Search

This note expands on the details of the relationship between 4d kination solutions in string cosmology and Kasner solutions of 10d general relativity. It extends previous analyses of this relationship in IIB string theory to other string theories and also to 11d M-theory, while also providing extensive detail on the relationship between perturbations of the 10d Kasner metric and the presence of radiation and matter backgrounds in the dimensionally reduced 4d kination theory.

We describe a new mechanism, whose ingredients are realised in string compactifications, for the formation of cosmic (super)string networks. Oscillating string loops grow when their tension $\mu$ decreases with time. If $2H + \dot{\mu}/\mu < 0$, where $H$ is the Hubble parameter, loops grow faster than the scale factor and an initial population of isolated small loops (for example, produced by nucleation) can grow, percolate and form a network. This condition is satisfied for fundamental strings in the background of a kinating volume modulus rolling towards the asymptotic large volume region of moduli space. Such long kination epochs are motivated in string cosmology by both the electroweak hierarchy problem and the need to solve the overshoot problem. The tension of such a network today is set by the final vacuum; for phenomenologically appealing Large Volume Scenario (LVS) vacua, this would lead to a fundamental string network with $G \mu \sim 10^{-10}$., Comment: 6 pages

This is a short account, based on a talk given at the 2024 Moriond Cosmology Conference, of where and why string theory matters in early universe cosmology. It is written for a cosmology audience predisposed to be at best sceptical, and at worst contemptuous, of the notion that either quantum gravity or string theory has any relevance for their discipline. I cover inflation, CMB tensor modes, extended kination or tracker epochs and reheating., Comment: 8 pages, based on talk at 2024 Moriond Cosmology Conference

New light hidden sector degrees of freedom represent one of the most approaches to going beyond the Standard Model. I give a short account of how such WISP candidates naturally appear in string compactifications and some descriptions of ways that they can affect early universe cosmology., Comment: 6 pages, contribution to the Proceedings of the 1st General Meeting of the COSMIC WISPers COST Action CA21106

We perform a detailed study of stringy moduli-driven cosmologies between the end of inflation and the commencement of the Hot Big Bang, including both the background and cosmological perturbations: a period that can cover half the lifetime of the universe on a logarithmic scale. Compared to the standard cosmology, stringy cosmologies motivate extended kination, tracker and moduli-dominated epochs involving significantly trans-Planckian field excursions. Conventional effective field theory is unable to control Planck-suppressed operators and so such epochs require a stringy completion for a consistent analysis. Perturbation growth in these stringy cosmologies is substantially enhanced compared to conventional cosmological histories. The transPlanckian field evolution results in radical changes to Standard Model couplings during this history and we outline potential applications to baryogenesis, dark matter and gravitational wave production., Comment: v1:61 pages, 2 figures, comments welcome. v2: References added, typos corrected

We review applications of string theory to cosmology, from primordial times to the present-day accelerated expansion. Starting with a brief overview of cosmology and string compactifications, we discuss in detail moduli stabilisation, inflation in string theory, the impact of string theory on post-inflationary dynamics (reheating, moduli domination, kination), dark energy (the cosmological constant from a string landscape and models of quintessence) and various alternative scenarios (string/brane gases, the pre big-bang scenario, rolling tachyons, ekpyrotic/cyclic cosmologies, bubbles of nothing, S-brane and holographic cosmologies). The state of the art in string constructions is described in each topic and, where relevant, connections to swampland conjectures are made. The possibilities for novel particles and excitations (axions, moduli, cosmic strings, branes, solitons, oscillons and boson stars) are emphasised. Implications for the physics of the CMB, gravitational waves, dark matter and dark radiation are discussed along with potential observational signatures., Comment: Invited Review for Physics Reports, 190 pages. V2: References added, typos corrected, minor additions

