Phenomenological Microscopic Motivation for Bulk Viscosity in the Informational Universe and Cosmological Tests with CLASS This work presents a phenomenological investigation of bulk viscosity arising within the Informational Universe framework, where physical reality is modeled as emerging from an underlying informational substrate. Building on a companion theoretical framework, the paper provides a microscopic motivation for bulk viscosity using relativistic kinetic theory and explores its cosmological implications. The informational field is treated as an effective scalar degree of freedom, and a temperature scaling of the bulk viscosity (ζ ∝ T⁴) is adopted based on dimensional arguments and early-universe consistency requirements. To ensure causal and stable dynamics, the analysis employs the Israel–Stewart second-order formalism. As a first quantitative test, the model is implemented in the CLASS Boltzmann solver using the Generalized Dark Matter module. Numerical simulations with a constant viscosity approximation demonstrate that a viscosity parameter of order c²ᵥis ≈ 10⁻⁴ leads to a reduction in the clustering parameter S₈, alleviating the known tension between cosmic microwave background predictions and weak lensing observations while remaining broadly consistent with ΛCDM large-scale behavior. The model predicts a scale-dependent suppression of the matter power spectrum at k ≳ 0. 1 h Mpc⁻¹, providing a clear and falsifiable observational signature. These results transform the informational hypothesis into a testable cosmological scenario and motivate future work involving full redshift-dependent implementations and comprehensive MCMC analyses against observational datasets. This study contributes to ongoing efforts to explore dissipative extensions of dark matter and highlights the potential role of informational physics as a bridge between microscopic dynamics and cosmological observables.
Mikheil Rusishvili (Sun,) studied this question.