2.5. When the Features become Problems
I have discussed three features of the SBB that are usually presented as ``problems''. What can make a feature a problem? The Flatness and Homogeneity features describe the need to give the SBB very special initial conditions in order to match current data. Who cares? If you look at the typical laboratory comparison between theory and experiment, success is usually measured by whether or not theoretical equations of motion correctly describe the evolution of the system. The choice of initial conditions is usually made only in order to facilitate the comparison. By these standards, the SBB does not have any problems. The equations, with suitably chosen initial conditions, do a perfectly good job of describing the evolution of the Universe.
However, there is something extremely strange about the initial conditions required by the SBB that makes us unable to simply accept the Flatness and Homogeneity features: The required initial conditions place the Universe very far away from where its equations of motion wish to take it. So much so that even today, 15 Billion years later, the Universe still has features (flatness and homogeneity) that the equations are trying to destroy.
I think a simple analogy is useful: Imagine that you come into my office and see a pencil balanced on its point. You notice it is still there the next day, the next week, and even a year later. You finally ask me ``what is going on with the pencil?'' to which I reply: ``yes, that's pretty interesting, but I don't have to explain it because it was that way when I moved into my office''. You would find such a response completely unreasonable. On the other hand, if the pencil was simply lying on my desk, you probably would not care at all how it got there.
The fact is that, like the fictitious balanced pencil, the initial conditions of the real Universe are so amazing that we cannot tear ourselves away from trying to understand what created them. It is at this point that the Flatness and Homogeneity features become ``problems''.
It is also at this point that the Horizon feature becomes a problem. The hope of a cosmologist who is trying to explain the special initial conditions is that some new physical processes can be identified which can ``set up'' these initial conditions. A natural place to hope for this new physics to occur is the Grand Unification (GUT) scale, where particle physics already indicates that something new will happen. However, once one starts thinking that way, one faces the fact that the Universe is compose of 1080 causally disconnected regions at the GUT epoch, and is seems impossible for physical processes to extend across these many disconnected regions and set up the special homogeneous initial conditions.