DARK energy, the mysterious stuff invoked to explain why the expansion of the universe is accelerating, could have a simple explanation. The energy may be coming from neutrinos that were created in copious quantities just after the big bang.
The leading candidate for dark energy is Einstein鈥檚 鈥渃osmological constant鈥�, which proposes that the vacuum of space has an inherent energy that counters gravity. But if you calculate the density of this energy using quantum theory, it works out at nearly 120 orders of magnitude greater than what would fit with cosmological observations. So physicists have proposed ever more exotic explanations for dark energy that require, for instance, the existence of extra dimensions.
Now a team of Italian physicists says the answer has been under our noses all along. Antonio Capolupo and Giuseppe Vitiello of the University of Salerno and Salvatore Capozziello at the University of Naples claim that dark energy can be explained by neutrinos, particles that have no charge and little or no mass. Vast numbers were created just after the big bang, and many remain today because they barely interact with matter.
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Neutrinos come in three 鈥渇lavours鈥�, and recent experiments have confirmed they can switch flavours. According to the researchers, this neutrino 鈥渕ixing鈥� contributes just the right amount of energy to the vacuum of space. 鈥淣eutrino mixing may solve the problem of dark energy,鈥� says Capolupo.
鈥淭he explanation for dark energy might have been under our noses, in a particle created in vast numbers by the big bang鈥�
Several years ago, the Italian trio, along with Massimo Blasone of the University of Salerno, developed a model to better explain neutrino mixing. Recently, the trio used the model to calculate how much energy the neutrinos contribute to the vacuum of space. The result fitted very well with the observational values for dark energy 鈥� assuming that they are the only significant contributors to the energy of vacuum ().
Intriguingly, there have been indications that every cubic centimetre of space contains about 300 neutrinos, and that the energy density of these particles is roughly equal to the energy density required to drive the acceleration caused by dark energy. However, no one could explain how the energy of neutrinos translates into the vacuum energy of space. The Italians鈥� model of neutrino mixing now provides a mechanism. The vacuum is thought to be a cauldron of particles that pop in and out of existence and the neutrinos are transmuted into energy by their interactions with the vacuum. 鈥淚t reveals an elegant, deep connection between particle physics and cosmology,鈥� says Capolupo.
Other cosmologists are cautious. 鈥淲e are far from a complete understanding of both dark energy and the problem of [neutrino] mixing,鈥� says Nikolaos Mavromatos at King鈥檚 College London. Scott Dodelson at particle research lab Fermilab in Chicago agrees. 鈥淚f their explanation works theoretically, it is extremely interesting,鈥� he says. 鈥淭heir model in principle is testable by looking at how dark energy evolves over time.鈥�
Once the team works out exactly how dark energy changes with time, projects such as the Supernova Cosmology Project can test the predictions. The model also predicts how neutrinos change flavours, which could be tested by experiments such as at the Sudbury Neutrino Observatory in Ontario, Canada.
