2025/12/18 by Chunyu Li, Xin Ren, Yuhang Yang +2
#astro-ph.CO #gr-qc
We investigate a perturbation-level modification of symmetric teleparallel gravity of the form f(Q)=F(Q)+M√(Q) and assess its potential to ease the σ8 tension. The square-root term leaves the background expansion unchanged at the level of FLRW cosmology, while modifying the effective gravitational coupling and thereby providing a decoupling between background evolution and structure-growth. Using the latest redshift-space distortion data, including DESI DR1 Full-Shape measurements, we constrain the model through its impact on the growth observable fσ8(z) across three representative backgrounds: ΛCDM, an H0-tension-reducing model, and a DESI-motivated dynamical dark energy scenario. In all cases, the square-root correction suppresses the growth of structure and induces a degeneracy with σ8, leading to weaker constraints from current data. This allows for a wider range of σ8 values consistent with Planck, with the effect being most pronounced in the H0-tension-oriented background model. However, because RSD data constrain fσ8 rather than σ8 directly, a residual degeneracy between M and σ8 remains, indicating that future multi-probe analyses combining lensing and full-shape clustering will be required to determine whether the √(Q) term represents a genuine signal of modified gravity.