We study runaway, kination-dominated epochs in string cosmology. We show how the apparent classical decompactification runaway of the volume modulus, described by a kination epoch in the 4-dimensional EFT, can be uplifted to a classical Kasner solution in 10d in which the non-compact dimensions collapse towards a Big Crunch. This can also be generalised for arbitrary spacetime and compactification dimensions. We conclude with some comments on how this picture is modified by quantum effects, and the need for both dynamical and kinematical Swampland constraints., Comment: 21 pages; v3. correction of section 3

We study the overshoot problem in the context of post-inflationary string cosmology (in particular LVS). LVS cosmology features a long kination epoch as the volume modulus rolls down the exponential slope towards the final minimum, with an energy density that scales as $m_s^4$. This roll admits attractor tracker solutions, and if these are located the overshoot problem is solved. We show that, provided a sufficiently large hierarchy exists between the inflationary scale and the weak scale, this will always occur in LVS as initial seed radiation grows into the tracker solution. The consistency requirement of ending in a stable vacuum containing the weak hierarchy therefore gives a preference for high inflationary scales -- an anthropic argument, if one likes, for a large inflation/weak hierarchy. We discuss various origins, both universal and model-dependent, of the initial seed radiation (or matter). One particularly interesting case is that of a fundamental string network arising from brane inflation -- this may lead to an early epoch in which the universe energy density principally consists of gravitational waves, while an LVS fundamental string network survives into the present universe., Comment: Added references, corrected typos and improved discussion on the validity of the EFT approach. Matches published version

This paper is concerned with large time behavior of the solution to a diffusive perturbation of the linear LSW model introduced by Carr and Penrose. Like the LSW model, the Carr-Penrose model has a family of rapidly decreasing self-similar solutions, depending on a parameter $\beta$ with $0<\beta\le 1$. It is shown that if the initial data has compact support then the solution to the diffusive model at large time approximates the $\beta=1$ self-similar solution. This result supports the intuition that diffusion provides the mechanism whereby the $\beta=1$ self-similar solution of the LSW model is the only physically relevant one., Comment: 44 pages

We give a concise argument that supersymmetric AdS type IIA DGKT flux vacua on general Calabi-Yaus have, interpreted holographically, integer conformal dimensions for low-lying scalar primaries in the dual CFT. These integers are independent of any compactification details, such as the background fluxes or triple intersection numbers of the compact manifold. For the K\"ahler moduli and dilaton, there is one operator with $\Delta = 10$ and $h^{1,1}_{-}$ operators with $\Delta =6$, whereas the corresponding axions have $\Delta = 11$ and $\Delta = 5$. For the complex structure moduli, the $h^{2,1}$ saxions have $\Delta =2$, and the axions $\Delta =3$. We give a tentative discussion of the origin of these integers and effects that would modify these results., Comment: 5 pages + references

We extend studies of holographic aspects of moduli stabilisation scenarios to both fibred versions of LVS and the type IIA DGKT flux vacua. We study the holographic properties of the low-energy moduli Lagrangian that describes both the AdS vacuum and also small perturbations about it. For type IIA vacua in the large-volume regime, the CFT data (operator dimensions and higher-point interactions) take a universal form independent of the many arbitrary flux choices, as was previously found for LVS stabilisation. For these IIA vacua the conformal dimensions of the dual operators are also, surprisingly, all integers, although we do not understand a deeper reason why this is so. In contrast to behaviour previously found for LVS and KKLT, the fibred models also admit cases of mixed double-trace operators (for two different axion fields) where the anomalous dimension is positive., Comment: 32 pages + references

This paper is concerned with the study of Green's functions for one dimensional diffusions with constant diffusion coefficient and linear time inhomogeneous drift. It is well know that the whole line Green's function is given by a Gaussian. Formulas for the Dirichlet Green's function on the half line are only known in special cases. The main object of study in the paper is the ratio of the Dirichlet to whole line Green's functions. Bounds, asymptotic behavior in the limit as the diffusion coefficient vanishes, and a log concavity result are obtained for this ratio., Comment: 60 pages

