Why is the Vacuum Energy Density Much Smaller Than the Zero-Point Energy Suggested by Quantum Field Theory?

Why is the Vacuum Energy Density Much Smaller Than the Zero-Point Energy Suggested by Quantum Field Theory?

The vacuum energy density problem, often called the cosmological constant problem, is one of the most perplexing mysteries in modern theoretical physics. According to quantum field theory (QFT), the vacuum is not empty but is filled with fluctuating energy, known as zero-point energy. These quantum fluctuations, if summed across all modes, predict an enormous vacuum energy density. However, observations of the large-scale structure and expansion rate of the universe indicate that the vacuum energy density, as inferred from the cosmological constant (Λ), is extraordinarily small in comparison. This discrepancy, sometimes referred to as a 120 orders of magnitude difference, poses a significant challenge for our understanding of the universe.

This article explores the vacuum energy density problem in detail by examining the concepts of zero-point energy, quantum field theory predictions, and why the observed cosmological constant is far smaller than what is expected from theoretical estimates.

1. The Concept of Zero-Point Energy

1.1. Quantum Fluctuations and Zero-Point Energy