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.
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
13 of the most incredible pictures from space
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Nasa's Hubble Space Telescope captured the tiny moon Phobos during its orbital trek around Mars. Because the moon is so small, it appears star-like in the Hubble pictures. The Hubble observations were intended to photograph Mars, and the moon's cameo appearance was a bonus. Phobos is one of the smallest moons in the solar system. It is so tiny that it would fit comfortably inside the Washington, DC. Courtesy: Nasa -
This is the first image of Saturn's ultraviolet aurora taken by the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST) in October 1998, when Saturn was a distance of 810 million miles (1.3 billion km) from Earth. Courtesy: Nasa -
Astronomers are using the Nasa / ESA Hubble Space Telescope to study auroras — stunning light shows in a planet’s atmosphere — on the poles of the largest planet in the solar system, Jupiter. Courtesy: Nasa -
This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. It is located in the constellation of Sagittarius (The Archer), roughly 6000 light-years away from us. Courtesy: Nasa -
This majestic false-colour image from Nasa's Spitzer Space Telescope shows 'mountains' where stars are born. These towering pillars of cool gas and dust are illuminated at their tips with light from warm embryonic stars. Courtesy: Nasa -
This Nasa Hubble Space Telescope photograph captures the chaotic activity atop a three-light-year-tall pillar of gas and dust that is being eaten away by the brilliant light from nearby bright stars. The pillar is also being assaulted from within, as infant stars buried inside it fire off jets of gas that can be seen streaming from towering peaks. This turbulent cosmic pinnacle lies within a tempestuous stellar nursery called the Carina Nebula, located 7,500 light-years away in the southern constellation Carina. Courtesy: Nasa -
These eerie, dark, pillar-like structures are actually columns of cool interstellar hydrogen gas and dust that are also incubators for new stars. The pillars protrude from the interior wall of a dark molecular cloud like stalagmites from the floor of a cavern. They are part of the Eagle Nebula (also called M16), a nearby star-forming region 7,000 light-years away, in the constellation Serpens. Courtesy: Nasa -
In this image provided by the Hubble Space Telescope (HST), the Omega Nebula (M17) resembles the fury of a raging sea, showing a bubbly ocean of glowing hydrogen gas and small amounts of other elements such as oxygen and sulphur. The nebula, also known as the Swan Nebula, is a hotbed of newly born stars residing 5,500 light-years away in the constellation Sagittarius. Courtesy: Nasa -
Nasa / ESA Hubble Space Telescope image reveals the heart of the Lagoon Nebula. Courtesy: Nasa -
The Cat's Eye Nebula, one of the first planetary nebulae discovered, also has one of the most complex forms known to this kind of nebula. Eleven rings, or shells, of gas make up the Cat's Eye. Courtesy: Nasa -
The Cone Nebula resides 2,500 light-years away in the constellation Monoceros. Radiation from hot, young stars (located beyond the top of the image) has slowly eroded the nebula over millions of years. Courtesy: Nasa -
This planetary nebula's simple, graceful appearance is thought to be due to perspective: our view from Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. Hot blue gas near the energising central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. Courtesy: Nasa -
This image shows the galaxy Messier 94, which lies in the small northern constellation of the Hunting Dogs, about 16 million light-years away. Within the bright ring around Messier 94 new stars are forming at a high rate and many young, bright stars are present within it – thanks to this, this feature is called a starburst ring. The cause of this peculiarly shaped star-forming region is likely a pressure wave going outwards from the galactic centre, compressing the gas and dust in the outer region. Courtesy: Nasa
