2.2.5. SMOOTHNESS PROBLEM
The horizon problem discussed above was in the context of the
large-scale homogeneity in the universe. A related problem arises when
we take into account the extraordinary smoothness of the MBR at small
angles (for a review see
32,
96,
128).
The difficulty for the standard
model comes from the chronology of the proposed scenario in which
discrete structures are supposed to evolve from primordial seeds. Most
theories of structure formation predict that some imprint of this
event remains on the MBR in the form of small fluctuations of the
order *T / T* ~
10^{-4} in the temperature over angular sizes ranging from a
few arc seconds to a few arc minutes. Contrary to these expectations
no such fluctuations have been detected. The current upper bounds are
*T / T* 10^{-5}. We discuss this point
at length in Section 2.3.