In What Sense Is the Early Universe Fine-Tuned?

TL;DR

The paper argues that the early universe's fine-tuning is real and better understood through a measure on cosmological trajectories, challenging traditional horizon and flatness problem interpretations.

Contribution

It offers a new perspective on cosmological fine-tuning by analyzing measures on trajectories, clarifying the nature of initial conditions beyond standard problems.

Findings

The fraction of smooth early universe histories is extremely small.

Traditional horizon and flatness problems are not the best explanations for fine-tuning.

A measure on trajectories provides a clearer understanding of initial condition requirements.

Abstract

It is commonplace in discussions of modern cosmology to assert that the early universe began in a special state. Conventionally, cosmologists characterize this fine-tuning in terms of the horizon and flatness problems. I argue that the fine-tuning is real, but these problems aren't the best way to think about it: causal disconnection of separated regions isn't the real problem, and flatness isn't a problem at all. Fine-tuning is better understood in terms of a measure on the space of trajectories: given reasonable conditions in the late universe, the fraction of cosmological histories that were smooth at early times is incredibly tiny. This discussion helps clarify what is required by a complete theory of cosmological initial conditions.