We present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Lyman-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI), to be released in the DESI Data Release 1. DESI BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1 < z < 4.2. To mitigate confirmation bias, a blind analysis was implemented to measure the BAO scales. DESI BAO data alone are consistent with the standard flat ΛCDM cosmological model with a matter density Ω_m=0.295±0.015. Paired with a baryon density prior from Big Bang Nucleosynthesis and the robustly measured acoustic angular scale from the cosmic microwave background (CMB), DESI requires H ₀=(68.52±0.62) km s^-1 Mpc^-1. In conjunction with CMB anisotropies from Planck and CMB lensing data from Planck and ACT, we find Ω_m=0.307± 0.005 and H ₀=(67.97±0.38) km s^-1 Mpc^-1. Extending the baseline model with a constant dark energy equation of state parameter w, DESI BAO alone require w=-0.99^+0.15 _-0.13. In models with a time-varying dark energy equation of state parametrised by w ₀ and w_a , combinations of DESI with CMB or with type Ia supernovae (SN Ia) individually prefer w ₀ > -1 and w_a < 0. This preference is 2.6σ for the DESI+CMB combination, and persists or grows when SN Ia are added in, giving results discrepant with the ΛCDM model at the 2.5σ, 3.5σ or 3.9σ levels for the addition of the Pantheon+, Union3, or DES-SN5YR supernova datasets respectively. For the flat ΛCDM model with the sum of neutrino mass ∑ m_ν free, combining the DESI and CMB data yields an upper limit ∑ m_ν < 0.072 (0.113) eV at 95% confidence for a ∑ m_ν > 0 (∑ m_ν > 0.059) eV prior. These neutrino-mass constraints are substantially relaxed if the background dynamics are allowed to deviate from flat ΛCDM.

Aims.We study the relationship between the local environment of galaxies and their star formation rate (SFR) in the Great Observatories Origins Deep Survey, GOODS, at . Methods.We use ultradeep imaging at 24 μm with the MIPS camera onboard to determine the contribution of obscured light to the SFR of galaxies over the redshift range . Accurate galaxy densities are measured thanks to the large sample of ~1200 spectroscopic redshifts with high (~70%) spectroscopic completeness. Morphology and stellar masses are derived from deep HST-ACS imaging, supplemented by ground based imaging programs and photometry from the IRAC camera onboard . Results.We show that the star formation-density relation observed locally was reversed at : the average SFR of an individual galaxy increased with local galaxy density when the universe was less than half its present age. Hierarchical galaxy formation models (simulated lightcones from the Millennium model) predicted such a reversal to occur only at earlier epochs () and at a lower level. We present a remarkable structure at , containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster. This structure illustrates how the individual SFR of galaxies increases with density and shows that it is the ~1-2 Mpc scale that affects most the star formation in galaxies at . The SFR of galaxies is found to correlate with stellar mass suggesting that mass plays a role in the observed star formation-density trend. However the specific SFR (=SFR/) decreases with stellar mass while it increases with galaxy density, which implies that the environment does directly affect the star formation activity of galaxies. Major mergers do not appear to be the unique or even major cause for this effect since nearly half (46%) of the luminous infrared galaxies (LIRGs) at present the HST-ACS morphology of spirals, while only a third present a clear signature of major mergers. The remaining galaxies are divided into compact (9%) and irregular (14%) galaxies. Moreover, the specific SFR of major mergers is only marginally stronger than that of spirals. Conclusions.These findings constrain the influence of the growth of large-scale structures on the star formation history of galaxies. Reproducing the SFR-density relation at is a new challenge for models, requiring a correct balance between mass assembly through mergers and in-situ star formation at early epochs.

This Special Issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multi--band imaging data obtained with the {\it Hubble Space Telescope} ({\it HST}) and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 square arcminutes in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the {\it Chandra} X-ray Observatory (\cxo) and ground-based facilities are supplemented with new, deep imaging in the optical and near-infrared from the European Southern Observatory (ESO) and from the Kitt Peak National Observatory (KPNO). Deep observations with the {\it Space Infrared Telescope Facility} (SIRTF) are scheduled. Reduced data from all facilities are being released worldwide within three to six months of acquisition. Together, this data set provides two deep reference fields for studies of distant normal and active galaxies, supernovae, and faint stars in our own galaxy. This paper serves to outline the survey strategy and describe the specific data that have been used in the accompanying letters, summarizing the reduction procedures and sensitivity limits.Comment: 19 pages, 4 figures. Accepted for publication in the Astrophysical Journal, Letters. See manuscript for the full authorship list. The data and the GOODS-group papers can be found at http://www.stsci.edu/science/goods