What is Dark Energy?   Effects of Dark Energy How is Dark Energy Detected?   What is a Type Ia Supernova?   Goals of SNfactory   SNfactory Collaboration   Role of CfCP

The use of Type Ia supernova as a tool to demonstrate the presence of dark energy was originally reported by two groups: the Supernova Cosmology Project (SCP) and the High-Z SN Team. Below is a simplified explanation of how SNe are used to determine the dark energy content in the universe.

The gravitational effects of the dark energy are too weak to detect in the solar system or even within the galaxy. These effects are observable only on cosmological scales of billions of light years. The most direct method uses Type Ia supernovae because they are nearly standard candles, that is, a class of objects with about the same intrinsic luminosity when they reach their brightest point. The plot below shows the brightness of a Type Ia SNe as a function of the number of days since peak brightness (data provided by SCP).

The measured redshift (z) gives the cosmological scale factor at the time of explosion. The scale factor "R" is 1 today, and is 1/(1+z) at the time of the SNe explosion. For example, a SNe with redshift z=0.5 corresponds to when the scale factor was 2/3, meaning that the distances between us and the SNe (at explosion time) was 2/3 of the distance today. This redshift is easily measured in SNe, as well as a variety of other cosmological objects such as galaxies, AGNs, and quasars. What make SNe Ia so special is that they also provide a calibrated cosmological time, something that no other object can provide.

To see very roughly how this cosmological time is measured, first consider a simple case with a flat, non-expanding universe; the time of the explosion (relative to today) is simply t = d/c ~ 1/, where L is the intrinsic SNe luminosity at peak brightness. Since L is the same for each SNe, we have a calibrated cosmological clock. In an expanding universe, the t vs. L relation is more complicated; the key point in this relation is that it also depends on the evolution of matter and dark energy densities. The basic idea is therefore to measure the expansion history of the universe; the scale factor from SNe Ia redshift and the cosmological time from peak-brightness are used to determine the cosmological parameters (matter and dark energy densities) that give the best description of the data.