We investigate whether Swampland constraints on the low-energy dynamics of weakly coupled, moduli-stabilised string vacua in AdS can be related to inconsistencies of their putative holographic duals or, more generally, recast in terms of CFT data. We find that various swampland consistency constraints are equivalent to a negativity condition on the sign of certain mixed anomalous dimensions. This condition is similar to well-established CFT positivity bounds arising from causality and unitarity, but not known to hold in general. The studied scenarios include LVS, KKLT, and both perturbative and racetrack stabilisation. Interestingly, the LVS vacuum (with $\Delta_{\varphi} = 8.038$) also appears to live very close to a critical value ($\Delta_{\varphi} = 8$) where the anomalous dimensions change sign. We finally point out an intriguing connection to the Swampland Distance Conjecture, both in its original and refined versions., Comment: 37 pages + references, 4 figures; v2 added references and corrected typos

This short note emphasises a potential tension between string models of inflation based on systems of branes and antibranes and the spectrum of strings in curved space, in particular the requirement that the leading Regge trajectory extends to the Planck scale allowing for the conventional string theory UV completion of gravity., Comment: 2 pages

The swampland program of delineating the space of effective field theories consistent with quantum gravity appears similar to the bootstrap program of delineating the space of quantum field theories consistent with conformal symmetry. With this in mind we rewrite the effective field theory of the Large Volume Scenario in AdS space solely in terms of $R_{AdS}$, in a form suitable for holographic analysis. This rewritten EFT takes a remarkably universal (and previously unnoticed) form, which is uniquely determined in the large-volume limit up to terms suppressed by $\mathcal{O} \left( 1/\ln R_{AdS} \right)$, with no reference to any of the fluxes, brane or instanton configurations that enter the microphysics of moduli stabilisation. The putative dual 3d CFT will have two low-lying single trace scalars, an even-parity scalar $\Phi$ dual to the volume modulus with $\Delta_{\Phi} = \frac{3}{2}\left( 1 + \sqrt{19} \right) \simeq 8.038$ and an odd-parity scalar $a$ dual to the volume axion with $\Delta_a = 3$. On the AdS side the higher-point interactions are likewise uniquely determined. As the AdS theory is both subject to swampland constraints and holographically related to a CFT, we argue that holography will lead to a `bootland' --- a map between swampland constraints on the AdS side and bootstrap constraints on the CFT side. We motivate this with a discussion of swampland quantum gravity constraints on the axion decay constant in the $\mathcal{V} \to \infty$ limit and the $\langle \Phi \Phi a a \rangle$ 4-point function on the CFT side., Comment: 30 pages; v2. published version

The main result of the paper is a global asymptotic stability result for solutions to the Lifschitz-Slyozov-Wagner (LSW) system of equations. This extends some local asymptotic stability results of Niethammer-Vel\'{a}zquez (2006). The method of proof is along similar lines to the one used in a previous paper of the authors. This previous paper proves global asymptotic stability for a class of infinite dimensional dynamical systems for which no Lyapounov function is (apparently) available., Comment: 60 pages

X-ray observations of bright AGNs in or behind galaxy clusters offer unique capabilities to constrain axion-like particles (ALPs). Existing analysis technique rely on measurements of the global goodness-of-fit. We develop a new analysis methodology that improves the statistical sensitivity to ALP-photon oscillations by isolating the characteristic quasi-sinusoidal modulations induced by ALPs. This involves analysing residuals in wavelength space allowing the Fourier structure to be made manifest as well as a machine learning approach. For telescopes with microcalorimeter resolution, simulations suggest these methods give an additional factor of two in sensitivity to ALPs compared to previous approaches., Comment: 8 pages, 4 figures

It has recently been conjectured that string theory does not admit de Sitter critical points. This note points out that in several cases, including KKLT or racetrack models, this statement is equivalent to the absence of supersymmetric Minkowski or AdS solutions. This equivalence arises from establishing the positivity of the potential in a large-radius limit, requiring a turnover of the potential before reaching an AdS vacuum. For example, this conjecture is incompatible with the simplest 1-modulus KKLT AdS supersymmetric solution., Comment: Prepared for submission to Int. Journ. Mod. Phys. A; v2. added references and more discussion

