Pasadena Institute Helps Unlock Secret of How Fast the Universe is Expanding

Published : Wednesday, December 12, 2018 | 5:50 AM

An artist's conception of what's called the cosmic distance ladder -- a series of celestial objects, including type Ia supernovae that have known distances and can be used to calculate the rate at which the universe is expanding. Image courtesy NASA/JPL-Caltech

An astronomy research institute in Pasadena has come up with improved calibrations that will likely make it easier and faster for scientists to measure cosmic distances and understand how fast the universe is expanding, and what role a mysterious force known as “dark energy” could be playing in driving that expansion.

The improved calibrations are the result of new work by researchers at the Carnegie Observatories in Pasadena and their peers at the Carnegie Institution for Science in Washington on the Carnegie Supernova Project.

The Project has been studying supernovae – giant explosions that occur when stars die – which are essential to understanding the chemical evolution of the universe.

Recently, the Project has focused on Type Ia supernovae which provides the most powerful tool for studying the expansion history of the universe and the nature of dark energy. This type of supernovae is also a vital tool that astronomers use as a kind of cosmic mile marker to infer the distances of celestial objects.

Type Ia supernovae are fantastically bright stellar phenomena. 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 brightness observed from Earth tells us 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. Thanks to the finite speed of light, not only can we measure how quickly the universe is expanding right now, but by looking farther and farther out into space, we see further back in time and can measure how fast the universe was expanding in the distant past. This led to the astonishing discovery in the late-1990s that the universe’s expansion is currently speeding up due to the repulsive effect of a mysterious “dark” energy. Improving the distance estimates made using type Ia supernovae will help astronomers better understand the role that dark energy plays in this cosmic 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 Alan Sandage and Wendy Freedman,” said Carnegie Observatories Director John Mulchaey.

However, the speed at which the brightness of type Ia supernova explosions fades away is not uniform. In 1993, Carnegie astronomer Mark Phillips showed that the explosions that take longer to fade away are intrinsically brighter than those that fade away quickly. This correlation, which is commonly referred to as the Phillips relation, allowed a group of astronomers in Chile, including Phillips and Texas A&M astronomer Nicholas Suntzeff, to develop type Ia supernovae into a precise tool for measuring the expansion of the universe.

Studying the supernovae using the near-infrared part of the spectrum was crucial to this finding. The light from these explosions must travel through cosmic dust to reach our telescopes, and these fine-grained interstellar particles obscure light on the blue end of the spectrum more than they do light from the red end of the spectrum in the same manner as smoke from a forest fire makes everything appear redder. This can trick astronomers into thinking that a supernova is farther away than it is. But working in the infrared allows astronomers to peer more clearly through this dusty veil.

“One of the Carnegie Supernova Project’s primary goals has been to provide a reliable, high-quality sample of supernovae and dependable methods for inferring their distances,” said lead author Burns.

“The quality of this data allows us to better correct our measurements to account for the dimming effect of cosmic dust” added Mark Phillips, an astronomer at Carnegie’s Las Campanas Observatory in Chile and a co-author on the paper.

The calibration of these mile markers is crucially important because there are disagreements between different methods for determining the universe’s expansion rate. The Hubble constant can independently be estimated using the glow of background radiation left over from the Big Bang. This cosmic microwave background radiation has been measured with exquisite detail by the Planck satellite, and it gives astronomers a more slowly expanding universe than when measured using type Ia supernovae.

“This discrepancy could herald new physics, but only if it’s real,” Burns explained. “So, we need our type Ia supernova measurements to be as accurate as possible, but also to identify and quantify all sources of error.”


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