PHYS08200605006 D.K. Hazra - Homi Bhabha National Institute
PHYS08200605006 D.K. Hazra - Homi Bhabha National Institute
PHYS08200605006 D.K. Hazra - Homi Bhabha National Institute
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Chapter 1<br />
Introduction<br />
At a first glance, the universe around us appears to be highly clumpy, with matter seemingly<br />
distributed in a rather non-uniform fashion. However, observations of millions of<br />
galaxies by different galaxy surveys such as the Two-degree-Field (2dF) galaxy redshift<br />
survey [1, 2] and the Sloan Digital Sky Survey (SDSS) [3, 4] indicate the universe to be<br />
homogeneous and isotropic on suitably large scales. These surveys suggest that the transition<br />
to homogeneity of the galaxy distribution occurs at length scales of around100 Mpc<br />
or so (in this context, see, for instance, Ref. [5]). On such large scales, it is General Relativity<br />
(GR) which is the theory that is expected to describe the universe. In GR, the fundamental<br />
quantity is the metric describing the spacetime, whose dynamics is determined<br />
by the matter through the Einstein field equations. The metric that describes the homogeneous<br />
and isotropic universe is referred to as the Friedmann-Lemaitre-Robertson-Walker<br />
(or, for convenience, simply Friedmann) line-element, which is essentially characterized<br />
by two quantities (see, for example, any of the following texts [6]). The first being the<br />
curvature of the spatial geometry of the universe, while the second is the scale factora(t),<br />
which allows one to account for the observed expansion of the universe. The quantity t<br />
denotes the cosmic time, i.e. the time as measured by clocks that are comoving with the<br />
expansion.<br />
The behavior of the scale factor a(t) is governed by the constituents of the universe<br />
through the Friedmann equations, which are the Einstein equations applied to the case of<br />
the Friedmann metric. A variety of observations point to the density of the universe today<br />
being rather close to the so-called critical density (with the dimensionless ratio of the<br />
actual density to the critical density deviating from unity by about one part in10 2 ), which<br />
corresponds to a geometry that is spatially rather flat. Till date, through different observations,<br />
we are aware of the fact that baryons (i.e. visible matter) and photons (viz. radiation)<br />
1