last year, the Zwicky Transient Facility telescope glimpsed a flash of light coming from the centre of a remote galaxy. Its timing coincided with the arrival of the gravitational waves from the black hole merger.

While it could be coincidence, some astronomers believe the flash could be the result of the newly-born black hole ploughing through the hot gas and dust at the heart of the distant galaxy.

Unfortunately, the flash wasn’t seen fast enough to carry out a detailed study. Even so, it has raised hopes that other black hole mergers will reveal their secrets through more than just faint ripples in the fabric of space-time.

Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK