Time is troubling to physicists because according to the equations, there’s no reason it shouldn’t go backwards. For instance, a pendulum’s swing is identical whether you start it on the left or the right. But we don’t see eggs spontaneously unsplatter, or ice unmelt into cubes in our lemonade.
Recently I have been pile-driving my way through Brian Greene’s The Fabric of the Cosmos. It’s a highly technical tour through modern frontiers of theoretical physics, and he gives a hefty 4 or 5 chapters discussing the nature of time. His proposed solution to the asymmetry of time has to do with entropy. For instance, if you shuffle a deck of cards there are many more ways for it to be disordered than in order, so it’s more likely to be out of order.
The initial conditions after the big bang were highly organized. The universe is far more chaotic today. At first, this argument did not make sense to me, but I realize it’s like a battery. The chemical reactions inside of a battery are all reversable. But the chemicals are arranged in a highly ordered state such to make the direction of reaction dominant. Thus the universe, the stars, and our animal metabolisms keep running off of the continual input of energy.