Cosmology is now stranger to large scale surveys.  The discipline prides itself on data collection, and when the data it is collecting is about galaxies that are billions of years old its easy to see why more data would be better.  Now, with a flurry of 29 new papers, the partial results from the largest cosmological survey ever – the Dark Energy Survey (DES) – have been released.  And it largely confirms what we already knew.

The DES took place over 6 years from 2013 to 2019, and looked at over 1/8th of the night sky for a total of 758 nights.  Results released on May 27th contain analysis of the data from the first half of that observational period, having already released results from the first year back in 2017.

Video describing the work by the DES.
Credit: NOIRLab YouTube Channel

Even using just half the data, the survey, which included 400 individual scientists from 25 institutions in 7 countries, observed over 226 million galaxies.  Observations were done with the Victor M Blanco telescope at Cerro Tololo Inter-American Observatory in Chile.  Measuring 4 meters in width, the Blanco telescope has a resolution of 570 megapixels – almost 50 times as much as a standard iPhone camera.

All that observational power is great to collect data, but the scientists need to know what to do with it when collected.  The goal of the survey was to “quantify the distribution of dark matter and the effect of dark energy” according to a press release by Fermilab, which built and tested the camera used in the survey.   These two poorly understood cosmic features make up 95% of all the known “stuff” in the universe.  


The Dark Energy Survey camera (DECam) at the SiDet clean room. The Dark Energy Camera was designed specifically for the Dark Energy Survey. It was funded by the Department of Energy (DOE) and was built and tested at DOE’s Fermilab.
Credit: NOIRLab

Despite their prevalence, they are very hard to detect, hence the name “dark”.  However, DES provides more insight that ever before into some characteristics  of these little understood phenomena.  In particular, two cosmological features were central to the survey’s efforts.  The first was the “cosmic web”, while the second were weak gravitational lenses.

The cosmic web is what cosmologists use to describe the structure of galaxies.  These massive clusters of gravitationally bound stars aren’t randomly distributed, as one might assume if the universe was all started from the same state.  They form a pattern, with clumps of galaxies banding together to form galaxy clusters. 


Maps of some recent work to map clustering due to dark matter.
Credit: Hong et. al., Astrophysical Journal

Cosmologists normally attribute those clumped up areas to the presence of higher densities of dark matter and, therefore, gravity.  Mapping where they occur in space provides insight into what areas of the galaxy might feature high concentrations of dark matter to study.  Results from universe growth models can then be compared to the cosmic web as a way to check their accuracy in predicting how the universe actually turned out.

Clustering isn’t the only way to detect dark matter though.  DES scientists also utilized a well-studied cosmological phenomena called gravitational lensing.  This effect happens when light is bent around areas of high gravity, which pockets of dark matter certainly are. Strong gravitational lensing, such as that around black holes, is a common enough feature of cosmology, producing features such as Einstein rings that look spectacular.

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