  The dynamics of particle, event, and apparent horizons in FLRW space are
discussed. The apparent horizon is trapping when the Ricci curvature is
positive. This simple criterion coincides with the condition for the
Kodama-Hayward apparent horizon temperature to be positive, and also
discriminates between timelike and spacelike character of the apparent horizon.
We discuss also the entropy of apparent cosmological horizons in extended
theories of gravity and we use the generalized 2nd law to discard an exact
solution of Brans-Dicke gravity as unphysical.
