Photons from a central galaxy of a galaxy cluster are gravitationally redshifted by a larger amount than other neighbouring galaxies (Figure 1). Measuring and modelling such a signal from galaxy clusters is a relatively new frontier in cosmology. Gravitational redshift is a prediction from General Relativity. Yan-Chuan Cai's homepage Gravitational Redshift from Galaxy Clusters Follow LiveScience for the latest in science news and discoveries on Twitter and on Facebook.
You can follow LiveScience senior writer Clara Moskowitz on Twitter ClaraMoskowitz. "Discussions of gravity's properties will continue, but Wojtak and colleagues' pioneering work gives a glimpse of the potential of new cosmological tests for achieving higher precision when millions of galaxy redshifts, from which gravitational redshifts can be extracted, become available in the future," physicist Gary Wegner of Dartmouth College in New Hampshire, who was not involved in the new research, wrote in an accompanying essay in the same issue of Nature. Ultimately, as more data is gathered about distant galaxies, such cosmic measurements should become even more accurate, and physicists may be able to distinguish better between the competing models. To preserve the theory, physicists would have to make some changes.
However, another alternative theory, called Tensor–vector–scalar gravity (TeVeS), does conflict with the new findings. This model, too, agrees with Wojtak and his colleagues' new measurements. One such competing theory is called the f(R) theory. But some researchers say these bizarre inventions aren't necessary if we simply tweak general relativity itself. Within the framework of general relativity, scientists have invented concepts called dark matter and dark energy, respectively, to deal with these problems. Yet there are still competing theories that have been proposed in recent years to accommodate the strange discovery that the universe seems to contain much more mass than simply the visible matter we can see, and that the cosmos seems to be accelerating in its expansion, propelled by an unknown force. The findings further support the already well-entrenched general theory of relativity, which has been successful in predicting many cosmic phenomena observed throughout the universe. We have a link between our local scale of the Earth and galaxy clusters." "This is the only general relativistic effect which has been observed and confirmed locally on the Earth and on the scale corresponding to the universe. "In our work we present for the first time the same effect but on a scale which is many orders of magnitude larger," Wojtak told LiveScience. Yet no one had managed to detect a proof of this prediction of general relativity on the cosmic scale, until now. Other studies confirmed the effect in the sun and in small nearby stars called white dwarfs. "This was a groundbreaking experiment," Wojtak said. This discovery came in 1959, when researchers measured the gravitational redshift in gamma-ray light emitted in a lab here on Earth. It took physicists 43 years to detect evidence of gravitational redshift. A decreased frequency is equivalent to a longer, or redder, wavelength. When that light travels to a new environment, say to a telescope on Earth, where there is comparatively lower gravity, and time moves more slowly, the light's frequency will decrease. Light emitted in this environment will have a certain frequency, which is related to the time scale (or the gravity strength) of the environment. Where you have a large mass like a galaxy cluster, there is strong gravity and space-time is severely warped, causing time to move more quickly. And mass, Einstein showed, affects space-time profoundly, by warping it. Specifically, it united the two concepts, which were thought to be independent, into one entity. General relativity, proposed by Einstein in 1916, revolutionized the way physicists think about space and time. Wojtak is lead author of a paper reporting the results in tomorrow's (Sept. "It agrees exactly with the measurements of this effect." "We have independent measurements of the cluster masses, so we can calculate what the expectation for gravitational redshift based on general relativity is," said University of Copenhagen astrophysicist Rados?aw Wojtak.
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