Above, a partial map of the distribution of galaxies in the Sloan Digital Sky Survey, going out to a distance of 7 billion light years. The amount of galaxy clustering that we observe today is a signature of how gravity acted over cosmic time, and allows as to test whether general relativity holds over these scales. (Credit: M. Blanton/Sloan Digital Sky Survey)

PRINCETON / UC BERKELEY (US)—An analysis of more than 70,000 galaxies demonstrates that the universe—at least up to a distance of 3.5 billion light years from Earth—plays by the rules set out 95 years ago by Albert Einstein in his General Theory of Relativity.

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, researchers have shown that Einstein’s theory explains the nearby universe better than alternative theories of gravity.

The results are important, the researchers say, because they shore up current theories explaining the shape and direction of the universe, including ideas about “dark energy,” and dispel some hints from other recent experiments that general relativity may be wrong.

“All of our ideas in astronomy are based on this really enormous extrapolation, so anything we can do to see whether this is right or not on these scales is just enormously important,” says James Gunn, the Eugene Higgins Professor of Astronomy, who is a member of the Princeton University team that led the study. “It adds another brick to the foundation that underlies what we do.”

“The nice thing about going to the cosmological scale is that we can test any full, alternative theory of gravity, because it should predict the things we observe,” says coauthor Uros Seljak, a professor of physics and of astronomy at the University of California, Berkeley, and a faculty scientist at Lawrence Berkeley National Laboratory who is on leave at the Institute of Theoretical Physics at the University of Zurich. “Those alternative theories that do not require dark matter fail these tests.”

In particular, the tensor-vector-scalar gravity (TeVeS) theory, which tweaks general relativity to avoid resorting to the existence of dark matter, fails the test.

The result conflicts with a report late last year that the very early universe, between 8 and 11 billion years ago, did deviate from the general relativistic description of gravity.

Findings are reported in the journal Nature. The study was led by researchers at Princeton University.

In recent years, several alternatives to Einstein’s General Theory of Relativity have been proposed. These modified theories of gravity depart from general relativity on large scales to circumvent the need for dark energy, an elusive force that must exist if the calculations of general relativity balance out. But because these theories were designed to match the predictions of general relativity about the expansion history of the universe, a factor that is central to current cosmological work, it has become crucial to know which theory is correct, or at least represents reality as best as can be approximated.

“We knew we needed to look at the large-scale structure of the universe and the growth of smaller structures composing it over time to find out,” says Reinabelle Reyes, a Princeton graduate student.

General relativity holds that gravity warps space and time, which means that light bends as it passes near a massive object, such as the core of a galaxy. Tests on a galactic or cosmic scale have been inconclusive.

“There are some crude and imprecise tests of general relativity at galaxy scales, but we don’t have good predictions for those tests from competing theories,” Seljak says.

Such tests have become important in recent decades because the idea that some unseen mass permeates the universe disturbs some theorists and has spurred them to tweak general relativity to get rid of dark matter.

TeVeS, for example, says that acceleration caused by the gravitational force from a body depends not only on the mass of that body, but also on the value of the acceleration caused by gravity.

The discovery of dark energy, an enigmatic force that is causing the expansion of the universe to accelerate, has led to other theories, such as one dubbed f(R), to explain the expansion without resorting to dark energy.

Tests to distinguish between competing theories are not easy, Seljak says. A theoretical cosmologist, he noted that cosmological experiments, such as detections of the cosmic microwave background, typically involve measurements of fluctuations in space, while gravity theories predict relationships between density and velocity, or between density and gravitational potential.

“The problem is that the size of the fluctuation, by itself, is not telling us anything about underlying cosmological theories. It is essentially a nuisance we would like to get rid of,” Seljak explains. “The novelty of this technique is that it looks at a particular combination of observations that does not depend on the magnitude of the fluctuations. The quantity is a smoking gun for deviations from general relativity.”

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