Annu. Rev. Astron. Astrophys. 1991. 29: 325-362 Copyright © 1991 by Annual Reviews. All rights reserved

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.