Type Ia Supernovae
Supernova Cosmology
Type Ia supernovae (SNe Ia) were used to discover the accelerated expansion of the universe. They also led to a drastic boost in the precision and accuracy of the local distance ladder measurements of the present day expansion rate, or the Hubble constant (see main Research page). SNe Ia are distance indicators, or standard candles--objects that always occur at the same brightness no matter where we observe the in the universe. At least, to a certain extent.
Measuring the shape (or "stretch") of a SN light curve allows us to better predict its luminosity, and thus its distance. This process is called "standardization," which, despite the fancy name, is just the standard process of adding parameters to one's model that is used to predict a Ia's luminosity., as shown below. SNe with wider light curves (higher stretch) are also observed to be intrinsically brighter. See Phillips+93, Riess+95, Perlmutter+96.
Perlmutter+98, Nature. (Top panel) Various high-redshift, SN Ia light curves, scaled to the same redshift. Notice the broader curves also reach their peaks at brighter magnitudes. (Bottom Panel) The same SN light curves, but after applying a linear correction to both the width (or "stretch") and the peak magnitude. The width-corrected magnitudes are several times more precise as distance indicators as the raw peak magnitudes of SNe Ia.
On top of the stretch, the color can be used to standardize further (commonly referred to as a Tripp relation).
Using spectra to model Type Ia Supernova Luminosity Variations
Upon my arrival our group had developed a new model for inferring the distances between Type Ia supernovae (SNe Ia) that is about twice as accurate as previous light-curve based models. In order to determine the present day expansion rate, or the Hubble Constant, we need to determine a zero point calibration for the model, which is how we bring the model's relative predictions between SNe onto an absolute system. That way, the SNe can tell us how fast the universe is expanding today, which also provides the boundary condition that sets the normalization of the universe's age and size.
