2.1 The cosmological principle
Our current Universe exhibits a wealth of nonlinear structures, but the zeroth order description of our
Universe is based on the assumption that the Universe is homogeneous and isotropic smoothed over
sufficiently large scales. This statement is usually referred to as the cosmological principle. In fact, the
cosmological principle was first adopted when observational cosmology was in its infancy; it
was then little more than a conjecture, embodying ‘Occam’s razor’ for the simplest possible
model.
Rudnicki [73] summarized various forms of cosmological principles in modernday language, which were
stated over different periods in human history based on philosophical and aesthetic considerations rather
than on fundamental physical laws:

The ancient Indian cosmological principle:

The Universe is infinite in space and time and is infinitely heterogeneous.

The ancient Greek cosmological principle:

Our Earth is the natural center of the Universe.

The Copernican cosmological principle:

The Universe as observed from any planet looks much the same.

The (generalized) cosmological principle:

The Universe is (roughly) homogeneous and isotropic.

The perfect cosmological principle:

The Universe is (roughly) homogeneous in space and time, and is isotropic in space.

The anthropic principle:

A human being, as he/she is, can exist only in the Universe as it is.
We note that the ancient Indian principle may be viewed as a sort of ‘fractal model’. The perfect
cosmological principle led to the steady state model, which although more symmetric than
the (generalized) cosmological principle, was rejected on observational grounds. The anthropic
principle is becoming popular again, e.g., in ‘explaining’ the nonzero value of the cosmological
constant.
Like with any other idea about the physical world, we cannot prove a model, but only falsify it. Proving
the homogeneity of the Universe is particularly difficult as we observe the Universe from one point in space,
and we can only deduce isotropy indirectly. The practical methodology we adopt is to assume homogeneity
and to assess the level of fluctuations relative to the mean, and hence to test for consistency with the
underlying hypothesis. If the assumption of homogeneity turns out to be wrong, then there are
numerous possibilities for inhomogeneous models, and each of them must be tested against the
observations.
For that purpose, one needs observational data with good quality and quantity extending up to high
redshifts. Let us mention some of those:

CMB fluctuations

Ehlers, Garen, and Sachs [18] showed that by combining the CMB isotropy with the Copernican
principle one can deduce homogeneity. More formally their theorem (based on the Liouville
theorem) states that “If the fundamental observers in a dust spacetime see an isotropic
radiation field, then the spacetime is locally given by the Friedman–Robertson–Walker (FRW)
metric”. The COBE (COsmic Background Explorer) measurements of temperature fluctuations
( on scales of ) give via the Sachs–Wolfe effect () and
the Poisson equation r.m.s. density fluctuations of on (see, e.g.,
[99]), which implies that the deviations from a smooth Universe are tiny.

Galaxy redshift surveys

The distribution of galaxies in local redshift surveys is highly clumpy, with the Supergalactic
Plane seen in full glory. However, deeper surveys like 2dF and SDSS (see Section 6) show that
the fluctuations decline as the lengthscales increase. Peebles [69] has shown that the angular
correlation functions for the Lick and APM (Automatic Plate Measuring) surveys scale with
magnitude as expected in a Universe which approaches homogeneity on large scales. While
redshift surveys can provide interesting estimates of the fluctuations on intermediate scales (see,
e.g., [72]), the problems of biasing, evolution, and correction would limit the ability of those
redshift surveys to ‘prove’ the cosmological principle. Despite these worries the measurement of
the power spectrum of galaxies derived on the assumption of an underlying FRW metric shows
good agreement with the CDM (cold dark matter) model.

Radio sources

Radio sources in surveys have a typical median redshift of , and hence are useful probes
of clustering at high redshift. Unfortunately, it is difficult to obtain distance information from
these surveys: The radio luminosity function is very broad, and it is difficult to measure optical
redshifts of distant radio sources. Earlier studies claimed that the distribution of radio sources
supports the cosmological principle. However, the wide range in intrinsic luminosities of radio
sources would dilute any clustering when projected on the sky. Recent analyses of new deep
radio surveys suggest that radio sources are actually clustered at least as strongly as local
optical galaxies. Nevertheless, on very large scales the distribution of radio sources seems nearly
isotropic.

Xray background

The Xray background (XRB) is likely to be due to sources at high redshift. The XRB
sources are probably located at redshift , making them convenient tracers of the
mass distribution on scales intermediate between those in the CMB as probed by COBE,
and those probed by optical and IRAS redshift surveys. The interpretation of the results
depends somewhat on the nature of the Xray sources and their evolution. By comparing the
predicted multipoles to those observed by HEAO1, Scharf et al. [75] estimate the amplitude
of fluctuations for an assumed shape of the density fluctuations. The observed fluctuations
in the XRB are roughly as expected from interpolating between the local galaxy surveys
and the COBE and other CMB experiments. The r.m.s. fluctuations on a scale of
are less than 0.2%.
Since the (generalized) cosmological principle is now well supported by the above observations, we shall assume
below that it holds over scales .
The rest of the current section is devoted to a brief review of the homogeneous and isotropic cosmological
model. Further details may be easily found in standard cosmology textbooks [96, 62, 69, 64, 10, 63].
The cosmological principle is mathematically paraphrased as that the metric of the Universe (in its
zeroth order approximation) is given by
where is the comoving coordinate, and where we use units in which the light velocity . The
above Robertson–Walker metric is specified by a constant , the spatial curvature, and a function of time
, the scale factor.
The homogeneous and isotropic assumption also implies that , the energymomentum tensor of the
matter field, should take the form of the ideal fluid:
where is the 4velocity of the matter, is the mean energy density, and is the mean
pressure.