Quantum chromodynamics (QCD), the theory that describes the strong nuclear force, is odd even by quantum mechanical standards. QCD dictates that, unlike pretty much any other particle, when you pull apart two quarks—the constituent particles of hadrons and one of the base particles of QCD—the force between them does not decrease. Instead, it increases, a property known as confinement. This means that if you pulled hard enough on two bound quarks, the energy between them could become so great that it would cause a quark-antiquark pair to pop into existence and alleviate the strain.

Quark-antiquark pairs are also thought to be a major component of the vacuum that pervades our Universe. Instead of being empty, the vacuum is thought to be teeming with a complex mix of these fundamental particles. However, a new paper suggests that this view of the Universe may have things wrong.

QCD is also an oddity in that, at higher and higher energies, the interactions between its constituent particles becomes weaker and weaker, a phenomenon known as asymptotic freedom. Given the (proven) existence of asymptotic freedom, physicists can directly model QCD at high energies.

The weakened interactions at high energies allows physicists to solve the complex mathematical equations through perturbative expansions. This is a technique where you can assume the answer has a simple algebraic form, plug it back into the original equation, drop all terms that are "complex," and solve the resulting, simplified equation. However, this only works when interactions are weak.

Unfortunately, at low energies (particularly the ground state), QCD is extremely difficult to comprehend or describe mathematically. Currently, physicists think that the ground state of QCD is a "vacuum quark condensate"—an incredibly complex sea of quark-antiquark pairs that is five times more dense than the matter at the center of a neutron star.

But the new paper, which will appear in an upcoming issue of Physical Review C, argues that the vacuum in the QCD ground state is actually empty. The authors, S.J. Brodsky, C.D. Roberts, R. Shrock, and P.C. Tandy, suggest that the quark condensate should be interpreted as a property of particles (the hadrons themselves)—it does not "leak" into the surrounding vacuum.

Their argument relies on QCD's unique property: the confinement of quark and gluons, which don't exist for any time outside of other particles. The authors suggest that this keeps all the mess they create contained in the pions, protons, neutrons, and other hadrons, so there can't be any form of quark-filled vacuum. This result goes against the commonly accepted idea that an extremely dense, spacetime-independent condensate permeates our universe.

In addition to changing how the vacuum is viewed, this model would also erase a discrepancy that plagues modern physics. The cosmological constant that's calculated when assuming a quark-gluon vacuum condensate is 45 order of magnitude (1045 times larger) than the one observed by astrophysicists. This new view of an empty vacuum would bring the theoretical prediction to something much closer to the one seen in the heavens.

Physical Review C, 2010. DOI: Upcoming.