The universe, as we understand it, started with a bang. Although we call it the Big Bang, the universe at this stage was actually pretty small—if we decide that something that encompasses all of space and time can ever be called small, then it was small. As the universe expands, the distance between any two points becomes longer, and since nothing can travel faster than the speed of light, regions of the universe are essentially independent of each other.
All of this leaves us with a problem, because we observe a universe that is much smoother than expected. To explain this, cosmologists posited that the early universe expanded much faster, meaning that the original smooth universe was preserved. This hypothesis, called inflation, predicts certain features in the cosmic microwave background radiation (CMB). Since these have been observed, inflation has become an accepted part of the universe's history. But what caused inflation?
No one really knows the answer to that. There are several ways to put inflation into cosmological models, but physicists would like to have inflation come out as a consequence of our physical understanding of the universe, rather having to bolt it onto the side. Currently, supersymmetry, loop quantum gravity, and string theory models all predict inflationary periods in the universe. Within these models certain parameters yield what are called cosmic strings, which are strings that are big enough to perturb the CMB.
Unfortunately, no one really knows exactly how they would perturb it, so researchers have been reduced to curve fitting. The idea is that inflation has set the basic smooth nature of the universe and cosmic strings have since altered this over the course of time since inflation ended. Now that the influence of inflation is reasonably well understood, we can attempt to see the difference between a universe with inflation and a universe with inflation and cosmic strings.
This is exactly what a recently published Physical Review Letter attempts to do and their conclusion was—wait for it—cosmic strings are a definite maybe. Their modeling shows that the inclusion of cosmic strings can improve their model's fit to the CMB compared to using inflation without strings. Once non-CMB based data is included, however, the two become indistinguishable. This constrains the properties of the strings to a reasonably narrow parameter range, but doesn't rule them out entirely.
Physical Review Letters, 2008, DOI: 10.1103/PhysRevLett.100.021301