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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.

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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)