Could ‘negative mass’ unify dark matter, dark energy?

The universe may be suffused with an invisible fluid that exerts negative gravity: repelling rather than attracting.

The theory has been published by Jamie Farnes from University of Oxford in England in the journal Astronomy and Astrophysics and if correct would singlehandedly explain two of the cosmos’s most mysterious phenomena: dark energy and dark matter.

Astronomers’ measurements of the cosmos suggest that visible material such as stars and gas account for only about 5% of the universe. For example, the way galaxies rotate shows gravitational effects from about five times as much matter as we can see: an invisible entity, dubbed dark matter. 

And the galaxies are moving away from each other with a speed that is increasing. The unexplained energy source for the accelerating expansion is termed dark energy.

In the decades since their discoveries, dark matter and dark energy have inspired many theories, but none have satisfactorily explained either phenomenon. Farnes’ theory seems elegant because it solves the two problems with a single solution – a repulsive dark fluid.

Such a fluid would explain dark energy, because its repulsive properties would push galaxies away from each other. It could also explain dark matter; if the repulsive fluid encircled a galaxy it would effectively compress it, propelling stars towards the galactic centre in the same way as additional mass within the galaxy would. {%recommended 6563%}

The symmetry of the solution appeals to Farnes because it gives gravity an opposite polarity that allows it to repel as well attract – much like the bipolarity of electromagnetic forces. 

Writing in The Conversation, he noted that “it therefore appears that a simple minus sign may solve one of the longest standing problems in physics”.

While Farnes suggests that negative mass is not as crazy as it might seem – for example, a bubble acts as if it possesses it – the theory is far from proven.

Krzysztof Bolejko, physicist at the University of Tasmania in Australia, says Farnes’ maths is fine, but points out that his dark fluid theory adds another complication.

“Farnes introduces matter that has negative mass, but in order to explain the accelerated rate of cosmic expansion this matter needs to be constantly created,” he says.

A font of constantly-created fluid contrasts with the most successful model to date of the cosmos, known as the Lambda Cold Dark Matter (CDM) model. In this construct, dark energy is modelled as a property of space, and cosmic expansion is a result of changes in the shape and curvature of the universe.

Farnes notes that his theory needs considerable work to explore all the implications of dark repulsive fluid theory. Whether it describes the universe better than the CDM model remains to be seen. The test will be where the two theories make differing predictions. The one that matches the observed behaviour for the universe will prevail.

Bolejko’s hunch is that the difference might show up cosmic voids: sparse regions of space that could fit hundreds of thousands of galaxies but instead harbour only a few.  

“We are starting to study cosmic voids. In galaxies there are so many processes it’s hard to tell what is caused by matter and what is from the dark sector,” he says.

“Inside cosmic voids the signal will be clearer and so it will be easier to distinguish between processes caused by dark energy and those caused by a constantly created matter with negative mass.”