Let me also say some elementary comments about the popularity of cosmic string models that decreased rapidly around 2000 (and may be revived now): the cosmic strings have also been proposed as alternatives against inflation to explain structure formation. And because their equation of state is roughly "pressure equals -1/3 of energy density" in average, you may imagine that they cause the acceleration of the Universe instead of the cosmological constant. Both of these applications now seem highly unlikely with WMAP: WMAP says "pressure is below -0.78 of energy density at 95% confidence level". Also, the inflationary predictions of non-uniformities of the cosmic microwave background (CMB) agree with WMAP while the cosmic-string-dominated models are more or less falsified - the latter would lead to a much smoother profile of the temperature variations.