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The FLAMINGO project: revisiting the S8 tension and the role of baryonic physics

Elbers, Willem; Helly, John; Frenk, Carlos; McCarthy, Ian G.; Salcido, Jaime; Schaye, Joop; Kwan, Juliana; Schaller, Matthieu; Braspenning, Joey; van Daalen, Marcel P.; Vandenbroucke, Bert; Conley, Jonah T.; Font, Andreea S.; Upadhye, Amol

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Willem Elbers
Postdoctoral Research Associate

Ian G. McCarthy

Jaime Salcido

Joop Schaye

Juliana Kwan

Joey Braspenning

Marcel P. van Daalen

Bert Vandenbroucke

Jonah T. Conley

Andreea S. Font

Amol Upadhye


A number of recent studies have found evidence for a tension between observations of large-scale structure (LSS) and the predictions of the standard model of cosmology with the cosmological parameters fit to the cosmic microwave background (CMB). The origin of this ‘S8 tension’ remains unclear, but possibilities include new physics beyond the standard model, unaccounted for systematic errors in the observational measurements and/or uncertainties in the role that baryons play. Here, we carefully examine the latter possibility using the new FLAMINGO suite of large-volume cosmological hydrodynamical simulations. We project the simulations onto observable harmonic space and compare with observational measurements of the power and cross-power spectra of cosmic shear, CMB lensing, and the thermal Sunyaev-Zel’dovich (tSZ) effect. We explore the dependence of the predictions on box size and resolution and cosmological parameters, including the neutrino mass, and the efficiency and nature of baryonic ‘feedback’. Despite the wide range of astrophysical behaviours simulated, we find that baryonic effects are not sufficiently large to remove the S8 tension. Consistent with recent studies, we find the CMB lensing power spectrum is in excellent agreement with the standard model, while the cosmic shear power spectrum, tSZ effect power spectrum, and the cross-spectra between shear, CMB lensing, and the tSZ effect are all in varying degrees of tension with the CMB-specified standard model. These results suggest that some mechanism is required to slow the growth of fluctuations at late times and/or on non-linear scales, but that it is unlikely that baryon physics is driving this modification.


Elbers, W., Helly, J., Frenk, C., McCarthy, I. G., Salcido, J., Schaye, J., …Upadhye, A. (2023). The FLAMINGO project: revisiting the S8 tension and the role of baryonic physics. Monthly Notices of the Royal Astronomical Society, 526(4), 5494–5519.

Journal Article Type Article
Acceptance Date Oct 8, 2023
Online Publication Date Oct 12, 2023
Publication Date 2023-12
Deposit Date Jan 29, 2024
Publicly Available Date Jan 29, 2024
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 526
Issue 4
Pages 5494–5519
Public URL


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