Mass is in line with a theory suggesting they could make up dark matter

Earlier this year, gravitational waves were found in what was called the 'discovery of the century'.

The Laser Interferometer Gravitational-Wave Observatory (Ligo) detected gravitational waves radiating from two black holes that crashed together about 1.3 billion years ago.

Now evidence is building that these black holes might be 'primordial' black holes, instead of traditional black holes.

If they are, they could be the answer to what makes up the mysterious dark matter, an unidentified substance that makes up 85 per cent of the mass of the universe, some researchers have said.

The Laser Interferometer Gravitational-Wave Observatory (Ligo) detected gravitational waves radiating from two black holes that crashed together about 1.3 billion years ago, simulation pictured. Researchers from Kyoto University have suggested the two black holes detected by Ligo could be 'primordial' black holes, instead of traditional black holes

WHAT ARE GRAVITATIONAL WAVES? The universe is made up of a fabric of space-time. This corresponds to Einstein's General Theory of Relativity, published in 1916. Objects in the universe bend this fabric, and more massive objects bend it more. Gravitational waves are considered ripples in this fabric. They can be produced, for instance, when black holes orbit each other or by the merging of galaxies. Gravitational waves are also thought to have been produced during the big bang. Advertisement

Researchers from Kyoto University have published a study in the journal Physical Review Letters today, suggesting the two black holes detected by Ligo could be 'primordial' black holes, instead of traditional black holes.

This means instead of being created when stars collapse at the end of their lives like most black holes, these might have formed from the extreme density of matter present soon after the big bang.

If further data support this observation, it could mark the first confirmed finding of a primordial black hole, guiding theories about the beginnings of the universe.

'The detected gravitational waves were created from a merger of two black holes thirty times the mass of the sun,' said study author Professor Takahiro Tanaka from Kyoto University.'

'It's extremely rare for such massive black holes to form in the present-day universe.'

'After this announcement, many astrophysicists started considering how such heavy black holes were created, and how such black hole binaries were formed.'

As a starting point, the team hypothesized that primordial black holes were distributed randomly in space.

Binary black holes recently discovered by the Ligo-Virgo collaboration could be primordial entities that formed just after the Big Bang. Primordial black hole binaries were discussed extensively in the 1990s. However, interest in them waned when observations implied that their number was limited

WHAT IS THE THEORY OF RELATIVITY? In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers - known as the theory of special relativity. This groundbreaking work introduced a new framework for all of physics, and proposed new concepts of space and time. He then spent 10 years trying to include acceleration in the theory, finally publishing his theory of general relativity in 1915. This determined that massive objects cause a distortion in space-time, which is felt as gravity. At its simplest, it can be thought of as a giant rubber sheet with a bowling ball in the centre. As the ball warps the sheet, a planet bends the fabric of space-time, creating the force that we feel as gravity. Any object that comes near to the body falls towards it because of the effect. Einstein predicted that if two massive bodies came together it would create such a huge ripple in space time that it should be detectable on Earth. Advertisement

'The universe was extremely hot and dense when it was first born. Primordial black holes came into being when gravitational collapse happened in regions which were especially dense,' said Professor Tanaka.

'They have a completely different origin from black holes that form from celestial bodies.'

Using general relativity, the team evaluated how often black holes merge in the present epoch.

They found that the Ligo-Virgo team's observational data on merger frequencies would fall in to place if the binaries were primordial, and if they constitute a thousandth of all dark matter in the universe.

Primordial black hole binaries were discussed extensively in the 1990s; however, interest in them waned when observations implied that their number was limited. To date, no one has found any primordial black holes, possibly making the LIGO-Virgo observations the first of their kind.

'Theoretical models about the beginnings of the universe are still hotly contested. Some models necessarily predict the existence of primordial black holes, so their discovery could help unlock important clues about the universe's early days,' said Professor Tanaka.

'When enough observational data related to black hole binaries has accumulated, it will become possible to confirm whether these are truly primordial.'

Ligo's proposed successor, the Einstein Telescope, will also be able to look at the two black holes as they swirl closer to each other, before the predicted head on collision. It might be able to give a more definitive answer.

The Einstein Telescope is a design concept for a third-generation gravitational-wave detector which will be 100 times more sensitive than current instruments.

There are a few ways these black hole mergers might be distinguished from traditional black hole mergers, including the shape of their orbits.

Primordial black holes would have an elliptical orbit, whereas traditional black hole orbits are circular.

Gravitational waves are considered ripples in this fabric of space time. Part of the experiment, this image shows gravitational waves as researchers saw them in the lab. They can be produced, for instance, when black holes orbit each other or by the merging of galaxies

Dr Simeon Bird and his colleagues at the John Hopkins University, Baltimore, have also suggested that the two black holes detected by Ligo (artist's impression shown) could be 'primordial' black holes, instead of traditional black holes. This means that they might have formed soon after the big bang

If these black holes exist today in large enough numbers, they could make up more dark matter, previous researchers suggested.

Earlier this year Dr Simeon Bird and his colleagues at the John Hopkins University, Baltimore also suggested Ligo's black holes could be primordial.

They said the black holes detected by Ligo have the perfect mass to fit with a theory that suggests dark matter is made up of these primordial black holes.

But so far there is no firm evidence for their existence.

'Interestingly enough, there remains a window for masses between 10 and 100 times the mass of the sun where primordial black holes may constitute the dark matter,' the researchers wrote in a paper.

WHAT IS DARK MATTER? Dark matter makes up 85 per cent of the universe, and is invisible because it does not reflect light. It can't be seen directly with telescopes but astronomers know it exists because of the gravitational effects it has on matter we see. The European Space Agency said: 'Shine a torch in a completely dark room, and you will see only what the torch illuminates. 'That does not mean that the room around you does not exist. 'Similarly we know dark matter exists but have never observed it directly.' Scientists are fairly sure it exists and is crucial to the universe, but they do not know what it looks like or where to find it. Dark matter is thought to be the gravitational 'glue' that holds the galaxies together, while just 5 per cent the universe consists of known material like atoms and subatomic particles. The existence of a mysterious invisible kind of matter in our universe called dark matter has been known about for almost 100 years, but we still do not know what the substance is made of. The favourite candidate for what makes dark matter is not a great fit Advertisement

The theory behind how it happens is that 'if two black holes in a galactic halo pass sufficiently close, they can radiate enough energy in gravitational waves to become gravitationally bound,' the researchers added.

'The bound black holes will then rapidly spiral inward due to emission of gravitational radiation and ultimately merge.'

Dark matter makes up 85 per cent of the mass of the universe, but it is invisible because it does not reflect light.

It cannot be seen directly with telescopes, but astronomers know it to be out there because of the gravitational effects it has on the matter.

The team wrote in the paper that their estimates raise the possibility that Ligo has detected primordial black hole dark matter.

If primordial black holes exist today in large enough numbers, they could make up dark matter, the researchers suggest. The black holes detected by Ligo have the perfect mass to fit with a theory that suggests dark matter is made up of these primordial black holes. Artist's impression of a black hole shown

Primordial black hole mergers are likely to be distributed spatially more like dark matter than luminous matter and have no optical nor neutrino counterparts, so would be invisible, they said.

'At the moment our idea is just an idea,' Dr Bird told MailOnline. 'It's possible, but we don't know whether or not it is really true...it's an exciting possibility!'