UCL Department of Physics and Astronomy


Dark Energy Spectroscopic Instrument enters construction phase

1 July 2016

A cell which will hold one of the DESI lenses (about 1 meter across), assembled at UCL’s Optical Laboratory


The Dark Energy Spectroscopic Instrument (DESI) has recently received formal approval from the US Department of Energy to move forward to the construction phase. DESI is a 3-D sky mapping project and will measure spectra of 35 million galaxies to provide new clues about Dark Energy. Installation of the project is set to begin next year at the Mayall 4-meter telescope in Kitt Peak National Observatory, Arizona, with observations starting up in early-2019.

UCL is one of the founding institutions of DESI, and part of the UK consortium within DESI that received funding from the Science and Technology Facilities Council (STFC) towards this project. A team of of UCL members of staff that includes Filipe Abdalla, David Brooks, Peter Doel, Jay Farihi, Andreu Font-Ribera, Ofer Lahav, Hiranya Peiris, Andrew Pontzen and Amelie Saintonge, together with their PhD students and post-docs, have been working on both the science preparations and on the instrumentation of the project.

The optical corrector, a system of 6 large lenses which is being assembled at UCL. This is based on UCL’s experience with an earlier project, the Dark Energy Survey (DES), which is now in full operation at the Blanco 4-meter in Chile, the twin telescope of the Mayall.

Prof. Peter Doel (UCL Physics & Astronomy), who leads the instrumentation work at UCL said: “This is a challenging project, which brings together state-of-the-art technologies. The lenses have to be polished and aligned very precisely, at the level of the width of a strand of human hair.”

Dark energy was originally discovered through its apparent influence on the increase in acceleration of expansion that started 7 billion years ago. Going from discovering dark energy to finding out what it is requires more detailed data on the cosmic expansion. “It’s not like the Higgs particle, that you know when you have it,” says project scientist Brenna Flaugher of Fermilab. “With dark energy, we don’t know what we have to measure to understand what it is. We are looking for hints.”

That’s where DESI comes in. By analysing the galaxy map data in two distinct ways—through baryon acoustic oscillations (BAO) and redshift-space distortions—scientists hope to learn how dark energy behaves, if not exactly what it is.

Prof. Ofer Lahav (UCL Physics & Astronomy), the UCL representative to DESI and member of the international DESI executive committee said: “The project will generate an impressive 3-D map of the galaxy distribution, with accurate distances to galaxies based on their spectroscopic redshifts. From this map we can infer what the universe is made of, in particular to test the current model of the universe and to characterise dark energy and dark matter."

In addition to the 35 million galaxies to be observed by DESI, about 700,000 of DESI’s targets will be bright, high-redshift quasars. They can be used as beacons to measure the absorption of hydrogen along the line of sight and map the distribution of hydrogen gas, thus providing another window onto cosmological structure and its evolution. The quasars’ brightness makes them DESI’s most distant tracers and allows cosmologists to measure the influence of dark energy back 12 billion years.

DESI will also be sensitive to neutrino mass. “Without massive neutrinos, the universe would be more blobby; with neutrinos, it is smoother,” says Lahav. “One dream we have with DESI is to measure the mass of neutrinos” by looking at the clustering patterns of galaxies.

After a proprietary period, the DESI data will be released to the wider astronomy community.



A cell which will hold one of the DESI lenses (about 1 meter across), assembled at UCL’s Optical Laboratory