Abstract
We assess two potential signals of the formation of our universe by the decay of a false vacuum. Negative spatial curvature is one possibility, but the window for its detection is now small. However, another possible signal is a suppression of the CMB power spectrum at large angles. This arises from the steepening of the effective potential as it interpolates between a flat inflationary plateau and the high barrier separating us from our parent vacuum. We demonstrate that these two effects can be parametrically separated in angular scale. Observationally, the steepening effect appears to be excluded at large l; but it remains consistent with the slight lack of power below l about 30 found by the WMAP and Planck collaborations. We give two simple models which improve the fit to the Planck data; one with observable curvature and one without. Despite cosmic variance, we argue that future CMB polarization and most importantly large-scale structure observations should be able to corroborate the Planck anomaly if it is real. If we further assume the specific theoretical setting of a landscape of metastable vacua, as suggested by string theory, we can estimate the probability of seeing a low-l suppression in the CMB. There are significant theoretical uncertainties in such calculations, but we argue the probability for a detectable suppression may be as large as O(1), and in general is significantly larger than the probability of seeing curvature.
Highlights
AND SUMMARYInflation [1,2,3,4] is a powerful framework for understanding the early universe, in particular the spectrum of density perturbations that led to structure formation and to anisotropies in the cosmic microwave background (CMB) [5,6,7,8,9]
Negative spatial curvature is one possibility, but the window for its detection is small. Another possible signal is a suppression of the cosmic microwave background (CMB) power spectrum at large angles
The steepening effect appears to be excluded at large l; but it remains consistent with the slight lack of power below l ≈ 30 found by the Wilkinson microwave anisotropy probe (WMAP) and Planck collaborations
Summary
Inflation [1,2,3,4] is a powerful framework for understanding the early universe, in particular the spectrum of density perturbations that led to structure formation and to anisotropies in the cosmic microwave background (CMB) [5,6,7,8,9]. The spatial curvature of the universe inside is always negative, which creates Hubble friction that prevents the field from overshooting the inflationary plateau [36] Motivated by this theoretical setting, but without committing to the string landscape, we will consider the phenomenological consequences of a first-order phase transition followed by inflation. (v) With a plausible prior for the distribution of the steepening feature among landscape vacua with slow-roll inflation, the probability for an observable effect to lie in the visible region can be as high as Oð1Þ.2. This is higher than the probability for seeing curvature under similar assumptions, which is Oð10%Þ. In the slow-roll approximation these power spectra are close to a power-law spectrum, which for Ps is often parametrized as k3Ps
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