Which explanation for the thermodynamic arrow of time — why time flows from past to future and entropy increases — will be validated as the correct scientific account?

rationale:

The thermodynamic arrow of time remains a profound unsolved problem in fundamental physics and the philosophy of physics. Currently, no single explanation achieves the decisive empirical validation and overwhelming consensus required for resolution. Instead, the estimates reflect the relative theoretical strengths of these programs and their trajectory toward ultimately commanding consensus.

The Past Hypothesis / Low-Entropy Big Bang (44%) This is the clear leading candidate and the de facto standard among specialists 2 sources. Time-symmetric microphysics alone—i.e., the Thermodynamic/Boltzmann account—inevitably predicts entropy increase in both temporal directions due to the reversibility objection 2 sources. To recover the macroscopic arrow of time, physicists and philosophers (such as Albert, Loewer, and Carroll) rely on the Past Hypothesis: the posit that the universe began in an extremely tiny, low-entropy region of phase space plato.stanford.edu. When combined with statistical mechanics, this forms the 'Mentaculus' framework, which successfully derives the causal and epistemic arrows, explaining fixed pasts and subjective flow 3 sources. Recent work extending this boundary condition into quantum frameworks, such as the Entanglement Past Hypothesis pmc.ncbi.nlm.nih.gov and Quantum Past Hypothesis philsci-archive.pitt.edu, further cements its status as the leading foundational assumption.

An entirely new physical law or framework (23%) While the Past Hypothesis is the leading operational assumption, it acts largely as a placeholder. It posits a highly specialized boundary condition but does not fundamentally explain why the initial state of the universe had such remarkably low entropy. For an explanation to be definitively 'validated' as the ultimate scientific account, it will likely require a foundational breakthrough in quantum cosmology or quantum gravity. The significant probability assigned to a new framework reflects the expectation that future physics will need to dynamically explain, derive, or supersede the Past Hypothesis rather than simply assuming it as a brute fact.

Secondary and Incomplete Accounts

• Thermodynamic/Boltzmann alone (8%): Widely recognized as incomplete because, without a specialized past boundary condition, it yields a two-sided entropy problem 2 sources.
• Quantum decoherence (8%): While an active research area producing important sub-arrows (e.g., in open quantum systems 2 sources), these models typically still require a low-entanglement initial condition [PMC11227449, d5bb93]. This renders decoherence a quantum reformulation or component of the Past Hypothesis rather than a standalone fundamental origin.
• Eternalism / Block Universe (7%): Though philosophically popular and motivated by relativity as a way to deny objective 'flow' plato.stanford.edu, it addresses the cognitive illusion of time rather than the physical mechanism of thermodynamic entropy increase.
• Cosmological expansion (6%): While correlated with the arrow of time, modern cosmological models (including bouncing cosmologies and dark energy models) generally still rely on special low-entropy or vacuum boundary conditions 2 sources.
• CPT/T-symmetry violation (2%) & Retrocausality (2%): Fundamental T-violation is real in weak interactions but is nearly universally dismissed as too tiny and disconnected to generate the macroscopic thermodynamic arrow pdg.lbl.gov. Retrocausality remains a fringe minority interpretation in quantum foundations plato.stanford.edu.