“The ‘Elephant’ in the Cosmos” –New Theory Says ‘No Need’ for Dark Energy

Dark Energy Survey


Dark energy has been described as everything from a fifth force to a new form of matter, but so far, no direct evidence has been found of its existence. Noble Prize winning physicist Adam Riess says: “I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative.”

“It’s the big elephant in the room,” says Claudia de Rham, a theoretical physicist at Imperial College about dark energy, the mysterious phenomena that pushes the cosmos to expand so rapidly and which is estimated to account for 70% of the contents of the universe. “It’s very frustrating.”

In her recent work, The gravitational rainbow beyond Einstein gravity, de Rham observes that recent direct detection of gravitational waves have been successfully used to examine the basic properties of the gravitational degrees of freedom setting an upper bound on their mass and constraining their speed of propagation with unprecedented accuracy. Within the current realm of observational and theoretical constraints, she explored the possibility for gravity to depart from general relativity (GR) in the infrared and its profound implications on our observable Universe.

De Rham has built on a radical theory proposed in 1939 by Swiss physicist Markus Fierz and Austrian theoretical physicist Wolfgang Pauli, one of the pioneers of quantum physics, that added mass to Einstein’s graviton. In Einstein’s general relativity, gravitational interactions are mediated by a massless spin-2 particle – the so-called massless graviton which gives it what gives it its enduring properties: long range and the universality.

“Dark Energy”– A Fifth Force or New Form of Matter?

If gravitons have a mass, reports The Guardian, then gravity is expected to have a weaker influence on very large distance scales, which could explain why the expansion of the universe has not been reined in. “One possibility is that you may not need to have dark energy – or rather, gravity itself fulfills that role,” says De Rham about her theory, known as massive gravity, that modifies Einstein’s general relativity, positing that the hypothetical particles (gravitons) that mediate the gravitational force themselves have a mass.

De Rham’s theory may signal a breakthrough in the century-long quest to build a working theory of massive gravity. Despite successive efforts, notes New Scientist “previous versions of the theory had the unfortunate feature of predicting the instantaneous decay of every particle in the universe – an intractable issue that mathematicians refer to as a ‘ghost’.”

“Very clever people had worked on this and the arguments were very compelling,” says De Rham. “People thought it would be impossible to make it work.” In 2011, when De Rham and her collaborators, Gregory Gabadadze from New York University and Andrew Tolley from Imperial College London, published a landmark paper on massive gravity, the response was swift and hostile.

The Chameleon –“Dark Energy is Hiding”

“People have their egos,” she told The Guardian. “If you say ‘Well actually what you did 40 years ago wasn’t quite right’, they’re not going to say ‘Let’s talk about it’.”

De Rham is quick to point out that at this stage, massive gravity is still just a theory: “It would be amazing if it was shown to be right,” she says. “That may or may not happen, but what will happen is that we’ll have a much better fundamental understanding of gravity and that’s just something so deep, it’s one of the big questions today.”

The Daily Galaxy, Max Goldberg, via The Guardian and Inspire

Image Credit: Dark Energy Survey Photo by Reidar Hahn, Fermilab