Up: At the cutting edge
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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.
Large scale bubble-like structures in the universe. The
image contains about 4000 galaxies each representing one luminous point.