DESI: The Dark Energy Spectroscopic Instrument

The Dark Energy Spectroscopic Instrument (DESI) is a massively multiplexed fibre-fed spectrograph that will make the next major advance in Fundamental Cosmology research in the timeframe 2019-2023. On the Mayall telescope, DESI will obtain spectra and redshifts for at least 18 million emission-line galaxies, 4 million luminous red galaxies and 3 million quasi-stellar objects. DESI will: (i) probe the effects of Dark Energy on the expansion history using baryon acoustic oscillations, (ii) measure the gravitational growth history through redshift-space distortions, (iii) measure the sum of neutrino masses, and (iv) investigate the signatures of cosmological inflation via primordial non-Gaussianity. The resulting 3-D galaxy maps at z < 2 and Lyman-alpha forest at z > 2 will make 1%-level measurements of the distance scale in 35 redshift bins; this will improve the precision of cosmological constraints by almost an order of magnitude compared to existing data, and be at least three times better than any result from currently planned competitor experiments. DESI will start before Euclid, will complement its redshift survey range by providing essential comparative lower and higher redshift data points, and will improve the Euclid calibrations of weak-lensing photometric redshifts. A wide range of additional science will also be possible using DESI including the study of galaxy evolution, galaxy cluster surveys, the structure of the Milky-Way galaxy and Galactic archaeology.

We propose that STFC enables a core group of UK scientists with relevant technical and scientific expertise to join the DESI collaboration and take a leading role in this experiment, using a similar strategy to that adopted to join the successful Dark Energy Survey (DES) project. DES will make complementary measurements to DESI, based on imaging data and photometric redshifts, which are significantly weaker than those based on spectroscopy. By building on acknowledged UK strengths in the technologies of large optics and highly multiplexed fibre systems, this core funding will enable leading UK technical roles in the optical corrector and fibre system work packages, from which strong leverage can be secured in the scientific leadership of the DESI project and the analysis of its survey data products. This is an area where UK groups have an established track record, for example in exploitation of the 2-degree Field Galaxy Redshift Survey.

DESI is an experiment that grew from the merger of the BigBOSS and DESpec studies, and will build upon STFC-funded feasibility studies for both projects. It brings together a team of people from both the particle physics and astronomy communities, with over 20 years of experience of working together on such projects. DESI is stronger than either the DES or the Baryon Oscillation Spectroscopic Survey (BOSS) pre-cursor experiments (which led to DESpec and BigBOSS respectively) and will utilise this heritage to organise the large collaboration of scientists and engineers involved, with UK scientists playing a key role in that organisation.

With this proposal, STFC has the opportunity to support key areas of UK astronomy technology, and to enable strategically important areas of UK scientific excellence in this unique cosmological experiment. We request support primarily for the technical work-packages led by UCL and Durham University on the optical corrector and optical fibre system, and for project-specific travel associated with the science working groups and DESI progress meetings. We will seek support for science exploitation separately through our STFC consolidated grants. For optimum benefit to the UK, we propose support at the level of £2.8M which will enable full participation from scientists in four of the leading UK groups involved in large-scale structure studies (Portsmouth, Durham, Edinburgh and UCL) and ensure that the survey science products are also well aligned with wider UK priorities.

DESI is a ground-based experiment to study the expansion rate of the universe and the growth of cosmological structure with a wide-angle spectroscopic galaxy and quasar redshift survey. The 5000-fibre DESI spectrograph will allow an order of magnitude improvement in the distance scale over the redshifts 0.8