New Difficulties for the Past Hypothesis

TL;DR

The paper critically examines the Past Hypothesis as an explanation for time asymmetry, highlighting new theoretical difficulties and arguing that any measure on universe models must break gauge symmetry, posing significant challenges.

Contribution

It introduces a systematic framework for evaluating the Past Hypothesis and extends criticism by showing measure constraints that threaten its viability.

Findings

Systematic framework for assessing the Past Hypothesis

Identification of three broad categories of criticism

Any gauge-invariant measure on universe models must break gauge symmetry

Abstract

Many macroscopic physical processes are known to occur in a time-directed way despite the apparent time-symmetry of the known fundamental laws. A popular explanation is to postulate an unimaginably atypical state for the early universe -- a "Past Hypothesis" (PH) -- that seeds the time-asymmetry from which all others follow. I will argue that such a PH faces serious new difficulties. First I strengthen the grounds for existing criticism by providing a systematic analytic framework for assessing the status of the PH. I outline three broad categories of criticism that put into question a list of essential requirements of the proposal. The resulting analysis paints a grim picture for the prospects of providing an adequate formulation for an explicit PH. I then provide a new argument that substantively extends this criticism by showing that any time-independent measure on the space of models of the universe must necessarily break one of its gauge symmetries. The PH then faces a new dilemma: reject a gauge symmetry of the universe and introduce a distinction without difference or reject the time-independence of the measure and lose explanatory power.