Einstein (Still) Rules The Universe
This composite image of the galaxy cluster Abell 3376 shows X-ray data from the Chandra X-ray Observatory and the ROSAT telescope in gold, an optical image from the Digitized Sky Survey in red, green and blue, and a radio image from the VLA in blue.
Credit: X-ray: NASA/CXC/SAO/A. Vikhlinin; ROSAT Optical: DSS Radio: NSF/NRAO/VLA/IUCAA/J.Bagchi

Two new studies that put Einstein's General Theory of Relativity to the test proved that even after 95 years, there's no messing with Albert.

The pair of independent studies each used observations from NASA's Chandra X-ray Observatory to test Einstein's theory of General Relativity, and to study the properties of gravity on cosmic scales. Both demonstrated that Einstein's theory continues to hold true almost a century after it was first published.

Each team of scientists took advantage of extensive Chandra observations of galaxy clusters, which are cosmic formations that can contain anywhere from 10 to thousands of galaxies. These clusters are the largest objects in the universe bound together by gravity.

Galaxy clusters are important objects that help researchers understand the universe as a whole. Because the observations of the masses of galaxy clusters are directly sensitive to the properties of gravity, they can provide crucial information.

The results of one study undercut a rival gravity model that challenged General Relativity, while the other showed that Einstein's theory is applicable over a vast range of times and distances across the cosmos.

Einstein takes all challengers

General relativity rocked the world of physics when Einstein first published his paper on the subject in 1915.

The theory built on the traditional idea of gravity based on Isaac Newton's laws, but added fundamentally new concepts like the notion that mass deforms the shape of space-time. This means that objects and even light that move through space near a large mass will travel on a curved path. Furthermore, it means that mass can stretch or shrink time as well.

The first new study's findings 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, Calif., who led the study. "Our work shows that the chances of its upsetting the champ are very slim."

In recent years, physicists have focused their attention on competing theories to General Relativity as a possible explanation for the accelerated expansion of the universe.

Currently, the most popular explanation for the acceleration is the so-called cosmological constant, which can be understood as vacuum energy that exists in empty space. This energy is referred to as dark energy to emphasize that it cannot be directly detected.

In the f(R) theory, the cosmic acceleration does not come from an exotic form of energy, but instead, from a modification of the gravitational force. This modified force also affects the rate at which small enhancements of matter can grow over the eons to become massive clusters of galaxies, opening up the possibility of a sensitive test of the theory.

Schmidt and his colleagues used mass estimates of 49 galaxy clusters in the local universe from Chandra, compared them with theoretical model predictions and studies of supernovas, the large-scale distribution of galaxies, and the cosmic microwave background, which is a form of electromagnetic radiation that fills the universe.

The researchers found that gravity on scales larger than 130 million light-years is no different from Einstein's geometric theory of gravitation.

"This is the strongest ever constraint set on an alternative to General Relativity on such large distance scales," said Schmidt. "Our results show that we can probe gravity stringently on cosmological scales by using observations of galaxy clusters."

Across the universe

The findings of the other, separate study bolstered Einstein's theory by directly testing it across cosmological distances and times.

Until now, General Relativity had only been verified using experiments from laboratory to solar system scales, leaving some to believe that the principles of General Relativity could break down when applied to much larger scales.

A research team at Stanford University compared Chandra observations of how rapidly galaxy clusters have grown over time to the predictions from General Relativity. They found that the Chandra observations were in almost complete agreement with Einstein's 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, who led the study at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University. "Excitingly and reassuringly, our results are the most robust consistency test of General Relativity yet carried out on cosmological scales."

Rapetti and his colleagues based their results on a sample of 238 galaxy clusters that were detected across the whole sky by the now-defunct ROSAT X-ray telescope. These observations were enhanced by detailed mass measurements for 71 distant from Chandra, and 23 relatively nearby clusters again using ROSAT.

Data was combined with studies of supernovas, the cosmic microwave background, the distribution of galaxies and estimates of the distance to galaxy clusters.

Observations of supernovas or the distribution of galaxies measure cosmic distances, which depend only on the expansion rate of the universe. In contrast, the cluster technique used by Rapetti and his colleagues also measure the growth rate of the cosmic structure, as driven by gravity.

"Cosmic acceleration represents a great challenge to our modern understanding of physics," said Rapetti's co-author Adam Mantz of NASA's Goddard Space Flight Center in Maryland. "Measurements of acceleration have highlighted how little we know about gravity at cosmic scales, but we're now starting to push back our ignorance."

The paper by Fabian Schmidt was published in Physics Review D, Volume 80 in October 2009 and is co-authored by Alexey Vikhlinin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Wayne Hu of the University of Chicago, Illinois. The paper by David Rapetti was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society and is co- authored by Mantz, Steve Allen of KIPAC at Stanford and Harald Ebeling of the Institute for Astronomy in Hawaii.