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Relativity, Gravitation and CosmologyA Basic Introduction$
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Ta-Pei Cheng

Print publication date: 2009

Print ISBN-13: 9780199573639

Published to Oxford Scholarship Online: February 2010

DOI: 10.1093/acprof:oso/9780199573639.001.0001

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The homogeneous and isotropic universe

The homogeneous and isotropic universe

(p.181) 9 The homogeneous and isotropic universe
Relativity, Gravitation and Cosmology

Ta-Pei Cheng

Oxford University Press

The universe, when observed on distance scales ≳100 Mpc, is homogeneous and isotropic. Hubble's discovery that the universe is expanding suggests strongly that it had a beginning. The estimate of the age of the universe by astrophysics from observation data is between 12 Gyr to 15 Gyr. The mass density of matter in the universe has around a quarter of the “critical density”. There is strong evidence showing that most of the mass in the universe does not shine and the nonrelativistic exotic dark matter has density ration of 0.21. A homogeneous and isotropic universe (the cosmological principle) is described by the Robertson-Walker metric in the comoving coordinates (the cosmic rest frame). In an expanding universe with a space that may be curved, any treatment of distance and time must be carried out with care.

Keywords:   homogeneous and isotropic universe, Hubble, age of the universe, critical density, baryonic matter, dark matter, cosmological principle, Robertson-Walker metric, distance in RW spacetime

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