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References

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anism and even discriminate among different candidates for its realization within a fundamental unified theory of all interactions as superstring theory and its supergravity descendants.

References

1.Crowe, M.J.: Theories of the World from Antiquity to the Copernican Revolution. Dover Publications, New York (1990)

2.Crowe, M.J.: Modern Theories of the World from Herschel to Hubble. Dover Publications, New York (1994)

3.Hubble, E.: A relation between distance and radial velocity among extra-galactic nebulae. Proc. Natl. Acad. Sci. USA 15, 168–173 (1929)

4.Lemaitre, G.: Un univers homogène de mass constant et de rayon croissant rendant compte de la vitesse radiale des nebuleuses extra-galactiques. Ann. Soc. Sci. Brux. Sér. a: Sci. Math. 47, 49–56 (1927)

5.Friedman, A.: Über die Krummung des Raumes. Z. Phys. 10(1), 377–386 (1922)

6.Gamow, G.: The origin of elements and the separation of galaxies. Phys. Rev. 74(4), 505–506 (1948)

7.Alpher, R.A., Herman, R.C.: On the relative abundance of elements. Phys. Rev. 74(12), 1737– 1742 (1948)

Chapter 5

Cosmology and General Relativity:

Mathematical Description of the Universe

Ma sedendo e mirando, interminati

Spazi di là da quella, e sovrumani

Silenzi, e profondissima quiete

Io nel pensier mi fingo; ove per poco

Il cor non si spaura. . .

Giacomo Leopardi

5.1 Introduction

Having completed in the previous chapter our historical review of Physical Cosmology, from its very beginning at the end of the XVIIIth century to the challenging discoveries that reshaped it at the beginning of the XXIst century, it is time to enter its rigorous mathematical formulation in terms of General Relativity, which constitutes the main goal of the present chapter. As we already extensively pointed out throughout Chap. 4, the two crucial issues in cosmology are those of homogeneity and isotropy, whose physical explanation is the goal of the inflationary theory. In order to understand and correctly utilize these two geometrical concepts within the context of our geometrical theory of gravitation we have to address in full some mathematical questions that were only touched upon in the first volume and in previous chapters of the second. These questions relate to the concept of isometries for Riemannian and pseudo-Riemannian manifolds. The proper treatment of isometries leads us to develop the Differential Geometry of Coset Manifolds and Symmetric Spaces which, besides being ubiquitous in Mathematical Physics, is also very relevant to our subsequent chapters devoted to an introduction to Supergravity, Branes and Supersymmetric Black-Holes.

After this preparatory step we address the mathematical description of the Universe by means of metrics that possess the two properties required by the Cosmological Principle: homogeneity and isotropy.

In order to clarify the independent role of the two symmetry requirements we begin by discussing homogeneous but not isotropic metrics and we present some examples. We discuss Kasner solutions and some more intriguing ones based on non-Abelian three dimensional groups. We outline the celebrated Bianchi classification of such homogeneous but not isotropic universes. We emphasize that the curious mechanisms associated with anisotropic homogeneous universes might play a

P.G. Frè, Gravity, a Geometrical Course, DOI 10.1007/978-94-007-5443-0_5,

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