Is the Hypothesis About a Low Entropy Initial State of the Universe
  Necessary for Explaining the Arrow of Time?
  According to statistical mechanics, micro-states of an isolated physical
system (say, a gas in a box) at time $t_0$ in a given macro-state of
less-than-maximal entropy typically evolve in such a way that the entropy at
time $t$ increases with $|t-t_0|$ in both time directions. In order to account
for the observed entropy increase in only one time direction, the thermodynamic
arrow of time, one usually appeals to the hypothesis that the initial state of
the universe was one of very low entropy. In certain recent models of
cosmology, however, no hypothesis about the initial state of the universe is
invoked. We discuss how the emergence of a thermodynamic arrow of time in such
models can nevertheless be compatible with the above-mentioned consequence of
statistical mechanics, appearances to the contrary notwithstanding.
