Scientists Cut Uncertainty of the Size of the Universe in Half


The night sky has captivated people for as long as we have told stories. For millennia, mankind has sat in the dark and taken in the glorious spectacle of the stars and planets as they have marched across the sky. While we now know that space goes on essentially forever, it's important to remember that star gazing is inherently a two-dimensional endeavor. To all intents and purposes, the sky is like a giant television screen. We can precisely know the position of celestial objects on the tapestry of the heavens, but knowing how far away these objects are is much harder.

In our common experience, we can determine the distance of objects using our binocular vision inherited from our tree-living ancestors, but even using our best space-based telescopes and most sophisticated techniques, this approach works for objects that are within a ten or twenty thousand light years. While impressive, that distance is just peanuts to space.

For distances greater than that, it is necessary to compare the absolute and observed brightness of well understood objects and to use the difference to determine the object's distance. Essentially, this approach is the same as when sailors use the apparent brightness of a known lighthouse to estimate how far away it is.

In astronomy, finding an object of known brightness is very difficult. After all, there are examples of both bright and dim stars and galaxies. Luckily there is one astronomical phenomenon for which it is possible to work out its absolute brightness. This object is called a supernova. Supernovae are the death throws of a star in the last minutes of its life. The star explodes with such violence that we can see the flash across the vast universe. The special class called a Type Ia supernova is thought to occur in binary star systems, when a white dwarf siphons off mass from its companion. This reproducible mechanism leads to a well determined brightness and has led scientists to term Type Ia supernovae as "standard candles." While Type Ia supernovae have been considered the gold standard for distant sources of light, they have a small residual variability that has limited our knowledge of the size of the universe.

A recent paper in Science describes the result of a study using Type Ia supernovae in young, star-forming, environments. Because of the environment in which these supernovae occur, astronomers have determined their intrinsic brightness (and therefore distance) with a precision of under four percent, which is about twice as good as earlier measurements.

The study used supernovae within about 400 million light years of Earth, which is a relatively small fraction of the size of the visible universe, but it is hoped that future studies will significantly improve our measurements of even more distant supernovae.