'Alien' black hole so massive it shouldn't exist sends wave through the universe

Andrew Griffin
Wednesday 02 September 2020 12:49 BST
Nasa gives a tour of dual black holes
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Astronomers have detected a gravitational wave from a black hole so massive that existing theories suggest it shouldn't exist.

The black hole that formed from the merger that sent the gravitational wave through space is 142 times the mass of the Sun, making it the most massive ever to be observed with gravitational waves. And the energy that it released through the universe is the equivalent to around eight solar masses, spreading out across the universe.

"This doesn't look much like a chirp, which is what we typically detect," said Nelson Christensen, a researcher at the French National Centre for Scientific Research who helped pick up the signal. "This is more like something that goes 'bang,' and it's the most massive signal LIGO and Virgo have seen."

Scientists are still unsure how the black hole could have been created, and how such a powerful blast might have been sent out, despite years of work spent hunting the black holes.

“The system we’ve discovered is so bizarre that it breaks a number of previous assumptions about how black holes form," said Karan Jani, a researcher from Vanderbilt University who was involved in the research, in a statement. "We took more than a year to confirm this alien black hole’s existence and are thrilled to be sharing this discovery with the world.”

One possibility is that the black hole was formed when two smaller black holes collided to make larger one, which then found a third black hole for yet another even larger collision. The chance of such an event happening is smaller than hitting a golf ball from China and another from Argentina and seeing them collide in the air.

The discovery could pose deep questions about fundamental questions of the universe, as well as giving clues into how the supermassive black holes that sit at the centre of some galaxies were able to form.

"This event opens more questions than it provides answers," said LIGO member Alan Weinstein, professor of physics at Caltech. "From the perspective of discovery and physics, it's a very exciting thing."

Since 2015, scientists have been observing black holes being born in the collision of two others using gravitational waves. As the powerful energy from the merger is sent out through the universe, it wobbles spacetime and can be picked up on Earth.

But the newly discovered signal is particularly special. It is named GW190521 and was spotted in May last year by LIGO and VIRGO, gravitational wave detectors in the US and Italy. The signal was made up of four short wiggles which lasted for less than a tenth of a second.

Whatever the conditions that formed the black hole, it has already broken a host of records. It is the most distant and oldest ever detected, with the gravitational waves taking seven billion years to arrive at Earth, and is also the most massive black hole ever observed through gravitational waves.

It also represents the first glimpse at "intermediate-mass" black holes. They are heavier than those that arise when massive stars collapse, but lighter than the supermassive ones that are at the centre of some galaxies.

Those intermediate-mass black holes – which have only ever been indirectly observed – could offer a hint at one of the biggest questions of astrophysics and cosmology, by allowing scientists to understand where those supermassive black holes come from. Researchers have suggested that those intermediate-mass black holes could keep merging until they grow into the kind of "cosmic monster" that sits at the middle of galaxies including our own.

It is not only the resulting black hole that is unique in its size. The two that merged to form it had masses around 65 and 85 times that of the Sun – but according to current theories, a collapsing star is unable to form black holes between 60 and 120 solar masses, since any star massive enough to do so would be blown apart by the supernova explosion that happens as they collapse.

That led to the suggestions that the black holes could already be the result of mergers that helped them grow more massive than expected. But it may also be a so-called "primordial" black hole, formed during the Big Bang.

The discovery is reported in two papers published today. One appears in Physical Review Letters and details the discovery of the gravitational wave signal itself, and another in Astrophysical Journal Letters explores the physical properties and implications of that signal.

The researchers note in the latter of those papers that the short nature of the signal means that it is harder to analyse, and that complexity could suggest that it came from a more exotic source. But they are resolute that the most likely explanation for such a powerful blast is the merging of two black holes.

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