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COLLEGE PARK, MD — An article published in the latest issue of the Astrophysical Journal lends strong support for a controversial theory that rejects the cold dark matter hypothesis central to what most scientists believe about the composition of the Universe.
In the October issue, which is now available online, University of Maryland astronomer Stacy McGaugh details cosmic microwave background predictions that he made last year and which subsequently proved correct. The cosmic microwave background is the faint radiation that scientists believe to be a remnant of the energy released in the Big Bang. Measurements of cosmic microwave background matching McGaugh’s 1999 prediction were reported in the journal Nature in March of this year by scientists conducting an experiment known as Boomerang.
The accuracy of his predictions, writes McGaugh, points to a universe that consists entirely of "ordinary" matter. This contradicts the widely held paradigm that 90 percent of the universe is made up of unseen matter, termed cold dark matter. Cold dark matter is widely thought to consist of a new kind of particle rather than the protons, neutrons, and other known particles that constitute ordinary matter.
"What I predicted correctly in an article in the October 1999 Astrophysical Journal is the amplitude of the second peak relative to the first peak in the power spectrum of the cosmic microwave background," McGaugh said.
Boomerang Results And Cold Dark Matter
In March, when the Boomerang results were announced, many cosmologists publicly rejoiced that the position of the first peak in the power spectrum of the microwave background indicated the universe was "flat," a key prediction of inflation, one of the central tenets of modern cosmology. However, cosmologists were puzzled by the small amplitude of the second peak relative to the first because it didn’t fit what they expected to see based on another key tenet, the theory of cold dark matter.
"On the other hand, the relative amplitudes were precisely what I had expected should cold dark matter not exist," said McGaugh.
According to McGaugh, the basis for his correct predictions lies in a little known alternative theory to dark matter called MOND, for modified Newtonian dynamics. "Until 1994, I was like most astronomers and didn’t think much of, or about, MOND," he said. "But that year a problem cropped up in my data for the rotation curves of low surface brightness galaxies. The data made no sense in the conventional dark matter context. I pounded my head against the wall for many months trying to make sense of it when by chance I attended a talk by Moti Milgrom, the Israeli physicist who conceived MOND.
"Without knowing who I was or what problem I was struggling with, he derived a series of predictions for how low surface brightness galaxies ought to behave in MOND. Everything that was so confusing in the dark matter context was actually a prediction of MOND."
"It was a classic example of the kind of hypothesis testing that forms the basis of science. In this case MOND’s predictions came true, cold dark matter’s did not," McGaugh said.
Modifying Newtonian Dynamics An Attention Getter
McGaugh’s newest article on modified Newtonian dynamics is attracting the attention and interest of many astronomers and physicists. But while acknowledging the accuracy and potential significance of his work, McGaugh says that even friends and colleagues on campus remain skeptical of MOND.
Cole Miller, an assistant professor of astronomy at Maryland who has had numerous friendly debates with McGaugh about MOND points out that there are possibilities that fit within the context of the theory of cold dark matter that could explain why the second cosmic microwave background peak is lower than cosmologists expected. "Though Stacy’s idea is very interesting, it’s not really possible to point to the Boomerang findings or to any current observation and conclusively say which theory [MOND or cold dark matter] is correct."
Miller said that for largely philosophical reasons most scientists are not likely to embrace Stacy’s position at this point. "MOND introduces a new fundamental constant in a way that seems ad hoc, and which is aesthetically displeasing to most cosmologists. On the other hand, cold dark matter postulates new and so far unobserved particles, so it’s good to keep an open mind about both possibilities," he said.
McGaugh said he agrees that it’s only natural to be skeptical of an idea as radical as MOND the first time you hear it. "But to be fair, I think we need to apply the same degree of skepticism to dark matter."