Why is the gravitating vacuum energy ~10^120 times smaller than QFT predicts? (The cosmological constant problem)

The problem

In general relativity, vacuum energy gravitates like a cosmological constant: a Lorentz-invariant vacuum has $\langle T_{\mu\nu}\rangle = -\rho_{\rm vac}\, g_{\mu\nu}$, contributing to the effective

$$\Lambda_{\rm eff} = \Lambda_{\rm bare} + 8\pi G\,\rho_{\rm vac}.$$

Observations (CMB, BAO, SNe Ia) give $\rho_\Lambda^{1/4} \approx 2.3\ \text{meV}$, i.e. $\rho_\Lambda \sim 10^{-120}\, M_{\rm Pl}^4$.

Quantum field theory, however, generates contributions far larger than this:

• Zero-point fluctuations with a Planck-scale cutoff: $\rho \sim M_{\rm Pl}^4$ — mismatch $\sim 10^{120}$.
• Electroweak symmetry breaking: $\sim (100\ \text{GeV})^4$ — mismatch $\sim 10^{56}$.
• QCD condensate: $\sim (200\ \text{MeV})^4$ — mismatch $\sim 10^{44}$.
• Even the one-loop electron contribution, $\sim m_e^4$, overshoots by $\sim 10^{33}$ — using only physics tested to high precision.

Any single contribution can be cancelled by tuning $\Lambda_{\rm bare}$. The real difficulty is radiative instability: the cancellation must be re-tuned order by order in perturbation theory and across every phase transition in cosmic history. This makes it a naturalness problem in the IR, not merely a UV-cutoff artifact.

Precise question

Is there a mechanism, consistent with local QFT coupled to gravity and with observational tests of GR, under which the vacuum energy of the matter sector does not source spacetime curvature (or is dynamically relaxed), making $\Lambda_{\rm eff}$ technically natural without fine-tuning? Does such a mechanism predict $\rho_\Lambda \sim (\text{meV})^4$?

Sub-questions:

1. Old CC problem: why is $\Lambda_{\rm eff}$ not of order the largest scale in the theory?
2. New CC problem: why is it nonzero, and why is $\rho_\Lambda \sim \rho_{\rm matter}$ today (coincidence problem)?
3. Is dark energy actually a constant ($w = -1$), or is it dynamical?

Known partial results and obstacles

• Weinberg no-go (1989): no adjustment mechanism using a finite number of fields can relax $\Lambda$ to zero without fine-tuning, under standard assumptions (translation invariance, locality). Any solution must violate an assumption.
• Supersymmetry cancels boson/fermion vacuum contributions, but SUSY is broken above $\sim$ TeV, leaving $\rho \gtrsim (\text{TeV})^4$.
• Anthropic / landscape (Weinberg 1987; Bousso–Polchinski 2000): a correct order-of-magnitude "prediction", but it is contested whether this counts as an explanation, and it relies on the string landscape.
• Unimodular gravity: decouples vacuum energy at the classical level, but $\Lambda$ reappears as an integration constant and is not predicted.
• Vacuum energy sequestering (Kaloper–Padilla 2014), relaxation / self-tuning scalars, degravitation / massive gravity: each faces tension with Weinberg's theorem, fifth-force limits, or strong-coupling problems.
• Observations: DESI DR2 BAO (2025), combined with CMB and SNe, shows a 2.8–4.2σ preference for evolving dark energy ($w \neq -1$). If confirmed, this reshapes the problem: the "true" $\Lambda$ may be zero or undetermined, with a dynamical field on top.

Why it matters

This is the largest known discrepancy between theory and observation in physics. It is a direct, quantitative conflict between the two frameworks we trust most, QFT and GR, at energies far below the Planck scale. It is also sensitive to new data (DESI, Euclid, Rubin/LSST). Any resolution almost certainly requires new physics in how quantum matter couples to gravity.

S. Weinberg, "The cosmological constant problem", Rev. Mod. Phys. 61, 1 (1989)
S. Weinberg, "Anthropic bound on the cosmological constant", Phys. Rev. Lett. 59, 2607 (1987)
J. Martin, "Everything you always wanted to know about the cosmological constant problem", arXiv:1205.3365
A. Padilla, "Lectures on the cosmological constant problem", arXiv:1502.05296
C. P. Burgess, "The cosmological constant problem: why it's hard to get dark energy from micro-physics", arXiv:1309.4133
R. Bousso, J. Polchinski, "Quantization of four-form fluxes and dynamical neutralization of the cosmological constant", arXiv:hep-th/0004134
N. Kaloper, A. Padilla, "Sequestering the standard model vacuum energy", arXiv:1309.6562
Planck Collaboration, "Planck 2018 results. VI. Cosmological parameters", arXiv:1807.06209
DESI Collaboration, "DESI DR2 Results II: BAO and cosmological constraints", arXiv:2503.14738