Document Type
Article
Publication Date
4-17-2012
Abstract
The expansion of the universe is often viewed as a uniform stretching of space that would affect compact objects such as atoms and stars, as well as the separation of galaxies. One usually hears that bound systems do not take part in the general expansion, but a much more subtle question is whether bound systems expand partially. In this paper, a definitive answer is given for a very simple system: a classical “atom” bound by electrical attraction. With a mathematical description appropriate for undergraduate physics majors, we show that this bound system either completely follows the cosmological expansion, or, after initial transients, completely ignores it. This all-or-nothing behavior can be understood using analysis techniques used in junior-level mechanics. We also demonstrate that this simple description is a justifiable approximation of the relativistically correct formulation of the problem.
Recommended Citation
Price, Richard H., and Joseph D. Romano. "In an expanding universe, what doesn’t expand?." American Journal of Physics 80.5 (2012): 376-381. https://doi.org/10.1119/1.3699245
Publication Title
American Journal of Physics
DOI
10.1119/1.3699245
Comments
© 2012 American Association of Physics Teachers. Original published version available at https://doi.org/10.1119/1.3699245