http://www.newscientist.com/article.ns?id=dn8082"YOUR waistline may be spreading but you can't blame it on the expansion of the universe." So says Richard Price, a physicist at the University of Texas at Brownsville, who has worked out that while some objects are stretched by cosmological expansion, others are not.
Cosmologists have long accepted that the universe is expanding, causing galaxies to spread apart like raisins in a rising loaf of bread, as the space between them stretches. But while Price was teaching a summer course, a question from a high-school student floored him. "He asked me if, as space expands, we all get bigger too," says Price. "I knew the standard answer was 'no', but I couldn't explain why not. And when I consulted my colleagues, neither could they."
Since atoms are made up mostly of empty space, with electrons "orbiting" the nucleus at distances typically many hundreds of times its diameter, it seemed reasonable to ask whether the electrons would be dragged away from the nucleus by the stretching of space. Price decided to examine the simplest system, that of a hydrogen atom, with one negative electron orbiting a positive proton. He found if the force involved - electromagnetic in the case of atoms - binding the system together is stronger than a certain critical value, the system will be entirely unaffected by the cosmological expansion (www.arxiv.org/abs/gr-qc/0508052).
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