3 research outputs found
NANOGrav signal from axion inflation
Several pulsar timing arrays including NANOGrav, EPTA, PPTA, and CPTA have
recently reported the observation of a stochastic background of gravitational
wave spectrum in the nano-Hz frequencies. An inflationary interpretation of
this observation is challenging from various aspects. We report that such a
signal can arise from the Chern-Simons coupling in axion inflation, where a
pseudoscalar inflaton couples to a (massive) gauge field, leading to
efficient production of a transverse gauge mode. Such tachyonic particle
production during inflation exponentially enhances the primordial perturbations
and leads to a unique parity-violating gravitational wave spectrum, that
remains flat near the CMB scales but becomes blue-tilted at smaller scales. We
identify the parameter space consistent with various cosmological constraints
and show that the resultant gravitational wave signals can provide extra
contribution on top of the standard astrophysical contribution from inspiraling
supermassive black hole binaries towards explaining the observed excess at
NANOGrav. The parity-violating nature of the signal can be probed in future
interferometers, distinguishing it from most other new physics signals
attempting to explain the NANOGrav result.Comment: 11 pages + references, 6 figures, added PTArcade analysis, modified
inflaton potential for improved fit to NANOGrav result, version submitted to
journa
Gauged Global Strings
We investigate the string solutions and cosmological implications of the
gauge global model. With two
hierarchical symmetry-breaking scales, the model exhibits three distinct string
solutions: a conventional global string, a global string with a heavy core, and
a gauge string as a bound state of the two global strings. This model reveals
rich phenomenological implications in cosmology. During the evolution of the
universe, these three types of strings can form a Y-junction configuration.
Intriguingly, when incorporating this model with the QCD axion framework, the
heavy-core global strings emit more axion particles compared to conventional
axion cosmic strings due to their higher tension. This radiation significantly
enhances the QCD axion dark matter abundance, thereby opening up the QCD axion
mass window. Consequently, axions with masses exceeding have the potential to constitute the whole dark matter abundance.
Furthermore, in contrast to conventional gauge strings, the gauge strings in
this model exhibit a distinctive behavior by radiating axions.Comment: 36 pages, 8 figure
Gauged global strings
Abstract We investigate the string solutions and cosmological implications of the gauge U(1)Z Γ global U(1)PQ model. With two hierarchical symmetry-breaking scales, the model exhibits three distinct string solutions: a conventional global string, a global string with a heavy core, and a gauge string as a bound state of the two global strings. This model reveals rich phenomenological implications in cosmology. During the evolution of the universe, these three types of strings can form a Y-junction configuration. Intriguingly, when incorporating this model with the QCD axion framework, the heavy-core global strings emit more axion particles compared to conventional axion cosmic strings due to their higher tension. This radiation significantly enhances the QCD axion dark matter abundance, thereby opening up the QCD axion mass window. Consequently, axions with masses exceeding ~ 10 β5 eV have the potential to constitute the whole dark matter abundance. Furthermore, in contrast to conventional gauge strings, the gauge strings in this model exhibit a distinctive behavior by radiating axions