Why does time flow forwards rather than backwards? Perhaps the answer to this long-standing mystery lies in what happened to the universe when gravity first took hold about 380,000 years after the big bang. We've known since the 19th century that entropy plays a role in the arrow of time. Eggs break rather than unbreak, coffee grows cold rather than hot, and people grow old rather than young. All these changes are associated with an increase in the entropy, or disorder, of a system. This happens because there are many more disordered than ordered states for a system to evolve into. That means by far the likeliest outcome is disorder and an increase in entropy. The trouble is there is scope for the universe to get more and more disordered only if it started off in a highly ordered state - a proposition physicists find hard to swallow because it is so unlikely. However, Lawrence Schulman of Clarkson University in New York state argues that the universe switched to a highly ordered state after 380,000 years - the first time it was cool enough for the constituents of atoms to combine.
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