Pretty big explosion: This false-colour X-ray image of the remains of Tycho's Supernova shows an expanding bubble of multimillion-degree debris (green and red) inside a more rapidly moving front of high-energy electrons (blue).
Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.
SYDNEY: A study of supernovae used as a 'standard candles' to measure the expansion of the universe has revealed how these exploding stars evolve. The results have major implications for studies of dark energy, researchers say.
The research, published last week in the Astrophysical Journal, looked at over 200 observations of 'Type Ia' supernovae over a period of five years.
Type Ia supernova are known as cosmological 'standard candles' because they are uniformly bright regardless of their distance from Earth. Supernovae are the extremely bright death throes of stars created as their cores collapse and they explode, and can be as bright as entire galaxies.
Type Ia supernovas were used in the discovery of dark energy, a mysterious force that plays a role in the expansion of the universe.
Standard candles
In 1998 studies of Type Ia supernova showed that galaxies within the universe were getting further and further apart. The two separate studies surprised astronomers who up until then had thought that gravity was slowing things down.
The research revealed that a mysterious 'repulsive' force – which became known as dark energy – took over from gravity about 6.3 billion years ago, causing the expansion of the universe to accelerate, rather than slow down.
Type Ia supernova were crucial evidence for this acceleration because of their known brightness. Because of the uniformity of the brightness of these supernovae, how bright or faint their light appears in observations from Earth can be directly correlated with how distant the supernovas are from us.
As the supernova recede from us, their light is stretched as it expands. Looking at how the wavelength of their light has stretched – a property known as the 'redshift' – has allowed us to infer how much the universe has expanded since the light left the supernova.
Slightly variable
However the brightness of Type Ia supernovae does slightly vary, and this latest study puts the standard candle theory to the test to find out how much it varies over time. If there were significant variations, it would have implications for the expanding universe scenario and our understanding of dark energy.
The new study, led by astronomer Ryan Foley from the University of California, Berkeley, was based on data from the ESSENCE Project, which used both ground-based telescopes and the Hubble Space Telescope to spot the supernovae. The researchers looked at the spectra from 88 distant Type Ia supernovae and more than 400 light spectra from supernovae in nearby galaxies.
Astronomer and team member Brian Schmidt from the Australian National University in Canberra said the study was "the most comprehensive job we could do" of seeing how the supernovas had evolved. "We want to make sure they're not changing over time... [and] as far as we can tell they're changing very little," he said.
Good news
The results are good news for how astronomers currently understand dark energy and also support Einstein's predictions about cosmological acceleration.
"Dark energy seems to get created as space gets created, which is what Einstein's view of dark energy – the cosmological constant – predicts," said Schmidt. "As near as we can tell, that theory is safe; Einstein's version of things is what is happening."
Astrophysicist Scott Croom, from the University of Sydney, who wasn't part of the study, said it was the "best bit of work to date" done on the evolution of Type Ia supernovae.
"The key reason there this much interest in Type Ia [supernovae] is because they are one of the primary ways to detect dark energy," said Croom. But he said that other methods of studying dark energy, such as looking at the cosmological background radiation and soundwaves leftover from the Big Bang, would probably be more useful for studying dark energy in the future.
Schmidt said he was inclined to agree: "If we want to push our understanding of dark energy forward, we need to use other methods – this study is pretty much as far as we can go [understanding dark energy] with supernova. It will be extremely difficult to make sure any mistakes don't creep in with further supernova studies."
