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.
When the acceleration of the expansion of the universe was initially discovered, scientists were very puzzled as to what force was pushing the galaxies apart at an ever-increasing rate. The most fundamental laws of physics were being violated unless some source of energy was causing this acceleration. Things moving faster means they have more energy, which has to come from somewhere. Thus, the concept of “dark energy” arose. It wasn’t all that difficult to estimate the energy necessary for this expansion. Because matter and energy are related by Einstein’s famous equation that E equals m c squared, this dark energy could be assigned a mass. This mass turned out to be greater than the mass of the visible universe and even more than the estimated mass of all of its dark matter plus visible matter.
Needless to say, scientists were not readily convinced in the 1990s of this posited dark energy. It took a great many more measurements of type 1a supernovae to convince them. After the Nobel Prize award, the scientific community was fairly united, as much as scientists ever are, about dark energy. Dark energy is now found in school textbooks.
I was not at all convinced that such a dramatic solution was really necessary. Not being a cosmologist, I had no alternatives to suggest, except for Einstein’s “cosmological constant” that he claimed was his biggest mistake but which still makes a comeback now and then in cosmological circles.
Therefore, I was pleased to see that we don’t have to have as much dark energy as once believed, that the amount will be much less. Quoting Dr. Milne, “We’re proposing that our data suggest there might be less dark energy than textbook knowledge, but we can’t put a number on it.” Could “less” be zero?
Two sets of discussions can arise from this new discovery. The first relates to whether it matters. After all, the impact of a greater or lesser acceleration of universe expansion will not be felt for trillions of years, long after our sun has used up its hydrogen fuel, swallowed up Mercury and Venus, fried the Earth, and become a cooled-off white dwarf. Who cares? The answer is that from these researches come strange and unexpected side discoveries. We just do not know if this particular discovery will someday impact our lives. For now, it’s just an interesting topic of conversation for all but a few scientists.
The second discussion is about the nature of science. Some will take this discovery as proving how fallible science is. After all, if we cannot even measure the expansion of the universe and estimate the amount of dark energy within a factor of two, how can scientists know that the Earth is 4.5 billion years old instead of the Biblically estimated 6,000 years? Yet, the results, as astonishing as they are, do not obviate the expansion of the universe from a very small beginning, the Big Bang. They do not change the research done on the life history of stars and their classification depending on size, color, and brightness. They have no impact at all on smaller phenomena such as solar system formation or even galaxy formation. They don’t affect the search for dark matter.
Science makes measurements and attempts to explain the data. It seeks out additional data to support or refute those explanations. Most often, the explanations are adjusted. Rarely, they must be tossed out completely, but that step is radical for science and is done only when the data against an explanation become overwhelming. Usually, data confirm or clarify existing explanations.
Were continuing investigation into the brightness of type 1a supernovae and measurements of distant ones to alter dark energy estimates significantly, which would be a startling result to many. If those estimates come to a range that includes zero, then Occam’s razor will force consideration of the possibility that dark energy does not exist after all. Even if this result does not come to pass, 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. Possibly, we only await a modern Count Rumford to figure out a better explanation.