The vast majority of the universe is made of stuff that we cannot see, since it's invisible to light, which has appropriately been named dark matter, and a force that repels gravity, named dark energy. Understanding their true nature is very difficult and while we can study the effects, we are yet to pin down the causes. Now a new theory proposes a unification of dark matter and dark energy into a single substance: a dark fluid with negative mass.
Published in Astronomy & Astrophysics, this dark fluid approach proposes a way to combine the two main explanations for dark matter and energy into a single substance.
"We now think that both dark matter and dark energy can be unified into a fluid which possesses a type of 'negative gravity,' repelling all other material around them," author Dr Jamie Farnes, of the University of Oxford, explained in a statement.
Negative mass particles have previously been ruled out as an explanation because as the universe expands, it is thought they would become less dense. But this new model suggests that these weird particles are continuously being created in the universe, and thus are not being diluted, delivering both exotic particles and changes to gravity.
“To my knowledge, this is the first theory that has incorporated both outcomes,” Dr Farnes told IFLScience. “As a bonus, the theory then seems to explain not only dark matter, but we also get a possible solution for dark energy – essentially for free. It seems that a simple minus sign may possibly be able to solve two of the biggest problems in physics”.
The model was tested using a simulation of the universe and it was able to create a virtual cosmos whose galaxy distribution formed as expected. It's an intriguing finding, though far from definite proof. But there are ways for this idea to be put to the test.
The model expects the distribution of galaxies to be changing in a very specific way over the ages of the universe. The Square Kilometer Array (SKA), a proposed radio telescope that if it goes ahead will be built in South Africa and Australia, would produce enough data on the subject that it should be able to prove if dark matter, modified gravity, or even this dark fluid is responsible.
The construction of the SKA will begin in 2019 and a more definite answer might come towards 2030. Since there’s time, Farnes is looking into past data to see if there are suggestions that negative mass particles might exist. He also suggested that CERN could spot them, if they do.
The model, as it currently stands, however, has no explanation for the nature of negative masses and their origin, which is an important limitation. Mass remains poorly understood in particle physics and more work is needed. Dr Farnes hopes that his idea is considered seriously even if it is unconventional.
“Of course, just because the theory is unconventional does not mean that it is correct! The data may possibly end up confirming that there are no negative masses, or possibly that we are completely surrounded by negative masses,” Dr Farnes stated. “Either way I am happy if we can move a little closer to understanding the true nature of our universe.”