The Co-evolution of Cosmic Entropy and Structures in the Universe
  According to the second law of thermodynamics, the arrow of time points to an
ever increasing entropy of the Universe. However, exactly how the entropy
evolves with time and what drives the growth remain largely unknown. Here, for
the first time, we quantify the evolving entropy of cosmic structures using a
large-scale cosmological hydrodynamical simulation. Our simulation starts from
initial conditions predicted by the leading LambdaCDM cosmology,
self-consistently evolves the dynamics of both dark and baryonic matter, star
formation, black hole growth and feedback processes, from the cosmic dawn to
the present day. Tracing the entropy contributions of these distinct components
in the simulation, we find a strong link between entropy growth and structure
formation. The entropy is dominated by that of the black holes in all epochs,
and its evolution follows the same path as that of galaxies: it increases
rapidly from a low-entropy state at high redshift until z~2, then transits to a
slower growth. Our results suggest that cosmic entropy may co-evolve with
cosmic structure, and that its growth may be driven mainly by the formation of
black holes in galaxies. We predict that the entropy will continue to increase
in the near future, but likely at a constant rate.
