Kavli Affiliate: Blake Sherwin
| First 5 Authors: Frank J. Qu, Qianjun Hang, Gerrit Farren, Boris Bolliet, Jessica Nicole Aguilar
| Summary:
We measure the growth of cosmic density fluctuations on large scales and
across the redshift range $0.3<z<0.8$ through the cross-correlation of the ACT
DR6 CMB lensing map and galaxies from the DESI Legacy Survey, using three
galaxy samples spanning the redshifts of $0.3 lesssim z lesssim 0.45$, $0.45
lesssim z lesssim0.6$, $0.6 lesssim z lesssim 0.8$. We adopt a scale cut
where non-linear effects are negligible, so that the cosmological constraints
are derived from the linear regime. We determine the amplitude of matter
fluctuations over all three redshift bins using ACT data alone to be
$S_8equivsigma_8(Omega_m/0.3)^{0.5}=0.772pm0.040$ in a joint analysis
combining the three redshift bins and ACT lensing alone. Using a combination of
ACT and textit{Planck} data we obtain $S_8=0.765pm0.032$. The lowest redshift
bin used is the least constraining and exhibits a $sim2sigma$ tension with
the other redshift bins; thus we also report constraints excluding the first
redshift bin, giving $S_8=0.785pm0.033$ for the combination of ACT and
textit{Planck}. This result is in excellent agreement at the $0.3sigma$ level
with measurements from galaxy lensing, but is $1.8sigma$ lower than
predictions based on textit{Planck} primary CMB data. Understanding whether
this hint of discrepancy in the growth of structure at low redshifts arises
from a fluctuation, from systematics in data, or from new physics, is a high
priority for forthcoming CMB lensing and galaxy cross-correlation analyses.
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