Cosmological constant problem

Determine a theoretical explanation for the observed small positive cosmological constant Λ that resolves the approximately 120-order-of-magnitude discrepancy between the quantum field theory estimate of the vacuum energy density (with a Planck-scale cutoff) and the dark-energy density inferred from cosmological observations, thereby enabling a consistent identification of Λ with vacuum energy.

Background

Within ΛCDM, the cosmological constant Λ is naturally associated with vacuum energy because the vacuum stress-energy tensor is proportional to the metric. However, computing the zero-point energy in quantum field theory yields a quartically divergent integral; imposing a Planck-scale cutoff gives a vacuum energy density of roughly 1074 GeV4, about 120 orders of magnitude larger than the observed dark-energy density inferred from cosmology. This enormous mismatch prevents a straightforward identification of Λ with vacuum energy.

The chapter presents this issue as one of the central theoretical open questions for ΛCDM and discusses it under “The cosmological constant problem,” emphasizing that progress likely requires improved understanding of quantum field theory in curved spacetime.

References

The formulation presented in Sect.~\ref{ssec:dyn}, while mathematically coherent, leaves some open questions. In general, these questions are connected to our ignorance about the nature of the constituents of the dark sector. In this section, two of these open questions are addressed: the cosmological constant problem; and the cosmic coincidence problem.

— Dark energy and cosmic acceleration  (2502.00923 - Marttens et al., 2 Feb 2025) in Subsection “Theoretical challenges of the ΛCDM model” (Section 3.4)

This is an important problem to fundamental physics, as to the date it is unknown what is the true nature of DE.

— Cosmology without the cosmological principle: A study of the large-scale structure effects in the background universe  (2608.15983 - Pastén, 17 Aug 2026) in Chapter 1, Section 1.1, subsection “Theoretical challenges for standard cosmology,” item 2, “The Nature of Dark Energy”

However, as Weinberg explained in , the problem is not solved completely, and this is an open issue in this regard.

— Non-minimal Unimodular Inflation  (2608.20095 - Malekpour et al., 20 Aug 2026) in Section 1, Introduction

In unimodular PGT, the central question is whether this contribution only shifts the cosmological integration constant or also enters the running of curvature- and torsion-dependent couplings.

— Towards Perturbative Unimodular Poincaré Gauge Theories with Propagating Torsion  (2608.22669 - Vieira et al., 24 Aug 2026) in Section 4, Discussions and Conclusions, paragraph beginning “The inclusion of fermionic matter is especially relevant”