We investigate possible evidence for a spectral dip around 3.5 keV in central cluster AGNs, motivated by previous results for archival Chandra observations of the Perseus cluster and the general interest in novel spectral features around 3.5 keV that may arise from dark matter physics. We use two deep Chandra observations of the Perseus and Virgo clusters that have recently been made public. In both cases, mild improvements in the fit ($\Delta \chi^2 = 4.2$ and $\Delta \chi^2 = 2.5$) are found by including such a dip at 3.5 keV into the spectrum. A comparable result ($\Delta \chi^2 = 6.5$) is found re-analysing archival on-axis Chandra ACIS-S observations of the centre of the Perseus cluster., Comment: 5 pages

If axion-like particles (ALPs) exist, photons can convert to ALPs on passage through regions containing magnetic fields. The magnetised intracluster medium of large galaxy clusters provides a region that is highly efficient at ALP-photon conversion. X-ray observations of Active Galactic Nuclei (AGNs) located within galaxy clusters can be used to search for and constrain ALPs, as photon-ALP conversion would lead to energy-dependent quasi-sinusoidal modulations in the X-ray spectrum of an AGN. We use Chandra observations of the central AGN of the Perseus Cluster, NGC1275, to place bounds on massive ALPs up to m_a ~ 10^-11 eV, extending previous work that used this dataset to constrain massless ALPs.

This paper concerns a Fokker-Planck equation on the positive real line modeling nucleation and growth of clusters. The main feature of the equation is the dependence of the driving vector field and boundary condition on a non-local order parameter related to the excess mass of the system. The first main result concerns the well-posedness and regularity of the Cauchy problem. The well-posedness is based on a fixed point argument, and the regularity on Schauder estimates. The first a priori estimates yield H\"older regularity of the non-local order parameter, which is improved by an iteration argument. The asymptotic behavior of solutions depends on some order parameter $\rho$ depending on the initial data. The system shows different behavior depending on a value $\rho_s>0$, determined from the potentials and diffusion coefficient. For $\rho \leq \rho_s$, there exists an equilibrium solution $c^{\text{eq}}_{(\rho)}$. If $\rho\le\rho_s$ the solution converges strongly to $c^{\text{eq}}_{(\rho)}$, while if $\rho > \rho_s$ the solution converges weakly to $c^{\text{eq}}_{(\rho_s)}$. The excess $\rho - \rho_s$ gets lost due to the formation of larger and larger clusters. In this regard, the model behaves similarly to the classical Becker-D\"oring equation. The system possesses a free energy, strictly decreasing along the evolution, which establishes the long time behavior. In the subcritical case $\rho<\rho_s$ the entropy method, based on suitable weighted logarithmic Sobolev inequalities and interpolation estimates, is used to obtain explicit convergence rates to the equilibrium solution. The close connection of the presented model and the Becker-D\"oring model is outlined by a family of discrete Fokker-Planck type equations interpolating between both of them. This family of models possesses a gradient flow structure, emphasizing their commonality., Comment: Minor revised version accepted for publication in Discrete & Continuous Dynamical Systems - A

This paper is concerned with establishing global asymptotic stability results for a class of non-linear PDE which have some similarity to the PDE of the Lifschitz-Slyozov-Wagner model. The method of proof does not involve a Lyapounov function. It is shown that stability for the PDE is equivalent to stability for a differential delay equation. Stability for the delay equation is proven by exploiting certain maximal properties. These are established by using the methods of optimal control theory., Comment: 49 pages

Galaxy clusters represent excellent laboratories to search for Axion-Like Particles (ALPs). They contain magnetic fields which can induce quasi-sinusoidal oscillations in the X-ray spectra of AGNs situated in or behind them. Due to its excellent energy resolution, the X-ray Integral Field Unit (X-IFU) instrument onboard the Athena X-ray Observatory will be far more sensitive to ALP-induced modulations than current detectors. As a first analysis of the sensitivity of Athena to the ALP-photon coupling $g_{a \gamma \gamma}$, we simulate observations of the Seyfert galaxy NGC1275 in the Perseus cluster using the SIXTE simulation software. We estimate that for a 200ks exposure, a non-observation of spectral modulations will constrain ${g_{a\gamma\gamma}\lesssim1.5\times10^{-13}\rm{GeV}^{-1}}$ for $m_a \lesssim 10^{-12} \rm{eV}$, representing an order of magnitude improvement over constraints derived using the current generation of satellites., Comment: 5 pages, 3 figures. Accepted to MNRAS

