EMPTY space may really be empty. Though quantum theory suggests that a vacuum should be fizzing with particle activity, it turns out that this paradoxical picture of nothingness may not be needed. A calmer view of the vacuum would also help resolve a nagging inconsistency with dark energy, the elusive force thought to be speeding up the expansion of the universe.
Quantum field theory tells us that short-lived pairs of particles and their antiparticles are constantly being created and destroyed in apparently empty space. A branch of the theory, called quantum chromodynamics (QCD) 鈥� which explains how gluons and quarks, the particles that make up protons and neutrons, behave 鈥� predicts that a vacuum should be awash with an interacting sea or 鈥渃ondensate鈥� of quarks and gluons. This picture helps to explain how particles made of quarks get most of their mass.
This condensate carries energy, so it might be thought to be a candidate for the mysterious source of dark energy, which can be described by a parameter called the cosmological constant. The trouble is that when physicists use QCD to estimate the condensate鈥檚 energy density, their calculations suggest it would pack a punch that is 1045 times the cosmological constant that we measure from observations of the universe鈥檚 expansion.
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Now of the SLAC National Accelerator Laboratory in Menlo Park, California, and colleagues have found a way to get rid of the discrepancy. 鈥淧eople have just been taking it on faith that this quark condensate is present throughout the vacuum,鈥� says Brodsky. Instead, his team have assumed that the condensate exists only inside protons, neutrons, pions and all other quark-containing particles, collectively known as hadrons (Physical Review C, ).
鈥淚n our picture, quarks and gluons can鈥檛 flutter in and out of existence unless they are inside hadrons,鈥� says team member of the Argonne National Laboratory in Illinois. As a result, the vacuum is much calmer and, crucially, the problem it poses for the cosmological constant is reduced.
鈥淚n our picture, quarks and gluons can鈥檛 flutter in and out of existence unless they鈥檙e inside hadrons鈥�
In 1974, of Tel Aviv University in Israel and , now at Stanford University in California, suggested that a condensate present only inside hadrons could give these particles mass. Brodsky and colleagues are the first to show that this idea also helps resolve the dark energy discrepancy.
of Ohio University in Athens is excited by the result, but says more work must be done to show that the condensate can鈥檛 leak out of hadrons and into the vacuum. He points out that the result doesn鈥檛 rule out the existence of a vacuum condensate. 鈥淚t just shows you don鈥檛 have to assume one.鈥�
Another issue is that the quark and gluon condensates predicted by QCD are not the only entities to jar with the observed cosmological constant. Other theories predict vacuum energies that also vastly exceed it (see 鈥淭he worst prediction physics ever made鈥�). 鈥淭o solve the cosmological constant problem you would have to eliminate all these contributions,鈥� says of the University at Buffalo in New York.
The worst prediction physics ever made
鈥淥ne down, three to go鈥� would be an appropriate mantra for anyone trying to explain the universe鈥檚 accelerating expansion in terms of the energy of quantum processes in empty space.
The problem is that all potential sources of this vacuum energy give values that far exceed the cosmological constant, an estimate of the universe鈥檚 energy density based on its observed expansion rate. A new study may have got rid of one source of excess energy (see main story), but there are other, even more problematic ones. The Higgs boson, thought to be partially responsible for giving other particles mass, has an associated field whose vacuum energy is 1056 times the observed cosmological constant. Meanwhile, the vacuum energy associated with grand unified theories that aim to unify electromagnetism and the nuclear forces gives a value 10110 times too big.
The biggest disparity of all comes from attempts to unify quantum mechanics and general relativity. Under so-called quantum gravity, the energy density is 10120 times too big. 鈥淭his is actually called the worst prediction that physics ever made,鈥� says Dejan Stojkovic of the University at Buffalo in New York.
