Inelastic dark matter (IDM) models feature an energy threshold for scattering with Standard Model particles, which enables their consistency with the increasingly stringent limits placed by direct detection experiments. In a typical construction, elastic scattering is absent at tree level, and a lighter dark matter state must first upscatter into a heavier state in order to interact with the nuclei in the detector. We model the excitation of IDM in the Earth followed by its downscattering inside a detector, and we show that considering this process markedly enhances the sensitivity of existing detectors. In particular, current limits based on XENON100 and XENON1T data can be extended to significantly larger mass splittings.

Models with axion like particles (ALPs) often predict the formation of a string-domain wall network in the early universe. We study how such networks of defects appear in the context of string theory, and discuss the conditions for their long-term stability. In a scenario with several axions, we show how a bias term in the potential arises naturally from the effects of multiple instantons, leading to the eventual decay of the domain walls. We find that the annihilation of the network leads to the generation of a stochastic gravitational wave background (SGWB) with a spectrum that has characteristic contributions from both walls and strings. The unique shape of the spectrum provides an opportunity to probe string theory axions at existing and upcoming observatories. The extinction of the network is also accompanied by the production of different axion mass eigenstates. In a region of the parameter space, the lightest eigenstate can be long lived and make up the dark matter in the universe.

Domain wall networks have attracted renewed interest, particularly in relation to the dynamics of network collapse. Accurately describing this process is challenging and typically requires large scale numerical simulations. Here we adopt a complementary approach by studying the collapse of individual closed domain walls, extending previous thin wall analyses and comparing them with adaptive mesh refinement field theory simulations. Firstly, we show that collapsing domain walls generically develop worldvolume singularities of two types: cuspidal edge singularities, consisting of one dimensional singular edges that propagate along the wall surface at the speed of light for a finite time, and cuspidal vertex singularities, which are spike like and instantaneous events where the wall moves momentarily at the speed of light. Both types of features arise generically from smooth initial conditions, and their formation and evolution follow the universal patterns of singularity theory. We show that these structures are captured both by the Nambu-Goto equations and by an eikonal like approximation valid in the relativistic regime. Furthermore, we demonstrate that the same singular structures are reproduced qualitatively in full field theory simulations, establishing that they are not artifacts of the thin wall approximation but robust features of realistic domain wall dynamics. Naturally, such focusing effects in the field theory simulations result in localized regions of high energy density. We briefly discuss possible phenomenological implications.

We study the full-sky distribution of the radio emission from the stimulated decay of axions which are assumed to compose the dark matter in the Galaxy. Besides the constant extragalactic and CMB components, the decays are stimulated by a Galactic radio emission with a spatial distribution that we empirically determine from observations. We compare the diffuse emission to the counterimages of the brightest supernovae remnants, and take into account the effects of free-free absorption. We show that, if the dark matter halo is described by a cuspy NFW profile, the expected signal from the Galactic center is the strongest. Interestingly, the emission from the Galactic anti-center provides competitive constraints that do not depend on assumptions on the uncertain dark matter density in the inner region. Furthermore, the anti-center of the Galaxy is the brightest spot if the Galactic dark matter density follows a cored profile. The expected signal from stimulated decays of axions of mass $m _{a} \sim 10 ^{-6}$ eV is within reach of the Square Kilometer Array for an axion-photon coupling $g _{a\gamma} \gtrsim (2-3) \times 10 ^{-11}$ GeV$^{-1}$.

The hot interiors of massive stars in the later stages of their evolution provide an ideal place for the production of heavy axion-like particles (ALPs) with mass up to O(100 keV) range. We show that a fraction of these ALPs could stream out of the stellar photosphere and subsequently decay into two photons that can be potentially detected on or near the Earth. In particular, we estimate the photon flux originating from the spontaneous decay of heavy ALPs produced inside Horizontal Branch and Wolf-Rayet stars, and assess its detectability by current and future X-ray and gamma-ray telescopes. Our results indicate that current and future telescopes can probe axion-photon couplings down to g_aγ∼ 4× 10^-11 GeV^-1 for m_a∼ 10-100 keV, which covers new ground in the ALP parameter space.

A network of cosmic strings (CS), if present, would continue emitting gravitational waves (GW) as it evolves throughout the history of the Universe. This results in a characteristic broad spectrum making it a perfect source to infer the expansion history. In particular, a short inflationary period caused by a supercooled phase transition would cause a drop in the spectrum at frequencies corresponding to that event. However, the impact on the spectrum is similar to the ones caused by an early matter-dominated era or from particle production, making it difficult to disentangle these different physical origins. We point out that, in the case of a short inflationary period, the GW spectrum receives an additional contribution from the phase transition itself. This leads to a characteristic imprint of a peak on top of a wide plateau both visible at future GW observatories.

The flux of high energy neutrinos and photons produced in a blazar could get attenuated when they propagate through the dark matter spike around the central black hole and the halo of the host galaxy. Using the observation by IceCube of a few high-energy neutrino events from TXS 0506+056, and their coincident gamma ray events, we obtain new constraints on the dark matter-neutrino and dark matter-photon scattering cross sections. Our constraints are orders of magnitude more stringent than those derived from considering the attenuation through the intergalactic medium and the Milky Way dark matter halo. When the cross-section increases with energy, our constraints are also stronger than those derived from the CMB and large-scale structure.

We study the radio signals generated when an axion star enters into the magnetosphere of a neutron star. As the axion star moves through the resonant region where the plasma-induced photon mass becomes equal to the axion mass, the axions can efficiently convert into photons, giving rise to an intense, transient radio signal. The energy released is determined by the axion star mass and conversion probability. Similarly, the peak frequency of the emitted radio signal is fixed by the axion mass, while cosmological redshift and Doppler shift could give rise to a wide range of frequencies. In particular, we show that a dense axion star with a mass $\sim 10^{-13}M_{\odot}$ composed of $\sim 10\: \mu$eV axions can account for most of the mysterious fast radio bursts in a wide frequency range.

