2025/08/26 by Pal, Sourav, Sarkar, Debanjan, Aviles, Alejandro
#Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #General Relativity and Quantum Cosmology (gr-qc)
paper · doi:10.48550/arxiv.2508.19106
We study the galaxy bispectrum multipoles in the Hu-Sawicki f(R) gravity model, where a scalar degree of freedom mediates a fifth force that is screened in high-density environments. The model is specified by fR0, the present-day background value of the scalar field, which controls the strength of deviations from General Relativity (GR). Using perturbation theory, we compute the redshift-space galaxy bispectrum with the full scale- and time-dependent second-order kernels, incorporating corrections from the scale-dependent growth rate and nonlinear screening. Expanding the bispectrum in spherical harmonics, we analyze the sensitivity of the multipoles to modified gravity and forecast their detectability in a Euclid-like survey. The monopole (B00) and quadrupole (B20) show the strongest signatures, with relative deviations of 2%-8% at z=0.7 and k1≃0.3 h \rm Mpc-1 (largest side of the triangle) for fR0=10-5. Higher multipoles provide weaker but complementary signals. For Euclid, we forecast signal-to-noise ratios up to ∼30 for the monopole and ∼15 for the quadrupole including the Finger-of-God damping and shot noise effect. These results demonstrate that bispectrum multipoles are a powerful probe of gravity, capable of breaking degeneracies with bias and velocity effects and strengthening constraints on deviations from ΛCDM.