Supernovae light the way to dark energy

Images from the Dark Energy Camera before (left) and after (right) a supernova explosion in a galaxy about 2 billion light-years away.

The Dark Energy Survey (DES) collaboration has captured images of 176 star explosions, called supernovae, including 16 that occurred farther than 7 billion light-years away and when the universe was only about half as old as it is today. A new type of CCD detector contained in the Dark Energy Camera enabled identification of the distant supernovae, making DECam about 10 times more sensitive than other optical cameras to the long-wavelength (red and near-infrared) light coming from these very distant explosions. This improved sensitivity will allow the DES collaboration to find more supernovae from this period in the history of the universe than any other project.

Our current understanding is that the universe is made up of about 70 percent dark energy and that this dark energy is causing the universe to expand at an accelerating rate. Measuring Type 1a supernovae is a way to study dark energy. The fainter the observed explosion, the further away it is, similar to the difference in brightness between nearby and distant candles. As the light of the explosion travels to us, it is stretched by the expansion of the universe and becomes redder. By combining the measured brightness and information about how much the light is stretched, cosmologists can calculate the expansion rate of the universe.

The amount and wavelength of a supernova’s light determines its age and type. Researchers use filters that divide optical light into four separate parts, with each filter allowing only certain wavelengths to pass through. We know these 16 supernovae are about 7 billion light-years away because most of the light was observed with the filter that allowed only the reddest light to pass through and be measured by the special red-sensitive detectors in the camera. Less sensitive cameras require time-consuming follow-up observations to determine the supernova age.

To search for supernovae, the DES observers take images of the same patch of sky every four to seven days. Then they subtract the images from each other and search for differences. Computers and teams of people looked at thousands of sets of DECam images to find the 176 candidate supernovae. So far five of the candidates have been followed up, and all five were confirmed to be type 1a supernovae.

The Dark Energy Survey will measure more than 3,000 type-1a supernovae in the next five years and provide new information about the mysterious nature of dark energy. For more information, see the Dark Energy Survey website.

—Brenna Flaugher

The Dark Energy Survey collaboration includes scientists, postdocs and graduate students from around the world, who worked together to build the camera, collect the images and identify the supernovae described in this result.