E. Alternatives to inflation
It is quite striking that we have a theory of initial conditions for the Universe, especially one that is testable and has met with some success. But the best way to measure the success of an idea is to have some real competition. So far the competition for inflation has been limited. (In the more narrow domain of the origin of perturbations, inflation has already vanquished a class of worthy competitors, the cosmic defect models [Albrecht(2000)].)
But competition does exist: one proposed alternative involves varying the speed of light (rather than the cosmic expansion) to resolve the horizon and other problems [Moffat(1993), Albrecht and Magueijo(1999)], but this idea still has to find a compelling foundation in fundamental physics (interesting efforts in this direction are ongoing [Alexander and Magueijo(2001), Moffat(2001)]. Another idea is connected with holography (the notion that the degrees of freedom of a gravitating system are much fewer than basic field theory suggests). It has been proposed that these limitations actually force the early universe to be highly homogeneous [Fischler and Susskind(1998), Kaloper and Linde(1999)]. But this idea has yet to take a concete form with any real predictive power. There is also the "ekpyrotic" scenario [Khoury et al.(2001)], which creates the start ofthe Big Bang as a collision between two "branes" in a higher dimensional space. This picture takes a very different view of explaining initial conditions from inflation. Rather than creating a situation in which many different initial conditions evolve dynamically into what we need, the ekpyrotic Universe needs to set up extremely special initial conditions to start with. But regardless of one's opinion of this alternative approach, efforts to explore the ekpyrotic scenario have led to exciting investigations into the nature of colliding branes [Khoury et al.(2001a)], work which may well elucidate interesting issues in brane physics.