University of Copenhagen | Niels Bohr Institute | 2011 Sept 28
All observations in astronomy are based on light emitted from stars and galaxies and, according to the general theory of relativity, the light will be affected by gravity. At the same time all interpretations in astronomy are based on the correctness of the theory of relatively, but it has never before been possible to test Einstein’s theory of gravity on scales larger than the solar system. Now astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have managed to measure how the light is affected by gravity on its way out of galaxy clusters. The observations confirm the theoretical predictions. The results have been published in the prestigious scientific journal, Nature.
Observations of large distances in the universe are based on measurements of the redshift, which is a phenomenon where the wavelength of the light from distant galaxies is shifted more and more towards the red with greater distance. The redshift indicates how much the universe has expanded from when the light left until it was measured on Earth. Furthermore, according to Einstein’s general theory of relativity, the light and thus the redshift is also affected by the gravity from large masses like galaxy clusters and causes a gravitational redshift of the light. But the gravitational influence of light has never before been measured on a cosmological scale.
"It is really wonderful. We live in an era with the technological ability to actually measure such phenomena as cosmological gravitational redshift”, says astrophysicist Radek Wojtak, Dark Cosmology Centre under the Niels Bohr Institute at the University of Copenhagen.
Galaxy clusters in the searchlight
Radek Wojtak, together with colleagues Steen Hansen and Jens Hjorth, has analysed measurements of light from galaxies in approximately 8,000 galaxy clusters. Galaxy clusters are accumulations of thousands of galaxies, held together by their own gravity. This gravity affects the light being sent out into space from the galaxies.
The researchers have studied the galaxies lying in the middle of the galaxy clusters and those lying on the periphery and measured the wavelengths of the light.
"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”, explains Radek Wojtak. Then he measured the entire galaxy cluster’s total mass and with that got the gravitational potential. By using the general theory of relativity he could now calculate the gravitational redshift for the different locations of the galaxies.
"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. 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. In that way our observations confirm the theory of relativity”, explains Radek Wojtak.
New light on the dark universe
The discovery has significance for the phenomena in the universe that researchers are working to unravel. It is the mysterious dark universe – dark matter and dark energy. In addition to the visible celestial bodies like stars, planets and galaxies, the universe consists of a large amount of matter, which researchers can work out that it must be there, but which cannot be observed as it neither emits nor reflects light. It is invisible and is therefore called dark matter. No one knows what dark matter is, but they know what the mass and thus the gravity must be. The new results for gravitational redshift do not change the researchers’ modelling for the presence of dark matter.
Another of the main components of the universe is dark energy, which according to the theoretical models acts like a kind of vacuum that causes the expansion of the universe to accelerate. According to the calculations, which are based on Einstein’s theory of relativity, dark energy constitutes 72 percent of the structure of the universe. Many alternative theories try to explain the accelerating expansion without the presence of dark energy.
Theory tested on a large scale
"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”, explains Radek Wojtak.
The new gravitation results thus contribute a new piece of insight to the understanding of the hidden, dark universe and provide a greater understanding of the nature of the visible universe.
Galaxy Clusters Validate Einstein's Theory
Science NOW | Yudhijit Bhattacharjee | 2011 Sept 28
Testing gravity is simple: walk out of a second-floor window and see what happens. It's a lot tougher to test Albert Einstein's theory of gravity—the general theory of relativity—which says that the gravity of an object warps space and time around it. Although researchers have proved general relativity on the scale of the solar system, validating it on cosmic scales has been more challenging. That's exactly what a group of astrophysicists in Denmark have now done.
The researchers, led by Radek Wojtak of the Niels Bohr Institute at the University of Copenhagen, set out to test a classic prediction of general relativity: that light will lose energy as it is escaping a gravitational field. The stronger the field, the greater the energy loss suffered by the light. As a result, photons emitted from the center of a galaxy cluster—a massive object containing thousands of galaxies—should lose more energy than photons coming from the edge of the cluster because gravity is strongest in the center. And so, light emerging from the center should become longer in wavelength than light coming from the edges, shifting toward the red end of the light spectrum. The effect is known as gravitational redshifting.
Wojtak and his colleagues knew that measuring gravitational redshifting within a single galaxy cluster would be difficult because the effect is very small and needs to be teased apart from the redshifting caused by the orbital velocity of individual galaxies within the cluster and the redshifting caused by the expansion of the universe. The researchers approached the problem by averaging data collected from 8000 galaxy clusters by the Sloan Digital Sky Survey. The hope was to detect gravitational redshift "by studying the properties of the redshift distribution of galaxies in clusters rather than by looking at redshifts of individual galaxies separately," Wojtak explains.
Sure enough, the researchers found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster, as predicted by general relativity. "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 says. The findings appear online today in Nature.
Besides confirming general relativity, the results strongly support the Lambda-Cold Dark Matter model of the universe, an already popular cosmological model according to which most of the cosmos is made up of invisible stuff that does not interact with matter constituting stars and planets. The test also lends support for dark energy, the mysterious force that appears to be pushing the universe apart.
David Spergel, an astrophysicist at Princeton University, compliments Wojtak and his colleagues on "cleverly combining" a large cluster data set to detect a "subtle effect." Spergel says, "This is another victory for Einstein. ... This cluster test suggests that we do live in a strange universe with dark matter and dark energy, but one in which Einstein's theory of gravity is valid on large scales."
Gravitational redshift of galaxies in clusters as predicted by general relativity - Radoslaw Wojtak, Steen H. Hansen, Jens Hjorth
- Nature 477 567 (29 Sept 2011) DOI: 10.1038/nature10445
A New Confirmation of General Relativity
Centauri Dreams | Paul Gilster | 2011 Sept 28