Credit: Photolibrary
AS CARL SAGAN SAID, "extraordinary claims require extraordinary evidence".
So here's an extraordinary claim for you: the universe does not consist primarily of stars or planets, nor hydrogen gas, nor even elusive dark matter, but an energy of an unknown kind that is undetectable except for its effect on the expansion of the universe.
And the evidence? Observations of a few hundred distant supernovae that appear to be further away than expected. This disparity between the scant substantiation and vast ramifications of dark energy has caused more than one scientist to pause to re-examine the evidence, and even our assumptions about the very laws of physics.
"Those scientists, including myself, who remain reluctant to accept this theory, feel that the model is based upon too many fundamental assumptions unverified in the laboratory," says astrophysicist Richard Lieu, from the University of Alabama, in the USA. "The universe…is not a place for inventing new physical laws."
The primary evidence for dark energy are observations of Type 1a supernovae, which serve as cosmic standard candles due to their predictable and consistent brightness. In 1998, two groups of astronomers independently mapped these distant supernovae to find the expansion of the universe was speeding up (see "Dark forces", Cosmos 16, p56).
However, the relationship between brightness and distance has only been established empirically. An error in our measurements could lead to false conclusions about the distance of the supernovae and the acceleration of the universe.
In fact, recent evidence has shown that the mass and brightness of supernovae are more variable than previously thought. In 2006 astronomer D. Andrew Howell, from the University of Toronto in Canada, found an exotic kind of Type 1a supernova.
Before exploding, this star acquired a much larger mass than previously thought possible. In addition to variations in mass, Type 1a supernovae are also brighter in younger galaxies where there is still star formation.
Yet these results do not deter Howell. "By identifying these oddballs and throwing them out of the sample of normal Type 1a supernovae, we can...make the remaining supernovae better standard candles," Howell says.
Astronomers have now studied over 300 Type 1a supernovae and are more confident in their results. "We all try to keep an open mind about alternatives, because dark energy is strange stuff, but each new result makes it harder and harder to explain things in terms of anything other than dark energy," Howell says.
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.
Observations continue to put alternative theories to the test, such as a recent survey of over 10,000 distant galaxies by the Franco-Italian VIMOS-VLT Deep Survey, reported in Nature in January 2008. This compared dark energy with a modified theory of gravity, and found that dark energy provided a slightly better fit to the observational data.
But the results are far from conclusive, and they don't take into consideration that we might be misinterpreting the observations, as suggested by Wiltshire.
What is clear is that our questions on the nature of dark energy are still far from being answered. But a number of new and ongoing experiments may help us gain more insight. Some are Earth-based galactic surveys, such as HETDEX (Hobby-Eberly Telescope Dark Energy Experiment) at the University of Texas in Austin, USA. Some are space-based, such as NASA's proposed JDEM (Joint Dark Energy Mission) to survey more Type 1a supernovae.
Any one of these may yield the evidence necessary to make us as confident of the theory of dark energy as we are of the Big Bang. And if that evidence is not forthcoming, it may take a re-evaluation of the way we're interpreting our observations of distant stars to make sense of our strange cosmos.
Jacqui Hayes is a sub-editor and staff writer with Cosmos in Sydney.
