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Third year student Sunayana Bhargava in front of telescope in Chile

After in-depth scanning of around a quarter of the world’s southern skies for six years and cataloguing hundreds of millions of distant galaxies, the Dark Energy Survey (DES) will log its last data today (Wednesday 9 January 2019).

Led by Sussex Professor of Astrophysics Kathy Romer, a team of 40 University of Sussex affiliated students, past and present, have contributed to some exciting scientific findings, helping to further unravel some of the universe’s cosmic mysteries.

The Survey is an international collaboration that began mapping a 5,000-square-degree area of the sky on 31 August 2013, in a quest to understand the nature of dark energy, the mysterious force that is accelerating the expansion of the universe.

Using the Dark Energy Camera, a 520-megapixel digital camera funded by the US Department of Energy Office of Science and mounted on the Blanco 4-meter telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory in Chile, scientists on DES took data on 758 nights over six years.

Over the course of the Survey, data from more than 300 million distant galaxies has been recorded. More than 400 scientists from over 25 institutions around the world have been involved in the project, which is hosted by the US Department of Energy’s Fermi National Accelerator Laboratory. The collaboration has already produced about 200 academic papers, with more to come.

Third year Astronomy PhD student and doctoral tutor at the University of Sussex, Sunayana Bhargava, says: “Working with the Dark Energy Survey (DES) has been both a privilege and immensely exciting. The Survey has provided us with very accurate measurements of the structure and fundamental properties of the universe. We have a precise number on the total amount of stars, galaxies and dark matter in our cosmos, in addition to a number which describes the elusive cosmic acceleration we observe today (the dark energy). The Dark Energy Survey has made tremendous advancements in cosmology, understanding our own galaxy, gravitational wave astronomy and much more. 

“The core of the Dark Energy Survey is the Dark Energy Camera. Observing in the visible part of the spectrum, the camera is essentially a gigantic eye staring into the dark Chilean sky to collect faint light from distant galaxies. Now, for the first time since August 2013, on January 9th, the Dark Energy Survey’s eye will collect its last photons, and finally get some sleep. 

“Fortunately, though the data collection is over, the science is not by any means. As the high-quality data is reduced and analysed, the scientific community can expect even more exciting results from DES in the months to come. Watch this space … literally.”

The Dark Energy Survey remains one of the most sensitive and comprehensive surveys of distant galaxies ever performed. The Dark Energy Camera is capable of seeing light from galaxies billions of light-years away and capturing it in unprecedented quality.

Scientific highlights from the Survey include:

• The most precise measurement of dark matter structure in the universe, allowing scientists to trace the evolution of the cosmos.
• More dwarf satellite galaxies orbiting our Milky Way, which provided insight into theories of dark matter.
• The creation of the most accurate dark matter map of the universe. 
• The spotting of the most distant supernova ever detected.
• 12 new moons of Jupiter were recently discovered.
• The detection of distant star-forming galaxies in the early universe, when the universe was only a few percent of its present age, has yielded new insights into the end of the cosmic dark ages.

Participating scientists also spotted the first visible counterpart of gravitational waves ever detected, a collision of two neutron stars that occurred 130 million years ago. The Dark Energy Survey was one of several sky surveys that detected this gravitational wave source, opening the door to a new kind of astronomy. 

The organisational structure of the Survey was also designed to give early-career scientists valuable opportunities for advancement and to participate in a historical international astronomy research project. Sunayana Bhargava, David Turner and Reese Wilkinson were three of the University of Sussex PhD students to go to Chile to participate in the research project. The record-breaking trio were not only the youngest observation team of three, but also clocked up the best ‘seeing’ – or clearest log of images - for a night of all scientific contributions across the six years of the Survey.

Professor Kathy Romer at the University of Sussex says: “Figuring out why the universe is behaving in a crazy way is a huge task and it’s taken more than a decade, hundreds of people, and many millions of pounds. Even though the data-taking phase is finished, the work to analyse this data will continue for years to come. My Gran would definitely approve of the Dark Energy Survey; as she used to say to me, 'If a job's worth doing, it's worth doing well!'

“At Sussex our specialism is marrying X-ray observations with DES data to get a better understanding of clusters of galaxies. Sussex students – including freshers – have been involved in DES science since it made its very first observations in 2012. Four Sussex students were able to go to Chile for DES Observing Runs as part of their PhD studies and I know that was a life-changing experience for all of them.”

Second year Astronomy PhD student and doctoral tutor at the University of Sussex, Reese Wilkinson says: ”I only recently moved into astronomy (coming from Theoretical Physics) and the best thing that DES has given me is the chance to travel to Chile, to go ‘Observing’ at the Dark Energy Camera site. This not only enabled me to understand exactly where the data is coming from, but provided me with an invaluable opportunity to speak to those that built the telescope and ask 101 questions about why they chose to do what they did and how to spot the quirks caused by the way the telescope was built.

“It's given someone who felt like a bit of an outsider and non-astronomer the feeling that they are actually becoming one. I think without DES being a part of my PhD, I’m not sure I would be as confident and as happy as I am now without the opportunities and experiences DES has provided.”

The Dark Energy Camera will remain mounted on the Blanco telescope at Cerro Tololo for another five to ten years and will continue to be a useful instrument for scientific collaborations around the world. Cerro Tololo Inter-American Observatory Director Steve Heathcote foresees a bright future for DECam. "Although the data-taking for DES is coming to an end, DECam will continue its exploration of the universe from the Blanco telescope and is expected remain a front-line engine of discovery for many years,” Heathcote said.

The DES collaboration – including the University of Sussex – will now focus on generating new results from its six years of data, including new insights into dark energy. With one era at an end, the next era of the Dark Energy Survey is just beginning.