Past hypothesis
The premise that the universe began in an extraordinarily low-entropy state, which grounds the thermodynamic arrow of time.
Definition
The past hypothesis is the proposition that the observable universe started, at the Big Bang, in a state of extraordinarily low entropy, and that everything since has been the slow relaxation toward equilibrium. It is the missing ingredient in Boltzmann's account of the arrow of time: the reversible laws cannot prefer a direction, but a low-entropy boundary condition in the past makes entropy increase toward the future a near-certainty.
On this view every familiar temporal asymmetry — that we remember the past and not the future, that causes precede effects, that stars shine, that life can metabolise — is downstream of one cosmological fact. The hypothesis raises its own deep question: a low-entropy beginning is fantastically improbable a priori, so why was the universe born that way? Roger Penrose has estimated the required fine-tuning at about one part in 10 raised to 10¹²³, which remains unexplained.
The past hypothesis converts the arrow of time from a law of physics into a fact of cosmic history — a statement about the universe's initial condition rather than its equations of motion.
History
Boltzmann speculated about cosmological initial conditions and low-entropy fluctuations in the 1890s. The modern term 'past hypothesis' was popularised by the philosopher David Albert in 'Time and Chance' (2000); the fine-tuning estimate is due to Roger Penrose (1979 onward).