The question you've raised is known as the entropy problem or the "arrow of time" problem. It relates to the apparent contradiction between the observed increase in entropy (a measure of disorder or randomness) over time and the existence of complex structures like galaxies, stars, and life forms in the universe.
According to the second law of thermodynamics, the entropy of a closed system tends to increase or, at best, stay the same over time. This law suggests that the universe should be moving toward a state of maximum entropy, where everything is evenly distributed and no gradients or structures exist. However, we observe the presence of highly ordered structures, which seems to contradict this idea.
There are a few key points to consider when addressing this issue:
The universe is not a closed system: The second law of thermodynamics specifically applies to closed systems, where no energy or matter can enter or leave. The universe, on the other hand, is an open system, constantly exchanging energy and matter with its surroundings. This exchange allows for the formation of structures and gradients, even as entropy increases in certain areas.
Initial conditions: The early universe, just after the Big Bang, had extremely low entropy. As the universe expanded and evolved, entropy started to increase, but the initial low-entropy state provided a starting point for the development of complexity. The fact that the universe began in such a highly ordered state allowed for the emergence of structures despite the overall trend of increasing entropy.
Localized entropy decreases: While entropy generally increases, there are local processes where entropy decreases. For example, stars and galaxies form through the gravitational collapse of matter, which leads to the concentration of energy and a decrease in entropy in those localized regions. Life itself is a prime example of a highly organized system that forms in certain conditions, even though it contributes to the overall entropy increase.
Time asymmetry: The arrow of time refers to the observation that entropy increases in one direction—forward in time. This time asymmetry is not yet fully understood, but it's thought to be related to the initial conditions of the universe and the nature of physical laws. Some theories propose that the arrow of time emerges from the low-entropy conditions of the early universe, creating a boundary between past and future.
In summary, the existence of ordered structures in the universe, despite the increase in entropy, can be explained by considering the universe as an open system, the influence of initial conditions, localized entropy decreases, and the time asymmetry inherent in physical laws. While the precise details and explanations are still subjects of ongoing research, these concepts provide a framework for understanding the apparent discrepancy between entropy and the existence of complexity in the universe.