Using Supernovae to Study Cosmology
The Redshift-Distance Test
As a function of redshift, the apparent magnitude of distant objects changes under different cosmologies, for two reasons:
This can be worked out under different cosmologies to derive a form akin to our regular magnitude-distance expression:
- The shape of space determines how photons spread out as they move outwards (the classic 1/d2 effect)
- The expansion history determines how the photons are redshifted.
m-M = 5logdL(z) -5
where dL(z) is the luminosity distance,
and depends on H0, OmegaM, OmegaL, and k. We typically plot this using
the distance modulus, not the distance, though (from Carroll and
If we had an object of fixed brightness -- a standard candle -- we
could plot its apparent magnitude as a function of distance and work
out the cosmology.
Remember Type Ia supernovae:
the explosion of a ~ 1.4 Msun white dwarf. These are pretty good
approximations to a standard candle, and they are extremely bright.
what we want to use for the redshift-distance test.
But are SN Ia's
Type Ia supernovae in galaxies w/ Cepheid distances
This translates to MB,max = -19.46 +/- 0.21. An
of 0.21 magnitudes translates to a distance uncertainty of 10%. Not
but there are some whopping outliers. Can we do better?
Yes. It turns out that the brighter SN Ia's
slowly than the fainter ones. Measure how bright they are and how fast
they fade, and you can make a correction. This reduces the dispersion
absolute magnitude to about 0.1 magnitudes.
But there's a significant drawback to using Type Ia
gotta find them...
Using supernovae to study cosmology
Take a BIG picture of the sky.
Come back next month and take the same picture.
Compare the two. Differences?
If you find a possible supernova, take a spectrum
of it and
make sure it is a Type Ia SNe.
Also take a spectrum of the galaxy it lives in, to
Watch the supernova as it fades, so we can get its
magnitude. This is important -- you probably didn't catch it when it
at its peak, so we need to fit it to a standard light curve to derive
Keep doing this so you have a big sample of high
supernovae. Then compare those supernovae to ones at lower redshift.
Some plots, courtesy of the supernovae
cosmology project at LBL and the high
z supernova search team at CfA:
More recent data: HST discovered supernovae, extending to higher redshift (Riess et al 2007).
In this plot mu=m-M. Curvature in the data is inconsistent with models
that use dust or evolution to explain faintness of high-z SNe; instead
it is indicative of the "jerk" in the expansion history when lambda
began to dominate and the universe went from decelerating to