The Dark Energy Survey: Uncovering the Invisible

Humanity’s journey to our present understanding of our Universe has been a long one. In the late 1600s, Issac Newton laid the foundations of classical mechanics by defining his three laws of motion. Over the centuries, however, observations began to emerge that could not be explained by classical mechanics. The orbit of Mercury, for example, doesn’t behave as expected, as it traces out a looping pattern that is inexplicable with only a Newtonian understanding of gravity.

In 1915 Albert Einstein published his general theory of relativity, which proposed an extraordinary explanation for Mercury’s baffling behavior: the curvature of the fabric of space and time (spacetime) itself. Fast forward to 1998 when two independent teams of astronomers attempted to measure the rate of expansion of the Universe, which should — according to both classical mechanics and general relativity — be slowing down. What they discovered though was quite the opposite. The Universe’s rate of expansion was not slowing down, or even stabilizing, but was actually increasing.

The ramifications of this discovery were colossal. Firstly, there was the possibility that general relativity was providing an incomplete picture of how gravity behaves on universal scales. Because general relativity had otherwise proved to be so robust, this was hard to accept. But the alternative solution was equally as wild: that there is an energy uniformly permeating our Universe that is impervious to the pull of gravity.

This notion would correlate with Einstein’s idea of a cosmological constant — some as yet undetected energy that suffuses the entire universe. This is a particularly fascinating proposition, despite Einstein himself dismissing it as ‘his greatest blunder’. Or perhaps this energy is sourced from an unknown form of matter, named quintessence by scientists. This unknown quantity has since been dubbed dark energy and is the mysterious part of the Universe’s matter and energy budget that the Dark Energy Survey (DES) seeks to better understand.

DES, funded by the US Department of Energy, the US National Science Foundation and several other partners, was a monumental project that carried out observations from 2013 to 2019, clocking an impressive 758 nights of observation during that time. The observations were taken using the specially made 570-megapixel US Department of Energy-fabricated Dark Energy Camera (DECam) which is mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), a Program of NSF’s NOIRLab. NOIRLab was a founding member of the DES Collaboration and a key player in bringing DES to life.

DES uses a four-pronged approach to studying dark energy; the observations of thousands of supernovae in order to quantify the history of expansion of the Universe; the observation of weak gravitational lensing and the observation of galaxy clusters to better understand how the Universe’s structure formed and to better determine the amount of matter in the Universe; and the measurement of the distribution of galaxies across the Universe with a technique called Baryon Acoustic Oscillations (BAO). All four of these techniques are more than deserving of their own blog-posts, but in brief they all allow for a very sophisticated observational analysis, which will improve our understanding of the history, structure and composition of our Universe.

DES completed its observation in 2019, and several early data releases have allowed astronomers to make use of the wealth of data as soon as possible. But the sheer volume of data collected means that the analysis is still ongoing, with the final results planned to be made available in 2024.

“Of course, getting the data is only the first step,” says Dr. Alistair Walker, DECam instrument scientist at CTIO. “The next stage is data analysis, which for these data requires new techniques to be developed and extraordinary care to be taken with systematics and statistics.”

Analyses of the early data releases have already yielded fascinating results, including the emergence of compelling evidence for a cosmological constant, suggesting that Einstein may have been right all along! They have also offered new insights into galaxies, supernovae, stellar evolution, celestial objects within the Solar System and the nature of gravitational wave events.

Perhaps most tantalizingly of all, the DES data have shown that galaxies seem to be distributed in a way that cannot be fully explained by the standard cosmological model (the model that underlies particle physics and describes all fundamental interactions, including gravity).

It’s important to note that the final DES data release will not signify the end of the search for dark energy. DES was a ‘Stage III’ program, as defined by the Dark Energy Task Force (yes, there was a Dark Energy Task Force!), meaning that it was intended to be a relatively small and fast program on which bigger, longer-term ‘Stage IV’ projects could be built with a view to Stage V projects in the 2030s and beyond. In this way, DES has set the stage for the ongoing development of humanity’s understanding of our Universe.