Question

As far as I know, the laws of physics are time-reversal invariant, which means there is no preferred direction of time. The arrow of time emerges with entropy which is a property of macrostates, not microstates. So, on a microscopic description of physical reality, why do we say the Big Bang happened first, or that it was the beginning of the Universe, if there is no preferred direction of time? Why can't we equally say the Big Bang happened last?

Answer 1

The Big Bang is considered the beginning of the universe since it corresponds to a low entropy state, from which the universe has been evolving towards higher entropy, providing a directional arrow of time. Despite microscopic laws being time-invariant, macroscopic phenomena like the Big Bang create an irreversible trajectory according to the second law of thermodynamics. This unfolding is supported by both theoretical and observational cosmology.

Step-by-step explanation:

While it is true that certain laws of physics are time-reversal invariant, suggesting that there is no preferred direction of time, the phenomenon of the Big Bang is associated with a specific direction of time because it coincides with the growth of entropy. This is consistent with the second law of thermodynamics, which dictates that the total entropy of an isolated system can never decrease over time. It provides us with the arrow of time, indicating a move from order to disorder. The Big Bang is perceived as the beginning because it represents a state of low entropy, which then increases over time, as is observed in the cosmic evolution.

In macroscopic systems with a vast number of particles, reversibility fades due to numerous interactions that prevent any return to initial conditions within a practical timeframe. The current state of the universe, indicated by the observable cosmic microwave background temperature, is a marker of its age and stage in its own thermodynamic evolution, stemming from the Big Bang's low entropy start toward eventual heat death - a state of maximum entropy.

Finally, both experimental physics and cosmology support the idea that the universe began in a Big Bang 13.8 billion years ago, a theory solidified by a wealth of evidence. Although on a microscopic level time might seem symmetrical, the Big Bang represents a low-entropy boundary condition which delineates a past profoundly different from the present. The arrow of time thus emerges due to the entropic processes that began with the Big Bang and continue to steer the evolution of our universe.