Limits of time in cosmology

TL;DR

This paper explores the fundamental limits of the concept of time in cosmology, identifying several physically motivated boundaries well before the Planck scale where the standard understanding of time becomes problematic.

Contribution

It introduces multiple new limits on the applicability of the cosmological time concept, extending well before the Planck scale, based on phase transitions and quantum issues in the early universe.

Findings

Multiple limits on cosmological time concept identified

Standard clocks become unreliable before the Planck scale

Quantum and phase transition issues undermine time at early epochs

Abstract

We provide a discussion of some main ideas in our project about the physical foundation of the time concept in cosmology. It is standard to point to the Planck scale (located at $\sim 10^{-43}$ seconds after a fictitious "Big Bang" point) as a limit for how far back we may extrapolate the standard cosmological model. In our work we have suggested that there are several other (physically motivated) interesting limits -- located at least thirty orders of magnitude before the Planck time -- where the physical basis of the cosmological model and its time concept is progressively weakened. Some of these limits are connected to phase transitions in the early universe which gradually undermine the notion of 'standard clocks' widely employed in cosmology. Such considerations lead to a 'scale problem' for time which becomes particularly acute above the electroweak phase transition (before $\sim 10^{-11}$ seconds). Other limits are due to problems of building up a cosmological reference frame, or even contemplating a sensible notion of proper time, if the early universe constituents become too quantum. This 'quantum problem' for time arises e.g. if a pure quantum phase is contemplated at the beginning of inflation at, say, $\sim 10^{-34}$ seconds.