Can measuring the positions and velocities of thousands of galaxies provide insight into the nature of dark energy? If so, we may have found a way to study what is perhaps the most puzzling question in astrophysics, the discovery that the expansion of the universe is proceeding faster today than it did in the past. Armchair theorists love dark energy because we know so little about it, and I routinely get e-mails offering to tell me exactly what dark energy is, few of which have any bearing on current observation or theory.
But that’s the way of mysteries — they incite comment — and as mysteries go, dark energy is a big one, perhaps the biggest now stirring the astrophysical cauldron. If we assume a dark energy producing a check on the gravitational pull of all matter in the cosmos, we’ve got the attention not just of cosmologists but propulsion theorists, who would love to find out how such a repulsive force might work. And if there is no such thing as dark energy, then determining why should tell us much about where and how we need to tweak current theories of gravitation, which also may have propulsion implications.
The European Southern Observatory’s Very Large Telescope array is at the heart of the latest dark energy work, looking at redshift distortions of distant galaxies by the thousands. The work relies on the fact that the expanding universe pushes galaxies away from each other, even as gravity tries to pull them together. Olivier Le Fevre, a member of the large international team behind this study, focuses on its technique:
“By measuring the apparent velocities of large samples of galaxies over the last thirty years, astronomers have been able to reconstruct a three-dimensional map of the distribution of galaxies over large volumes of the Universe. This map revealed large-scale structures such as clusters of galaxies and filamentary superclusters. But the measured velocities also contain information about the local motions of galaxies; these introduce small but significant distortions in the reconstructed maps of the Universe. We have shown that measuring this distortion at different epochs of the Universe’s history is a way to test the nature of dark energy.”
With thirteen thousand spectra in a field of view twenty times the size of the full Moon now available, the team can compare its result to the 2dF Galaxy Redshift Survey of the ‘local’ universe, assessing what the comparison tells us about dark energy. What seems to be emerging thus far is a confirmation of the technique, which will now require a set of future measurements to be extended over an area ten times larger than the current field. We are, in other words, still at the point of shaping our tools, and unable to make the definitive call on dark energy vs. competing explanations for what we observe. But shaping our tools is simply part of the necessary and painstaking preliminaries that make all science work.
The paper is Guzzo et al. (and I do mean ‘et al.,’ as there are 51 authors listed!), “A test of the nature of cosmic acceleration using galaxy redshift distortions,” Nature 451 (31 January 2008), pp. 541-544 (abstract). Also be aware of Strauss, “Cosmology: An ancient view of acceleration,” in the same issue (more on this one when I have time to study it).