Credit: Photolibrary
However, the scientific community is far from reaching a consensus. Some cosmologists think even our understanding of the underlying physics is flawed and have tried to modify the laws of gravity, appealing to higher-dimensional spaces, exotic brane worlds, and string theory in attempt to explain the supernovae observations.
An alternative approach is taken by David Wiltshire, a theoretical cosmologist from the University of Canterbury in New Zealand. In 2007 he worked out a solution to Einstein's general relativity, which far from being radical, he claims is "radically conservative".
This is because, unlike other cosmological theories, it does not require altered gravity or other exotic factors. And it doesn't need dark energy.
According to general relativity, the presence of mass slows the rate at which time passes. So, for example, clocks on satellites tick very slightly faster than those on Earth.
Wiltshire suggests that the distribution of matter in the universe leads to larger differences in the flow of time than previously considered. It is this oversight, he claims, that leads to the false conclusion that the universe is expanding at an accelerating rate: "Such acceleration is fundamentally not there; it is an illusion brought on by trying to fit an over-simplified model of the universe."
Wiltshire's theory also accounts for another quirk. According to the Standard Model, the universe has not always expanded at a uniform rate. After an initial period of rapid expansion called 'inflation', it slowed down. Then, approximately 6.3 billion years ago, the expansion started to speed up again.
According to Wiltshire's model, time was synchronised just after the Big Bang, as matter was uniformly distributed. Some time later, dust, stars, and galaxies started to evolve and voids started to open up. At this point, the flow of time started to diverge.
This also implies that the age of the universe varies. In matter-rich sections, such as our galaxy, the universe is 14.7 billion years old, Wiltshire claims. But in a less crowded void – where time is not slowed by mass – the universe is 18.6 billion years old.
"A huge area of general relativity is still largely unexplored, and this area is the crucial one for understanding dark energy in cosmology," Wiltshire says.
Thomas Buchert, from Claude Bernard University in Lyon, France, has been working on the problem of a lumpy universe for over a decade. Like Wiltshire, he believes it is time to rethink the explanations for dark energy and the standard model of the universe, but he's not convinced the answer has been found yet.
"Wiltshire…contributes to this new and lively discussion, but [he] does not provide an uncontroversial solution to the problem of dark energy," Buchert says.
