Holography, the Cosmological Constant and the Upper Limit of the Number of e-foldings

TL;DR

This paper explores the theoretical upper limit on the number of inflationary e-foldings in the universe, using holographic principles and cosmological models, and compares different bounds to understand their implications.

Contribution

It derives an expression for the upper limit of inflation e-foldings based on holographic bounds and cosmological parameters, extending previous arguments.

Findings

Holographic D-bound yields higher upper limits than entropy thresholds.

Upper limit can reach 146 under extremal conditions.

For typical cosmological assumptions, the limits are around 65-85, consistent with solving flatness and horizon problems.

Abstract

If the source of the current accelerating expansion of the universe is a positive cosmological constant, Banks and Fischler argued that there exists an upper limit of the total number of e-foldings of inflation. We further elaborate on the upper limit in the senses of viewing the cosmological horizon as the boundary of a cavity and of the holographic D-bound in a de Sitter space. Assuming a simple evolution model of inflation, we obtain an expression of the upper limit in terms of the cosmological constant, the energy density of inflaton when the inflation starts, the energy density as the inflation ends, and reheating temperature. We discuss how the upper limit is modified in the different evolution models of the universe. The holographic D-bound gives more high upper limit than the entropy threshold in the cavity. For the most extremal case where the initial energy density of inflation is as high as the Planck energy, and the reheating temperature is as low as the energy scale of nucleosynthesis, the D-bound gives the upper limit as 146 and the entropy threshold as 122. For reasonable assumption in the simplest cosmology, the holographic D-bound leads to a value about 85, while the cavity model gives a value around 65 for the upper limit, which is close to the value in order to solve the flatness problem and horizon problem in the hot big bang cosmology.