Astronomers have discovered a cluster of young galaxies (quasars) that stretches four billion light years across! See here and here. The abstract of their paper says:
This new, Huge-LQG appears to be the largest structure currently known in the early Universe. Its size suggests incompatibility with the Yadav et al. scale of homogeneity for the concordance cosmology, and thus challenges the assumption of the cosmological principle.
The paper itself says:
The cosmic microwave background (CMB) is usually considered to provide the best evidence for isotropy, and hence of homogeneity too, given the assumption of isotropy about all points. Nevertheless, there do appear to be large-scale features in the CMB that may challenge the reality of homogeneity and isotropy – see Copi et al. (2010) for a recent review. More recently still than this review, Rossmanith et al. (2012) find further indications of a violation of statistical isotropy in the CMB. Furthermore, Yershov, Orlov & Raikov (2012) find that the supernovae in the redshift range 0.5–1.0 are associated with systematic CMB temperature fluctuations, possibly arising from large-scale inhomogeneities. Observationally, for SDSS DR7 galaxies with 0.22 < z < 0.50, Marinoni, Bel & Buzzi (2012) find that isotropy about all points does indeed apply on scales larger than ∼210 Mpc.
The occurrence of structure on Gpc-scales from the Huge-LQG and from galaxies implies that the Universe is not homogeneous on these scales. Furthermore, if we accept that homogeneity refers to any property of the Universe then an intriguing result is that of Hutsemékers et al. (2005), who found that the polarization vectors of quasars are correlated on Gpc scales. Similarly, the existence of cosmic flows on approximately Gpc scales (e.g. Kashlinsky et al. 2010), regardless of their cause, is itself implying that the Universe is not homogeneous.
Of course, history and, most recently, the work of Park et al. (2012) indicate that one should certainly be cautious on the question of homogeneity and the cosmological principle. The SGW (Gott et al. 2005) – and before it, the Great Wall (Geller & Huchra 1989) – was seen as a challenge to the standard cosmology and yet Park et al. (2012) show that, in the ‘Horizon Run 2’ concordance simulation of box-side 10 Gpc, comparable and even larger features can arise, although they are of course rare. Nevertheless, the Huge-LQG presented here is much larger, and it is adjacent to the CCLQG, which is itself very large, so the challenges still persist.
This structure challenges isotropy and the cosmological principle – so how much do our current models of reality really know?