Disentangling constraints on S8 from the high and low redshift CMB
Abstract
The clustering of matter in the universe is quantified by σ8, the RMS fluctuations today in linear theory in spheres of radius 8h−1 Mpc, where h parametrizes the Hubble constant. Surveys of gravitational lensing and galaxy clustering best constrain S8≡ (Ωm/0.3)0.5 σ8, where Ωm is the energy density of matter in units of the critical density. Comparing this late time measurement to the value of S8 inferred from the cosmic microwave background (CMB), one can test the ΛCDM prediction for the growth of clustering from the primordial universe to now. Several analyses report hints that CMB measurements of S8 are larger than those from galaxy surveys. However, CMB measurements have information from both high and low redshift due to lensing, and these tests traditionally conflate the two. Here, we present a technique to extract constraints on the cosmological parameters from each redshift component separately, enabling a high-z vs. low-z clustering comparison that was not previously possible. We marginalize over lensing information using a principal component analysis, and show that the primordial CMB from combined Planck 2018, ACT DR6, and SPT-3G D1 prefers S8= 0.795±0.022, in agreement with the recent determination from the Dark Energy Survey (DES) of S8=0.789±0.012. The clustering inferred from CMB lensing is larger. However, both the level of excess lensing and increase in agreement with DES depends on the prior on the sum of neutrino masses, ∑ mν. We find that removing lensing brings the CMB towards agreement with DES in any case.
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BibTeX
@misc{jones2026disentangling,
title = {{Disentangling constraints on $S_8$ from the high and low redshift CMB}},
author = {Joann Jones and Ualype de Andrade Uchôa and Scott Dodelson},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37752},
eprint = {2610.37752},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37752},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Disentangling constraints on S_8 from the high and low redshift CMB
AU - Joann Jones
AU - Ualype de Andrade Uchôa
AU - Scott Dodelson
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37752
UR - https://arxiv.org/abs/2610.37752
AB - The clustering of matter in the universe is quantified by σ_8, the RMS fluctuations today in linear theory in spheres of radius 8h^−1 Mpc, where h parametrizes the Hubble constant. Surveys of gravitational lensing and galaxy clustering best constrain S_8≡ (Ω_m/0.3)^0.5 σ_8, where Ω_m is the energy density of matter in units of the critical density. Comparing this late time measurement to the value of S_8 inferred from the cosmic microwave background (CMB), one can test the ΛCDM prediction for the growth of clustering from the primordial universe to now. Several analyses report hints that CMB measurements of S_8 are larger than those from galaxy surveys. However, CMB measurements have information from both high and low redshift due to lensing, and these tests traditionally conflate the two. Here, we present a technique to extract constraints on the cosmological parameters from each redshift component separately, enabling a high-z vs. low-z clustering comparison that was not previously possible. We marginalize over lensing information using a principal component analysis, and show that the primordial CMB from combined Planck 2018, ACT DR6, and SPT-3G D1 prefers S_8= 0.795±0.022, in agreement with the recent determination from the Dark Energy Survey (DES) of S_8=0.789±0.012. The clustering inferred from CMB lensing is larger. However, both the level of excess lensing and increase in agreement with DES depends on the prior on the sum of neutrino masses, ∑ m_ν. We find that removing lensing brings the CMB towards agreement with DES in any case.
ER -