Abstract
In addition to spectacular signatures such as black hole superradiance and the rotation of CMB polarization, the plenitude of axions appearing in the string axiverse may have potentially dangerous implications. An example is the cosmological overproduction of relic axions and moduli by the misalignment mechanism, more pronounced in regions where the signals mentioned above may be observable, that is for large axion decay constant. In this work, we study the minimal requirements to soften this problem and show that the fundamental requirement is a long period of low-scale inflation. However, in this case, if the inflationary Hubble scale is lower than around O(100) eV, no relic DM axion is produced in the early Universe. Cosmological production of some axions may be activated, via the misalignment mechanism, if their potential minimum changes between inflation and today. As a particular example, we study in detail how the maximal-misalignment mechanism dilutes the effect of dangerous axions and allows the production of axion DM in a controlled way. In this case, the potential of the axion that realises the mechanism shifts by a factor ∆θ = π between the inflationary epoch and today, and the axion starts to oscillate from the top of its potential. We also show that axions with masses ma ∼ O(1 − 100) H0 realising the maximal-misalignment mechanism generically behave as dark energy with a decay constant that can take values well below the Planck scale, avoiding problems associated to super-Planckian scales. Finally, we briefly study the basic phenomenological implications of the mechanism and comment on the compatibility of this type of maximally-misaligned quintessence with the swampland criteria.
Highlights
In addition to spectacular signatures such as black hole superradiance and the rotation of CMB polarization, the plenitude of axions appearing in the string axiverse may have potentially dangerous implications
An example is the cosmological overproduction of relic axions and moduli by the misalignment mechanism, more pronounced in regions where the signals mentioned above may be observable, that is for large axion decay constant
We show that axions with masses ma ∼ O(1 − 100) H0 realising the maximal-misalignment mechanism generically behave as dark energy with a decay constant that can take values well below the Planck scale, avoiding problems associated to super-Planckian scales
Summary
In this work we tackle both issues — that is, the DM axion overabundance and the apparent difficulty in finding consistent axion quintessence — together in the axiverse. These axions have masses and decay constants determined exclusively by geometric reasons, allowing to understand in a transparent way why their abundance is troublesome in compactifications that realise gauge sectors with couplings comparable to the SM gauge couplings. S ∼ 2π/α is the action of the string instanton or dark Yang-Mills (YM) sector that dominates the axion mass and Λ4 ∼ m2susyMP2 , with msusy the scale of SUSY breaking These geometric axions saturate the bound of the Weak Gravity Conjecture [64] and, while it is true that the decay constant can be reduced in warped or highly anisotropic compactifications, as we will see they are a good guide to illustrate the issue of overabundance. We will study the minimal requirements of this kind of solutions and, later, study in detail a particular model based on the maximal-misalignment mechanism presented in [62]
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