Fundamental Aspects of the Expansion of the Universe and Cosmic Horizons

TL;DR

This paper investigates the fundamental aspects of the universe's expansion, cosmic horizons, and thermodynamics within the LambdaCDM model, revealing counter-intuitive phenomena and testing the generalized second law's validity across various cosmological scenarios.

Contribution

It clarifies conflicts in the literature, derives new dynamics for non-comoving galaxies, and explores the implications of varying fundamental constants on black hole thermodynamics.

Findings

Objects at constant proper distance can have non-zero redshift.

The generalized second law of thermodynamics generally holds for cosmological horizons.

Variations in fundamental constants could lead to violations of the GSL, offering a way to test theories.

Abstract

In the context of the new standard LambdaCDM cosmology we resolve conflicts in the literature regarding fundamental aspects of the expansion of the universe and cosmic horizons and we link these concepts to observational tests. We derive the dynamics of a non-comoving galaxy and use this to demonstrate the counter-intuitive result that objects at constant proper distance can have a non-zero redshift. Receding galaxies can be blueshifted and approaching galaxies can be redshifted, even in an empty universe for which one might expect special relativity to apply. We then test the generalized second law of thermodynamics (GSL) and its extension to incorporate cosmological event horizons. In spite of the fact that cosmological horizons do not generally have well-defined thermal properties, we find that the GSL is satisfied for a wide range of models. We explore in particular the relative entropic 'worth' of black hole versus cosmological horizon area. An intriguing set of models show an apparent entropy decrease but we anticipate this apparent violation of the GSL will disappear when solutions are available for black holes embedded in arbitrary backgrounds. Recent evidence suggests a small increase in the fine structure constant (alpha =e^2/hbar c) over cosmological time scales. This raises the question of which fundamental quantities are truly constant and which might vary. We show that black hole thermodynamics may provide a means to discriminate between alternative theories invoking varying constants, because some variations in the fundamental 'constants' could lead to a violation of the generalized second law of thermodynamics.