The universe, perhaps surprisingly, is not comprised of galaxies randomly distributed throughout space; that is, it is not very homogeneous. Instead, its galaxies are clustered into distinct structures that harbor dark matter, typically gigantic filaments separated by vast voids—the “large-scale structure (LSS),” an architecture whose discovery and mappings were pioneered by Harvard-Smithsonian Center for Astrophysics astronomers about thirty years ago.
Astronomers have combined LSS maps with results from the cosmic microwave background radiation ( CMBR)and ideas about the inflationary big bang to assemble a remarkably consistent picture of the universe, its origins and its evolution.
The artist’s impression shows the evolution of the universe, beginning with the Big Bang (left) and the appearance of the cosmic microwave background. The formation of the first stars ended the cosmic dark ages, followed by the formation of galaxies. Artwork by M. Weiss/CfA.
Mysteries remain, for example dark matter, which is also expected to gather in large-scale structures. CfA astronomers David James and Tony Stark were members of a large international team that used photons from galaxies in the early universe (“tracer galaxies”) to probe the LSS in more detail.
“The nature of dark matter is one of the biggest mysteries in science and we need to use any related new data to tackle it,” says Avi Loeb of the CfA.
As these photons traverse the universe on their way to us, their paths are perturbed by the gravitational influences of the LSS, including in particular the effects of gravitational lensing. The apparent placements of young galaxies as projected on the sky and their statistical distributions are sensitive both to the current and the evolving geometry and structure of matter in the universe.
The astronomers recognized that although the details of the projected mass distribution are extremely complex, using the ratios of some parameters could obviate some uncertainties, enabling them to obtain important constraints on the current models of cosmic evolution.
The team combined observations from the Dark Energy Survey (an optical survey that has mapped millions of galaxies), the South Pole Telescope (a submillimeter-wave facility studying the CMBR and early galaxies), and the Planck mission (a far infrared and millimeter survey spacecraft).
One particularly valuable advantage of this approach is that it does not require knowing the distances to the tracer galaxies (distances would require their being able to measure the faint spectroscopic redshifts). The scientists were able to obtain constraints with a precision of about ten percent on some of the detailed parameters of current cosmological models, and they forecast that with further research these techniques will even enable them to constrain some of the essential features of dark matter, like its equatrion of state, and properties that have so-far remained elusive.
The Daily Galaxy, Max Goldberg, via Harvard-Smithsonian Center for Astrophysics