cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...

, the past hypothesis is a fundamental law of

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

that postulates that the universe started in a low-

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

state, in accordance with the

second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unles ...

. The second law states that any closed system follows the

arrow of time The arrow of time, also called time's arrow, is the concept positing the "one-way direction" or "asymmetry" of time. It was developed in 1927 by the British astrophysicist Arthur Eddington, and is an unsolved general physics question. This ...

, 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,See

Ludwig Boltzmann Ludwig Eduard Boltzmann (; 20 February 1844 – 5 September 1906) was an Austrian physicist and philosopher. His greatest achievements were the development of statistical mechanics, and the statistical explanation of the second law of ther ...

''Vorlesungen über Gastheorie'' ("Lectures on Gas Theory", 1896) but the term "past hypothesis" was coined by philosopher

David Albert David Z. Albert (born 1954) is Frederick E. Woodbridge Professor of Philosophy and Director of the MA Program in The Philosophical Foundations of Physics at Columbia University in New York. Education and career He received his bachelor's degree ...

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 The principle of indifference (also called principle of insufficient reason) is a rule for assigning epistemic probabilities. The principle of indifference states that in the absence of any relevant evidence, agents should distribute their cre ...

, according to which every possible microstate within a certain

macrostate In statistical mechanics, a microstate is a specific microscopic configuration of a thermodynamic system that the system may occupy with a certain probability in the course of its thermal fluctuations. In contrast, the macrostate of a system refe ...

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 Inflationism is a heterodox economic, fiscal, or monetary policy, that predicts that a substantial level of inflation is harmless, desirable or even advantageous. Similarly, inflationist economists advocate for an inflationist policy. Mainstream ec ...

models or the anthropic principle. The Weyl curvature hypothesis, an alternative model by Roger Penrose, 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 f ...

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

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