M. C. David Marsh

Excess Astrophysical Photons from a 0.1–1 keV Cosmic Axion Background

Primordial decays of string theory moduli at z ∼ 10 naturally generate a dark radiation Cosmic Axion Background (CAB) with 0.1 1 keV energies. This CAB can be detected through axionphoton conversion in astrophysical magnetic fields to give quasi-thermal excesses in the extreme ultraviolet and soft X-ray bands. Substantial and observable luminosities may be generated even for axion-photon couplings ≪ 10GeV. 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.

Charting an Inflationary Landscape with Random Matrix Theory

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.

Soft X-ray excess in the Coma cluster from a Cosmic Axion Background

We show that the soft X-ray excess in the Coma cluster can be explained by a cosmic background of relativistic axion-like particles (ALPs) 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 ga???~?2???10-13?Ge -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 (3?kpc) scales over those with most power on (12?kpc) scales. Within this scenario, we bound the mean energy of the axion background to 50?eV??Ea???250?eV, the axion mass to ma??10-12?eV, and derive a lower bound on the axion-photon coupling ga?????(0.5/??Neff)?1.4???10-13?Ge -1.

The cosmophenomenology of axionic dark radiation

A bstractRelativistic 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 ECoM ≫ Tγ, 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 over-production of 4He 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 Tγ ≪ Tfreezeout. Most axions — or any other dark radiation candidate from modulus decay — still linger today as a Cosmic Axion Background with Eaxion ~

Decoupling and de Sitter Vacua in Approximate No-Scale Supergravities

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Observational consistency and future predictions for a 3.5 keV ALP to photon line

(100) eV, and a flux of ~ 106 cm−2 s−1.

Simple Emergent Power Spectra from Complex Inflationary Physics

A bstractWe study N

Galaxy cluster thermal x-ray spectra constrain axionlike particles

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