Carnegie Science | Spring 2019 6 New work from the Carnegie Supernova Project provides the best-yet calibrations for using Type la supernovae to measure cosmic distances to determine how fast the universe is expanding and how dark energy may drive this process. Led by Carnegie astronomer Chris Burns, the team’s findings were published in The Astrophysical Journal. Type la supernovae are fantastically bright, violent explosions of a white dwarf, a stellar remnant that has exhausted its nuclear fuel, that is part of a binary system with another star. Type la supernovae are vital tools that astronomers use as cosmic mile markers to infer the distances of celestial objects. While the precise details of the explosion are still unknown, it is believed that they are triggered when the white dwarf approaches a critical mass, so the brightness of the phenomenon is predictable from the energy of the explosion. The difference between the predicted brightness and the observed brightness reveals the distance to the supernova. Astronomers employ these precise distance measurements, along with the speed at which their host galaxies are receding, to determine the rate at which the universe is expanding, a measure called the Hubble Constant. Thanks to the finite speed of light, we can measure how quickly the universe is expanding now and, by looking farther into space, we can see and measure how fast the universe was expanding in the distant past. In the late 1990s, this led to the astonishing discovery that the universe’s expansion is accelerating due to the repulsive effect of mysterious “dark” energy. Improving the distance estimates using Type la supernovae will help astronomers better understand the role that dark energy plays in this expansion. “Beginning with its namesake, Edwin Hubble, Carnegie astronomers have a long history of working on the Hubble Constant, including vital contributions to our understanding of the universe’s expansion made by Allan Sandage and Wendy Freedman,” said Observatories director John Mulchaey. However, the speed at which the brightness of Type la supernova explosions fade away is not uniform. In 1993, Carnegie astronomer Mark Phillips showed that the explosions that take longer to fade are intrinsically brighter than those that fade quickly. This correlation, which is called the Phillips relation, allowed a group of astronomers in Chile, including Phillips and Texas A&M astronomer Nicholas Suntzeff, to develop Type la supernovae into a precise tool for measuring the expansion of the universe. Calibrating Cosmic Mile Markers This artist’s concept of the cosmic distance ladder shows a series of celestial objects, including Type la supernovae that have known distances and can be used to calculate the rate at which the universe is expanding. Image courtesy NASA/JPL-Caltech 6 Carnegie Science | Spring 2019