The Past Hypothesis Revisited
The classic Past Hypothesis (PH) claims the universe began in an extraordinarily low‑entropy state to explain the arrow of time. It describes the condition but never explains why entropy was low.
Axiom of the Immutable Past
Once an event is archived, it never changes.
Immutability means absolute stasis inside the past slice of spacetime.
Temperature Collapses to Zero
Thermodynamic temperature (T) measures the rate of micro‑state change. If no change occurs, then T = 0.
(Conceptually: T ∝ ∂E / ∂S; with ∂E = 0, temperature vanishes.)
Zero Temperature ⇒ Zero Entropy
With T = 0 and no heat flow (dQrev = 0), the entropy integral collapses: Spast = 0.
In plain language: no change → no temperature → no entropy.
How Immutable Past Improves the Past Hypothesis
| Past Hypothesis | Immutable Past Theory |
|---|---|
| Posits a “special” low‑entropy start without mechanism. | Derives S = 0 directly from immutability; no fine‑tuning required. |
| Leaves starting temperature undefined. | Explains T = 0 as a natural consequence of frozen dynamics. |
| Time arrow grounded in assumption. | Arrow follows naturally: entropy can only rise away from a zero‑entropy past. |
Implications
- Cosmology — Simulations can set initial
T = 0,S = 0exactly, eliminating arbitrary “smoothness” parameters. - Quantum gravity — Continuous wave‑function collapse by spacetime curvature “freezes” history, enforcing
T = 0. - Statistical mechanics — Entropy increase is simply the opening of new micro‑states in the present slice while the past remains locked at zero.
Bottom line: Low entropy is no longer a mysterious starting condition; it is the unavoidable thermodynamic shadow of an unchanging past.