We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO). Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as g aγ ∼ 10 −12 GeV −1 , or to electrons g ae ∼10 −13 , IAXO has the potential to find the QCD axion in the 1 meV∼1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

We construct a class of random potentials for N 1 scalar fields using non-equilibrium random matrix theory, and then characterize multifield inflation in this setting. By stipulating that the Hessian matrices in adjacent coordinate patches are related by Dyson Brownian motion, we define the potential in the vicinity of a trajectory. This method remains computationally efficient at large N , permitting us to study much larger systems than has been possible with other constructions. We illustrate the utility of our approach with a numerical study of inflation in systems with up to 100 coupled scalar fields. A significant finding is that eigenvalue repulsion sharply reduces the duration of inflation near a critical point of the potential: even if the curvature of the potential is fine-tuned to be small at the critical point, small cross-couplings in the Hessian cause the curvature to grow in the neighborhood of the critical point.

We show that the soft X-ray excess in the Coma cluster can be explained by a cosmic background of relativistic axions converting into photons in the cluster magnetic field. We provide a detailed self-contained review of the cluster soft X-ray excess, the proposed astrophysical explanations and the problems they face, and explain how a 0.1 − 1 keV axion background naturally arises at reheating in many string theory models of the early universe. We study the morphology of the soft excess by numerically propagating axions through stochastic, multi-scale magnetic field models that are consistent with observations of Faraday rotation measures from Coma. By comparing to ROSAT observations of the 0.2 − 0.4 keV soft excess, we find that the overall excess luminosity is easily reproduced for g aγγ ∼ 2 × 10 −13 GeV −1 . The resulting morphology is highly sensitive to the magnetic field power spectrum. For Gaussian magnetic field models, the observed soft excess morphology prefers magnetic field spectra with most power in coherence lengths on O(3 kpc) scales over those with most power on O(12 kpc) scales. Within this scenario, we bound the mean energy of the axion background to 50 eV E a 250 eV, the axion mass to m a 10 −12 eV, and derive a lower bound on the axion-photon coupling g aγγ 0.5/∆N eff 1.4 × 10 −13 GeV −1 .In the enlightening case of sufficiently high axion energies or small ambient electron densities, the conversion probability for a fixed domain is given bywhere B ⊥ denotes the magnetic field component transverse to the axion velocity and L denotes the corresponding coherence length [11]. This conversion allows the potential detection of a CAB through axion-photon conversion.Galaxy clusters support magnetic fields that are modest in magnitude (B ≈ µG) but are extended over megaparsec distances and have kiloparsec coherence scales, allowing observationally significant axion-photon conversion probabilities. In [1], a crude single-domain model with a fixed magnitude and coherence length for the magnetic field was used to estimate the axion-photon coupling M that would be required to reproduce the soft excess in Coma from a CAB, finding M ≈ 10 13 GeV.In this paper we continue the study of axion-photon conversion in the Coma cluster using a far more detailed model of the Coma magnetic field. This model was constructed in [12] to fit rotation measure (RM) observations of seven polarised light sources, using the Coma cluster as a Faraday screen. The model describes the central Mpc 3 of Coma (see also [13] for a magnetic field model describing the region 1.5 Mpc to the southwest of the cluster centre). We review the observational evidence for cluster magnetic fields in section 4 and describe the model for the Coma magnetic field in detail in section 4.2. Using this stochastic model, we construct a numerical simulation of the magnetic field in the central region of the cluster, propagate axions through it and quantitatively study the resulting predictions for the soft excess morphology. This paper is organised as follo...

