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.
