Planck evidence for a closed Universe and a possible crisis for cosmology

Abstract

The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in CMB power spectra compared to that predicted in the standard ΛCDM model.A closed universe can provide a physical explanation for this effect, with the Planck CMB spectra now preferring a positive curvature at more than 99% C.L. Here we further investigate the evidence for a closed universe from Planck, showing that positive curvature naturally explains the anomalous lensing amplitude and demonstrating that it also removes a well-known tension within the Planck data set concerning the values of cosmological parameters derived at different angular scales. We show that since the Planck power spectra prefer a closed universe, discordances higher than generally estimated arise for most of the local cosmological observables, including BAO. The assumption of a flat universe could therefore mask a cosmological crisis where disparate observed properties of the Universe appear to be mutually inconsistent.Future measurements are needed to clarify whether the observed discordances are due to undetected systematics, or to new physics, or simply are a statistical fluctuation.The recent Planck Legacy 2018 release of observations of Cosmic Microwave Background (CMB) anisotropies (PL18, hereafter) has reported some unexpected results, revealing the possibility for new physics beyond the so-called ΛCDM standard cosmological model 1, 2 . Indeed, while the inflationary predictions for coherent acoustic oscillations have been fully confirmed, a preference for a higher lensing amplitude A lens than predicted in the base ΛCDM at about 3 standard deviations 1 arXiv:1911.02087v1 [astro-ph.CO] 5 Nov 2019 has been found in the temperature and polarization angular spectra. We argue that the "A lens " anomaly has profound implications for some extensions to ΛCDM such as the curvature of the universe. The constraints from the PL18 CMB spectra on curvature, parameterized through the energy density parameter Ω K , are indeed quite surprising, suggesting a closed universe at 3.4 standard deviations (−0.007 > Ω K > −0.095 at 99% C.L. 1-3 ).As is well known, inflation theory naturally predicts a flat universe 4, 5 . However, inflationary models with Ω K < 0 6-8 are relatively simple to build, with primordial homogeneity and isotropy easier to achieve than in open models. An issue for closed inflation models is that to obtain Ω K ∼ −0.1, fine-tuning at a few per cent level is needed 7 . This does not sound very compelling, but it may still be acceptable given the presence of a far more finely-tuned cosmological constant.Closed models could also lead to a large-scale cut-off in the primordial density fluctuations, around the curvature scale R c = (c/H 0 )|Ω K | −0.5 ∼ 10 Gpc, in agreement with the observed low CMB anisotropy quadrupole 7, 9 . Confirmation of a positive spatial curvature would also have several implications for inflationary theory, and, for example, severely challenge models of eternal inflation 10, 11 .In this Letter, we show that, i...

Four-volume cutoff measure of the multiverse

“…This distribution is almost indistinguishable from that in the scale factor cutoff measure [12] which is shown by a dashed curve. The Planck data favors a slightly negative value of k [19] but is consistent with a spatially flat universe within 2σ [20]. The observationally allowed range within 3σ is about |Ω k | 0.01 or |k| 3 × 10 −2 , which is indicated by shading in the figure.…”

Distant foreground and the Planck-derived Hubble constant

“…Di Valentino, Melchiorri & Silk (2019) have found that the ΛCDM parameters derived from the Planck data can be brought into agreement with each other and with the Cepheid-based H0 value by assuming a closed Universe with the curvature parametrised by the energy density parameter ΩK = −0.091 ± 0.037. According to these authors, such a fit requires drastic changes in the ΛCDM model, unless there exists an obvious possible solution to this problem in the form of hitherto undetected systematics in the CMB data.…”

Consequences of Massive Neutrino to Astrophysics and Cosmology

“…It turns out that the global structure of the universe is still an open problem. For example, a recently published article addresses this issue and argues that gravitational curvature renders a closed universe [41]. Another discussion of the structure of a closed universe can be found in the literature [42].…”