Luminous Galaxy Clusters Confirm Einstein’s Theory of Relativity

Light emitted from galaxy clusters is affected by gravity, supporting General Relativity—Albert Einstein’s theory of gravitation—on a scale larger than our solar system, according to research published in Nature on Sept. 28.

Large distances in the universe can be measured using redshift, where light’s wavelength moves more and more towards the red with a galaxy’s remoteness. When it reaches Earth, this reveals how much more the universe has expanded since the light was emitted.

Based on Einstein’s General Relativity, redshift is affected by gravity from large masses, such as galaxy clusters—thousands of galaxies held together by their own gravity.

However, until now, this gravitational influence has not been measured beyond our solar system.

“It is really wonderful,” said Radek Wojtak of the University of Copenhagen in a press release. “We live in an era with the technological ability to actually measure such phenomena as cosmological gravitational redshift.”

Wotjak’s team looked at data from about 8,000 galaxy clusters, and measured redshift wavelengths in galaxies at cluster centers and those at the edges.

“We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to ‘crawl’ out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge,” Wojtak explained.

By measuring each cluster’s total mass, the researchers found its gravitational potential and calculated the gravitational redshift at different locations using General Relativity.

“It turned out that the theoretical calculations of the gravitational redshift based on the general theory of relativity was in complete agreement with the astronomical observations,” Wojtak said.

“Our analysis of observations of galaxy clusters show that the redshift of the light is proportionally offset in relation to the gravitational influence from the galaxy cluster’s gravity,” he added.

“In that way our observations confirm the theory of relativity.”

These findings provide compelling evidence for the existence of dark energy, a mysterious force originally hypothesized by Einstein to explain the ever-accelerating expansion rate of our universe.

Dark energy can only be seen through its effect, because it does not emit or reflect light. Based on Einstein’s theory of relativity, dark energy comprises 72 percent of the universe’s structure.

“Now the general theory of relativity has been tested on a cosmological scale and this confirms that the general theory of relativity works and that means that there is a strong indication for the presence of dark energy,” Wojtak concluded.