Speaker
Description
The $\Lambda$CDM cosmological model faces persistent challenges, most notably the statistically significant discrepancies in the Hubble constant ($H_0$) and the amplitude of matter fluctuations ($S_8$). In this context, we study the cosmological effects of a Bergmann-Wagoner scalar-tensor model with cold dark matter (ST-CDM for short) featuring an oscillatory gravitational strength, characterized by an early-time enhancement and late-time suppression of gravity. The model conditions are chosen such that the field-dependent Newton's constant $G(\phi)$ recovers the standard local value at low redshifts ($z<2$) to satisfy late-time observational constraints. To analyze the perturbations and test the theoretical predictions, we used a modified version of hi_class cosmological code and the Cobaya MCMC sampler for a Bayesian statistical analysis. For early-universe constraints, we use the full Planck 2018 CMB likelihood and the joint likelihood of Planck 2018 and ACT DR6, finding that a greater value of $G(\phi)$ at early-times effectively decreases the sound horizon by increasing the expansion rate. We also discuss the imprints of the model on structure formation.