3,128 research outputs found
Relaxing Isocurvature Bounds on String Axion Dark Matter
If inflation scale is high, light scalars acquire large quantum fluctuations
during inflation.If sufficiently long-lived, they will give rise to CDM
isocurvature perturbations, which are highly constrained by the Planck data.
Focusing on string axions as such light scalars, we show that thermal inflation
can provide a sufficiently large entropy production to dilute the CDM
isocurvature perturbations. Importantly, efficient dilution is possible for the
string axions, because effectively no secondary coherent oscillations are
induced at the end of thermal inflation, in contrast to the moduli fields. We
also study the viability of the axion dark matter with mass of about 7 keV as
the origin of the 3.5 keV X-ray line excess, in the presence of large entropy
production.Comment: 18 pages, 2 figure
Adiabatic suppression of the axion abundance and isocurvature due to coupling to hidden monopoles
The string theory predicts many light fields called moduli and axions, which
cause a cosmological problem due to the overproduction of their coherent
oscillation after inflation. One of the prominent solutions is an adiabatic
suppression mechanism, which, however, is non-trivial to achieve in the case of
axions because it necessitates a large effective mass term which decreases as a
function of time. The purpose of this paper is twofold. First, we provide an
analytic method to calculate the cosmological abundance of coherent oscillation
in a general situation under the adiabatic suppression mechanism. Secondly, we
apply our method to some concrete examples, including the one where a string
axion acquires a large effective mass due to the Witten effect in the presence
of hidden monopoles.Comment: 30 pages, 3 figure
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