We extend previous searches for X-ray spectral modulations induced by ALP-photon conversion to a variety of new sources, all consisting of quasars or AGNs located in or behind galaxy clusters. We consider a total of seven new sources, with data drawn from the Chandra archive. In all cases the spectrum is well fit by an absorbed power-law with no evidence for spectral modulations, allowing constraints to be placed on the ALP-photon coupling parameter $g_{a\gamma\gamma}$. Two sources are particularly good: the Seyfert galaxy 2E3140 in A1795 and the AGN NGC3862 within the cluster A1367, leading to 95\% bounds for light ALPs ($m_a \lesssim 10^{-12} {\rm eV}$) of $g_{a\gamma\gamma} \lesssim 1.5 \times 10^{-12} {\rm GeV}^{-1}$ and $g_{a\gamma\gamma} \lesssim 2.4 \times 10^{-12} {\rm GeV}^{-1}$ respectively., Comment: 17 pages

Highly spinning Kerr black holes with masses $M = 1 - 100\ M_{\odot}$ are subject to an efficient superradiant instability in the presence of bosons with masses $\mu \sim 10^{-10} - 10^{-12}\ {\rm eV}$. We observe that this matches the effective plasma-induced photon mass in diffuse galactic or intracluster environments ($\omega_{\rm pl} \sim 10^{-10} - 10^{-12}\ {\rm eV}$). This suggests that bare Kerr black holes within galactic or intracluster environments, possibly even including the ones produced in recently observed gravitational wave events, are unstable to formation of a photon cloud that may contain a significant fraction of the mass of the original black hole. At maximal efficiency, the instability timescale for a massive vector is milliseconds, potentially leading to a transient rate of energy extraction from a black hole in principle as large as $\sim 10^{55} \ {\rm erg \, s}^{-1}$. We discuss possible astrophysical effects this could give rise to, including a speculative connection to Fast Radio Bursts., Comment: 5 pages; v2. accepted version

\emph{Hitomi} observations of Perseus with the Soft X-ray Spectrometer (SXS) provide a high-resolution look at the 3.5 keV feature reported by multiple groups in the Perseus cluster. The \emph{Hitomi} spectrum -- which involves the sum of diffuse cluster emission and the point-like central Active Galactic Nucleus (AGN) -- does not show any excess at $E \sim 3.5 {\rm keV}$, giving an apparent inconsistency with previous observations of excess diffuse emission. We point out that 2009 \emph{Chandra} data reveals a strong dip in the AGN spectrum at $E = (3.54 \pm 0.02) {\rm keV}$ (cluster frame) -- the identical energy to the diffuse excess observed by \emph{XMM-Newton}. Scaling this dip to the 2016 AGN luminosity and adding it to the diffuse \emph{XMM-Newton} excess, this predicts an overall dip in the SXS field of view of $(-5.9 \pm 4.4) \times 10^{-6} \, {\rm ph} \, {\rm cm}^{-2} \, {\rm s}^{-1}$ at $E= 3.54$ keV -- a precise match to the \emph{Hitomi} data when broadened by the dark matter virial velocity. We describe models of Fluorescent Dark Matter that can reproduce this physics, in which dark matter absorbs and then re-emits 3.5 keV photons emitted from the central AGN., Comment: v2. 7 pages; v3. as accepted in Phys. Rev. D

