How the collision of two black holes may solve one of the universe's biggest mysteries

Invisible particles that clump together to create 'boson stars' with massive gravitational force could be the key to unlocking the mysteries of dark matter

epa08723695 (FILE) - A handout photo made available by the European Southern Observatory (ESO) on 01 October 2020 shows an artist's impression of the web of the supermassive black hole astronomers have found with the help of ESO's Very Large Telescope (VLT) (reissued 06 October 2020). 
The joint winners of the 2020 Nobel Prize in Physics are Roger Penrose, Reinhard Genzel and Andrea Ghez.  EPA/ESO/L. Calçada / HANDOUT  HANDOUT EDITORIAL USE ONLY/NO SALES HANDOUT EDITORIAL USE ONLY/NO SALES *** Local Caption *** 56387184

A year after scientists witnessed the "mind-boggling" collision of two giant black holes, they now suspect the event actually involved two even more bizarre objects – and could answer one of the biggest mysteries about our universe.

What could be weirder than black holes?

Astronomers have long suspected that our universe contains vast quantities of a substance unlike anything observed on Earth.

Studies of galaxies hint at the presence of invisible “dark matter” lurking between the visible stars, revealing its presence through its gravitational pull.

Quite what this dark matter consists of is one of the biggest of all cosmic mysteries. Now an international team of scientists think they may have an answer.

After re-examining what was thought to be the collision of two huge black holes announced last September, the scientists think the event was actually the merger of two colossal clumps of invisible dark matter known as boson stars.

This illustration provided by LIGO/Caltech in September 2020 depicts two black holes of about 66 and 85 solar masses spiralling into each other to form the GW190521 black hole. Gravitational waves from the merger were detected by the LIGO and Virgo observatories in May 2019. (LIGO/Caltech/MIT/R. Hurt (IPAC) via AP)

What on Earth are boson stars?

All the matter we are familiar with consists of electrons, protons and neutrons – particles collectively known as fermions. All other particles are bosons, and they are best-known for transmitting forces like gravity and electromagnetism.

But attempts to solve the dark matter mystery have led theorists to suggest another type of boson might exist. Known as an ultralight boson, it would have a mass far below even that of the electron. But if such particles gather together in sufficient numbers they would form vast, invisible star-sized clumps called “boson stars”.

Spread across the whole cosmos, these could invisibly exert enough gravitational pull to account for dark matter.

So why do scientists now think boson stars exist?

Last September’s announcement of a collision between two giant black holes made headlines because even astronomers were shocked by what they had witnessed. Ripples in the very fabric of space and time were picked up by observatories in the US and Europe, which appeared to come from the collision of two black holes halfway across the universe.

Dozens of such events have been detected before, but the sheer size of the black holes involved made this one different – and perplexing.

Black holes are typically formed when huge stars run out of nuclear fuel and collapse under their own gravity. But calculations suggest that stars over 65 times the mass of the sun explode before they can form black holes. Last September’s event involved black holes 66 and 85 times the mass of the sun – both breaking this limit.

Now a team led by Dr Juan Calderón Bustillo of the University of Santiago de Compostela, Spain, believe the mystery can be solved if the collision actually involved two boson stars.

In research published in the leading physics journal Physical Review Letters, the team compare the observations with predictions of what would happen if two such objects collided. They found boson stars gave a better fit than assuming black holes were involved.

What are the implications?

Literally cosmic. First, the boson star explanation means there is no need to worry about how two “impossibly” massive black holes came to exist.

Second, the team calculates that the collision of the two boson stars would still have created a huge black hole around 250 times the mass of the sun. This opens up the possibility that truly colossal black holes may be formed by colliding clumps of dark matter, rather than lots of smaller black holes.

But most important of all, the boson star explanation would mean scientists have had their first-ever glimpse of what dark matter really is. Calculations by the team suggest the two objects each contained around 100 billion billion billion tonnes of dark matter in the form of ultralight vector bosons, particles each around a billion billion times lighter than the electron.

What happens next?

When two huge boson stars collide, they affect the surrounding space differently from colliding black holes – giving clues to their true identity. Astronomers will now be looking out for these in future events. Laboratory experiments have been set up to detect dark matter particles on Earth – so far without success. Now scientists have a better idea of what they may be looking for, this will help tune their detectors and design new experiments.

The hope is that, almost a century after the mystery of dark matter reared its head, scientists may soon have a good answer when schoolchildren ask: “What is the universe made of?”

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

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