Axionic Dark Radiation and the Milky Way's Magnetic Field

Recently it has been suggested that dark radiation in the form of axions produced during the decay of string theory moduli fields could be responsible for the soft x-ray excess in galaxy clusters. These soft X-ray photons come about due to the conversion of these axions into photons in the magnetic fields of the clusters. In this work we calculate the conversion of axionic dark radiation into X-ray photons in the magnetic field of our own Galaxy. We consider Delta N ~ 0.5 worth of dark radiation made up of axions with energy of order 0.1-1 keV. We show that it is possible, if a little optimistic, to explain the large regions of X-ray emission located above and below the centre of the Galactic plane detected in the 3/4 keV ROSAT all sky map completely due to the conversion of dark radiation into photons with an inverse axion-photon coupling of M ~ 3e13 GeV and an axion mass of m < 1e-12 eV. Different parameter values could explain both these features and the 3/4 keV X-ray background. More conservatively, these X-ray observations are a good way to constrain such models of axionic dark radiation.Comment: 15 pages, 5 figure

Cosmic acceleration in models with density dependent moduli

The effective equation of state of normal matter is changed in theories where the size of the compact space depends upon the local energy density. In particular we show how the dilution of a fluid due to the expansion of the universe can be compensated by an increase of the effective coupling of that fluid to gravity in the presence of a potential which acts to reduce the size of the compact space. We estimate how much cosmic acceleration can be obtained in such a model and comment on the difficulties faced in finding an appropriate potential.Comment: 16 pages, 2 figures. Revised to match published versio

Constraining the Equation of State of Dark Energy with Gamma Rays

Starlight in the Universe impedes the passage of high energy (e.g. TeV) gamma rays due to positron-electron pair production. The history of this stellar radiation field depends upon observations of star formation rate which themselves can only be interpreted in the context of a particular cosmology. For different equations of state of dark energy, the star formation rate data suggests a different density of stellar photons at a particular redshift and a different probability of arrival of gamma rays from distant sources. In this work we aim to show that this effect can be used to constrain the equation of state of dark energy. The current work is a proof of concept and we outline the steps that would have to be taken to place the method in a rigorous statistical framework which could then be combined with other more mature methods such as fitting supernova luminosity distances.Comment: 6 pages, 4 figure

Complementarity of Dark Matter Searches at Resonance

We consider models of dark matter where the couplings between the standard model and the dark sector fall at resonance due to kinematics and direct detection experiments become insensitive. To be specific, we consider a simple model of 100 GeV - TeV scale dark matter coupled to the standard model via a vector boson. We explore whether it will be possible to exclude such regions of the parameter space using future observations of dijet rates at the LHC and CTA and AMS observations of the Galactic Centre.Comment: 7 pages, 9 figure

Singlet Fermionic Dark Matter and the Electroweak Phase Transition

We consider a model with a gauge singlet Dirac fermion as a cold dark matter candidate. The dark matter particle communicates with the Standard Model via a gauge singlet scalar mediator that couples to the Higgs. The scalar mediator also serves to create a tree-level barrier in the scalar potential which leads to a strongly first order electroweak phase transition as required for Electroweak Baryogenesis. We find a large number of models that can account for all the dark matter and provide a strong phase transition while avoiding constraints from dark matter direct detection, electroweak precision data, and the latest Higgs data from the LHC. The next generation of direct detection experiments could rule out a large region of the parameter space but can be evaded in some regions when the Higgs-singlet mixing is very small.Comment: 9 pages, 6 figures. v2: figure captions updated, colour confusion fixed, no other changes. v3: references added, some clarification, now matches version published in JHE

Capture of Inelastic Dark Matter in White Dwarves

We consider the capture of inelastic dark matter in white dwarves by inelastic spin-independent scattering on nuclei. We show that if the dark matter annihilates to standard-model particles then, under the assumption of primordial globular cluster formation, the observation of cold white dwarves in the globular cluster M4 appears inconsistent with explanations of the observed DAMA/LIBRA annual modulation signal based on spin-independent inelastic dark matter scattering. Alternatively if the inelastic dark matter scenario were to be confirmed and it was found to annihilate to standard-model particles then this would imply a much lower dark matter density in the core of M4 than would be expected if it were to have formed in a dark matter halo. Finally we argue that cold white dwarves constitute a unique dark matter probe, complementary to other direct and indirect detection searches.Comment: 8 pages, 5 figures. Typos corrected. References and discussion added. Figures updated. Main conclusions unchanged. v3 Journal versio