By Harry Keller
Editor, Science Education
Dr. Peter A. Milne and his associates have found an unexpected and, to the cosmological community, startling result from their surveys of supernovae. This result illustrates both the consistent and varying nature of science at the same time.
We know from a great many astronomical observations that the universe has been expanding for a little short of 14 billion years and continues to expand. Because of gravity, everyone expected that this expansion was slowing over time with theories and measurements suggesting that this expansion would eventually coast to a very dilute universe drifting apart at ever slower speeds.
In the 1990s, some astronomers separately discovered that the universe is expanding ever more rapidly instead of the expected opposite slowing of expansion using measurements of he brightness of very distant supernovae. They received the Nobel Prize in physics for this work in 2011.
Stars can explode. One common explosion is called a nova. A much more cataclysmic and extremely brighter explosion is a supernova. Supernovae shine with a brightness that can exceed that of all of the hundred billion or so stars in its galaxy. For this reason, we can see them in distant galaxies that are barely visible in our best telescopes. A supernova is a rare event occurring about three times a century in a galaxy the size of our Milky Way. With hundreds of billions of galaxies, however, it’s not too hard to find hundreds each year using modern astronomical equipment.
A special sort of supernova created when the two stars in a binary star system go through a specific series of interactions is known as a type 1a supernova. Because of the steps required to reach supernova status, the brightness of these type 1a supernovae has been considered to be a constant that can be used to estimate distances to very distant galaxies. Brightness declines with distance in a very precise manner.
There remains the possibility that acceleration of very distant bodies in our universe away from each other is a basic property of our space-time structure not detectable at smaller distances of only millions or even tens of millions of light-years, that “dark energy” is just an attempt to recast a phenomenon into understandable terms, just as the caloric theory of heat was long ago. -HK
The measurements of these supernovae were the reason to believe that the expansion of the universe was accelerating. We are seeing these very distant supernovae with light that started its journey over ten billion years ago when the universe was very young. Dr. Milne has discovered that type 1a supernovae are not all the same but fall into two categories of different brightness. Furthermore, the supernovae from the early universe are, on average, less bright than those in the more recent universe.
The lower brightness of the distant supernovae may well be due to less inherent brightness instead of greater distance. This finding destroys a fair piece of that Nobel Prize discovery. Dr. Milne still attests that the universe’s expansion is accelerating, just not so fast, but the vast number of recalculations being done to account for this new discovery will take some time. Continue reading