Entropy trend of the universe: Is the entropy of the universe continually increasing according to the second law?

Introduction / Context:
The second law of thermodynamics, in its broad cosmological interpretation, states that the total entropy of the universe (system plus surroundings) does not decrease. This idea underlies irreversibility and the direction of natural processes.

Given Data / Assumptions:

• Universe modeled as system + surroundings.
• Real processes involve irreversibilities (friction, mixing, heat transfer across finite ΔT, chemical reaction, etc.).
• Entropy is a state property; changes are path-independent.

Concept / Approach:
For any real process, ΔS_universe = ΔS_system + ΔS_surroundings ≥ 0. Equality holds only for the idealized limit of a reversible process. Thus, as processes unfold spontaneously, the universe’s entropy increases or, in the reversible limit, remains constant—never decreases.

Step-by-Step Solution:

Verification / Alternative check:
Clausius inequality for any cycle, ∮(δQ/T) ≤ 0, is equivalent to ΔS_universe ≥ 0 for real processes, confirming the directionality of time in thermodynamics.

Why Other Options Are Wrong:

• Incorrect: Violates the second law.
• True only for closed systems / reversible cycles: The statement applies generally; reversible cycles give ΔS_universe = 0, not < 0.

Common Pitfalls:
Interpreting “always increasing” as strictly increasing in every infinitesimal step; the rigorous statement is “never decreases,” with equality for reversible processes.

Final Answer:
Correct