Multi-field inflation, non-Gaussianity, Planck and a new paper

Earlier this year, the first, long awaited, analysis of cosmological data from the Planck satellite was released. In a series of papers, the Planck team told us the outcome of the years the satellite spent observing the cosmic microwave background (CMB). In particular, they greatly improved constraints on the statistical properties of primordial perturbations. These perturbations imprint themselves on the CMB as temperature fluctuations. To those of us who study inflation, which is thought to produce these perturbations, and who research methods to better predict the statistics a given model of inflation gives rise to, it was a disappointment to learn that there was no detection confirming the statistics are non-Gaussian. Instead the bounds on how much the three-point correlation function, the first non-Gaussian statistic, can vary from zero were considerably tightened. This is summarized by the constraints on the $f_{\rm nl}$ parameter which parametrises the amplitude of the three-point function: $f_{\rm nl} = 2.3 \pm 5.8$. We see that $f_{\rm nl}$ is consistent with zero.

The simplest models of inflation — one field with standard kinetic energy and a smooth potential — predict $f_{\rm nl} \ll 1$, while models with more than one field can produce a much larger $f_{\rm nl}$ that would have been detected by Planck. It has been argued, therefore,  that the Planck results for $f_{\rm nl}$, in combination with results on other parameters, support single field models. In a new paper, myself and collaborators Joe Elliston and Reza Tavakol argue something slightly different: the results tell us that there is no pressure to consider multi-field models, but that such models are still consistent for large regions of their feasible initial condition and parameter spaces. Moreover, there are other reasons that might lead us to consider multi-field models rather than single field ones, such as asking where in fundamental physics the inflaton field comes from, and whether it is likely that just one field contribute to inflation in that context. Continue reading