Modified cosmology through generalized mass-to-horizon entropy: implications for structure growth and primordial gravitational waves

Abstract: In the framework of entropic cosmology, entropic forces arising at the cosmological horizon have been proposed as an alternative mechanism to explain the Universe's current accelerated phase. However, recent studies have shown that, under the Clausius relation and assuming a linear mass-to-horizon (MHR) relation, all entropic force models reduce to the original Bekenstein-Hawking formulation, regardless of the specific form of the horizon entropy. As a result, they inherit the same observational limitations in accounting for cosmic dynamics. To address this issue, a generalized MHR has been introduced, providing the foundation for a modified cosmological scenario rooted in the gravity-thermodynamics conjecture. In this work, we explore the implications of this generalized framework for early-Universe dynamics. Specifically, we analyze the growth of matter perturbations within the spherical Top-Hat formalism in the linear regime, showing that the density contrast profile is significantly influenced by the modified background dynamics predicted by the model. Moreover, considering the sensitivity of upcoming gravitational wave detectors in the sub-$10^3\,\mathrm{Hz}$ range, we examine the impact on the relic abundance of Primordial Gravitational Waves (PGWs), identifying parameter regions where deviations from standard cosmology may arise through an enhanced PGW spectrum.