Relaxing the cosmological moduli problem by low-scale inflation

Abstract

We show that the cosmological abundance of string axions is much smaller than naive estimates if the Hubble scale of inflation, H inf , is sufficiently low (but can still be much higher than the axion masses) and if the inflation lasts sufficiently long. The reason is that the initial misalignment angles of the string axions follow the Bunch-Davies distribution peaked at the potential minima. As a result, the cosmological moduli problem induced by the string axions can be significantly relaxed by low-scale inflation, and astrophysical and cosmological bounds are satisfied over a wide range of the mass without any fine-tuning of the initial misalignment angles. Specifically, the axion with its decay constant f φ = 10 16 GeV satisfies the bounds over 10 −18 eV m φ 10 TeV for H inf 10 keV − 10 6 GeV. We also discuss cases with multiple axions and the QCD axion.

Dark Matter from Scalar Field Fluctuations

“…Similar ideas have recently been studied in the literature, including e.g. self-interacting [8][9][10][11][12] or non-minimally coupled DM [13][14][15], DM coupled to the inflaton [16], or * ttenkan1@jhu.edu axion DM [17][18][19][20]. In this paper we show that even in the simplest possible case the scalar field can successfully constitute all DM without being in conflict with the CMB data.…”

Misalignment & Co.: (pseudo-)scalar and vector dark matter with curvature couplings

“…The effective mass is then typically of the order of the Hubble scale, leading to a non-trivial evolution of both the field and the fluctuations during inflation. This evolution during inflation impacts all three possible non-thermal scenarios for the production of the dark matter density: misalignment [1][2][3][5][6][7], stochastic [38][39][40][41] and fluctuation [42][43][44][45][46][47] production. After inflation, the Ricci scalar vanishes during radiation domination and the evolution proceeds in the standard way.…”

“…Furthermore, they are naturally produced via the misalignment mechanism [1][2][3][5][6][7] and as such they generally contribute to at least a fraction of the observed dark matter density. The initial (homogeneous) field value in our observable Universe can simply correspond to the initial misalignment of the field away from the vacuum, or it can arise "stochastically" [36][37][38][39][40][41] from the accumulated effect of fluctuations during a long phase of inflation. Additional contributions may arise from production via inflationary fluctuations [42][43][44][45][46][47] as well as from decays of precursor particles [48][49][50][51][52][53].…”

QCD axion on hilltop by a phase shift of π.

“…Lastly, let us emphasize that the low-scale inflation satisfying H inf 1 GeV and N H 2 inf /m 2 a enables the QCD axion to explain DM for both f a 10 12 GeV [17,18,141] and f a 10 12 GeV. In the former, θ i 1 is realized by the BD distribution around θ i = 0, while in the latter, θ i ≈ π is realized by the πnflation mechanism.…”