Explicit integration of Friedmann's equation with nonlinear equations of state

Shouxin Chen, Gary W. Gibbons, Yisong Yang

Research output: Contribution to journalArticlepeer-review


In this paper we study the integrability of the Friedmann equations, when the equation of state for the perfect-fluid universe is nonlinear, in the light of the Chebyshev theorem. A series of important, yet not previously touched, problems will be worked out which include the generalized Chaplygin gas, two-term energy density, trinomial Friedmann, Born-Infeld, two-fluid models, and Chern-Simons modified gravity theory models. With the explicit integration, we are able to understand exactly the roles of the physical parameters in various models play in the cosmological evolution which may also offer clues to a profound understanding of the problems in general settings. For example, in the Chaplygin gas universe, a few integrable cases lead us to derive a universal formula for the asymptotic exponential growth rate of the scale factor, of an explicit form, whether the Friedmann equation is integrable or not, which reveals the coupled roles played by various physical sectors and it is seen that, as far as there is a tiny presence of nonlinear matter, conventional linear matter makes contribution to the dark matter, which becomes significant near the phantom divide line. The Friedmann equations also arise in areas of physics not directly related to cosmology. We provide some examples ranging from geometric optics and central orbits to soap films and the shape of glaciated valleys to which our results may be applied.

Original languageEnglish (US)
Article number020
JournalJournal of Cosmology and Astroparticle Physics
Issue number5
StatePublished - May 13 2015


  • cosmological applications of theories with extra dimensions
  • dark energy theory
  • physics of the early universe

ASJC Scopus subject areas

  • Astronomy and Astrophysics


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