The Evolving Search For The Nature Of Dark Energy: Part 1, Supernovae as Standard Candles

Dark energy appears to account for over three-quarters of the stuff in the Universe, and it’s pushing all the rest – ordinary matter and dark matter – farther apart at an ever-increasing rate. But what is dark energy? Although theories abound, the short answer is that nobody knows.

We know it exists because of an experimental technique that uses specific types of exploding stars, or supernovae, as “standard candles.” A dozen years ago measurements of these supernova at increasing distances from Earth led to the unexpected discovery of dark energy; observations of supernovae continue to increase in power and precision in ongoing studies.

Independent evidence from measurements of the cosmic microwave background and other estimates of the matter density of the Universe provided early support for the radical idea of dark energy. Newer and quite different techniques, including weak lensing and baryon acoustic oscillations, are now poised to offer unique insights into what Nobel Prize-winner Frank Wilczek has called “the most fundamentally mysterious thing in basic science.”

Type Ia Supernovae: The Best Standard Candles

During the 1980s and 90s, the Supernova Cosmology Project (SCP), co-founded by Saul Perlmutter and Carl Pennypacker and based at Berkeley Lab, demonstrated that Type Ia supernovae were excellent standard candles for measuring the expansion history of the Universe. Although the idea had been circulating within the astronomical community for years, says Perlmutter, a Berkeley Lab astrophysicist and professor of physics at UC Berkeley, “In the early days, people thought measuring expansion with supernovae would be too hard.”

The SCP went on to show that distant supernovae, short-lived and unpredictable as they are, can nevertheless be collected “on demand,” allowing observers to schedule telescope time in advance and accumulate enough data to make confident estimates of expansion.

“In retrospect it seems obvious, but we realized that the whole process could be systematized,” Perlmutter explains. “By searching the same group of galaxies three weeks apart, we could find supernovae candidates that had appeared in the meantime. We could guarantee four to eight supernovae each time, and all of them would be on the way up” growing brighter instead of already fading.

Type Ia supernovae are among the brightest things in the Universe; what’s more, they are all almost the same brightness, with differences that can be standardized to less than 10 percent. Thus a supernova’s apparent brightness shows how far away it is and, because light takes time to travel, how far back in time it exploded.

The supernova’s redshift – the shifting of spectral lines (signals of specific elements in the exploding star) toward the red end of the spectrum – is a direct measure of how much the space through which the light has traveled has stretched.

The idea is simple on paper: by comparing brightness to redshift for numerous Type Ia supernovae, from nearby to very distant, an observer can tell how the rate of expansion of the Universe has changed over time.