Many physicists expect to understand the nature of dark matter and dark energy in due course. However, others believe that these concepts are merely symptoms of an ailing theory and are looking at alternative models of gravity that can explain observations without invoking dark matter or dark energy. One alternative is modified Newtonian dynamics (MOND), and its generalized partner tensor–vector–scalar (TeVeS) theory, which is supposed to obviate the need for dark matter. Another is f(R) gravity, which does away with dark energy.
Now, Radoslaw Wojtak and colleagues at the University of Copenhagen have used data from the Sloan Digital Sky Survey to test these theories against one another. The study focuses on the gravitational redshift of galaxies within galaxy clusters. This quantity describes how much energy it costs photons to leave a cluster. As they leave and lose energy, the photon wavelengths stretch to the red side of the spectrum. Importantly, the different models of gravity predict different amounts of redshift.
Unfortunately, measuring the gravitational redshift is not easy. There are other sources of redshift including the universe's expansion and the individual motions of galaxies within a cluster. Wojtak and colleagues therefore calculated the average redshift as a function of distance from the cluster's centre – a process that should exclude these other sources.
