Dark Energy vs. Dark Matter
What is Dark Energy?
How Can We Find It?
Studying Supernovae
Sound Waves Method
Gravitational Lensing

Exploding Stars

Supernovae illuminate the expanding universe

standard candles

If you know the true brightness of a distant light bulb, you can calculate its distance by measuring how bright it appears. The reduction in brightness follows a well-defined rule: The brightness diminishes by the square of the distance. If you move a bulb twice as far away, it will appear just one-fourth as bright; move it three times farther, and it's just one-ninth as bright (left). The same thing happens with supernovae. Astronomers know a supernova's true brightness, so measuring its apparent brightness reveals its distance, and therefore the distance to its parent galaxy (right). [Tim Jones]

Astronomers discovered dark energy when they noticed that some exploding stars, called supernovae, were fainter than expected. The finding showed that the universe is expanding faster as it ages.

Several research teams will use this same technique to probe larger areas of the universe over a greater span of time — from the modern day back to perhaps one or two billion years after the Big Bang.

The key to this technique is a type of exploding star called a Type Ia supernova — the complete destruction of a white dwarf star.

Such an explosion is visible across great cosmic distances, making Type Ia supernovae good "standard candles" for measuring the scale of the universe. That’s because Type Ia supernovae brighten and fade in a predictable way, so measuring how long it takes one of them to brighten and then fade reveals its true brightness. By comparing a star’s true brightness to how bright it appears, astronomers can find its distance.

Astronomers then measure how fast the star is moving away from Earth by measuring its redshift — a stretching of its light waves caused by the expansion of the universe itself.

Teams of astronomers are using this technique to study many more supernovae in different regions of the universe. Some use small, automated telescopes to scan large regions of the sky on different nights. By comparing images on different nights, they can detect supernovae, which flare to life in just a few hours. Once they spot a supernova, astronomers can then monitor it with larger telescopes to plot its lightcurve — the way it brightens and fades — and its spectrum.

supernova explosion

A white dwarf begins its explosion as a Type Ia supernova in this artist’s concept. The white dwarf, at lower right, explodes because it has been stealing gas from a bloated companion star (left). When enough gas builds up it triggers an explosion that blasts the star to bits. Such a supernova can be seen across billions of light-years. [ESO]

Each supernova will be at a different distance from Earth, which means we see it at a different time. A supernova that is one billion light-years away, for example, actually exploded one billion years ago, so its light gives us a view of the universe as it was one billion years ago. The supernova surveys will find exploding stars at many different times — a span of several billion years.

These observations will allow astronomers to determine how fast the universe was expanding at different times in its history. Each dark-energy theory makes different predictions about how the expansion rate is changing, so actually measuring that rate will strengthen or weaken different ideas.

Some supernova searches are underway now, with bigger surveys using larger telescopes planned for 2010 or later. Ultimately, an orbiting satellite dedicated to the supernova search may join the hunt, providing sharper views of the sky than are possible from the ground. This clear view will yield more precise lightcurves and redshifts, and therefore better plots of the expansion history of the universe.

Sound Waves Method

scaling the universe