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The Universe through a magnifying glass: Precision cosmology with CMB lensing



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Qu, Frank Jiatianfu 


The cosmic microwave background provides a unique back-light for illuminating the growth of structures in our universe. Measuring the arcminute-scale lensing deflections experienced by the CMB photons as they travel from the last scattering surface to our telescopes enables the mapping of the matter distribution to very high redshifts. This lensing signal provides a clean window for constraining fundamental physics, such as the sum of neutrino masses, structure growth, and the nature of dark energy. Among other applications, precisely measuring this lensing signal will enable robust tests of the standard cosmological model via the comparison of high-precision measurements of structure growth at late times with predictions.

This thesis explores several uses and applications of CMB lensing. On the theory side, we construct novel statistical methods to measure lensing, such as using the shear estimator on the full sky to be more robust to extragalactic foregrounds in Chapter 4. Chapter 3 exploits the synergy between CMB lensing and other probes of structure growth. It discusses the method of combining CMB lensing map with appropriately scaled large-scale structure tracers to construct a high-redshift mass map and leverage more robust inference of cosmological parameters and sample variance cancellation.

On the data side, the main section of the thesis focuses on CMB lensing measurements obtained from data release 6 of the Atacama Cosmology Telescope. This work provides a state-of-the-art lensing power spectrum measurement at a significance of 43σ and an associated signal-dominated lensing mass map that enable a host of cosmological and astrophysical science goals. This lensing measurement, largely independent of measurements from Planck or galaxy survey data, provides a novel avenue to obtain information about large-scale growth and new insight into potential tensions in structure formation.

The thesis also discusses novel methods to tackle key systematics affecting precision ground-based CMB lensing. These include using cross-correlation-based lensing estimators robust to noise modelling and a repertoire of foreground mitigation techniques for suppressing the contamination from extragalactic foregrounds. Two hundred null tests accompany the analysis to ensure the measurement is free from unmitigated systematic effects. The lensing analysis and pipeline used here provide a foundation for high-resolution, ground-based lensing measurements covering a significant portion of the sky. This framework will be used for ongoing analyses of ACT data incorporating day-time observations from 2017-2022 and night-time data recorded in 2022. Moreover, the analysis presented here paves the way for upcoming surveys like the Simons Observatory.





Sherwin, Blake


CMB, cosmology, Gravitational lensing


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
FJQ acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 851274). FJQ further acknowledges the support from a Cambridge Trust international scholarship and the Trinity College Henry Barlow honorary scholarship.