THE biggest camera ever put into space could solve what has become the most important problem in cosmology: the nature of the mysterious dark energy which appears to be speeding up the Universe鈥檚 expansion. It will do this by measuring the brightness and distance of 2000 supernovae, helping determine which model of dark energy seems most accurate.
The concept of dark energy emerged in 1998, when it was discovered that distant supernovae of known brightness were fainter than expected. The favoured explanation for this is that the expansion rate of the Universe must have increased since the supernovae exploded, pushing them farther away and so making them fainter. If this is so, some previously unseen force must be driving the galaxies, and this force is associated with something dubbed dark energy.
The proposed giant camera forms the heart of the SuperNova Acceleration Probe (SNAP), which the US Department of Energy has just confirmed as one of its top scientific priorities. 鈥淪NAP is one of the most important scientific projects of the coming decade,鈥� says astronomer Max Tegmark at the University of Pennsylvania. Current plans are for the probe to be launched in 2008.
Advertisement
SNAP will use an on-board spectrograph to measure the red shifts of supernovae, which depend on their distance.
A leading contender for what constitutes dark energy is the idea that it is associated with the 鈥渃osmological constant鈥�, a repulsive force pervading empty space proposed by Einstein. Its central characteristic is that its energy density is constant at all times in the Universe鈥檚 history. Another front runner is 鈥渜uintessence鈥�, and this type of energy can vary both over time and space. 鈥淲e want to see how the dark energy changes with look-back time, so we can distinguish between different models of what it is,鈥� says Greg Aldering of the Lawrence Berkeley Laboratory in California, one of SNAP鈥檚 originators.
SNAP will use a 2-metre telescope to gather light from a patch of sky 1.5掳 across 鈥� that鈥檚 6000 times the area Hubble can see. It will capture images using a 600-megapixel camera containing two types of detectors: CCD sensors for visible light, and mercury-cadmium-telluride detectors for infrared. Hubble has only a 2-megapixel CCD visible-light camera and a 65,000-pixel camera to capture infrared.
Building the camera will throw up several technical challenges, including dealing with waste heat from the camera鈥檚 electronics. Heat doesn鈥檛 dissipate easily in the vacuum of space, so it tends to heat up the sensors and damage them. To deal with the problem, customised low power, low-heat microchips are being developed by aerospace firm Rockwell.
The camera is expected to cost a few hundred million dollars 鈥� a fraction of Hubble鈥檚 $1.5-billion price tag. The 1.6-tonne SNAP probe will measure 6 metres long and 2.5 metres in diameter and will be launched in a highly elliptical 3-day orbit. 鈥淭he closest point needs to be at roughly the same longitude as Berkeley, so we can download the huge amount of data with an 11-metre radio dish at the Berkeley Space Science Lab,鈥� says Aldering.