## Abstract

OBSERVATIONAL estimates suggest that the cosmological density parameter Ω, defined as the ratio of the density of the Universe ρ_{o} and the critical density ρ_{crit} which divides open from closed universes, is ≤0.5. According to the standard Friedmann models, any value of Ω ¬ 1 diverges away from one as the Universe evolves. The cosmological Ω-problem is that, on the one hand, making the present value of Ω close to but not equal to one requires an extraordinarily precise adjustment of the initial conditions of the Universe^{1}, but on the other hand inflationary models, which were devised in part to avoid this fine-tuning, predict that Ω should differ from one only by an exponentially small amount^{2}. Conventional inflationary models thus appeal to the existence of 'missing' matter or a non-zero cosmological constant to make up the density deficit. Here it is shown that 'extended inflation'^{3,4}, a recent variation on inflationary cosmology, accommodates a range of initial conditions which lead to Ω ≲ 0.5. The parameter range is narrow, perhaps finely tuned, but non-zero.

Original language | English (US) |
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Pages (from-to) | 47-49 |

Number of pages | 3 |

Journal | Nature |

Volume | 345 |

Issue number | 6270 |

DOIs | |

State | Published - 1990 |

Externally published | Yes |

## All Science Journal Classification (ASJC) codes

- General