Why Does the Universe Expand ?

TL;DR

This paper explores the fundamental reasons behind the universe's expansion, critiques existing explanations, and proposes a novel instability-based perspective linked to the universe's increasing classicality and thermodynamic time arrows.

Contribution

It introduces a new instability argument related to the kinetic energy sign in the Hilbert action and connects it with the universe's classical evolution and thermodynamic time direction.

Findings

Conventional explanations are inadequate.

Instability due to the kinetic term sign may drive expansion.

A thermodynamic perspective links the arrow of time with cosmic evolution.

Abstract

The purpose of the paper is five-fold: (a) Argue that the question in the title can be presented in a meaningful manner and that it requires an answer. (b) Discuss the conventional answers and explain why they are unsatisfactory. (c) Suggest that a key ingredient in the answer could be the instability arising due to the `wrong' sign in the Hilbert action for the kinetic energy term corresponding to expansion factor. (d) Describe how this idea connects up with another peculiar feature of our universe, viz. it spontaneously became more and more classical in the course of evolution. (e) Provide a speculative but plausible scenario, based on the thermodynamic perspective of gravity, in which one has the hope for relating the thermodynamic and cosmological arrows of time.