Can Cosmology Test String Theory?

Today, I listened a talk by Richard Easther of Yale with the above title. I think their approach is certainly worth a try. Rather than looking specific string-cosmology like brane world scenarios or stringy inflation, they pursued a search by an effective field theory approach. They took a reasonable initial condition anzats with a minimum length scale and evolved it outside to the horizon. When it comes back to the horizon, we observe it as the CMB fluctuations. Since in any inflation model today’s fluctuation length scales were beyond quantum gravity scales, it is plausible that we can see some effects, but are they big enough?

Their anzats, which seems reasonable in for string theory, modulates the power spectrum and for some values in the parameter space it is observable. To quote from their paper astro-ph/0412613: "For the specific case we consider, we conclude that if the tensor to scalar ratio, r~0.15, the ratio between the inflationary Hubble scale H, and the scale of new physics M has to be on the order of 0.004 if the modulation is detectable at the 2σ level. For a lower value of r, the bound on H/M becomes looser." This means if the universe is very very kind to us, we can see something even with the second year WMAP data. (By the way, does anybody know what's going on in WMAP? Third year data should be almost ready but they haven't announced the second year.)

I want to mention one point. I couldn't see any point in this approach very specific to string theory. Any theory with a minimum scale is equally fine. Even if we could see a significant signal, string theory still has to predict a specific scenario.

Today I also learned that fitting the cosmological parameters is not being done by brute fore n dimensional grid search anymore. There is a well working Bayesian algorithm called Monte Carlo Markov Chain, first proposed for CMB analysis in astro-ph/0006401.

I hope coming years will be full of surprises in cosmology.