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. “I have absolutely no clue what dark energy is,” says Nobel-Prize winning physicist Adam Riess . “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 the mysterious phenomenon of dark energy that supposedly pushes the cosmos to expand and is estimated to account for 68% of the universe. “It’s very frustrating.”
Profound implications on our observable Universe
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 infrared observational and theoretical constraints, de Rham explored the possibility for gravity to depart from general relativity (GR), which would have profound implications on our observable Universe such as the formation and mass content of nearby galaxies. A departure from standard General Relativity would also have important consequences on our understanding of cosmology, structure formation in the early universe, and the ultimate fate of the cosmos.
Foundations of de Rham’s Theory
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 graviton. Like photons, massless gravitons would travel at the speed of light across the observable universe without decaying.
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.
“Gravity fulfills role of dark energy”
“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. Massive gravity modifies Einstein’s general relativity, positing that the hypothetical particles (gravitons) that mediate the gravitational force themselves have a mass.
Could gravitons have the same story as neutrinos?
Gravitons are not the first supposedly massless particles hypothesized to actually have some mass. According to the original Standard Model of particle physics, neutrinos were expected to have zero mass. However, experiments in the late 1990s demonstrated that neutrinos can oscillate between three flavors and therefore must have a very small mass. The lightest of the three neutrinos is six million times less massive than the electron. The 2015 Nobel Prize in physics was awarded for the discovery that neutrinos oscillate and have mass.
A breakthrough on the theory of massive gravity?
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.
“Still just a hypothesis”
“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 hypothesis: “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.”
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