How Gravity Works: 'Chameleon Theory' Could Provide Alternative to Einstein's General Theory of Relativity

In 1915, Albert Einstein published his pioneering theory of general relativity (GR) which beautifully describes the nature of gravity and its effect on large objects in space. Since then, this work has formed the bedrock of scientists' understanding of our universe.

But despite its importance, general relativity is not the only explanation for the workings of gravity. In fact, various scientists have proposed alternative theories over time, though it should be noted that none of these are consistent with experimental data to the extent that GR is.

Now, a team of scientists from the University of Durham, U.K., has published a study in the journal Nature Astronomy which suggests that one of these alternate theories—known as f(R) gravity or "chameleon theory"—could actually explain how gravity works in some situations.

"Chameleon theory is a class of gravity theories alternative to Einstein's general relativity," Baojiu Li, a co-lead author of the paper from Durham's Institute for Computational Cosmology (ICC,) told Newsweek. "In this theory, in addition to the standard gravitational force, there is an extra force—the so-called 'fifth force' because it is different from the other four types of fundamental interactions."

"The theory has non-linear dynamics, which enables the additional force to be conveniently 'hidden' in dense regions such as our solar system, where experiments have found no evidence for it," Li said. "In low-density regions, however, experimental data is scarce and the additional force can be at work, which will affect how things gravitate. Such an environment-dependent behavior of the theory earns it the name 'Chameleon.'"

For their study, the team ran advanced supercomputer simulations which showed that galaxies could still form in a universe where the principles of chameleon theory applied.

"What we have found is that realistic galaxies like our Milky Way can form, with properties as observed, even with the complicated behavior of the fifth force—this is by no means guaranteed," Li said. "It also shows that chameleon theory can make certain distinct predictions on the evolution of the large-scale structures in the universe from GR."

The latest findings could have significant implications for our understanding of phenomena such as black holes and the role they play in galaxy formation, as well as the nature of so-called "dark energy"—the mysterious force or substance that scientists use to explain the accelerating expansion rate of the universe. The researchers say that the latest findings could help to cast new light on the properties of dark energy, for example.

"General relativity has been very successful in explaining many phenomena, and is one of the foundations of modern cosmology," Li said. "However, one thing that has been puzzling cosmologists is the observational evidence that the expansion of the universe is accelerating. In the framework of GR, the expansion rate is determined by the matter density in the universe, and by accounting for all known matter species, including dark matter, the theory of GR predicts that this rate must be ever decelerating."

"Therefore, one has to assume that there are other—so far undetected—materials present in the universe which speeds up its expansion rate," Li said. "A so-called cosmological constant is one of the simplest possibilities, but its origin and value have proven to be difficult to explain, which motivates other, alternative, explanations, such as dark energy."

Chameleon theory is an example of a different class of alternative explanations, known as modified gravity theories, in which the speeding up of the universe's expansion is assumed to be due to a modified gravitational law compared to GR.

"The accelerated expansion is one of the biggest questions in modern cosmology, and both dark energy and modified gravity models are being actively studied in order to find a satisfactory answer," Li said. "Chameleon theory is a popular modified gravity model."

The researchers stress that their results don't prove that general relativity is necessarily wrong, just that it doesn't have to be the only way to explain gravity's role in the universe's evolution.

"These results on their own do not suggest any problems with GR," Li said. "The success of GR in explaining small-scale—such as in the Solar System—behaviors of gravity is not spoiled. On the other hand, the experimental or observational confirmation of GR on much larger—such as cosmological—scales and for much larger systems is less well established, leaving the possibility of alternative theories."

"This study is the first time to demonstrate that an alternative gravity theory, despite its complicated behavior of gravitational force, can still make realistic galaxies," Li said. "Studies like this will help us check the feasibility of a theory, as well as identifying places where we may test the different theories of gravity using future observational data. Of course, much more work is needed to reach a final conclusion."

The next step for the Durham team is to test their findings using real world experimental data. However, they will have to wait until next year's opening of the Square Kilometre Array in South Africa—what will be the world's largest radio telescope—in order to conduct this research.

This article was updated to include additional comments from Baojiu Li.

Milky Way
The Milky Way galaxy appears in the dark night sky over hoodoos in the Alberta Badlands at Dinosaur Provincial Park. Christopher Morris/Corbis via Getty Images
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