Speaker
Description
The notion of time arises in two fundamentally distinct frameworks: physical time, grounded in operational standards such as atomic transitions or electromagnetic oscillations, and geometrical time, typically defined through coordinate choices and metric structures. This distinction becomes particularly problematic in the early universe, where traditional physical clocks—based on stable, observable periodic processes—are unavailable. In such regimes, theorists rely on mathematically defined constructs like conformal time $\eta=\int \frac{d t}{a(t)}$ to describe temporal evolution. While conformal time clarifies causal structure and underpins key predictions in inflationary cosmology, its status as a physically interpretable clock—connected to observable processes—remains deeply unclear. This ambiguity complicates efforts to define observables, specify initial conditions, and evaluate the singularity structure of early-universe models.
Christof Wetterich (2024) recently proposed photon-based clocks as physically meaningful, frame-invariant measures that seemingly bridge these two conceptions. However, under the extreme conditions of the early universe—where quantum gravitational effects become significant—can clocks grounded solely in classical geometry and quantum fields on fixed backgrounds remain coherent operational standards?
This paper examines this tension, arguing that a consistent and physically meaningful definition of early-universe clocks requires a quantum gravity context. Photon-based clocks, though attractive for their theoretical consistency, fundamentally depend on the assumption that photons maintain stable frequencies and coherent propagation even in regimes approaching quantum-gravitational breakdown. This assumption, I argue, is neither trivial nor necessarily justified within a semi-classical approximation that couples quantum electrodynamics to a classical gravitational background. Without direct linkage to empirical observables—such as primordial fluctuations or model-independent correlations—neither conformal nor field-based clocks meet the criteria of physical measurement.
I contend that this dilemma drives cosmologists toward two paths: either developing new methods for empirically grounding theoretical clocks or reconceiving cosmological clocks as inferential constructs—epistemic tools embedded within theoretical models rather than direct standards of measurement. This discussion is relevant not only for theoretical cosmologists seeking operational robustness but also for philosophers of cosmology concerned with how time is defined, measured, and conceptualized in the absence of observers, instruments, or classical reference systems.
| Presenter's Name | Asya Ciftci |
|---|---|
| Presenter's Email Address | [email protected] |
| Keywords | early universe cosmology, physical time, conformal time, photon-clocks, quantum gravity |
| Recording Permission | YES |
| Virtual Audience Permission | YES |
| Event Photography | I understand |
