Since the big bang, a cosmic battle has been under way between matter (both dark matter and ordinary) and dark energy.
The gravitational force that draws massive objects such as galaxies towards each other works against the expansion of the universe. But astronomical observations show that the universe’s expansion has, oddly, been speeding up. This led scientists to conclude that dark energy, a mysterious force acting as a sort of anti-gravity, permeates the entirety of space. And dark energy appeared to be at a significant advantage in the cosmic tug of war.
As space expands, matter becomes more spread out, so its influence weakens. By contrast, dark energy is thought to be a property of space itself. So as space expands, it is constantly filled up with more dark energy, so its strength remains constant and it becomes the runaway winner. That, at least, has been the prediction of the most widely accepted theoretical model of recent decades.
“Eventually you have this picture where more distant regions are receding from us faster than light and they become, in principle, untouchable. That loss of contact grows and grows until even the most nearby current galaxies are too far away,” said Prof John Peacock, a cosmologist at the University of Edinburgh and a collaborator with the Dark Energy Spectroscopic Instrument (Desi), based in Arizona. “It ends in an awful loneliness where we are left isolated because everything is being whisked away too fast.”
This “big freeze” scenario is predicted by the current leading theoretical model of the universe, which has assumed dark energy to be constant and which suggests that dark energy accounts for 70% of the content of the universe, with dark matter comprising 25% and ordinary matter just 5%.
The most recent Desi results present the latest in a series of challenges to this picture. Measurements of 15m galaxies spanning 11bn years of cosmic history reveal patterns that scientists say are most readily interpreted as the result of dark energy that has evolved over time.
The data is best explained, according to the Desi analysis, if dark energy peaked when the universe was about 70% of its current age and is now on the decline. If confirmed, this reopens the question of whether dark energy is destined to dominate the fate of the universe.
If dark energy were, in future, to decline beyond zero and become negative (effectively switching teams and joining forces with gravity) the expansion could be sent into reverse, resulting in an ultimate “big crunch”.
“We could be back to one of those comforting old solutions where the universe might recollapse – and perhaps start again,” said Prof Carlos Frenk, a cosmologist at Durham University and a Desi team member. “I was always disturbed by a universe that keeps expanding forever … to become a dark, frozen expanse. I’m much more at ease with the possibility of new universes emerging in due course.”
But this is just one of a series of possibilities. If dark energy declined to zero and settled there, expansion would continue and the universe would have a quieter ending, in which galaxies remain in view until stars run out of fuel and flicker out of existence.
“The fate of the universe depends on what the dark energy does in the future,” said Peacock. “For things to recollapse it would have to change its actual sign. In principle, it could. Whether it’s going to, we have no idea.”
