The imprint of gravitational waves created shortly after the big bang may offer direct evidence for inflation theory, according to a discovery by the BICEP2 experiment at the South Pole and released today. What this means for those alternatives theories to inflation such as string gas cosmology and the ekpyrotic universe? What would happen with these two cases particularly?
Alan Guth summed it up nicely (emphasis mine):
Measuring the amplitude, or strength, of the primordial B modes at different angular scales tells you how the inflationary expansion rate changed with time during the period of inflation. Understanding the extent to which it varies is an important clue to determining details of what drove the inflation. Alternative models to inflation, like the ekpyrotic models that Paul Steinhardt and Neil Turok are pushing, predict incredibly small levels of gravitational waves, essentially zero on any measurable scale. This would, at face value, completely rule out those models.
In regards to String Gas Cosmology (SGC), this 2011 paper (arXiv link) by Robert Brandenberger suggests that gravitational wave perturbations would also exist in the SGC model. Thus, I don't believe that this particular non-inflationary model would be ruled out (but I'm not declaring this with any level of certainty).