Entropic cosmology for a generalized black-hole entropy
  An entropic-force scenario, i.e., entropic cosmology, assumes that the
horizon of the universe has an entropy and a temperature. In the present study,
in order to examine entropic cosmology, we derive entropic-force terms not only
from the Bekenstein entropy but also from a generalized black-hole entropy
proposed by C. Tsallis and L.J.L. Cirto [Eur. Phys. J. C \textbf{73}, 2487
(2013)]. Unlike the Bekenstein entropy, which is proportional to area, the
generalized entropy is proportional to volume because of appropriate
nonadditive generalizations. The entropic-force term derived from the
generalized entropy is found to behave as if it were an extra driving term for
bulk viscous cosmology, in which a bulk viscosity of cosmological fluids is
assumed. Using an effective description similar to bulk viscous cosmology, we
formulate the modified Friedmann, acceleration, and continuity equations for
entropic cosmology. Based on this formulation, we propose two entropic-force
models derived from the Bekenstein and generalized entropies. In order to
examine the properties of the two models, we consider a homogeneous, isotropic,
and spatially flat universe, focusing on a single-fluid-dominated universe. The
two entropic-force models agree well with the observed supernova data.
Interestingly, the entropic-force model derived from the generalized entropy
predicts a decelerating and accelerating universe, as for a fine-tuned standard
$\Lambda$CDM (lambda cold dark matter) model, whereas the entropic-force model
derived from the Bekenstein entropy predicts a uniformly accelerating universe.
