arXiv subjects

Antonios Kyriazis

Publications and source records attributed to Antonios Kyriazis.

5 recordsLinked to original sources

arXiv2026

Topics in the phenomenology of axions: the cases of cosmic strings and superradiance

Originally proposed as a solution to the strong CP problem and later understood to be a good dark matter candidates, axions have appeared in a variety of beyond-the-standard-model theories and are imbued with rich phenomelogical consequences. In this dissertation, we will examine two of these consequences: the emission of axions from cosmic strings and the superradiant mechanism in black hole physics. The spontaneous symmetry breaking of a global symmetry in the early universe can give rise to a network of cosmic strings, which emit ultra light, axion-like particles that can contribute to the dark matter density. We will discuss how the associated density fluctuations of these particles can be computed from first principles by treating them as a collection of plane waves. We will then calculate the density power spectrum and compare it to well-established cosmological observables, as well as to observables from future surveys, to derive constraints on the mass of the particles and the symmetry breaking scale. These light axion-like particles can also form a gravitational atom around a spinning black hole through the superradiance process. Considering the black hole to be part of a binary system, the tidal potential of the companion periodically perturbs the gravitational atom such that an atomic transition occurs between two of its energy states. Gravitational waves are emitted by the cloud during this transition. We will derive the analytical formulae of both the strain waveform and frequency spectrum of the signal and identify the systems that would be the most promising for detecting it in future, space-based gravitational wave observatories.

hep-ph↗

arXiv2026

Echoes of Global Cosmic Strings

If the Universe underwent a cosmic phase transition, it may have left behind a network of cosmic strings. When these strings arise from the breaking of a gauge symmetry, their decay produces a significant stochastic background of gravitational waves. In contrast, if they originate from the breaking of a global symmetry, their decay predominantly yields Nambu-Goldstone bosons, which can persist as dark matter or dark radiation. In this work, we assess the detectability of this particle spectrum using a range of cosmological probes. We employ semi-numerical methods to estimate the resulting energy density and compute the associated matter power spectrum. We then compare these predictions with observations of the cosmic microwave background, Lyman-$α$ forest, large-scale structure surveys, and the UV luminosity function, thereby deriving constraints on the Nambu-Goldstone boson mass and the symmetry-breaking scale. Finally, we present projections for the sensitivity of upcoming cosmic microwave background missions.

hep-ph↗

arXiv2026

Heating Up the Black Hole X-ray Binary Accretion Disk by Superradiance

A superradiant cloud of ultralight axions around a black hole, that is part of an X-ray binary system, can heat up its accretion disk and be detected by the thermal X-ray spectrum emitted by the disk. We consider a derivative coupling of the axions to the plasma fermions and calculate the emissivity of the inverse bremsstrahlung process that results in a temperature fluctuation of the disk. Based on the thin-disk model and the multicolor disk model, we derive the thermal spectrum with axion heating, which shows an enhanced thermal photon flux and a red-/blue- shifted peak spectral frequency. A single bump hunting search of the axion heating signature in the thermal spectrum of a $10M_\odot$ black hole X-ray binary with a spectral measurement sensitivity of 10\% (1\%) can derive the constraint on axion-electron coupling $|g_{ae}|\gtrsim7.5\times 10^{-12} ~(2.4 \times 10^{-12})$ for axion mass $m_a=5.2\times 10^{-12}\,$eV in a saturated $|211\rangle$ state, and $|g_{ae}|\gtrsim4.5\times 10^{-12} ~(1.4 \times 10^{-12})$ for axion mass $m_a=1.0\times10^{-11}\,$eV in a saturated $|322\rangle$ state. The projected sensitivities are competitive with those from XENONnT. A detailed continuum fitting can further improve the detectability and provide a complementary bound to the black hole spin-down measurement.

astro-ph.HE↗

arXiv2025

Gravitational Waves from Resonant Transitions of Tidally Perturbed Gravitational Atoms

Light bosons can form a gravitational atom (GA) around a spinning black hole through the superradiance process. Considering the black hole to be part of a binary system, the tidal potential of the companion periodically perturbs the GA such that an ``atomic'' transition occurs between two of its energy eigenstates. The resonant transition is modeled by the Landau-Zener system, where the orbital frequency of the companion determines the relevant transition. In this work, we study a novel quasi-monochromatic gravitational wave signal originating directly from the level transition of the GA in a binary system. We derive the analytical formulae of both the strain waveform and frequency spectrum of the signal. We further investigate the GA-binary systems that can have a large signal-to-noise ratio in the milli-Hz to deci-Hz frequency band. Using the future space-based gravitational wave observatory DECIGO, we find the signal-to-noise ratio is $\mathcal{O}(10-200)$ for the fine-structure constant $α\simeq 0.3$, host black hole mass $M= 150M_\odot$ and boson mass $μ\simeq 10^{-13} \rm eV$ at a distance within 100 kpc. Given astrophysical uncertainties about the black hole's initial spin, the degeneracy with other monochromatic signals and the small merger rate at those distances, we conclude that the detection of the signal would be challenging.

hep-ph↗

arXiv2025

Effects of a dark matter caustic passing through the Oort cloud

We investigate the effect of a dark matter caustic passing through the Solar System. We find, confirming a previous result, that the Sun tracks the caustic surface for some time. We integrate numerically the equations of motion of the Sun and a comet for a large number of initial conditions and of caustic passage properties. We calculate the probability for the comet to escape the Solar System and the probability for it to fall within 50 A.U. of the Sun, given the initial semimajor axis and eccentricity of its orbit. We find that the average probability for a comet to fall within 50 A.U. of the Sun is of order $3 \times 10^{-4}$ and that comets which are initially at a distance larger than about $10^5$ A.U. have a probability of order one to be ejected from the Solar System.

astro-ph.CO↗