We present tomographic measurements of structure growth using
cross-correlations of Atacama Cosmology Telescope (ACT) DR6 and Planck CMB
lensing maps with the unWISE Blue and Green galaxy samples, which span the
redshift ranges 0.2≲z≲1.1 and 0.3≲z≲1.8, respectively. We improve on prior unWISE cross-correlations not just by
making use of the new, high-precision ACT DR6 lensing maps, but also by
including additional spectroscopic data for redshift calibration and by
analysing our measurements with a more flexible theoretical model. An extensive
suite of systematic and null tests within a blind analysis framework ensures
that our results are robust. We determine the amplitude of matter fluctuations
at low redshifts (z≃0.2−1.6), finding S8≡σ8(Ωm/0.3)0.5=0.813±0.021 using the ACT cross-correlation alone and S8=0.810±0.015 with a combination of Planck and ACT cross-correlations; these
measurements are fully consistent with the predictions from primary CMB
measurements assuming standard structure growth. The addition of Baryon
Acoustic Oscillation data breaks the degeneracy between σ8 and
Ωm, allowing us to measure σ8=0.813±0.020 from the
cross-correlation of unWISE with ACT and σ8=0.813±0.015 from the
combination of cross-correlations with ACT and Planck. These results also agree
with the expectations from primary CMB extrapolations in ΛCDM
cosmology; the consistency of σ8 derived from our two redshift samples
at z∼0.6 and 1.1 provides a further check of our cosmological model.
Our results suggest that structure formation on linear scales is well described
by ΛCDM even down to low redshifts z≲1.Comment: 73 pages (incl. 30 pages of appendices), 50 figures, 16 tables, to be
submitted to ApJ. Watch G. S. Farren and A. Krolewski discuss the analysis
and results under https://cosmologytalks.com/2023/09/11/act-unwis