ARE THERE OTHER KINDS OF

The universe’s expansion is accelerating under the influence of a mysterious force, which could emerge from multiple sources

Reported by Robert Lea

© Tobias Roetsch

T

he expansion of the universe is a phenomenon that can only really be measured by observing galaxies separated by massive gulfs of space. The further apart, the faster they race away from each other. And that effect is speeding up. Dark energy is the name given to the hypothetical force that is causing this acceleration. Think of it as gravity’s ‘evil’ counterpart, providing a negative pressure that fills the universe and drives objects apart at an increasingly rapid rate. But dark energy doesn’t seem to work on objects as gravity does. Instead it preys on the very fabric of the cosmos itself – expanding the very space between objects.

Despite the fact it accounts for roughly 69 per cent of the universe’s total energy, we know very little about this mysterious, repulsive energy. We aren’t completely in the dark about dark energy’s ‘secret identity’, however. Cosmologists are aware of a number of ‘prime suspects’ that could account for its effects. “The leading candidate is the cosmological constant, or lambda, which we associate with the vacuum space’s energy,” Luz Ángela García, a physicist at Universidad ECCI, Bogotá, Colombia, explains. “Other possible candidates are quintessence fields, proposed in the early 21st century to establish a ‘fifth force’ responsible for the negative pressure that might cause the accelerated expansion of the universe.”

García, whose work involves finding alternative models to describe dark energy, as well as testing characteristics of this mysterious force, continues: “From this fifth force, many flavours of these fields have appeared in the literature, like K–essence, tachyonic fields, phantom fields and galileons, among many others.” With so many potential candidates for dark energy, it’s reasonable to ask if dark energy might arise from more than one source. Yet this is a question that cosmologists are only really beginning to consider. “There is no direct evidence for dark energy emerging from more than one source,” García posits. “But there is no reason to discharge such a possibility either.”

The Higgs boson should make a far greater contribution to the cosmological constant than measured

©CERN

Part of why scientists may have only just begun to consider a multi-source approach to dark energy could be the fact that ever since it was observed, the expansion of the universe has continued to catch researchers off guard. The discovery in the early 20th century by Edwin Hubble that the universe is expanding came as a shock to the scientific community at large, but really caught Albert Einstein flat-footed. Mainly because he’d already accidentally predicted and dismissed it.