Axions and axion-like particles (ALPs) are a well motivated extension of the Standard Model and are generic in String Theory. The X-ray transparency of the magnetized intracluster medium (ICM) in galaxy clusters is a powerful probe of very light ALPs (masses 0 < m a < 10 −11 eV); as X-ray photons propagate through the magnetic field of the ICM, they may undergo energy-dependent quantum mechanical conversion into ALPs (and vice versa), imprinting distortions on the observed X-ray spectrum. We present new Chandra data for the active galactic nucleus NGC 1275 at the center of the Perseus cluster. Employing the High-Energy Transmission Gratings (HETG) with a 490 ks exposure, we obtain a high-quality 1-9 keV spectrum free from photon pileup and ICM contamination. Apart from iron-band features, the spectrum is accurately described by a power-law continuum, with any spectral distortions at the < 3% level. We compute photon survival probabilities as a function of ALP mass m a and ALP-photon coupling constant g aγ for an ensemble of ICM magnetic field models, and then use the NGC 1275 spectrum to derive constraints on the (m a , g aγ )-plane. Marginalizing over the magnetic field realizations, the 99.7% credible region limits the ALP-photon coupling to g aγ < 6 − 8 × 10 −13 GeV −1 (depending upon the magnetic field model) for masses m a < 1 × 10 −12 eV. These are the most stringent limit to date on g aγ for these very light ALPs, and have already reached the sensitivity limits of next-generation helioscopes and light-shining-through-wall experiments. We highlight the potential of these studies with the next-generation X-ray observatories Athena and Lynx, but note the critical importance of advances in relative calibration of these future X-ray spectrometers.

Axion-like particles (ALPs) and photons can quantum mechanically interconvert when propagating through magnetic fields, and ALP-photon conversion may induce oscillatory features in the spectra of astrophysical sources. We use deep (370 ks), short frame time Chandra observations of the bright nucleus at the centre of the radio galaxy M87 in the Virgo cluster to search for signatures of light ALPs. The absence of substantial irregularities in the X-ray power-law spectrum leads to a new upper limit on the photon-ALP coupling, g aγ : using a very conservative model of the cluster magnetic field consistent with Faraday rotation measurements from M87 and M84, we find g aγ < 2.6 × 10 −12 GeV −1 at 95% confidence level for ALP masses m a ≤ 10 −13 eV. Other consistent magnetic field models lead to stronger limits of g aγ 1.1-1.5 × 10 −12 GeV −1 . These bounds are all stronger than the limit inferred from the absence of a gamma-ray burst from SN1987A, and rule out a substantial fraction of the parameter space accessible to future experiments such as ALPS-II and IAXO.

Relativistic axions are good candidates for the dark radiation for which there are mounting observational hints. The primordial decays of heavy fields produce axions which are ultra-energetic compared to thermalised matter and inelastic axion-matter scattering can occur with E CoM ≫ T γ , thus accessing many interesting processes which are otherwise kinematically forbidden in standard cosmology. Axion-photon scattering into quarks and leptons during BBN affects the light element abundances, and bounds on overproduction of 4 He constrain a combination of the axion decay constant and the reheating temperature. For supersymmetric models, axion scattering into visible sector superpartners can give direct non-thermal production of dark matter at T γ ≪ T f reezeout . Most axions -or any other dark radiation candidate from modulus decay -still linger today as a Cosmic Axion Background with E axion ∼ O(100) eV, and a flux of ∼ 10 6 cm −2 s −1 .

Galaxy clusters can efficiently convert axion-like particles (ALPs) to photons. We propose that the recently claimed detection of a 3.55-3.57 keV line in the stacked spectra of a large number of galaxy clusters and the Andromeda galaxy may originate from the decay of either a scalar or fermionic 7.1 keV dark matter species into an axion-like particle (ALP) of mass ma 6 · 10 −11 eV, which subsequently converts to a photon in the cluster magnetic field. In contrast to models in which the photon line arises directly from dark matter decay or annihilation, this can explain the anomalous line strength in the Perseus cluster. As axion-photon conversion scales as B 2 and cool core clusters have high central magnetic fields, this model can also explain the observed peaking of the line emission in the cool cores of the Perseus, Ophiuchus and Centaurus clusters, as opposed to the much larger dark matter halos. We describe distinctive predictions of this scenario for future observations.

Primordial decays of string theory moduli at z~10(12) naturally generate a dark radiation cosmic axion background with 0.1-1 keV energies. This cosmic axion background can be detected through axion-photon conversion in astrophysical magnetic fields to give quasithermal excesses in the extreme ultraviolet and soft x-ray bands. Substantial and observable luminosities may be generated even for axion-photon couplings <<10(-11) GeV(-1). We propose that axion-photon conversion may explain the observed excess emission of soft x rays from galaxy clusters, and may also contribute to the diffuse unresolved cosmic x-ray background. We list a number of correlated predictions of the scenario.