There must therefore be good reason to believe in the uniformity of these supernovas, as well as some reason to doubt it. Along this line, there's an interesting article in Science, 2 July 1999, p. 19, "Has a Cosmic Standard Candle Flickered?" This article contains the following quotation:
Type Ia supernovas are believed to arise in interacting binaries from the thermonuclear explosion of a carbon-oxygen white dwarf pushed beyond the Chandrasekhar limit by mass transfer from a neighbouring companion star. In the process a fixed amount of radioactive nickel-56 is believed to be produced, whose subsequent decay into cobalt-56 and then to stable iron-56 is thought to power the entire light curve in these events. As a consequence of the uniformity of the underlying processes, type Ia supernovas serve, in principle, as excellent "standard candles" to obtain extragalactic distances. In practice, the uniformity of the underlying conditions has been questioned as being controversial.(from the online Encyclopedia Britannica article, "Cosmos: The extragalactic distance scale and Hubble's constant")
So again we have some evidence that these supernovas are uniform, since each group seems to have similar behavior, and some evidence that they are not, since the more distant group has different behavior.
"A team of astronomers says it has found slight, previously unnoticed variations among the exploding stars called type Ia supernovae. These explosions, thought to flare up to roughly the same brightness each time, have served the crucial role of "standard candles" whose apparent brightness, as seen from Earth, can serve as a measure of their distance. ..."
"The team found that the time it took the explosions to reach their peak brightness was more than 2 days longer than the average for supernovae billions of light-years away as measured by a second group. `They're pretty strongly discordant with one another,' says Riess of the two data sets. `If it's true, it's extremely interesting,' says Eddie Barron, an astrophysicist ... "
This is very interesting. This difference could mean that these explosions apparently occur in slow motion when they are further away. This would be consistent with a slowdown in the speed of light, which would make distant objects appear in slow motion relative to nearby ones. If light were traveling faster when it left an object than when it arrived on earth, events that occurred when the light left the object would appear in slow motion to us. A change in the speed of light might affect the properties of matter in many ways, but it could be that a residual slowdown in apparent explosion time would remain.
This possibility can be tested by seeing if this pattern of slowdown with distance is confirmed by more measurements in many more locations in the universe. If so, then it is hard to see how there could be a systematic variation in such supernovas with distance from the earth. The only reasonable explanation would be some change in the physical constants with time, such as a decrease in the speed of light.
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