Cosmology from SKA
Chairs: Changbom Park (KIAS) and David Parkinson (KASI)
The goal is to discuss the cosmological surveys that will be done by SKA and the pathfinders need, their needs and possible enhancements. This discussion will motivate people to get involved in science with SKA, and identify which science area they are interested in and can contribute to.
There will be a single talk given by David Parkinson, focussing on the Cosmology Red Book. Discussion and interuption is encouraged.
Please add any comments or annotations to this Google doc during the presentations.
The Cosmology Red book for SKA (https://arxiv.org/abs/1811.02743) really focussed on cosmology from wide area surveys, looking at the cosmology that can be done through the clustering statistics of:
- Continuum galaxies (angular clustering), which are effective for large-scale cosmology, such as early Universe, inflation, and the cosmological principle
- HI galaxies (3D clustering, at low-redshift), which is good for the BAO and growth of structure
- HI intensity mapping (3D clustering at higher redshift, but at low angular resolution), also good for BAO, growth of structure, and can cover a very wide volume
- Weak lensing shear (angular correlations of ellipticities), good for dark energy and modified gravity, through the lensing potential
However, by the time the SKA comes on line (late 2020s), a large number of optical surveys will have covered and measured similar area and redshift range to the planned cosmology surveys. DESI for example will have 3D power spectrum measurements out to z~1 and above, and QSO clustering at z~3.
What then, for SKA? It seems that a lot of what SKA has to offer as an advantage over optical surveys are a lack of systematics, or at least very different systematics to optical surveys. I would suggest that cross-correlation and cross-identification are the key to any successful radio cosmology survey.
- Cross-correlation is very powerful for HI intensity mapping, which has so far only been measured in cross-correlation (the WiggleZ Dark Energy Survey cross-correlated with the Green Bank Telescope). The question of the 21cm foregrounds makes detection of the auto-correlation of HI difficult, but cross-correlation easier and less susceptible to systematics. This is being shown by the HIR4 project, with papers by Asorey et al (2020), and Shi et al (2020, in prep).
- Cross-identification is very useful for Continuum clustering and weak lensing shear. It allows for photometric redshifts to be used to split the sample up into redshift bins, and allows for the multi-tracer approach (for example radio AGN cross-correlated with radio star-forming galaxies) to reduce the effect of cosmic variance.
What facilities can Korea provide to 'value-add', or make possible cosmology surveys with SKA?
- High-resolution radio — if there was a VLBI facility in the south, could it be used to provide classification of radio galaxies, allow the continuum sample to be sub-divided
- Optical photometric — is there a role for KMTNet in cross-identifying bright, close radio sources
- Optical spectroscopic — can Korea help provide optical spectroscopic follow-up of radio sources in the southern hemisphere, to enchance photometric/clustering redshifts for continuum sources
What large-volume simulations are needed?
- What role can HR5 (and HR6, 7, etc) play in modelling the cosmological measurements that SKA will do? HIR4 was already somewhat successful for Tianlai simulation. Can we do it again for continuum sources?
- Can we improve the way that peculiar velocities are estimated for the TF relation?
- What other kind of probes are possible with the radio that are impossible, or very hard, in the optical?