A tantalizing hint that dark matter could be slowly changing into dark energy has been uncovered by a team of cosmologists in the UK and Italy. While the specific nature of the interaction driving the conversion is not known, the process could be responsible for slowing the growth of galaxies and other large-scale structure in the universe across the past eight billion years. If the conversion continues at the current rate, the universe's ultimate fate as a cold, dark and empty place could come sooner than expected.

Since the accelerating expansion of the universe was discovered in 1998, the best model of the evolution of the universe involves a cosmological constant (Λ) – which describes the accelerating expansion – along with cold dark matter (CDM). CDM comprises slow-moving particles that do not interact with electromagnetic radiation and are extremely long-lived. The particles account for about 85% of the matter in the universe and therefore their gravitational forces dominate the formation of large-scale structure.

While the ΛCDM model is supported by many different observations, several inconsistencies have come to light recently. Using data on cosmic-microwave-background radiation acquired in 2013 by the Planck space telescope, the ΛCDM model has been used to predict the rate at which large-scale structure should grow across the history of the universe. However, several different studies suggest that the rate at which structure is forming is slower than predicted by Planck/ΛCDM, which could mean that CDM is disappearing from the universe.

Cosmologists have tried to tackle these discrepancies by making slight modifications to the ΛCDM model – allowing Λ to change with time, for example, or having CDM decay into normal matter and energy – but these changes seem to create as many questions as answers.

Now, Valentina Salvatelli, Najla Said and Alessandro Melchiorri at the University of Rome, along with David Wands and Marco Bruni at the University of Portsmouth, have come up with a new way of reconciling these observations. They have extended the ΛCDM model to allow dark matter to decay to create dark energy. To check their model against observations, Wands and colleagues compared the evolution of large-scale structure as predicted by Planck/ΛCDM with large-scale-structure observations derived from measurements of redshift space distortions in galaxy surveys.

The researchers divided the history of the universe into four time bins of roughly equal length, and found that their proposed interaction becomes significant in the third and fourth bins – that is, between about eight billion years ago and the present day.

"It appears that the standard [ΛCDM] model is no longer sufficient to describe all of the data. We think we've found a better model of dark energy," explains Wands.

Although the rate at which CDM appears to be converted into dark energy is very slow, Wands says that if it continues at the current rate, all the CDM in the universe will be gone in about 100 billion years. "If the dark energy is growing and dark matter is evaporating, we will end up with a big, empty, boring universe with almost nothing in it," explains Wands.

Catherine Heymans of the University of Edinburgh describes the research as "a fascinating result", and points out that it is part of a larger effort to reconcile discrepancies (or "tensions") between measurements made by Planck and those from other telescopes. "Other researchers have said that this tension can be resolved if the dark-matter particle is a sterile neutrino," she explains, adding, "others still are looking at different modified-gravity theories to explain the result."

Heymans also points out that the tension could be the result of systematic errors in how one or more of the observations are made. "More data and further meticulous analysis of those data and the systematics that might be associated with them are the way to find out if this fascinating theory could be true," she cautions.

The research is described in Physical Review Letters.

  • There is much more about dark matter and dark energy in this feature-length article by Jeff Forshaw at the University of Manchester: "Theories of the dark side". This is one of several articles about dark matter and dark energy in the July 2014 issue of Physics World, which focuses on "The dark universe". You can access the July issue using the Physics World digital app by joining the Institute of Physics as an IOPimember