We propose a mechanism to generate primordial black holes (PBHs) that is independent of cosmological inflation and occurs slightly below the QCD phase transition. Our setup relies on the collapse of long-lived string-domain wall networks and is naturally realized in QCD axion models with domain wall number N_{DW}>1 and Peccei-Quinn symmetry broken after inflation. In our framework, dark matter is mostly composed of axions in the meV mass range along with a small fraction, Ω_{PBH}≳10^{-6}Ω_{CDM} of heavy M∼10^{4}-10^{7}  M_{⊙} PBHs. The latter could play a role in alleviating some of the shortcomings of the standard cosmological model on subgalactic scales. The scenario might have distinct signatures in ongoing axion searches as well as gravitational wave observatories.

We study the generation of magnetic field seeds during a first-order electroweak phase transition, by numerically evolving the classical equations of motion of the bosonic electroweak theory on the lattice. The onset of the transition is implemented by the random nucleation of bubbles with an arbitrarily oriented Higgs field in the broken phase. We find that about 10% of the latent heat is converted into magnetic energy, with most of the magnetic fields being generated in the last stage of the phase transition when the Higgs oscillates around the true vacuum. The energy spectrum of the magnetic field has a peak that shifts towards larger length scales as the phase transition unfolds. By the end of our runs the peak wavelength is of the order of the bubble percolation scale, or about a third of our lattice size.

We consider a gauge-singlet complex scalar field Phi with a global U(1) symmetry that is spontaneously broken at some high energy scale f(a). As a result, the angular part of the Phi-field becomes an axion-like particle (ALP). We show that if the Phi-field has a non-zero coupling kappa to the Standard Model Higgs boson, there exists a certain region in the parameter space where the global U(1) symmetry-breaking induces a strongly first order phase transition, thereby producing stochastic gravitational waves that are potentially observable in current and future gravitational-wave detectors. In particular, we find that future gravitational-wave experiments such as TianQin, BBO and Cosmic Explorer could probe a broad range of the energy scale 10(3) GeV less than or similar to f(a) less than or similar to 10(8) GeV, independent of the ALP mass. Since all the ALP couplings to the Standard Model particles are proportional to inverse powers of the energy scale f(a) (up to model-dependent O (1) coefficients), the gravitational-wave detection prospects are largely complementary to the current laboratory, astrophysical and cosmological probes of the ALP scenarios.

TeV emission from blazars can be used to probe the intergalactic magnetic fields and measure their intensity, coherence length, and helicity. Intergalactic magnetic fields deflect the electron-positron pairs produced by very-high-energy gamma-rays from blazars, resulting in broadened beams of cascade gamma-rays developing along the projected direction of the blazar jet. We present an analysis that uses for the first time the jet orientation of 12 high-synchrotron-peaked (HSP) BL~Lac-type blazars from VLBA radio observations to search for signatures of pair haloes in the Fermi-LAT data. Our search improves the sensitivity of previous studies by taking the asymmetry of the pair haloes into account, increasing the signal to noise and reducing the possibility of systematics in determining the point spread function. Although there is no significant detection, a hint for an offset halo with a global significance of 2$\sigma$ is found in the $30-300$ GeV energy range, corresponding to an intergalactic magnetic field with $\sim 10^{-15}$ Gauss, consistent with the inferred field from a prior study using stacked HSP BL Lac objects.

This white paper summarizes the workshop "U.S. Cosmic Visions: New Ideas in Dark Matter" held at University of Maryland on March 23-25, 2017.

We investigate if the topology of pure gauge fields in the electroweak vacuum can play a role in classical dynamics at the electroweak phase transition. Our numerical analysis shows that magnetic fields are produced if the initial vacuum has nontrivial Chern-Simons number, and the fields are helical if the Chern-Simons number changes during the phase transition.

The growth of a massive black hole will steepen the cold dark matter density at the center of a galaxy into a dense spike, enhancing the prospects for indirect detection. We study the impact of black hole spin on the density profile using the exact Kerr geometry of the black whole in a fully relativistic adiabatic growth framework. We find that, despite the transfer of angular momentum from the hole to the halo, rotation increases significantly the dark matter density close to the black hole. The gravitational effects are still dominated by the black hole within its influence radius, but the larger dark matter annihilation fluxes might be relevant for indirect detection estimates.

We present a novel method that allows to derive an upper limit on the scattering cross section of dark matter with nucleons which is independent of the velocity distribution. To this end, we combine null results from direct detection experiments and neutrino telescopes, and use the fact that taken together, these classes of experiments probe the whole range of possible dark matter velocities. The resulting halo-independent upper limits on the dark matter scattering cross section are remarkably strong, and can be used to robustly rule out models of dark matter, without the need to invoke specific assumptions about the local velocity distribution.

We report on the results of a search for γ-ray pair halos with a stacking analysis of low redshift blazars using data from the Fermi Large Area Telescope. For this analysis we used a number of a priori selection criteria, including the spatial and spectral properties of the Fermi sources. The angular distribution of ~1 GeV photons around 24 stacked isolated high-synchrotron-peaked BL Lacs with redshift z<0.5 shows an excess over that of pointlike sources. A frequentist test yields a p value of p~0.01 for the extended emission against the point-source hypothesis. A Bayesian estimation provides Bayes factors log_{10}B_{10}>2, consistent with expectations for pair halos produced in the intergalactic magnetic fields with strength B_{IGMF}~10^{-17}-10^{-15} G.