Homogeneity and isotropy

  One of the central simplifying assumptions of Einstein's cosmology is that, on average, the universe is the same in every direction (isotropy) and in every location (homogeneity). This does not mean, however, that the universe is a boring tapioca-like thing. The distribution of galaxies is far from smooth, with most of them concentrated in relatively narrow sheets separated by large voids, see Fig. 8.22. The situation is reminiscent of a series of soap bubbles where the soapy water corresponds to the galaxies, the air inside the bubbles to the voids.

There are a few hypotheses which explain the origin of this type of structure. These must account not only for the voids, but also for the inhomogeneities in the comsic background radiation; and they must also predict a reasonable time-line for the development of galaxies. All these constraints are difficult to satisfy, making this an area of very active current research.

  

Figure 8.22: Large scale bubble-like structures in the universe. The image contains about 4000 galaxies each representing one luminous point.