A thermodynamic approach to gravity could explain cosmic acceleration without dark energy - Physics - General Physics - Quantum Physics

A small causal diamond used as a tiny local laboratory for deriving gravity from thermal physics. Heat flows in and out across the light-like boundaries of the diamond, allowing the authors to ask what kind of gravitational theory emerges from a more general thermodynamic process that might happen inside the diamond. Credit: Isichei and Magueijo / Physical Review Letters.

Gravity, the force that attracts objects toward each other, is currently framed by Albert Einstein's theory of general relativity. This framework describes gravity as the curvature of spacetime, the invisible four-dimensional fabric of the universe.

While general relativity is now the central theory of gravity, it fails to explain some cosmological phenomena and mysteries, such as the so-called cosmological constant problem. This is the unexplained mismatch between the observed energy of empty space and the far greater values predicted by quantum theories.

In a recent paper published in Physical Review Letters, researchers at Imperial College London tried to frame gravity using thermodynamics, the framework that describes how energy and heat transform. Their study builds on a seminal paper by theoretical physicist Ted Jacobson, published more than three decades ago.

"I first came across Jacobson's seminal 1995 work when I was just out of my Ph.D., and I found the idea fascinating," João Magueijo, senior author of the paper, told Phys.org.

"He inverted the logic of Hawking and Bekenstein's arguments that Einstein gravity has temperature and entropy and instead used thermal physics to derive Einstein gravity. I wanted to do something with this idea for years, but all my attempts failed miserably. Then last year, while on holiday on a remote Greek island, part of which has no internet, which may have helped, I realized that most previous work had tried to retrofit existing theories of gravity into Jacobson's construction." 

The thermodynamic cycle proposed in the Letter. Standard Einstein gravity corresponds to the degenerate case in which only heat-flow legs are present. Allowing the additional work-producing legs opens the door to new gravitational theories, including ones in which matter-energy conservation is modified. Credit: Isichei and Magueijo / Physical Review Letters.

Building on this realization, Magueijo started exploring the possibility of describing gravity starting from thermal physics alone, without trying to determine what type of gravity theory would emerge. His hope was that this process would lead to entirely new theories of gravity that no one had thought of before.

Linking gravity, thermodynamics and the expanding universe

To further develop the ideas he had been contemplating, Magueijo started collaborating with Ray Isichei, a Ph.D. student he was supervising at Imperial College. Together, the two researchers started examining gravity from a thermodynamic standpoint, specifically framing it as an Otto cycle, a thermodynamic construct that describes how gasoline engines work.

"We asked what happens if the thermodynamic process behind gravity is not just heat flow," Magueijo explained. "In ordinary thermodynamics, heat is almost never the whole story: There may also be chemical reactions, expansion against a piston, work being done or other contributions. So, we added this missing 'something else' to the argument, without prejudice regarding what would come out the other side."

To their surprise, the researchers found that the gravitational theory they derived allowed matter and energy to be created or destroyed. This was a total shock, as the conservation of energy and matter is a fundamental physical principle. The fact that it could be violated almost prompted them to abandon their theory altogether.

"The idea did not end up in the garbage bin because we realized that, when applied to the universe as a whole, it could reproduce the observed acceleration of cosmic expansion without having to posit dark energy, a cosmological constant, or any of the usual ingredients invoked to explain it," Magueijo said.

"Normal matter should pull back and decelerate the expansion of the universe, but that assumes the usual conservation laws. In this model, normal matter whose conservation law is modified (allowing for continuous creation) can instead drive acceleration."

Fueling new theoretical studies

The team's study offers a fresh and unconventional theory of gravity, suggesting that Einstein's theory of relativity could also potentially be framed as a thermodynamic process. This theoretical framework could eliminate the need for a conventional cosmological constant, potentially helping to tackle a long-standing issue in cosmology.

While the new theory devised by Magueijo and Isichei is intriguing, it is still speculative and in its early stages. The researchers are now planning further studies aimed at developing it further and comparing its predictions with available cosmological evidence and experimental results.

"A lot of work now needs to be done comparing the model in detail with cosmological observations," Magueijo added. "When I started my Ph.D., back in 1990, you could still say almost anything in cosmology, because the paucity of data allowed it. Cosmology has since become a high-precision, data-driven subject. Any new idea now must pass the gauntlet of observation." 

Source: A thermodynamic approach to gravity could explain cosmic acceleration without dark energy