Axion-like particles (ALPs) can induce localised oscillatory modulations in the spectra of photon sources passing through astrophysical magnetic fields. Ultra-deep Chandra observations of the Perseus cluster contain over $5 \times 10^5$ counts from the AGN of the central cluster galaxy NGC1275, and represent a dataset of extraordinary quality for ALP searches. We use this dataset to search for X-ray spectral irregularities from the AGN. The absence of irregularities at the O(30%) level allows us to place leading constraints on the ALP-photon mixing parameter $g_{a\gamma\gamma} \lesssim 1.4 - 4.0 \times 10^{-12} {\rm GeV}^{-1}$ for $m_a \lesssim 10^{-12}$ eV, depending on assumptions on the magnetic field realisation along the line of sight., Comment: v1. 37 pages, 17 figures; v2. references added, additional discussion of gain and bounds; v3. final version accepted by ApJ

We prove that for an open domain $D \subset \mathbb{R}^d $ with $d \geq 2 $ , for every (measurable) uniformly elliptic tensor field $a$ and for almost every point $y \in D$ , there exists a unique Green's function centred in $ y $ associated to the vectorial operator $ -\nabla \cdot a\nabla $ in D. In particular, when $d > 2$ this result also implies the existence of the fundamental solution for elliptic systems, i.e. the Green function for $ -\nabla \cdot a\nabla $ in $ \mathbb{R}^d $. Moreover, introducing an ensemble $\langle\cdot \rangle$ over the set of uniformly elliptic tensor fields, under the assumption of stationarity we infer for the fundamental solution $G$ some pointwise bounds for $\langle |G(\cdot; x,y)|\rangle$, $\langle|\nabla_x G(\cdot; x,y)|\rangle$ and $\langle |\nabla_x\nabla_y G(\cdot; x,y)|\rangle$. These estimates scale optimally in space and provide a generalization to systems of the bounds obtained by Delmotte and Deuschel for the scalar case., Comment: 47 pages, new version with a new introduction

It is unknown whether a bound on axion field ranges exists within quantum gravity. We study axion field ranges using extended supersymmetry, in particular allowing an analysis within strongly coupled regions of moduli space. We apply this strategy to Calabi-Yau compactifications with one and two K\"ahler moduli. We relate the maximally allowable decay constant to geometric properties of the underlying Calabi-Yau geometry. In all examples we find a maximal field range close to the reduced Planck mass (with the largest field range being 3.25 $M_P$). On this perspective, field ranges relate to the intersection and instanton numbers of the underlying Calabi-Yau geometry., Comment: 28 pages, 6 figures

Axion-like particles (ALPs) and photons inter-convert in the presence of a magnetic field. At keV energies in the environment of galaxy clusters, the conversion probability can become unsuppressed for light ALPs. Conversion of thermal X-ray photons into ALPs can introduce a step-like feature into the cluster thermal bremsstrahlung spectrum, and we argue that existing X-ray data on galaxy clusters should be sufficient to extend bounds on ALPs in the low-mass region $m_a \lesssim 1 \times 10^{-12}\,{\rm eV}$ down to $M \sim 7\times 10^{11}\, {\rm GeV}$, and that for $10^{11}\, {\rm GeV} < M \lesssim 10^{12}$ GeV light ALPs give rise to interesting and unique observational signatures that may be probed by existing and upcoming X-ray (and potentially X-ray polarisation) observations of galaxy clusters., Comment: 5 pages

This paper is concerned with the study of a diffusive perturbation of the linear LSW model introduced by Carr and Penrose. A main subject of interest is to understand how the presence of diffusion acts as a selection principle, which singles out a particular self-similar solution of the linear LSW model as determining the large time behavior of the diffusive model. A selection principle is rigorously proven for a model which is a semi-classical approximation to the diffusive model. Upper bounds on the rate of coarsening are also obtained for the full diffusive model., Comment: 53 pages

