In cosmology, the past hypothesis is a fundamental law of physics that postulates that the universe started in a low- entropy state, in accordance with the second law of thermodynamics. The second law states that any closed system follows the arrow of time, meaning its entropy never decreases. Applying this idea to the entire universe, the hypothesis argues that the universe must have started from a special event with less entropy than is currently observed, in order to preserve the arrow of time globally. This idea has been discussed since the development of

statistical mechanics In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. It does not assume or postulate any natural laws, but explains the macroscopic be ...

,See Ludwig Boltzmann ''Vorlesungen über Gastheorie'' ("Lectures on Gas Theory", 1896) but the term "past hypothesis" was coined by philosopher David Albert in 2000. Philosophical and theoretical efforts focus on trying to explain the consistency and the origin of the postulate. The past hypothesis is an exception to the principle of indifference, according to which every possible

microstate A microstate or ministate is a sovereign state having a very small population or very small land area, usually both. However, the meanings of "state" and "very small" are not well-defined in international law.Warrington, E. (1994). "Lilliputs ...

within a certain macrostate would have an equal probability. The past hypothesis allows only those microstates that are compatible with a much-lower-entropy past, although these states are assigned equal probabilities. If the principle of indifference is applied without taking into account the past hypothesis, a low- or medium-entropy state would have likely evolved both from and toward higher-entropy macrostates, as there are more ways statistically to be high-entropy than low-entropy. The low- or medium-entropy state would have appeared as a "statistical fluctuation" amid a higher-entropy past and a higher-entropy future. Common theoretical frameworks have been developed in order to explain the origin of the past hypothesis based on inflationary models or the

anthropic principle The anthropic principle, also known as the "observation selection effect", is the hypothesis, first proposed in 1957 by Robert Dicke, that there is a restrictive lower bound on how statistically probable our observations of the universe are, beca ...

. The Weyl curvature hypothesis, an alternative model by

Roger Penrose Sir Roger Penrose (born 8 August 1931) is an English mathematician, mathematical physicist, philosopher of science and Nobel Laureate in Physics. He is Emeritus Rouse Ball Professor of Mathematics in the University of Oxford, an emeritus fello ...

, argues a link between entropy, the arrow of time and the curvature of spacetime (encoded in the

Weyl tensor In differential geometry, the Weyl curvature tensor, named after Hermann Weyl, is a measure of the curvature of spacetime or, more generally, a pseudo-Riemannian manifold. Like the Riemann curvature tensor, the Weyl tensor expresses the tidal forc ...

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Philosophy of thermal and statistical physics Philosophy of time {{physical-cosmology-stub

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