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Supernova shift may distort dark-energy readings
A. BURROWS/ARIZONA UNIVERSITY/SPL
This week–
Supernova blow to dark energy studies DAVID SHIGA
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’s history, astronomers have treated them as “standard candles”. 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. 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’s 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 darkenergy measurements. Astronomers making these calculations already allow for the
Full burn ahead keeps mice free of diabetes RAMPING up fat metabolism doesn’t just stop weight gain – it could also prevent type 2 diabetes. Previous studies had shown that mice engineered to lack an enzyme called acetyl-CoA carboxylase 2 (ACC2) deposited less fat in their tissues, despite eating up to 40 per cent more than normal mice. Because fatty deposits around the liver can lead to insulin resistance and type 2 diabetes, removing ACC2 should 14 | NewScientist | 13 October 2007
also protect mice from diabetes. There was a catch, however. For years, researchers had thought that burning more fat meant less carbohydrate would be used up. “This is the Randle hypothesis,” says James Ntambi at the University of WisconsinMadison. “In one metabolic pathway you generate intermediates that inhibit the enzymes of the other metabolic pathway.” If this was the case, removing
–How bright is it really?–
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. “If 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,” he says. “We haven’t
discovered enough supernovae yet to be able to tell if that kind of precision is achievable.” 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. ●
ACC2 could cause carbohydrate levels to rise – leading to excessively high blood sugar, insulin resistance and the onset of type 2 diabetes. Now, Gerald Shulman of Yale University and Salih Wakil at the Baylor College of Medicine in Houston, Texas, have found that this doesn’t happen after all. “The old concept that fat oxidation lowers glucose oxidation doesn’t seem to hold in the ACC2 knockout mice,” says Wakil. “Both fat and carbohydrate
oxidation are increased.” That means the mice are well protected against insulin resistance and type 2 diabetes, he says (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0706794104). Since ACC2 is also found in humans, the mouse result could have implications for diabetes research, as well as leading to new drugs to combat obesity. Ntambi thinks there is still work to be done, though. ACC comes in two forms. While removing ACC2 protects mice from insulin resistance, removing ACC1 is lethal. “It’s important to get an inhibitor for ACC2 alone,” says Ntambi. “That’s the challenge.” Colin Barras ●
“The result could have implications for diabetes research, as well as leading to new drugs to combat obesity”
www.newscientist.com
This week–
Supernova blow to dark energy studies DAVID SHIGA
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’s history, astronomers have treated them as “standard candles”. 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. 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’s 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 darkenergy measurements. Astronomers making these calculations already allow for the
Full burn ahead keeps mice free of diabetes RAMPING up fat metabolism doesn’t just stop weight gain – it could also prevent type 2 diabetes. Previous studies had shown that mice engineered to lack an enzyme called acetyl-CoA carboxylase 2 (ACC2) deposited less fat in their tissues, despite eating up to 40 per cent more than normal mice. Because fatty deposits around the liver can lead to insulin resistance and type 2 diabetes, removing ACC2 should 14 | NewScientist | 13 October 2007
also protect mice from diabetes. There was a catch, however. For years, researchers had thought that burning more fat meant less carbohydrate would be used up. “This is the Randle hypothesis,” says James Ntambi at the University of WisconsinMadison. “In one metabolic pathway you generate intermediates that inhibit the enzymes of the other metabolic pathway.” If this was the case, removing
–How bright is it really?–
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. “If 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,” he says. “We haven’t
discovered enough supernovae yet to be able to tell if that kind of precision is achievable.” 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. ●
ACC2 could cause carbohydrate levels to rise – leading to excessively high blood sugar, insulin resistance and the onset of type 2 diabetes. Now, Gerald Shulman of Yale University and Salih Wakil at the Baylor College of Medicine in Houston, Texas, have found that this doesn’t happen after all. “The old concept that fat oxidation lowers glucose oxidation doesn’t seem to hold in the ACC2 knockout mice,” says Wakil. “Both fat and carbohydrate
oxidation are increased.” That means the mice are well protected against insulin resistance and type 2 diabetes, he says (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0706794104). Since ACC2 is also found in humans, the mouse result could have implications for diabetes research, as well as leading to new drugs to combat obesity. Ntambi thinks there is still work to be done, though. ACC comes in two forms. While removing ACC2 protects mice from insulin resistance, removing ACC1 is lethal. “It’s important to get an inhibitor for ACC2 alone,” says Ntambi. “That’s the challenge.” Colin Barras ●
“The result could have implications for diabetes research, as well as leading to new drugs to combat obesity”
www.newscientist.com