In my previous post, I mentioned that I would go further into the scientific details of the 2011 Nobel Prize in Physics. As a reminder, the winners of the prize, Saul Perlmutter, Brian Schmidt, and Adam Reiss were awarded for their discovery of the accelerating expansion of the universe. This was actually a surprising result that came out of their data, since what was believed for the last century was that the expansion of the universe was decelerating due to the inward pull of gravity.

To measure this supposed deceleration, they needed something known as a standard candle, an object that always shines with the same brightness. For their standard candles, they used Type Ia Supernovae. A supernova is the explosion of a star when it reaches the end of its life. Regular supernovae explode with different brightness depending on the surrounding conditions, making them unreliable standard candles. But Type Ia Supernovae have an interesting property that they always explode with the same brightness.

Type Ia Supernovae are the explosions of white dwarf stars. Near the ends of its lifetime, a star’s outer layer becomes very gaseous and it expands. What remains in the center is a very hot, dense, and white core composed of carbon and oxygen. Water has a density of 1 gram per cubic cm. A white dwarf has a density of 3 million grams per cubic cm. Our sun’s outer layer one day will expand and its circumference would engulf the orbit of the earth and it too will become a white dwarf. Type Ia Supernovae occur in binary star systems, a star system where two stars orbit each other. Since the white dwarf is so dense, its gravity becomes so strong that it sucks the material of its companion star causing the white dwarf to grow. When it grows and reaches the Chandresekhar limit (1.4 times the size of our sun), the white dwarf rips apart and explodes in a blast so powerful it can outshine an entire galaxy. It is believed that white dwarfs get their intrinsic brightness because they only explode when they reach this characteristic limit.

The laureates used two pieces of information of the supernova in order to determine the expansion rate: the brightness and the cosmological redshift. Redshift is a phenomenon where the wavelength of light gets elongated causing it to appear red (red light has long wavelengths, blue light has short). When light travels through space, it gets elongated because space itself is expanding. This causes it to appear red and thus this is cosmological redshift.

By comparing the observed brightness to the intrinsic brightness of the supernova, they determined how far away it was. And using the redshift, they determined how much the universe has expanded since that supernova occurred (as you look farther out in space, you look further back in time). Their plan was to observe supernovae at different distances, therefore different times, and compare them to each other to see how the expansion rate has changed. As you move standard candles farther away from you, they get less bright (imagine a flashlight moving away from you). Since they expected that the universe was decelerating, they also expected that the brightness of the supernova would get less bright more slowly. But what they found was that as they looked further out in space, the brightness became less much faster than they expected. Therefore the only conclusion they could reach was that the universe was expanding at an accelerating rate!