Speaker
Description
Dark matter bulk viscosity is frequently proposed as a unified phenomenological mechanism to drive the late-time accelerated expansion of the Universe without a cosmological constant. In this work, we employ a general, parameter-independent dynamical systems approach to rigorously assess the macroscopic viability of such dissipative models across the entire cosmic history. We establish a topological No-Go theorem demonstrating that the inclusion of any thermodynamically consistent bulk viscosity—strictly constrained by the Second Law of Thermodynamics—fundamentally forbids the existence of a stable, physically viable radiation-dominated early universe. To explicitly verify this theoretical breakdown, we analyze a standard power-law parametrization for the bulk viscosity ($\xi \propto H \, \Omega_m^s$), which reveals an inescapable parametric dichotomy. For exponents $s \geq 1$, preserving the linear growth of large-scale structures demands a deeply suppressed viscosity, which completely expels the late-time viscous attractor from the physical phase space and renders the model indistinguishable from standard $\Lambda$CDM. Conversely, for fractional exponents ($s < 1$), the late-time viscous attractor is physically accessible, but the backward integration inevitably triggers a fatal past-time mathematical singularity that severs the cosmic history and prevents the existence of a primordial radiation epoch. We conclude that dark matter bulk viscosity is topologically and dynamically incapable of serving as a viable alternative to dark energy.