Tycho Supernova Remnant

Type Ia Supernovas

Starting in the mid-1990s, a promising (and important) standard candle has been used, Type Ia Supernovas (SNe). Type Ia SNe are very bright and can stand out against the backdrop of a galaxy, even though the light from a galaxy is produced by hundreds of billions of stars (see upper left panel). Type Ia SNe are produced by the explosion of an accreting massive white dwarf star as the white dwarf nears the Chandrasekhar Mass Limit (see upper right panel). This allows Type Ia SNe to be observed to large redshift z > 1.

For further information on Type Ia SN, see the nice NASA review of Type Ia SN on here.

Standard Candles?

Because all Type Ia SNe were thought to originate under similar conditions, it was suggested that their outbursts may be very similar and, in fact, maybe their outburst luminosities would be identical making them powerful Standard Candles because of their huge luminosities. Unfortunately, it was discovered that although they were similar, there were important differences in their light curves (see the left panel).

Standardizable Candles

However, an interesting correlation was discovered where it was found that Type Ia SNe which declined in brightness slower were intrinsically brighter. The intrinsic Type Ia SN luminosity and the rate of decline were well-defined so that the rate of decline could be used to infter the intrinsic luminosity of the event (much as the Period-Luminosity relation for Cepheids allows one to use the period of pulsation to determine the average intrinsic luminosity of a Cepheid). Making this correction, all Type Ia SN do appear similar (see the right panel).

Oh my, notice that the authors chose to make their Hubble Law plot in a different way. They used the magnitude difference on the vertical axis (that is, they placed DISTANCE on the vertical axis) and placed redhsift z on the horizontal axis. Nonetheless, their plot shows the SN results in comparision to various models. Their data suggests that Ω is nearly 1 with a substantial Dark Energy component (the upper solid curve). A universe with Ω = 1 with no Dark Energy does not fit as well (the lower dashed curve).

To remind you, the traditional form for the plot when there is no dark energy is shown above. The redshift (velocity) plotted against distance. Notice that all slowing down universes show larger redshift at large distances. The open universe shows the least amount of slowing down at large distances than do either the flat or closed universes.What kind of behavior would a universe with dark energy show at large distance?

The Type Ia SNe results offered the first credible evidence that Ω = 1 and that the Universe was made primarily of Dark Energy. This was astonishing as it suggested that the expansion rate of the Universe was increasing with time. The later work on the Cosmic Microwave Background Radiation, CMB, by the Wilkinson Microwave Anisotropy Probe, WMAP group, really nailed it, however.

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