EFFORTS to discover the nature of the mysterious force known as dark energy have been thrown into disarray by the discovery that supernovae are not as predictable as had been assumed.
Because the average brightness of the stellar explosions known as type Ia supernovae was thought to stay the same over the universe鈥檚 history, astronomers have treated them as 鈥渟tandard candles鈥�. In other words, they have used their apparent brightness as seen from Earth as a yardstick for measuring how far away they are, and from this they have estimated the rate of expansion of the universe.
Now an investigation of supernovae by Andrew Howell of the University of Toronto, Canada, and colleagues has thrown the basis for such measurements into doubt.
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The researchers examined data from the Supernova Legacy Survey and the Hubble Higher-z Supernova Search. This showed that type Ia supernovae, thought to signal the deaths of white dwarf stars, were about 12 per cent brighter 8 billion years ago than they are now (The Astrophysical Journal Letters, vol 667, p 37). Although Howell鈥檚 finding does not affect the conclusion that dark energy exists, it will require astronomers to adjust their calculations.
Even small variations in the brightness of a source being treated as a standard candle could make it impossible to be certain whether dark energy was stronger or weaker in the past. Knowing whether dark energy changes with time, and if so in what way, is crucial to deciding between competing ideas for what dark energy actually is.
Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, led one of the two teams that independently discovered dark energy in 1998 using calculations based the brightness of supernovae. He says a change in average supernova brightness could affect dark-energy measurements. Astronomers making these calculations already allow for the fact that longer-lasting supernovae are brighter than their more fleeting counterparts. But these corrections could be imperfect. Now they might have to make further corrections.
Howell says it is not clear whether precise enough corrections can be made. 鈥淚f we are going to make the next leap to measuring changes in [dark energy] with time, it requires the correction to be better than 2 per cent,鈥� he says. 鈥淲e haven鈥檛 discovered enough supernovae yet to be able to tell if that kind of precision is achievable.鈥�
Why the early universe had more of the brighter type Ia supernovae remains a mystery. However, the brighter ones seem to occur more often in places where there is a high rate of star formation. Star formation was more vigorous in the early universe, so this could explain why there were more of the brighter supernovae then.
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