From Landau two-fluid model to de Sitter Universe

Condensed matter analogs are useful when considering phenomena related to the quantum vacuum.This is because, in condensed matter, we know the physics both in the infrared and in the ultraviolet limits, while, in particle physics and gravity, the physics on the trans-Planckian scale are unknown.One of the cornerstones of the connections between nonrelativistic condensed matter and modern relativistic theories is the two-fluid hydrodynamics of superfluid helium, which was developed by Landau and Khalatnikov (Isaak Markovich Khalatnikov later founded and headed the Landau Institute).The dynamics and thermodynamics of the de Sitter state of the expansion of the Universe bear some features of the multi-fluid system.There are actually three components: the quantum vacuum, the gravitational component, and relativistic matter.The expanding de Sitter vacuum serves as a thermal bath with local temperature, which is twice the Gibbons±Hawking temperature related to the cosmological horizon.This local temperature leads to the heating of the matter component and the gravitational component, which behaves like Zel'dovich stiff matter and represents dark matter.