Cosmology with Phase 1 of the Square Kilometre Array; Red Book 2018: Technical specifications and performance forecasts

Abstract: We present a detailed overview of the cosmological surveys that will be carried 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 sqdeg; a wide and deep continuum galaxy and HI intensity mapping survey over 20,000 sqdeg from z = 0.35 - 3; and a deep, high-redshift HI intensity mapping survey over 100 sqdeg from z = 3 - 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to z ~ 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 surveys like LSST and Euclid, 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.

• The paper demonstrates how SKA1's survey strategies forecast cosmological parameters and constrain dark energy and the growth rate.
• It outlines three survey methods, including galaxy redshift, HI intensity mapping, and continuum surveys across diverse redshifts.
• The work emphasizes addressing systematics such as foreground contamination and calibration errors through synergy with optical datasets.

Cosmology with Phase 1 of the Square Kilometre Array

The academic paper under discussion delineates the cosmological potential of Phase 1 of the Square Kilometre Array (SKA1). It provides detailed technical specifications and scientific forecasts for various cosmological surveys to be conducted with SKA1, particularly focusing on the science made possible by the three key surveys.

Survey Strategies and Objectives

1. Medium-Deep Continuum and Spectroscopic HI Galaxy Survey: This survey covers 5,000 deg² and aims to study weak lensing through continuum emissions and conduct a low-redshift spectroscopic HI galaxy survey.
2. Wide and Deep Continuum Galaxy and HI Intensity Mapping Survey: Spanning 20,000 deg², this survey will facilitate continuum galaxy analysis and HI intensity mapping between redshifts 0.35 and 3.
3. High-Redshift HI Intensity Mapping Survey: Targeting a smaller region of 100 deg², this survey aims to perform deep observations within the high-redshift range of 3 to 6.

These surveys are designed to achieve several scientific goals, including meticulously measuring the dark energy equation of state out to $z \sim 3$ via precision cosmic expansion measurements, investigating potential deviations from General Relativity on a cosmological scale, and acquiring insights into the Universe's isotropy, homogeneity, and non-Gaussianity. Furthermore, the combination of radio clustering and weak lensing measurements, alongside optical datasets like those from LSST and Euclid, is cited as having the possibility of significantly augmenting cosmological constraints beyond the capabilities of individual optical or radio surveys.

HI Galaxy Redshift Survey Implications

The HI galaxy redshift survey within SKA1 provides prospects for elucidating the evolution of neutral hydrogen in galaxies, thereby contributing vital data for galaxy formation and evolution models. It predicts substantial constraints on the linear growth rate, $f\sigma_8$, at low redshifts, complementing existing optical surveys such as DESI and allowing a refined analysis of dark energy dynamics.

HI Intensity Mapping Prospects

The HI intensity mapping surveys are anticipated to primarily leverage SKA1's potential in measuring high-fidelity cosmological signals across an extensive range of redshifts. The forecasting results indicate that the SKA1 can offer competitive constraints on the cosmological parameters, particularly for the cosmic expansion rate ($H(z)$) and angular diameter distance ($D_A(z)$), at mid-to-high redshifts when benchmarked against upcoming spectroscopic surveys. Additionally, these surveys have the potential to advance our grasp of ultra-large scale structures, gauge primordial non-Gaussianity, and address General Relativity effects on cosmic scales. Importantly, intensity mapping can also play a significant role in synergy with optical surveys, enhancing redshift estimation, and cross-calibration endeavors.

Challenges and Systematics

The paper also outlines the prospective challenges and systematic uncertainties that need to be carefully managed to exploit SKA1's full cosmological potential. These include dealing with strong foreground contamination, $1/f$ noise, bandpass calibration issues, and interference from terrestrial and satellite sources.

Conclusion and Future Directions

In conclusion, SKA1 holds the promise of significantly enriching the standard cosmological model, allowing more detailed studies into dark energy, cosmic inflation, and neutrino masses. The collaborative nature envisioned with other optical surveys is expected to leverage each dataset's unique strengths, ensuring robust cross-verification and mitigation of systematic errors. As such, SKA1 is poised to cement its role as a critical instrument not only for radio astronomy but also for broader cosmological research.