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Gee, Dad, It's Really Dark Out There.
on March 19, 2009
In 1970 many physicists and cosmologists thought that we would soon know all the basic physical principles governing the universe. The Big Bang was largely confirmed by cosmic background radiation and we knew about expansion. Quantum mechanics ruled the small while the large was the domain of relativity. The two theories weren't linked yet, but it was merely a matter of a little more time and work. Success would bring an explanation unifying the four fundamental physical forces of the universe: Electromagnetism, the strong force (binding protons and neutrons together in atomic nuclei), the weak force (governing certain kinds of particle decay) and gravity. Surely the "theory of everything" was not far off.
Wrong. Observations in 1970 revealed that gravitational motions of gas clouds in the Andromeda galaxy were occurring at speeds far greater than the entire observed mass of that galaxy could account for. Similar problems detected in the 1930's involving motions of entire galaxies had long been disregarded. Soon other observations confirmed that so-called "ordinary matter" is insufficient to account for observed gravitational effects in the cosmos. Thus the universe must contain huge amounts of "dark matter," that we cannot observe and the composition of which we do not know (it is not made of the particles that constitute ordinary matter).
Then in 1998 reports of observations of distant supernovae revealed that the expansion of the universe was not slowing, as would be expected from long-term effects of gravity, but was instead accelerating. Something was overcoming the gravitational power of all of the matter in the universe. The acceleration, moreover, has not been present from the Big Bang on. For billions of years the speed of expansion slowed. Then, about 5 billion years ago, acceleration began. Obviously energy--a lot of it--- was required to explain these phenomena. This is "dark energy." We cannot detect it and currently know almost nothing about it.
Today scientists believe that 5% of the universe consists of "ordinary" [observable] matter, 23% of "dark" matter and 72% of "dark energy." So in about 40 years we have gone from thinking that we knew almost everything about the essentials of cosmology to actually knowing something about only 5% of the universe, very little about an additional 23% and almost nothing about 72% of it.
But author Gates (herself a theoretical physicist by training) is energized rather than discouraged. In this book she discusses fully the problems noted above (and more), explains their significance and outlines in detail the methods that are being used or planned to attack them. The book's title comes from one of the major investigational tools: Use of relativistic spacetime itself as an observational device. General relativity teaches that the presence of mass warps spacetime. As light travels through the universe it follows a curved path through these "dimples" in spacetime. Because these warps bend light, it is as if a giant lens has been dropped into space, magnifying and displacing light from more distant sources behind it. When Earth is aligned with such a "lens," it allows us to detect and analyse vastly more distant light sources otherwise not observable. This is useful in searching for dark matter because it allows "maps" to be made showing where dark matter exists in distant sources. Gates does an outstanding job of describing this and other significant techniques for making these inquiries, noting the strengths and weaknesses of each method in investigating certain types of phenomena. The use of different techniques should permit the capture of disparate forms of data and lead, we hope, to detection and analysis of dark matter and dark energy.
This is an exciting time for cosmology and physics. The results of these inquiries will, at the least, radically change our view of the cosmos. Depending on results, it may be necessary to modify Einstein's theory of general relativity to account for the observed actions of gravity in the universe. Gates is also quite good at conveying the excitement now animating these scientific fields. Her writing is clear and readable, if seldom compelling. Overall this book is recommended for anyone interested in today's leading cosmological puzzles.