General Relativity fights off competing theory

Posted by Emma Woollacott

Einstein famously said that no amount of experimentation would prove him right. But two new studies have backed up his General Theory of Relativity, and one significantly undermines a rival theory.

Both teams took advantage of observations from the Chandra X-ray Observatory of galaxy clusters, the largest objects in the universe bound together by gravity.

The first finding significantly weakens a competitor to General Relativity known as 'f(R) gravity'.

"If General Relativity were the heavyweight boxing champion, this other theory was hoping to be the upstart contender," said Fabian Schmidt of the California Institute of Technology in Pasadena, who led the study. "Our work shows that the chances of its upsetting the champ are very slim."

In recent years, some physicists have suggested competing theories to General Relativity to explain the accelerated expansion of the universe.  Currently, the most popular explanation is the so-called cosmological constant, which can be understood as energy that exists in empty space. This energy is referred to as dark energy as it can't be directly detected.

In the f(R) theory, the cosmic acceleration comes not from an exotic form of energy but from a modification of the gravitational force. This also affects the rate at which  massive clusters of galaxies form, opening up the possibility of a sensitive test of the theory.

Schmidt and colleagues used mass estimates of 49 galaxy clusters and compared them with theoretical model predictions and studies of supernovas, the cosmic microwave background, and the large-scale distribution of galaxies.

They found no evidence that gravity is different from General Relativity on scales larger than 130 million light years.

A second study also bolsters General Relativity by directly testing it across cosmological distances and times. Until now, General Relativity had been verified only up to solar system scales, leaving  the possibility that it breaks down on much larger scales.

To ckeck this out, a group at Stanford University compared Chandra observations of how rapidly galaxy clusters have grown over time to the predictions of General Relativity. The result is nearly complete agreement between observation and theory.

"Einstein's theory succeeds again, this time in calculating how many massive clusters have formed under gravity's pull over the last five billion years," said David Rapetti of Stanford University and SLAC National Accelerator Laboratory, who led the study.

"Excitingly and reassuringly, our results are the most robust consistency test of General Relativity yet carried out on cosmological scales."