Motivated by the possibility of explaining the 3.5 keV line through dark matter decaying to axion-like particles that subsequently convert to photons, we study ALP-photon conversion for sightlines passing within 50 pc of the galactic centre. Conversion depends on the galactic centre magnetic field which is highly uncertain. For fields at low or mid-range of observational estimates (10--100 $\mu$G), no observable signal is possible. For fields at the high range of observational estimates (a pervasive poloidal mG field over the central 150 pc) it is possible to generate sufficient signal to explain recent observations of a 3.5 keV line in the galactic centre. In this scenario, the galactic centre line signal comes predominantly from the region with $z > 20$ pc, reconciling the results from the Chandra and XMM-Newton X-ray telescopes. The dark matter to ALP to photon scenario also naturally predicts the non-observation of the 3.5 keV line in stacked galaxy spectra. We further explore predictions for the line flux in galaxies and suggest a set of galaxies that is optimised for observing the 3.5 keV line in this model., Comment: 21 pages + Appendix, 5 figures

We further study a scenario in which a 3.55 keV X-ray line arises from decay of dark matter to an axion-like particle (ALP), that subsequently converts to a photon in astrophysical magnetic fields. We perform numerical simulations of Gaussian random magnetic fields with radial scaling of the magnetic field magnitude with the electron density, for both cool-core `Perseus' and non-cool-core `Coma' electron density profiles. Using these, we quantitatively study the resulting signal strength and morphology for cool-core and non-cool-core clusters. Our study includes the effects of fields of view that cover only the central part of the cluster, the effects of offset pointings on the radial decline of signal strength and the effects of dividing clusters into annuli. We find good agreement with current data and make predictions for future analyses and observations., Comment: 14 pages, 6 figures

It was recently found that the soft X-ray excess in the center of the Coma cluster can be fitted by conversion of axion-like-particles (ALPs) of a cosmic axion background (CAB) to photons. We extend this analysis to the outskirts of Coma, including regions up to 5 Mpc from the center of the cluster. We extract the excess soft X-ray flux from ROSAT All-Sky Survey data and compare it to the expected flux from ALP to photon conversion of a CAB. The soft X-ray excess both in the center and the outskirts of Coma can be simultaneously fitted by ALP to photon conversion of a CAB. Given the uncertainties of the cluster magnetic field in the outskirts we constrain the parameter space of the CAB. In particular, an upper limit on the CAB mean energy and a range of allowed ALP-photon couplings are derived., Comment: 21 pages, 7 figures

We further explore a scenario in which the recently observed 3.55 keV photon line arises from dark matter decay to an axion-like particle (ALP) of energy 3.55 keV, which then converts to a photon in astrophysical magnetic fields. This ALP scenario is well-motivated by the observed morphology of the 3.55 keV flux. For this scenario we study the expected flux from dark matter decay in the galactic halos of both the Milky Way and Andromeda (M31). The Milky Way magnetic field is asymmetric about the galactic centre, and so the resulting 3.55 keV flux morphology differs significantly from the case of direct dark matter decay to photons. However the Milky Way magnetic field is not large enough to generate an observable signal, even with ASTRO-H. In contrast, M31 has optimal conditions for ALP to photon conversion and the intrinsic signal from M31 becomes two orders of magnitude larger than for the Milky Way, comparable to that from clusters and consistent with observations., Comment: 15 pages, 7 figures Version accepted for publication in JCAP

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 $m_{a} \lesssim 6\cdot 10^{-11}~{\rm eV}$, 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 explains 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., Comment: 6 pages

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 $g_{a\gamma\gamma} \sim 2 \times 10^{-13}$ 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 ${\cal O}(3 {\rm ~kpc})$ scales over those with most power on ${\cal O}(12 {\rm ~kpc})$ scales. Within this scenario, we bound the mean energy of the axion background to $50\, {\rm eV}\lesssim \langle E_a \rangle \lesssim 250\, {\rm eV}$, the axion mass to $m_a \lesssim 10^{-12}\,\hbox{eV}$, and derive a lower bound on the axion-photon coupling $g_{a\gamma\gamma} \gtrsim \sqrt{0.5/\Delta N_{\rm eff}}\, 1.4 \times 10^{-13}$ GeV$^{-1}$., Comment: 43 pages, 11 figures

