Atacama Cosmology Telescope DR6 and DESI: Structure growth measurements from the cross-correlation of DESI legacy imaging galaxies and CMB lensing from ACT DR6 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>P</mml:mi><mml:mi>l</mml:mi><mml:mi>a</mml:mi><mml:mi>n</mml:mi><mml:mi>c</mml:mi><mml:mi>k</mml:mi></mml:math> PR4

Abstract

We measure the growth of cosmic density fluctuations on large scales and across the redshift range <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mn>0.3</a:mn><a:mo>&lt;</a:mo><a:mi>z</a:mi><a:mo>&lt;</a:mo><a:mn>0.8</a:mn></a:math> through galaxy clustering and the cross-correlation of the ACT data release 6 cosmic microwave background (CMB) lensing map and galaxies from the Dark Energy Spectroscopic Instrument Legacy Survey, using three galaxy samples spanning the redshifts of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>0.3</c:mn><c:mo>≲</c:mo><c:mi>z</c:mi><c:mo>≲</c:mo><c:mn>0.45</c:mn></c:math>, <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mn>0.45</e:mn><e:mo>≲</e:mo><e:mi>z</e:mi><e:mo>≲</e:mo><e:mn>0.6</e:mn></e:math>, <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mn>0.6</g:mn><g:mo>≲</g:mo><g:mi>z</g:mi><g:mo>≲</g:mo><g:mn>0.8</g:mn></g:math>. We adopt a scale cut where nonlinear 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 Atacama Cosmology Telescope (ACT) data alone to be <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>S</i:mi><i:mn>8</i:mn></i:msub><i:mo>≡</i:mo><i:msub><i:mi>σ</i:mi><i:mn>8</i:mn></i:msub><i:mo stretchy="false">(</i:mo><i:msub><i:mi mathvariant="normal">Ω</i:mi><i:mi>m</i:mi></i:msub><i:mo>/</i:mo><i:mn>0.3</i:mn><i:msup><i:mo stretchy="false">)</i:mo><i:mn>0.5</i:mn></i:msup><i:mo>=</i:mo><i:mn>0.772</i:mn><i:mo>±</i:mo><i:mn>0.040</i:mn></i:math> in a joint analysis combining the three redshift bins and ACT lensing alone. Using a combination of ACT and data we obtain <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:msub><n:mi>S</n:mi><n:mn>8</n:mn></n:msub><n:mo>=</n:mo><n:mn>0.765</n:mn><n:mo>±</n:mo><n:mn>0.032</n:mn></n:math>. The lowest redshift bin used is the least constraining and exhibits a <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mo>∼</p:mo><p:mn>2</p:mn><p:mi>σ</p:mi></p:math> tension with the other redshift bins; thus we also report constraints excluding the first redshift bin, giving <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:msub><r:mi>S</r:mi><r:mn>8</r:mn></r:msub><r:mo>=</r:mo><r:mn>0.785</r:mn><r:mo>±</r:mo><r:mn>0.033</r:mn></r:math> for the combination of ACT and . This result is in excellent agreement at the <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:mn>0.3</t:mn><t:mi>σ</t:mi></t:math> level with measurements from galaxy lensing, but is <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mn>1.8</v:mn><v:mi>σ</v:mi></v:math> lower than predictions based on 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.