Einstein's General Theory of Relativity passes biggest cosmic test

Washington, March 11 : Albert Einstein's General Theory of Relativity has passed its biggest test yet, with an analysis of more than 70,000 galaxies by scientists, supporting the hypothesis.

By calculating the clustering of these galaxies, which stretch nearly one-third of the way to the edge of the universe, and analyzing their velocities and distortion from intervening material, the researchers have shown that Einstein's theory explains the nearby universe better than alternative theories of gravity.

General relativity has been rigorously tested within the Solar System, where it explains the motion of planets with precision.

But its reach between galaxies has been harder to verify and should not be taken for granted, according to cosmologist Alexie Leauthaud, at the Lawrence Berkeley National Laboratory in California.

If general relativity does break down at large scales, it could help cosmologists to explain away one of their biggest headaches: dark energy, the origin of which remains a mystery.

Dark energy is a force that makes up almost three-quarters of the matter and energy in the Universe and pushes it apart.

The accelerated expansion could be explained without dark energy, however, if general relativity is wrong and gravity weakens at cosmic scales.

Several candidate 'modified gravity' theories take this line but, until now, no one has come up with a way to test them at large scales.

Now, Reina Reyes at Princeton University in New Jersey and her colleagues have compared some of the models using data on the position, velocity and apparent shape of 70,000 distant galaxies mapped by the Sloan Digital Sky Survey.

The rival theories make different predictions about the degree to which light travelling to us from distant galaxies will be bent by the gravity of intermediate galaxies.

This process, called 'gravitational lensing', distorts the apparent shape of the galaxies.

The theories also make different predictions for both how fast galaxies grow and how they cluster together.

The team had to combine all three measures — gravitational lensing, growth and clustering — into one ratio, called EG, in such a way that any uncertainty introduced by dark matter assumptions cancels out.

The team found a value for EG of about 0.39 - a good match to the general relativistic prediction of around 0.4.

Scott Dodelson at the Fermilab Center for Particle Astrophysics in Batavia, Illinois, who was part of the team that proposed the EG test in 2007, said that the work is "exciting and important".

"It's impressive that the team has shown we can rigorously test general relativity at large scales with data we have now," he said.