The cyclic model simplified

Paul J. Steinhardt, Neil Turok

Research output: Contribution to journalShort surveypeer-review

73 Scopus citations


The Cyclic Model attempts to resolve the homogeneity, isotropy, and flatness problems and generate a nearly scale-invariant spectrum of fluctuations during a period of slow contraction that precedes a bounce to an expanding phase. Here we describe at a conceptual level the recent developments that have greatly simplified our understanding of the contraction phase and the Cyclic Model overall. The answers to many past questions and criticisms are now understood. In particular, we show that the contraction phase has equation of state w > 1 and that contraction with w > 1 has a surprisingly similar properties to inflation with w < -1/3. At one stroke, this shows how the model is different from inflation and why it may work just as well as inflation in resolving cosmological problems.

Original languageEnglish (US)
Pages (from-to)43-57
Number of pages15
JournalNew Astronomy Reviews
Issue number2-6 SPEC. ISS.
StatePublished - May 2005

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Cyclic model
  • Density perturbations
  • Flatness problem
  • Horizon problem
  • Inflation


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