A study conducted at the University of Rome and the University of Portsmouth is suggesting that the amount of dark matter in the cosmos, the catalyst that facilitates the creation of new stars and galaxies, is decreasing as it interacts with dark energy. If this is true it would mean that, as time passes, the Universe could be destined to end up a desolate and nearly featureless place (even more so than it already is).
Since the days of Newton, we've known that gravity attracts ordinary matter closer together. Applied to the vastness of space, this means that the stars and galaxies in the Universe, though they are still traveling further and further apart in the wake of the Big Bang, should gradually slow down, come to a stop and eventually start collapsing toward each other.
But in 1998 when astronomers set out to measure the rate at which the expansion was slowing, they were in for a shock: the Universe, it turns out, is in fact increasing its rate of expansion – a discovery for which the scientists involved were later awarded the Nobel Prize in Physics.
The simplest (though not only) explanation, which is part of the currently accepted standard model of cosmology, is that the vacuum of space features an energy density that is constant both in space and time, a "dark energy" which is gravitationally repulsive and therefore counteracts the attractive effects of gravity, causing the cosmos to expand ever faster.
However, over the past few months, several cosmological surveys have cast doubts on the validity of this model. Cosmic microwave background experiments, such as ESA's Planck space telescope, have been able to precisely measure the parameters at the basis of the currently accepted model, and they have highlighted a few discrepancies. For instance, these parameters overestimate the speed at which galaxy clusters are growing.
Other galaxy surveys, such as the Sloan Digital Sky Survey, are measuring the gravitational potential of distant celestial bodies using so-called "redshift space distortions" and have cast further doubt on the validity of the currently accepted model.
Different theories have been advanced to resolve these discrepancies. One way to explain them would be to consider dark energy that changes slowly over time but does not interact with dark matter. A second possibility could be that neutrinos, electrically neutral subatomic particles, have a larger than expected mass. However, neither approach seems to provide a convincing solution.
Now, a new model advanced by researchers based in Italy and the UK appears to be the most promising yet.
"We found that the data prefer a model where the dark matter slowly loses energy to the dark energy," Prof. David Wands, who led the study, told Gizmag. "This naturally suppresses the growth of structure at late time. Our model gives a better fit to the redshift space distortions."
Prof. Wands tells us that dark matter is usually assumed to be non-interacting with ordinary matter and falls readily into initially overdense regions of the Universe, further increasing their density. Ordinary matter then falls into those same overdensities to form stars and galaxies, although the gravitational potential on large scales is dominated by dark matter, which makes up approximately 27 percent of the mass-energy in the Universe, versus the five percent of ordinary matter. In a sense, then, dark matter is the catalyst that allows new stars and galaxies to form.
"What we are seeing here, in these findings, suggests that dark matter is evaporating, slowing that growth of structure," says Wand. "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."
The new model introduces a parameter to describe the transfer of energy from dark matter to dark energy, but does not attempt to explain why this might be happening.
"It remains an open question as to why this occurs, though there have been many attempts to model possible interactions in the dark sector and we could now be seeing evidence of this."
The team's findings appear in a study published in the journal Physical Review Letters.
Source: University of Portsmouth