Accurate cosmology from the Dark Energy Survey

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

Dark matter and dark energy cannot be directly observed with any telescope, but are of great interest because they make up 96% of the mass-energy in the Universe. Neither dark matter nor dark energy is fully predicted or described by any current physical theory. The Dark Energy Survey (DES), being conducted on the 4-meter Blanco telescope in Chile, is designed to measure the properties of these dark components to high precision, and determine whether General Relativity is the correct theory of gravity on large scales. This project will focus on solving three problems that the DES faces: uniformly calibrating the 100,000 galaxy images obtained by the survey (providing an improvement of 10x); providing accurate redshifts (based on images only, not spectroscopy); and measuring the subtle distortions of the shapes of galaxies to high accuracy to determine the distribution of dark matter. This work will then enhance the quality of the DES observations so that scientists can better measure the properties of dark matter and energy, as well as those of the other astronomical objects being observed. Besides these scientific objectives, the project will work with undergraduates (from a variety of educational backgrounds) to develop STEM capabilities. In addition, this program includes a new partnership with the Franklin Institute for outreach to community-based organizations and middle-school teachers in Philadelphia through Franklin's "City Skies" traveling astronomy program and hosted workshops for teachers. City Skies has a focus on historically underserved inner-city neighborhoods. The City Skies project targets development of curricular materials and presentations on astronomical topics. Support from the current project will allow the PI to extend work with Franklin. This project clearly relates to NSF's mission to promote the progress of science. In addition, the "Broader Impact" portion of the project does this, and advances the national health, prosperity and welfare by helping to develop interest in, and educating, STEM activities among the next generation of diverse individuals. DES is collaborative international effort of more than 200 scientists to image one quarter of the Southern sky in 5 colors using a DOE-funded 500 Mpixel camera installed on the NSF-funded Blanco 4-meter telescope on Cerro Tololo, Chile. How can these images measure things that neither emit nor absorb light? First, the dark matter's gravity bends the light emitted by the 5% of the Universe that is visible, causing subtle distortions in the shapes of visible objects. The final DES data will use the "weak gravitational lensing" (WL) signature on the shapes of 200 million galaxies to measure the clustering of dark matter to ~1% accuracy. A second gravitational signature of dark matter and energy is that they drive the motions of the visible galaxies, so that the statistics of their positions and redshifts (distances) constrain the dark components; this is especially powerful in combination with the WL information. This proposal focuses on solving three of the most difficult problems that DES faces in reaching its full potential for these measurements. The first challenge is uniform calibration of 100,000 different images taken over 5 years: it will be improved from the current 1% state of the art to near the 0.1% level that was achieved for single nights of the survey using previous NSF support. A second challenge is to measure the WL signature to part-per-thousand accuracy: the proposers have developed the first practical algorithm to attain this accuracy on simulated data, and propose to improve it and apply to the DES images. The third difficult part of the survey is assigning accurate galaxy redshifts without spectroscopic observations of each source. It is proposed to integrate the photometric redshift estimation and lensing measurement into a single process that rigorously treats noise and selection effects to produce a single joint redshift/lensing posterior probability. This in turn enables a new technique for analyzing DES data by creating an ensemble of maps of dark matter density that are consistent with all of the observed positions, shapes, and colors of the galaxies.