As light from galaxies billions of light-years away travels to us, it is subtly deflected by the gravitational influence of massive structures along its path. This effect, called weak gravitational lensing, encodes important information about the way the universe expanded and how structure within it grew in the past. This information is key to unlocking the biggest mystery in cosmology, the nature of the accelerated expansion of the universe, an effect called dark energy. The Dark Energy Survey, or DES, seeks answers to this mystery by mapping an eighth of the night sky.
DES measures weak gravitational lensing signals by correlating the shapes of hundreds of millions of galaxies. The subtle weak lensing deflections by large-scale structure shear the shapes of the galaxies. This effect is tiny — a very small “stretch” to galaxy images that already come in a wide variety of shapes and sizes — and it is only by comparing and correlating these large numbers of galaxies that we can beat down the noise. Even worse, the Earth’s atmosphere and the telescope optics distort the images even more than the signal we are looking for, and these distortions must be carefully removed to uncover the weak lensing signals.
But if we can beat these challenges, then the coherent pattern of galaxy stretching will provide a map and a history of gravity and the growth of structure in the universe that tells the story of the last 8 billion years of the cosmos.
Precise shape measurements are not the only requirement for learning about dark energy from DES. The survey must also estimate the distances to all its galaxies, which is done by measuring their redshifts, the fractional stretching of their light due to the expansion of the universe. Because DES takes images in broad color filters, it can see only a very coarse spectrum of the light from each galaxy and so can get only very approximate distances to each galaxy. These photometric redshifts, as they’re called, have their own complexities that must be carefully controlled so that distance errors don’t ruin the constraints on dark energy from DES data.
This month DES released a collection of papers making these high-precision galaxy shape measurements, understanding the redshifts of the galaxies and using this information to constrain cosmology. This early data set is sensitive mainly to two numbers: the amount of matter in the universe and how much that matter has pulled together gravitationally into the structures that form the skeleton of galaxies and galaxy clusters. Two of the most powerful existing cosmology surveys, the Planck Satellite and Canada-France-Hawaii Telescope Lensing Survey, seem to disagree about these quantities, and the DES measurements sit squarely half way between them.
Despite containing millions of galaxies, the data that went into this analysis is only a tiny fraction of the full survey, just a few percent. The final DES data set will be more than 30 times bigger, requiring even more accurate galaxy shape and redshift measurements than what was achieved for this first analysis. When completed and analyzed, DES data will provide powerful new information about the history, contents and likely future of the universe.
—Matthew R. Becker, Stanford University, and Joe Zuntz, University of Manchester for the Dark Energy Survey