# The DES and DESI projects

## Introduction

The Dark Energy Survey started in 2013 and is the foremost photometric galaxy survey in this decade, imaging 5000 sq. deg. (an octant of the sky) in five optical and near-infrared bands (grizY) to unprecedented depths ($i_{AB}$ ~ 24), and measuring the position on the sky, distance and shape of almost 300 million galaxies up to a redshift z ~ 1.4. Starting in 2019, the Dark Energy Spectroscopic Instrument (DESI) will collect spectra for over 30 million galaxies and quasars covering the whole redshift range 0.2 < z < 3.5, and will become the foremost spectroscopic galaxy survey in the next decade. IFAE is a founding member of both collaborations. In DES, IFAE designed and produced a large fraction of the readout electronics of the DES camera, while in DESI, IFAE has designed and is producing the 10 GFA cameras necessary for the guiding, focusing and alignment of the 5000 fibers in the DESI focal plane. Both surveys have as their main goal to unveil the nature of the mysterious dark-energy component of the Universe that powers its current accelerated expansion.

## The Dark Energy Survey spots the optical counterpart of a merger of two neutron stars

On August 17, 2017, the LIGO-VIRGO collaboration reported the first observation of a gravitational-wave signal from the merger of two neutron stars, GW170817. Contrary to the merger of two black holes,the merger of two neutron stars merge produces a signal across the electromagnetic spectrum. Within hours of the announcement, DES had identified the optical counterpart of GW170817 (fig. 1). This heralded the birth of multi-probe astronomy including gravitational waves, which will enable a wealth of astrophysical and fundamental physics studies. As a first example, putting together the measured amplitude of the gravitational wave signal, which, as a standard siren, provides the distance to the merger, and the measured redshift of the galaxy where the merger took place, one can obtain a measurement of the expansion rate of the , independent of all other measurements, which rely on either standard candles or standard rulers (B.P. Abbott et al. 2017, “A gravitational-wave standard siren measurement of the Hubble constant”, Nature 551, 85. While the precision of the current measurement $H_{0}=70^{+12}_{-8}$ km/s/Mpc, based on a single event, is rather limited, the technique will become very competitive with more events.

## The Dark Energy Survey Year 1 Cosmology Results

In summer 2017, DES published a series of 12 papers containing the main cosmological results from the analysis of the first-year data, covering some 1500 sq. deg. The most stringent cosmological constraints come from the combination of measurements of the galaxy clustering in a sample of Luminous Red Galaxies (LRG), the shear-shear correlations in a weak-lensing sample of 35 million galaxies, and the tangential shear of the galaxies in the weak-lensing sample around the galaxies in the LRG sample (galaxy-galaxy lensing). This combination maximizes the constraining power of the data, in particular by breaking the degeneracy between the galaxy bias of the LRG sample and the amplitude of the fluctuations in the underlying matter distribution.

## In 2017, the group at IFAE led two of the 12 DES papers containing the cosmological results obtained from the analysis of the first-year data of the survey

Two PhD students at IFAE led the galaxy-galaxy lensing analysis, which was published in J. Prat, C. Sánchez et al. (DES Collaboration) 2017, “Dark Energy Survey Year 1 Results: Galaxy-Galaxy Lensing”, arXiv:1708.01537 [astro-ph.CO]. Another two PhD students led the development of a method to constraint the redshift distribution of the weak-lensing sample using the angular cross-correlations between the positions of the galaxies in that sample and those in the LRG sample with well-known redshifts. This was published in M. Gatti, P. Vielzeuf et al. (DES Collaboration) 2017, “Dark Energy Survey Year 1 Results: Cross-Correlation Redshifts - Methods and Systematics Characterization”, arXiv:1709.0092 [astro-ph.CO]. Both articles have been recently accepted for publication in PRD and MNRAS, respectively.
The result of the overall combination of the three two-point correlation functions is presented in Fig. 3, which shows the 68% and 95% allowed contours in the $S_8-\Omega_m$ plane. Here $S_8$ measures the inhomogeneity of the matter distribution now: it is related to the standard deviation of the matter density fluctuations in spheres of 8 Mpc/h radius, while $Ω_m$ is the fraction of matter in the total matter-energy of the Universe now. In Fig. 2, we can see that the DES results are in fair agreement with those from the Planck satellite. Planck measures the tiny inhomogeneity of the early Universe at redshift ~1100, and extrapolates to the current epoch assuming the ΛCDM cosmology. The agreement with DES and Planck is, then, a test of the validity of ΛCDM. This is the first time a measurement of the matter distribution now can compete in precision with those coming from the CMB. When DES will finalize the analysis of its full data sample, the area of its ellipse in Fig. 2 will decrease by a factor of five or more, providing a very stringent test of ΛCDM.

## GFA design and production for DESI

During 2017, the IFAE group finished the development of the second iteration of the GFA cameras and started their production, complete with mechanical enclosures, filters, CCDs, readout electronics, thermal control, etc. Figure 3 shows the final version of the readout electronics for one GFA camera, composed of three stacked, interconnected cards.