Black holes are among the most fascinating and hotly debated objects in the universe.
They have captured the public imagination for decades, thanks in part to the late Stephen Hawking, who transformed them from a hard-to-understand scientific theory into a source of mystical wonder.
They have also permeated popular culture through sci-fi magazines, Star Trek and Hollywood blockbusters.
But what are the five most bizarre and captivating theories about black holes that are so inscrutable that they boggle the mind?
Here MailOnline takes a look.
1. They are surrounded by a ‘ring of fire’
In 2019, astronomers took the first ever image of a black hole located in a distant galaxy.
Described by scientists as ‘a monster’, it is three million times the size of Earth.
The image shows an intensely bright ‘ring of fire’, as scientists described it, surrounding a perfectly circular dark hole.
“It feels like looking at the gates of hell,” said Heino Falcke of Radboud University in Nijmegen, the Netherlands.
As black holes consume matter that gets too close, they squeeze it into a superheated disc of glowing gas.
In the image of the giant black hole at the heart of the nearby galaxy Messier 87 (M87), the bottom of the ring appears bright because the gases there are being whipped towards Earth.
The black hole also bends light around it, which is what creates the circular shadow.
2. They have ‘hair’
In 2015, the late physicist Professor Stephen Hawking suggested that black holes were not the ‘eternal prisons’ many believe them to be, adding that it was possible for data to escape from the abyss.
A year later, he expanded on the theory by saying that the answer lies in the zero-energy particles, or ‘soft hairs’, that sit on the black hole’s horizon.
It suggests that particles sitting at the event horizon, the boundary of the black hole, would consist of photons and gravitons, which are subatomic packets of light and gravitational energy.
These quantum particles of very low or even zero energy deposited on the edge of the black hole can capture and store information removed from the particles falling into the black hole.
This effectively means that while the particles falling into the black hole may be gone, their information continues to linger on the edge of oblivion in this ‘soft hair’ of quantum particles.
The theoretical physicist compared the return of information to a burned encyclopedia, where information would technically not be lost, but would be incredibly difficult to decipher.
The hypothesis has not been proven, but may help resolve a long-standing paradox about what happens to gas and dust that has fallen into a black hole.
3. They emit fountains of gas
A black hole’s powerful gravitational grip means that nothing can escape if it gets extremely close to the edge of the hole.
But many of these mysterious objects are actually surrounded by an accumulation of gas and dust that orbit black holes a bit like water going down a drain.
According to a 2018 study, this build-up of material is a three-step process.
First, the cold gas forms a disk near the plane of rotation that heats up until the molecules break down.
Some of these molecules are ejected above and below the disk, which then recedes to create a fountain-like structure.
Alternative observations also suggest that this motion produces curved rings surrounding the inner columns of matter, which shoot straight into the air and strongly resemble fountains.
4. They are the source of dark energy
Last month, scientists from Imperial College London made an exciting announcement about black holes.
They excitingly revealed that the objects could actually be the source of unknown energy known as Dark Energy.
Basically, the Big Bang theory of the creation of our universe originally predicted that its expansion would slow down – or even begin to contract – due to gravity.
But in 1998, astronomers were surprised to discover that not only was the universe still expanding, this expansion was also accelerating.
To account for this discovery, it was proposed that a ‘dark energy’ was responsible for pushing things apart more strongly than gravity.
This was linked to a concept Einstein had proposed but later discarded – a ‘cosmological constant’ that counteracted gravity and prevented the universe from collapsing.
However, black holes posed a problem – their extremely strong gravity is hard to resist, especially at their centers, where everything seems to collapse into a phenomenon called a ‘singularity’.
To dig deeper into the problem, a team of 17 researchers from nine countries studied nine billion years of black hole evolution.
They observed ancient and dormant galaxies and found that black holes gain mass in a way consistent with containing vacuum energy, or dark energy.
In fact, the size of the universe at various times closely matched the mass of supermassive black holes at the heart of galaxies.
In other words, the amount of dark energy in the universe can be explained by black hole vacuum energy – meaning that black holes are the source of dark energy.
5. They can be ‘back doors’ to other parts of the universe
Deep inside a black hole is the gravitational singularity, where space-time curves toward infinity and whatever passes through can survive.
Or so it has always been believed.
But in a recent study, researchers suggested that there may actually be a way out through a wormhole at the center of the black hole, which acts as a ‘back door’.
According to this theory, anything traveling through the black hole would be ‘spaghettified’, or stretched to the limit, but return to its normal size when it emerges in another region of the universe.
Although it is unlikely that a human would survive this process, the researchers say that the matter inside the black hole would not be lost forever, as previously thought, and would instead be ejected into another region of the universe.
And the researchers say there would be no need for ‘exotic’ energy to generate the wormhole, as Einstein’s theory of gravity suggests.