Little Ado About Everything: an overview of the ηCDM model, a stochastic cosmology from structure formation

Abstract

The ηCDM model is an extension of the standard cosmological framework that revisits the cosmological principle in a Universe undergoing structure formation by adopting a stochastic perspective. It aims to address the physical origin of late-time cosmic acceleration, to solve the coincidence problem, and to alleviate current cosmological tensions without introducing a fundamental dark-energy field.

In this talk I will offer an overview of the ηCDM model, focusing on its theoretical formulation, on its performance against a broad set of cosmological observations, and on promising directions for future investigations.

The key idea of ηCDM is to treat the evolution of spatially-averaged, coarse-grained regions of the Universe on cosmic-web scales as a stochastic process. This is meant to capture unresolved, small-scale degrees of freedom related to boundary mass fluxes, curvature fluctuations and correlations between density and velocity fields. Stochasticity across different regions is encoded via a multiplicative noise term ('a little ado') in the cosmological equations, while the global background evolution is operationally defined as an ensemble average over such regions.

I will show that the renormalized background dynamics in ηCDM corresponds to an effective spatially flat cosmology with an emergent dark-energy component characterized by a time-dependent equation of state, which naturally leads to late-time acceleration with the right timing and with a stable asymptotic behavior in the infinite future.

I will further showcase the excellent performance of ηCDM in confronting recent type-Ia supernovae, baryon acoustic oscillation, CMB, and structure growth rate data, highlighting its ability to simultaneously relieve the H0 and fsigma8 tensions.

Finally, I will discuss how high-order cosmographic analyses and diagnostic tests such as the Linder's diagram may provide powerful model selection tools to distinguish ηCDM from the standard scenario by using data at intermediate redshifts from current and upcoming wide galaxy surveys.