Inflationary paradigm in trouble after Planck2013

Abstract

Recent results from the Planck satellite combined with earlier observations from WMAP, ACT, SPT and other experiments eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported by the Planck Collaboration. More important, though, is that all the simplest inflaton models are disfavored statistically relative to those with plateau-like potentials. We discuss how a restriction to pl ateau-like models has three independent serious drawbacks: it exacerbates both the initial conditions problem and the multiverse-unpredictability problem and it creates a new diffi culty that we call the inflationary “unlikeliness problem.” Finally, we comment o n problems reconciling inflation with a standard model Higgs , as suggested by recent LHC results. In sum, we find that recent ex perimental data disfavors all the best-motivated inflation ary scenarios and introduces new, serious diffi culties that cut to the core of the inflationary paradigm. For thcoming searches for B-modes, nonGaussianity and new particles should be decisive. The Planck satellite data reported in 2013 [1] shows with high precision that we live in a remarkably simple universe. The measured spatial curvature is small; the spectrum of fluc tuations is nearly scale-invariant; there is a small spectr al tilt, consistent with there having been a simple dynamical mechanism that caused the smoothing and flattening; and the fluctuations are nearly Gaussian, eliminating exotic and complicated dynamical possibilities, such as inflationary models with n oncanonical kinetic energy and multiple fields. (In this Lette r, we will not discuss the marginal deviations from isotropy on large scales reported by the Planck Collaboration [2].) The results not only impose tight quantitative constraints on all cosmological parameters [3], but, qualitatively, they call for a cos mological paradigm whose simplicity and parsimony matches the nature of the observed universe. The Planck Collaboration attempted to make this point by describing the data as supporting the simplest inflationary models [4, 5, 6]. However, the models most favored by their data (combined with earlier results from WMAP, ACT, SPT and other observations [7]) are simple by only one criterion: an inflat on potential with a single scalar field su ffi ces to fit the data. By several other important criteria described in this Letter, the favored models are anything but simple: Namely, they suffer from exacerbated forms of initial conditions and multiverse problems, and they create a new diffi culty that we call the inflationary “unlikeliness problem.” That is, the favored inflaton potentia ls are exponentially unlikely according to the logic of the inflati onary paradigm itself. The unlikeliness problem arises even if we ∗ Corresponding author