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Gravitational waves point to primordial black holes

Published: 28th July, 2016 at 12:00
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New theory suggests LIGO-Virgo discovered gravitational waves came from mysterious black holes during the Big Bang.

It took 100 years for us to finally discover the gravitational waves that Albert Einstein predicted in his Theory of General Relativity, but since the LIGO-Virgo detected the faint ripples in spacetime, scientists around the world have pondered where they came from. A new theory, published in Physical Review Letters by astrophysicists at Japan’s Kyoto University, could point to one of the most mysterious phenomena in the Universe – primordial black holes.


What is a primordial black hole?

The black holes that we normally think about are stellar black holes, which form when a massive star collapses in on itself and forms an incredibly dense ball around 10 miles across. Primordial black holes on the other hand are far more mysterious and up until this point are thought to be so rare that astronomers aren’t really searching for them. They are believed to have formed 13.8 billion years ago when the Universe was still young, and unlike stellar black holes, primordial black holes form by the incredible heat and pressure created by expansion following the Big Bang, causing matter to combine into small, but incredible heavy black holes.

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"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," explains study author Takahiro Tanaka. "They have a completely different origin from black holes that form from celestial bodies."

Making waves

When LIGO-Virgo discovered the gravitational waves, many scientists pondered how two black holes that were thirty times the mass of the Sun could have formed when it is extremely rare that they should form in the present-day Universe at all.

By estimating an even distribution of primordial black holes throughout the Universe, the Tanaka’s team used Einstein’s theory of General Relativity to estimate how frequently black holes would merge in the current epoch. They found that the gravitational wave signal discovered by LIGO-Virgo had to be formed by two colliding primordial black holes.

“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," says Tanaka.

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

This is the first time anyone has showed the possible existence of primordial black holes, which means it is another incredible discovery made by the LIGO-Virgo collaboration – they’re really making waves now!

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Alexander McNamaraOnline Editor, BBC Science Focus

Alexander is the former Online Editor at BBC Science Focus.


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