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See that bullet-shaped golden object in the galaxy cluster? That's Albert Einstein shooting down the competing theories to his General Theory Of Relativity. (Actually, it's X-ray data showing material flowing into a galaxy cluster, as a result of a merger.) This composite image includes X-ray data from Harvard's Chandra Observatory (in gold); optical images from the Digitized Sky Survey in red, green and blue; and radio images from the Very Large Array in blue. The combined image allows scientists to study how gravity works at cosmic scales. The growth of galaxy clusters like this one, Abell 3376, is influenced by the expansion rate of the universe, which in turn is influenced by dark matter and dark energy. Two new theories claim that gravity should act differently than Albert Einstein's General Relativity predicted, at massive scales — larger than 130 million light years. According to Chandra's press release:
In the first of the new studies of gravity, an alternative theory to General Relativity called "f(R) gravity" was tested. In this theory, the acceleration of the expansion of the universe does not come from an exotic form of energy but from a modification of the gravitational force. Mass estimates of galaxy clusters in the local universe were compared with model predictions for f(R) gravity. Data from geometrical studies, such as supernova work, were also used. Using this comparison between theory and observation, no evidence was found that gravity is different from General Relativity on scales larger than 130 million light years. This limit corresponds to a hundred-fold improvement on the bounds of the modified gravitational force's range that can be set without using the cluster data.
In the second study, a comparison was made between X-ray observations of how rapidly galaxy clusters have grown over cosmic time to the predictions of General Relativity. Once again, data from geometrical studies such as distances to supernovas and galaxy clusters were incorporated. Nearly complete agreement was seen between observation and theory, arguing against any alternative gravity models with a different rate of growth. In particular "DGP gravity" (named after its inventors Gia Dvali, Gregory Gabadadze, and Massimo Porrati) predicts a slower rate of cluster growth than General Relativity, because gravity is weakened on large scales as it leaks into an extra dimension. Like f(R) gravity, the DGP model is designed to avoid the need for an exotic form of energy causing cosmic acceleration.