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In 2000, cosmic shear, namely the gravitational lensing effect generated by the large-scale structure, was first measured by photometric galaxy surveys. Since then, it has been playing an increasingly important role in modern cosmology, providing tight constraints on the matter density parameter (Omega_m) and fluctuation amplitude parameter (S8). With the improvement in image quality, sky coverage, and survey depth, tensions have emerged between the S8 constraints from galaxy surveys and the CMB survey by the Planck satellite. Combined analysis of cosmic shear, galaxy-galaxy lensing, and galaxy clustering from recent "Stage-III" surveys (KiDS, DES, and HSC), namely the 3x2pt, has confirmed this tension to a significance of ~3sigma.

The final cosmic shear results from the Kilo-Degree Survey (KiDS), however, show consistency with Planck (0.73σ), driven largely by improved redshift calibration. This highlights the critical role of redshift calibration in future surveys. To further improve precision, a 6×2pt analysis has been proposed, combining correlations among galaxy positions and shapes from photometric surveys and galaxy positions from spectroscopic surveys. This approach strengthens cosmological constraints while mitigating redshift uncertainties through self-calibration. A key challenge for 6×2pt analyses is the variable depth of photometric galaxy samples, caused by non-uniform selection effects. It introduces an additional correlation signal and bias in parameter constraints. By applying an unsupervised machine learning method, we can obtain "organized randoms" that can account for the selection effects.

In this talk, I will review the current status of cosmology with galaxy surveys, with an emphasis on cosmic shear and 6×2pt analyses using KiDS-Legacy data. These efforts set the stage for upcoming Stage-IV surveys and promise to sharpen our understanding of the Universe.

Papers for reference:

https://www.aanda.org/articles/aa/abs/2024/06/aa46730-23/aa46730-23.html (KiDS DR5 data descriptions);

https://arxiv.org/abs/2503.19441 (KiDS-Legacy cosmological constraints);

https://www.aanda.org/articles/aa/abs/2025/07/aa52466-24/aa52466-24.html (6x2pt forecast paper);

https://www.aanda.org/articles/aa/abs/2025/02/aa52808-24/aa52808-24.html (Organized randoms)

BIO

I finished my bachelor's study at the Department of Physics, Tsinghua University in 2015, and then I earned a PhD at the University of British Columbia in 2021. I am a postdoc fellow at the German Centre for Cosmological Lensing based at Ruhr University Bochum, until October 2025, and will move to Nagoya University in November. I am an active member of the Kilo-degree Survey and a full member of the Dark Energy Science Collaboration. My research work focuses on analysing multiple large-scale structure tracers and developing tools and pipelines for future cosmological surveys.

Host: Cheng Zhao