Axion stars are coherent clumps of wavelike dark matter which are expected to form abundantly in dense structures like axion miniclusters. Due to their large central densities of up to $\rho_\star \sim 10^{23}\,$GeV$\,$cm$^{-3}$ at a QCD axion mass of $m_a=50\,\mu$eV, it has been conjectured that they can give rise to a number of different observable signatures. Among these signatures are the...
Axion-like particles, which we call axions, are promising dark matter candidates and may form substructures such as miniclusters and axion stars. When axions couple to photons, this interaction sets a critical axion star mass, which we call decay mass, above which parametric resonance of photons occurs. We consider the accretion of axion stars at decay mass within our galaxy and estimate the...
In this talk, we discuss the unphysical behavior that arises when addressing infrared (IR) divergences in particle production rates in cosmology, particularly focusing on axion production coupled to photons. This unphysical behavior manifests as negative production rates for soft axion momentum.
To address this, we calculate the axion production rate at finite temperature. Initially, we use...
Does a QCD axion have room for enough Peccei-Quinn symmetry violation to impact the misalignment mechanism? Constraints from the neutron electric dipole moment tell us that the QCD axion must very nearly conserve PQ symmetry, but we also know that the symmetry is not perfect. Even a small amount of PQ breaking can have interesting phenomenology. We show how temperature-dependent PQ breaking...
Axion Kinetic Misalignment can be generated by higher-order operators that explicitly break the PQ symmetry at very high energies. This results in a kick in the angular direction of the PQ field, causing a delay in the onset of axion oscillations. For the higher-order operator to be relevant, the PQ radial mode must reach very high values. We explore the possibility that a non-minimally...
High-frequency gravitational waves ($f\gtrsim1\,$MHz) are a smoking gun for the existence of exotic physics. Indeed, GW backgrounds generated in the early Universe could be characterized by high-frequency signals, allowing one to probe inflation, first-order phase transitions, topological defects and primordial black holes. The lack of current and future gravitational waves experiments...
Ultralight Axion-Like Dark Matter (ALDM) is characterized by its wave-like nature on astronomical scales and has a potential to address small-scale structure problems in local galaxies. As linearly polarized pulsar light travels through the ALDM galactic halo, its position angle can oscillate due to cosmic birefringence induced by the ALDM Chern-Simons coupling to electromagnetic field. Pulsar...
Anisotropies play a central role in distinguishing between cosmological and astrophysical sources of the GWB, as detectable anisotropies are expected for a GWB from a population of supermassive black hole binaries (SMBHBs) but not for cosmological sources. A search for anisotropies in the NANOGrav 15-year dataset resulted in a null detection. We show that this null detection is not yet in...
After the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) reported evidence of an isotropic background of gravitational waves (GWB) at nanohertz in its 15-year data set, the next important step is to understand where this signal might come from. One way to do this is to look for the presence of anisotropies in it: this would help to distinguish an astrophysical origin,...
Recent PTA analyses show strong evidence for a SGWB with the characteristic Hellings-Downs inter-pulsar correlations. The observed signal may stem from supermassive black hole binary mergers or early universe phenomena. These two scenarios can differ significantly in the expected level of anisotropy, which could be used to discriminate between astrophysical and cosmological origin of the...
