Physics Division Research Highlights

Void that is truly empty may help solve dark energy puzzle

The universe is expanding at an ever-increasing rate. Theoretical nuclear physics is trying to explain this in terms of the energy of quantum processes in empty space. The problem is that hitherto all potential sources of this so-called vacuum energy give values that far exceed the cosmological constant, an estimate of the universe’s energy density based on its observed expansion rate. This has been described as the worst physics prediction ever made. However, a new study eliminates a large part if not all of this discrepancy.

For almost fifty years it has been held that the ground-state – the vacuum – of the strong-interaction piece of the Standard Model; namely, quantum chromodynamics (QCD), is enormously complicated, being filled by a sea of quark-antiquark pairs, a condensate which is five-times more dense than matter at the core of a neutron star. The condensate carries energy and so, if it exists, it must contribute to the universe’s dark energy, which is expressed in the computed value of the cosmological constant.

The new study proves that there is an alternative to this conventional picture; namely, the ground-state is empty! Owing to the remarkable property of quark and gluon confinement, the condensate is entirely contained within the pions, protons and other states that constitute the hadron spectrum. Using rigorous tools in nonperturbative QCD, the study proves that the tumultuous vacuum fluctuations do not leak from hadrons, and thus, contrary to conventional belief, there are no space-time-independent condensates permeating the universe.

Hence, quarks and gluons cannot flutter in and out of existence unless they are inside hadrons. As a result, the vacuum that fills spacetime is much calmer. This paradigmatic shift has a huge impact on the cosmological constant paradox: it reduces the mismatch between theory and observation by a factor of 10⁴⁵! Its implications for the other source of dark energy within the Standard Model are actively being explored.

• New perspectives on the quark condensate, S.J. Brodsky, C.D. Roberts, R. Shrock and P.C. Tandy, Phys. Rev. C 82, 022201 (Rapid Comm.) (2010).

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