The magnitude of the flux superpotential $W_{flux}$ plays a crucial role in determining the scales of IIB string compactifications after moduli stabilisation. It has been argued that values of $W_{flux}$ much less than one are preferred, and even required for physical and consistency reasons. This note revisits these arguments. We establish that the coupling (g) of heavy Kaluza-Klein modes to light states scales as ${M_{KK} / M_{Pl}}$ (hence is suppressed by two third powers of the inverse volume of compactification) and argue that consistency of the superspace derivative expansion requires $gF/M^2 \sim m_{3/2}/ M_{KK} << 1$, where $F$ is the auxiliary field of the light fields and $M$ the ultraviolet cutoff. This gives only a mild constraint on the flux superpotential, $W_{flux} << V^{1/3}$ (where V is the volume of the compactification), which can be easily satisfied for order one values of $W_{flux}$. This regime is also statistically favoured and makes the Bousso-Polchinski mechanism for the vacuum energy hierarchically more efficient., Comment: 14 pages

In large volume models reheating is driven by the decays of the volume modulus to the visible sector, while the decays to its axion partners result in dark radiation. In this article we discuss the impact of loop corrections on the only model-independent visible decay channel: the decay into Higgs pairs via a Giudice-Masiero term. Including such radiative effects leads to a more precise determination of the relative fraction of dark radiation, since by contrast all loop corrections to the volume axion decay mode are Planck suppressed. Assuming an MSSM spectrum and that the Giudice-Masiero coupling is fixed at the string scale by a shift symmetry in the Higgs sector, we arrive at a prediction for the effective number of neutrinos. The result turns out to be too large to be consistent with data, highly disfavouring the minimal model., Comment: 14 pages, 3 figures; v2: Version to appear in JHEP. Expanded introduction to clarify arguments, assumptions and applicability of results; added references

Primordial decays of string theory moduli at z \sim 10^{12} naturally generate a dark radiation Cosmic Axion Background (CAB) with 0.1 - 1 keV energies. This CAB can be detected through axion-photon 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 \ll 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., Comment: 4 pages, 2 figures. References and figures updated; v3. matches published version

This paper is concerned with the study of solutions to discrete parabolic equations in divergence form with random coefficients, and their convergence to solutions of a homogenized equation. In [11] rate of convergence results in homogenization and estimates on the difference between the averaged Green's function and the homogenized Green's function for random environments which satisfy a Poincar\'{e} inequality were obtained. Here these results are extended to certain environments in which correlations can have arbitrarily small power law decay. Similar results for discrete elliptic equations were obtained in [12]., Comment: 29 pages

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 $E_{CoM} \gg T_{\gamma}$, 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 $T_{\gamma} \ll T_{freezeout}$. Most axions --- or any other dark radiation candidate from modulus decay --- still linger today as a Cosmic Axion Background with $E_{axion} \sim \mathcal{O}(100) eV$, and a flux of $\sim 10^6 cm^{-2} s^{-1}$., Comment: 25 pages, 9 figures. Published version

We study soft supersymmetry breaking terms for anisotropic LARGE volume compactifications, where the bulk volume is set by a fibration with one small four-cycle and one large two-cycle. We consider scenarios where D7s wrap either a blow-up cycle or the small fibre cycle. Chiral matter can arise either from modes parallel or perpendicular to the brane. We compute soft terms for this matter and find that for the case where the D7 brane wraps the fibre cycle the scalar masses can be parametrically different, allowing a possible splitting of third-generation soft terms., Comment: 21 pages, 3 figures; references added

We consider reheating driven by volume modulus decays in the LARGE Volume Scenario. Such reheating always generates non-zero dark radiation through the decays to the axion partner, while the only competitive visible sector decays are Higgs pairs via the Giudice-Masiero term. In the framework of sequestered models where the cosmological moduli problem is absent, the simplest model with a shift-symmetric Higgs sector generates 1.56 < N_{eff} - N_{eff,SM} < 1.74. For more general cases, the known experimental bounds on N_{eff} strongly constrain the parameters and matter content of the models., Comment: 6 pages; v2. refs updated