Is the Universe Homogeneous? (On Large Scales)

Abstract

I critically discuss in a pedagogical and phenomenological way a few crucial tests challenging the claims by Pietronero and collaborators that there is no evidence from available galaxy catalogues that the Universe is actually homogeneous above a certain scale. In a series of papers, these authors assert that observations are consistent with a fractal distribution of objects extending to the limit of the present data. I show that while galaxies are indeed clustered in a scale-free way on small and intermediate scales, this behaviour does not continue indefinitely. Although the specific wavelength at which the galaxy distribution apparently turns to homogeneity is dangerously close to the size of the largest samples presently available, there are serious hints suggesting that this turnover is real and that its effects are detected in the behaviour of statistical estimators. The most recent claims of a continuing fractal hierarchy up to scales of several hundreds Mpc seem to be abscribable to the use of incomplete samples or to an improper treatment of otherwise high- quality data sets. The fractal perspective, nevertheless, can represent a fruitful way to look at the clustering properties of galaxies, when properly coupled to the traditional gravitational instability scenario. In the last part of this paper I therefore try to clarify, at a very simple level, some of the confusion existing on the actual scaling properties of the galaxy distribution. In this context, I shall discuss how these can provide hints on the evolution of the large-scale structure of the Universe, when one properly takes into account the effect of both initial conditions and nonlinear gravitational growth in producing the presently observed distribution.