Yesterday’s trip to the dark side involved the so-called ‘dark flow,’ the apparent motion of galactic clusters along a path in the direction of the constellations Centaurus and Hydra. Today we look at two other dark conjectures — dark matter and dark energy. Are both a part of the universe we observe, or can we do away with them by clever manipulation of Einstein’s theory of general relativity? The latest word, from an international team of researchers studying the clustering of more than 70,000 galaxies, is that GR seems to have passed yet another test. This is useful stuff, because one of the implications is that dark matter is the most likely explanation of the movement of galaxies and galaxy clusters as they seem to respond to an unseen mass.
The possibility of dark matter was noted as long ago as 1933 by Fritz Zwicky, who studied the average mass of galaxies within the Coma cluster and obtained a value much higher than expected from their luminosity. Later studies of individual galaxies made it clear that a halo of dark matter would explain anomalous galactic rotations. But all that assumed no changes to general relativity at cosmological scales.
Image: A partial map of the distribution of galaxies in the Sloan Digital Sky Survey, going out to a distance of 7 billion light years. The amount of galaxy clustering that we observe today is a signature of how gravity acted over cosmic time, and allows us to test whether general relativity holds over these scales. Credit: M. Blanton, Sloan Digital Sky Survey.
Confirming general relativity is old news on the level of the Solar System, but tests on the galactic level has proven inconclusive. Theories like tensor-vector-scalar gravity (TeVeS) have emerged that avoid the presence of dark matter by applying changes to general relativity. The theory was developed by Jacob Bekenstein and could account for observed galactic rotations as well as gravitational lensing, but it has remained controversial and the new work seems to rule it out. According to this news release from the University of California at Berkeley, TeVeS posits that acceleration caused by the gravitational force from a body depends not only on the mass of that body, but also on the value of acceleration caused by gravity.
Uros Seljak (UC Berkeley), a co-author of the paper on this work, notes the value of testing general relativity at cosmological distances:
“The nice thing about going to the cosmological scale is that we can test any full, alternative theory of gravity, because it should predict the things we observe. Those alternative theories that do not require dark matter fail these tests.”
As to the tests themselves, they revolve around a quantity known as EG, which is based on the amount of clustering in observed galaxies and the distortion of light produced by its passage through intervening matter. Pengjie Zhang (Shanghai Observatory) explains EG this way:
“Put simply, EG is proportional to the mean density of the universe and inversely proportional to the rate of growth of structure in the universe. This particular combination gets rid of the amplitude fluctuations and therefore focuses directly on the particular combination that is sensitive to modifications of general relativity.”
Seljak notes that cosmological experiments usually involve measuring fluctuations in space, while gravity theories predict relationships between density and velocity, or between density and gravitational potential:
“The problem is that the size of the fluctuation, by itself, is not telling us anything about underlying cosmological theories. It is essentially a nuisance we would like to get rid of,” Seljak said. “The novelty of this technique is that it looks at a particular combination of observations that does not depend on the magnitude of the fluctuations. The quantity is a smoking gun for deviations from general relativity.”
The new study also questioned theories like f(R), a mechanism for explaining the accelerated expansion of the universe without resorting to dark energy. The Wikipedia offers up a look at these alternate theories in its section on dark matter, also going into variations of Modified Newtonian Dynamics (MOND), one of which is Bekenstein’s TeVeS. The researchers worked with data from the Sloan Digital Sky Survey to calculate EG and compare it to the predictions of TeVeS as well as f(R) and the cold dark matter model of general relativity as enhanced with a cosmological constant to explain the universe’s accelerated expansion.
The result: General relativity fits within the experimental error, while the EG predicted by f(R) is also within the margin of error, but TeVeS is not. We seem to be making progress, but it’s worth remarking that Seljak is looking toward expanding the analysis to as many as a million galaxies with the advent of the Sloan Digital Sky Survey III’s Baryon Oscillation Spectroscopic Survey, which will be finished in about five years. We can also hope for data from ESA’s Euclid mission along with NASA’s Joint Dark Energy Mission, but for both we’ll have a wait of at least a decade. That gives us time as well for the unfolding of direct detection experiments, which may one day tell us more about the exact identity of dark matter and dark energy.
The paper is Reyes et al., “Confirmation of general relativity on large scales from weak lensing and galaxy velocities,” Nature 464 (11 March 2010), pp. 256-258 (abstract).