The Cosmological Constant Problem arises because the quantum field theory predicts an enormous vacuum energy density, whereas astronomical observations show that the actual value driving cosmic acceleration is incredibly small.
The theoretical vacuum energy density from the Planck-era quantum field theory is
But the observed value from recent astronomical measurements is
This astonishingly huge mismatch – by roughly – is the largest known discrepancy between theory and measurement in Physics.
It suggests that something deep in our understanding of vacuum energy, gravity, or both is missing.
The Casimir effect complicates this puzzle, because it proves that vacuum fluctuations are not just mathematical bookkeeping. It shows that zero‑point energy has tangible, testable consequences in the lab, making it harder to dismiss vacuum energy as something that can simply be “renormalized away” without physical implications.
Yet the Casimir effect can only measure differences in vacuum energy between configurations, not the absolute value that gravity responds to. General Relativity couples to the total vacuum energy density, while Casimir experiments reveal only how that energy shifts when boundaries or materials modify the quantum fields.
So although the Casimir effect confirms the reality of vacuum fluctuations, it does not tell us why the Universe’s vacuum energy is so small.
This leaves a conceptual tension: laboratory physics insists that vacuum fluctuations are real and energetic, while cosmology insists that the vacuum’s gravitational effect is tiny.
Reconciling these two facts remains one of the most profound open problems in Theoretical Physics, hinting that our understanding of spacetime, quantum fields, or both may require a radical revision.