We present a detailed overview of the cosmological surveys that we aim to carry out with Phase 1 of the Square Kilometre Array (SKA1) and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5 000 deg2; a wide and deep continuum galaxy and HI intensity mapping (IM) survey over 20 000 deg2 from $z = 0.35$ to 3; and a deep, high-redshift HI IM survey over 100 deg2 from $z = 3$ to 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to $z \sim 3$ with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to $z = 6$ . These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical and near-infrared (NIR) surveys like Euclid, LSST, and WFIRST leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

Angular fluctuations of the Near InfraRed Background (NIRB) intensity are observed up to scales < ∼ 1 • . Their interpretation is challenging as even after removing the contribution from detected sources, the residual signal is > 10 times higher than expected from distant galaxies below the detection limit and first stars. We propose here a novel interpretation in which early, intermediate mass, accreting direct collapse black holes (DCBH), which are too faint to be detected individually in current surveys, could explain the observed fluctuations. We find that a population of highly obscured (N H > ∼ 10 25 cm −2 ) DCBHs formed in metal-free halos with virial temperature 10 4 K at z > ∼ 12, can explain the observed level ≈ 10 −3 (nW m −2 sr −1 ) 2 of the 3.6 and 4.5 µm fluctuations on scales > 100 ′′ . The signal on smaller scales is instead produced by undetected galaxies at low and intermediate redshifts. Albeit Compton-thick, at scales θ > 100 ′′ DCBHs produce a CXB (0.5-2 keV)-NIRB (4.5µm) cross-correlation signal of ≃ 10 −11 erg s −1 cm −2 nW m −2 sr −1 slightly dependent on the specific value of the absorbing gas column (N H ≈ 10 25 cm −2 ) adopted and in agreement with the recent measurements by Cappelluti et al. (2012a). At smaller scales the cross-correlation is dominated by the emission of high-mass X-ray binaries (HMXB) hosted by the same low-z, undetected galaxies accounting for small scale NIRB fluctuations. These results outline the great potential of the NIRB as a tool to investigate the nature of the first galaxies and black holes.

It has been proposed that the first, intermediate-mass (≈ 10 5−6 M ⊙ ) black holes might form through direct collapse of unpolluted gas in atomic-cooling halos exposed to a strong Lyman-Werner (LW) or near-infrared (NIR) radiation. As these systems are expected to be Compton-thick, photons above 13.6 eV are largely absorbed and reprocessed into lower energy bands. It follows that direct collapse black holes (DCBHs) are very bright in the LW/NIR bands, typically outshining small high-redshift galaxies by more than 10 times. Once the first DCBHs form, they then trigger a runaway process of further DCBH formation, producing a sudden rise in their cosmic mass density. The universe enters the "DCBH era" at z ≈ 20 when a large fraction of atomic-cooling halos are experiencing DCBH formation. By combining the clustering properties of the radiation sources with Monte Carlo simulations we show that in this scenario the DCBH mass density rises from ∼ 5 M ⊙ Mpc −3 at z ∼ 30 to the peak value ∼ 5×10 5 M ⊙ Mpc −3 at z ∼ 14 in our fiducial model. However, the abundance of active (accreting) DCBHs drops after z ∼ 14, as gas in the potential formation sites (unpolluted halos with virial temperature slightly above 10 4 K) is photoevaporated. This effect almost completely suppresses DCBH formation after z ∼ 13. The DCBH formation era lasts only ≈ 150 Myr, but it might crucially provide the seeds of the supermassive black holes (SMBHs) powering z ∼ 6 quasars. LWBackground LW intensity from normal galaxies J BH LW Background LW intensity from DCBHs J crit LW Critical LW intensity for DCBH formation f gal ⋆ Star formation efficiency of normal galaxies f pop3 ⋆ Star formation efficiency of Pop III stars M pop3 crit,0 Critical halo mass for galaxy/DCBH formation under photoevaporation feedback ρ pop3 Mass density of Pop III stars ρ BH Mass density of active DCBHs ρ cum BH Cumulative DCBH mass density p g Probability of genetic enrichment p J Probability that a halo sees a super-critical LW intensity p W Probability that a halo being enriched by metals carried by galactic winds p J−WProbability that a halo sees a super-critical LW intensity but without being polluted by galactic winds P (> z, M |M 0 , z 0 ) For a halo with mass M 0 at z 0 , the probability that its most massive progenitor with mass M formed before z dP dz (z, M |M 0 , z 0 ) Redshift derivative of above probability