LIGHT is the bedrock of astronomy, so it may come as a surprise that astronomers don’t have a very good handle on measurements of brightness. That is set to change, however, as the antiquated brightness scale undergoes a long-overdue upgrade that could help to reveal the true nature of dark energy.
More than 2000 years ago, the Greek astronomer Hipparchus devised a scale ranking the apparent brightness of different stars. Today, astronomers use much the same system, measuring brightness relative to a handful of standard reference stars. The trouble is, the reference stars’ brightness is not known very accurately, and measurements of it have not kept pace with developments in detector technology. For example, the most accurate measurements of the bright star Vega date back to the 1970s. “It’s surprising. There has been relatively little work on that in the past couple of decades,” says Gary Bernstein of the University of Pennsylvania in Philadelphia.
To redress this, a team led by Mary Elizabeth Kaiser of Johns Hopkins University in Baltimore, Maryland, is planning to launch a rocket-borne telescope to make the most accurate measurements yet of the reference stars’ brightness (arxiv.org/abs/1001.3925). Called the Absolute Color Calibration Experiment for Standard Stars (ACCESS), the NASA-funded mission will lift off in a year or two and make four suborbital flights, each taking it above Earth’s distorting atmosphere for a few minutes at a time.
During these brief jaunts, ACCESS will gauge the brightness of four common reference stars – the sky’s brightest star, Sirius; Vega; and a couple of much dimmer ones – to a precision of 1 per cent or better. That is twice the accuracy of current measurements, an advance that will be possible thanks to the calibration of the telescope’s sensors with artificial light sources before launch.
The measurements ACCESS makes will serve as a benchmark to calibrate the observations of other telescopes. This will allow the brightness of supernovae and other objects to be measured more accurately.
Such precision will be key to finding out the secrets of dark energy, a mysterious entity that is causing the universe to expand at an ever faster rate. The existence of dark energy was deduced in 1998 when astronomers noticed that distant supernovae were fainter – and thus farther away – than expected.
Astronomers still don’t know where dark energy comes from. It could spring from a fundamental new force, or it might point to a flaw in our understanding of gravity. To better understand it, researchers are examining the history of cosmic expansion, searching for slight variations in the expansion rate over time. This requires more accurate measurements of the brightness of supernovae at different cosmic epochs.
ACCESS team member Adam Riess of Johns Hopkins University, one of dark energy’s discoverers, says subtle errors can crop up when combining brightness data from multiple telescopes, potentially misleading astronomers about the nature of the acceleration. “You could think that dark energy is changing with scale or time, but it’s only an artefact of the fact that your observatories have not all used the same reference point,” he told New Scientist.
The ACCESS mission will help astronomers avoid this pitfall, he says. “It doesn’t measure dark energy itself but it makes your scale more accurate.” …