It’s long been realized that one kind of supernova – the explosion of white dwarfs that have too much mass dumped on them – should have consistent explosions, in much the same way that blowing up to identical sticks of dynamite should create identical explosions. As with so many things, that word “should” is the kicker. If the supernovae do all have the exact same energy, we can use them to measure distance by looking at the relationship between known luminosity and measured brightness. By then measuring the motions of these supernovae, we can get at the expansion rate of the universe. 

It was from supernovae that in 1998 folks discovered our universe is accelerating, and that is an uncomfortable realization. If it turns out that the supernovae simply change over time, with distant supernovae being a slightly different luminosity, then our data will artificially look like the universe is doing something it is not.

Since 1998, a whole lot of effort has gone into trying to understand how local and distant supernovae may or may not differ from one another. In two new papers in The Astrophysical Journal, both led by Kyle Boone, researchers use data from the Nearby Supernova Factory, a survey of supernovae explosions. With this data, they can make detailed comparisons of the supernova spectra, as a function of time. According to Boone: Conventional measurement of supernova distances uses light curves – images taken in several colors as a supernova brightens and fades. Instead, we used a spectrum of each supernova. These are so much more detailed, and with machine-learning techniques it then became possible to discern the complex behavior that was key to measuring more accurate distances.”

In this immense data set, pairs of identical-looking supernovae at near and far distances could be found. They also found that once they’d corrected their data for brightness and color, variability in supernovae was restricted to specific spectral lines, and by taking into consideration those specific differences, they could explain almost 90% of the observed variation in supernovae.

This work gave them two different pathways to test our understanding of the expansion of the universe. Since they had found identical-looking supernovae at very different distances, they could assume the supernovae with identical spectra also had identical luminosities and calculate their relative distances. With enough twins, more and more of space can be compared. They were also able to take into consideration how the observed differences affect the supernovae’s luminosity and make corrections. They estimate their results are accurate to within a remarkably low 3%. According to co-author Greg Aldering: …not only is this distance measurement technique more accurate, it only requires a single spectrum, taken when a supernova is brightest and thus easiest to observe – a game changer!

Our ability to take spectra of supernovae is only limited by the size of the telescopes we are using. It doesn’t require that much light to use filters and measure an object’s brightness in red, blue, and ultraviolet or some other broad set of colors. It is much harder to spread that light out into a detailed rainbow and get enough light in each color that a camera can measure. As larger and larger telescopes are built in the coming years, we will be able to press out further into the universe as we use this new technique to expand our understanding of the universe’s growth.

More Information

Berkeley Lab press release

“The Twins Embedding of Type Ia Supernovae. I. The Diversity of Spectra at Maximum Light,” K. Boone et al., 2021 May 6, The Astrophysical Journal