text: The phrase 'arrow of time' refers to any process that is assymetrical w.r.t. time. I.e. the process happens in one direction and not in the other direction (or more correctly, the reverse process is exceedingly improbable).\n\nThe main arrow of time that we are aware of is related to the physical quantity called 'entropy' and the law of the universe that governs it, the 2nd Law of Thermodynamics.  It would take some space to describe this properly, but to summarize, 'entropy' can be thought of as the amount of disorder or randomness in a closed system (one that is not receiving heat energy from an outside source).  The 2nd Law says that the entropy (disorder or randomness) of that closed system can only go up over time.  I.e. things don't get 'less random' unless some outside energy is available to cause that order.  The 2nd Law is perhaps the most profound law of the universe, and the only one we know of that introduces a time assymetry (an arrow of time).\n\nThe wiki page (see Sources), provides a nice list of different concepts of the arrows of time:\n- The thermodynamic arrow of time (entropy)\n- The cosmological arrow of time\n- The radiative arrow of time\n- The causal arrow of time\n- The weak arrow of time\n- The quantum arrow of time\n- The psychological/perceptual arrow of time\n\nIt sounds like you're interested in the cosmological arrow, and there are two possibilities:\n\nThe first is entropy (the thermodynamic arrow), and the idea that the entropy at the Big Bang was very low, and has slowly been increasing, and will eventually and inevitably lead to the heat death of the universe.\n\nThe second arrow is related to the expansion of the universe, and if gravity eventually causes the universe to stop expanding and begin contracting, the time arrow will be reversed.  The idea that the 2nd Law itself would be reversed is pretty radical.  I.e. ... that as gravity pulls everything together, things will become more and more ordered, and entropy will be *decreasing*.  \n\nIt's not clear how this second arrow will affect perception by intelligent beings.  It may be that we can't tell the difference ... i.e. that mental processes are bound to the thermodynamic arrow (entropy) ... so if this is reversed by the contraction of the universe, we won't know the difference and the universe will always appear (to us) to be expanding.  Maybe that's what has occurred!  :-)\n\nHope that helps.