Dark goo: Bulk viscosity as an alternative to dark energy

Understand

We present a simple (microscopic) model in which bulk viscosity plays a role in explaining the present acceleration of the universe.

• The effect of bulk viscosity on the Friedmann equations is to turn the pressure into an "effective" pressure containing the bulk viscosity.
• For a sufficiently large bulk viscosity, the effective pressure becomes negative and could mimic a dark energy equation of state.
• Our microscopic model includes self-interacting spin-zero particles (for which the bulk viscosity is known) that are added to the usual energy content of the universe.