Abstract
Realising de Sitter vacua in string theory is challenging. For this reason it has been conjectured that de Sitter vacua inhabit the Swampland of inconsistent low-energy effective theories coupled to gravity. Since de Sitter is an attractor for ΛCDM, the conjecture calls ΛCDM into question. Reality appears sympathetic to this idea as local measurements of the Hubble constant H0 are also at odds with ΛCDM analysis of Planck data. This tension suggests that the de Sitter state is unstable, thereby implying a turning point in the Hubble parameter. We present a model relieving this tension, which predicts a turning at small positive redshift z⁎ that is dictated by present-day matter density ωm. This feature is easily identified by homogeneous surveys covering redshifts z≤0.1. We comment on the implications for the Swampland program.
Highlights
String theory enjoys a billing as the leading candidate for a theory of quantum gravity
Recalling that one realisation of de Sitter is an asymptotic future attractor for the ΛCDM cosmological model, and ΛCDM captures beautifully measurements of the Cosmic Microwave Background (CMB) anisotropies due to Planck [16], one can recognise that the conjecture is in obvious conflict with ΛCDM
In this note we assume the conjecture is correct and attempt a reconciliation with ΛCDM. To this end there are two possibilities: either a deviation from ΛCDM happens in the past at positive redshift (z > 0), so that it is observable and of interest to science, or it happens in the future (z < 0) and is the subject of science fiction
Summary
String theory enjoys a billing as the leading candidate for a theory of quantum gravity. In light of the recent tension in the Hubble constant H0 between its local value H0 = 73.48 ± 1.66 km s−1 Mpc−1 [17] and the Planck result based on ΛCDM H0 = 66.93 ± 0.62 km s−1 Mpc−1 [19], it is natural to entertain the prospect that some departure happens at positive redshift. Note that at low redshift, z < 2, physics is dominated by only two parameters from the six of ΛCDM: the Hubble constant H0 and matter density ωm at redshift z = 0.
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