Axion landscape and natural inflation

Abstract

Multiple axions form a landscape in the presence of various shift symmetry breaking terms. Eternal inflation populates the axion landscape, continuously creating new universes by bubble nucleation. Slowroll inflation takes place after the tunneling event, if a very flat direction with a super-Planckian decay constant arises due to the alignment mechanism. We study the vacuum structure as well as possible inflationary dynamics in the axion landscape scenario, and find that the inflaton dynamics is given by either natural or multi-natural inflation. In the limit of large decay constant, it is approximated by the quadratic chaotic inflation, which however is disfavored if there is a pressure toward shorter duration of inflation. Therefore, if the spectral index and the tensor-to-scalar ratio turn out to be different from the quadratic chaotic inflation, there might be observable traces of the bubble nucleation. Also, the existence of small modulations to the inflaton potential is a common feature in the axion landscape, which generates a sizable and almost constant running of the scalar spectral index over CMB scales. Non-Gaussianity of equilateral type can also be generated if some of the axions are coupled to massless gauge fields.

Natural Inflation on the brane with a TeV-scale gravity: Parameter constraints after Planck 2015

“…We can determinate numerically from Eqs. (34) and (35) that, by fixing N , the tensor-to-scalar ratio decreases as the parameter α is increasing. On the other hand, after to reach a maximum value, the scalar spectral index begins to decrease as α is increasing.…”

Transplanckian axions!?

“…The 'empirical' case-by-case realization that this requirement is not obviously realized in string theory compactifications [7] (which provide a template for a quantum gravity framework), motivated the construction of models with multiple axions and potentials from gauge instantons, in which some axion linear combination effectively hosts a transplanckian field range in its basic period, even if the original periodicities are taken sub-planckian [8,9] (see [10][11][12][13][14][15][16][17][18][19] for recent works). These models are formulated in purely phenomenological field theory terms, and therefore there remains the question of whether their features survive in actual embeddings in theories including quantum gravitational corrections.…”

Adiabatic suppression of the axion abundance and isocurvature due to coupling to hidden monopoles

“…[6] for a review and refs. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] for recent works.) 2 Thermal inflation is another possible solution to the cosmological moduli problem, where the moduli density is diluted by a mini-inflation around the TeV scale [23][24][25][26].…”