## Abstract

A large class of cosmological solutions (of the Einstein equations) in string theory, in the presence of Maxwell fields, is obtained by O(d, d) transformations of simple backgrounds with d toroidal isometries. In all examples in which we find a (closed) expanding universe, such that the universe admits a smooth, complete initial value hypersurface, a naked singularity may form only at the time when the universe collapses. The discrete symmetry group O(d, d, Z) identifies different cosmological solutions with a background corresponding to a (relatively) simple CFT, and therefore, may be useful in understanding the properties of naked singularities in string theory.

Original language | American English |
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Pages (from-to) | 162-166 |

Number of pages | 5 |

Journal | Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics |

Volume | 294 |

Issue number | 2 |

DOIs | |

State | Published - 12 Nov 1992 |

Externally published | Yes |

### Bibliographical note

Funding Information:We thank K. Bardakci, C. Nappi and E. Witten for useful discussions. We would like to acknowledge the hospitality of the Theoretical Physics Group at Lawrence Berkeley Laboratory where part of this work was done. The work of A.G is supported in part by DOE grant no. DE-FG02-90ER40542. The research of A.P. is supported by an INFN fellowship and partially by the NSF grant PHY90-21984.