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We present constraints on the axion-photon coupling scale M for light axion-like particles by combining recent Supernova Type Ia data with the latest measurements of the Hubble expansion at redshifts between 0 and 2. Allowing for a coupling between axions and photons leads to a modification of the inferred luminosity distances for supernovae due to the conversion of photons to axions in the presence of intergalactic magnetic fields. We constrain such couplings by considering deviations from the luminosity-angular diameter distance relation dL dA(1+z)2. We find that for intergalactic magnetic fields of order 1 nG, current supernova and Hubble expansion data rule out a region in the coupling scale M between 1010 and 1011 GeV.

We construct a wide class of black hole solutions to the general theory of ghost free multi-metric gravity in arbitrary spacetime dimension, extending and generalising the known results in 4-dimensional dRGT massive gravity and bigravity. The solutions are split into three generic classes based on whether the metrics can be simultaneously diagonalised - one of which does not exist in dRGT massive gravity nor bigravity, and is only possible when one has more than two interacting metric fields. We also linearise the general multi-metric theory to determine the dynamics of the massive spin-2 modes, including examples where this can be done analytically, and use the linear theory to discuss the stability of the 4-dimensional multi-Schwarzchild and multi-Kerr solutions. We explain how the instabilities that plague these solutions in dRGT massive gravity and bigravity carry across to the general multi-metric theory, touching upon ideas of dimensional deconstruction to make sense of the results.

The higher order generalisation of the clockwork mechanism to gravitational interactions provides a means to generate an exponentially suppressed coupling to matter from a fundamental theory of multiple interacting gravitons, without introducing large hierarchies in the underlying potential and without the need for a dilaton, suggesting a possible application to the hierarchy problem. We work in the framework of ghost free multi-gravity with \"nearest-neighbour\" interactions, and present a formalism by which one is able to construct potentials such that the theory will always exhibit this clockwork effect. We also consider cosmological solutions to the general theory, where all metrics are of FRW form, with site-dependent scale factors/lapses. We demonstrate the existence of multiple deSitter vacua where all metrics share the same Hubble parameter, and we solve the modified Einstein equations numerically for an example clockwork model constructed using our formalism, finding that the evolution of the metric that matter couples to is essentially equivalent to that of general relativity at the modified Planck scale. It is important to stress that while we focus on the application to clockwork theories, our work is entirely general and facilitates finding cosmological solutions to any ghost free multi-gravity theory with \"nearest-neighbour\" interactions. Moreover, we clarify previous work on the continuum limit of the theory, which is generically a scalar-tensor braneworld, using the Randall-Sundrum model as a special case and showing how the discrete-clockwork cosmological results map to the continuum results in the appropriate limit.

In determining the gravitational signal of cusps from a network of cosmic strings loops, a number of key parameters have to be assumed. These include the typical number of cusps per period of string oscillation and the typical values of the sharpness parameters of left and right moving waves on the string, evaluated at the cusp event. Both of these are important, as the power stored in the gravitational waves emitted from the loops of string is proportional to the number of cusps per period, and inversely proportional to the product of the sharpness parameters associated with the left and right moving modes on the string. In suitable units both of these quantities are usually thought to be of order unity. In order to try and place these parameters on a more robust footing, we analyse in detail a large number of randomly chosen loops of string that can have high harmonics associated with them, such as one might expect to form by chopping off an infinite string in the early universe. This allows us to analyse tens of thousands of loops and obtain detailed statistics on these crucial parameters. While we find in general the sharpness parameters are indeed close to unity, as assumed in previous work (with occasional exceptions where they can become \\(O(10^{-2})\\)), the cusp number per period scales directly with the number of harmonics on the loop and can be significantly larger than unity. This opens up the possibility of larger signals than would have otherwise been expected, potentially leading to tighter bounds on the dimensionless cosmic string tension \\(G\\mu\\).

Based on a binary tree model for the self intersection of cosmic string loops containing high harmonics we estimate the number of self-intersections of the parent and daughter loops and the associated cusp production to determine the most likely number of cusp events per period on the resultant non-self intersecting loops, and provide an updated calculation for the gravitational wave signal that arrives on Earth from cusps on such loops. This is done for different numbers of cusps supported from the cosmic strings of the network, and for different harmonic distributions on the loops. We plot our results of the event rate of gravitational waves emanating from the cusps in terms of redshift, having fixed the value of \\(G\\mu\\) and the received frequency of the signal, and compare our results to those in [1, 2].

We consider the higher order clockwork theory of gravitational interactions, whereby a number of gravitons are coupled together with TeV strength, but nevertheless generate a Planck scale coupling to matter without the need for a dilaton. It is shown that the framework naturally lends itself to a five-dimensional geometry, and we find the 5D continuum version of such deconstructed 4D gravitational clockwork models. Moreover, the clockwork picture has matter coupled to particular gravitons, which in the 5D framework looks like a braneworld model, with the Randall-Sundrum model being a special case. More generally, the gravitational clockwork leads to a family of scalar-tensor braneworld models, where the scalar is not a dilaton.

Far-from-equilibrium phenomena unveil the intricate principles of complex systems, including snowflake growth and fluid turbulence, with broad applications ranging from foreign exchange trading to climate modeling. A recurring feature across these systems is the emergence of a spectral cascade, where energy is transferred across the system's length scales, following a simple power law. The statistical theory of weak wave turbulence, in which only leading order interactions are considered, successfully predicts scaling laws for stationary states in idealised scenarios. Realistic conditions, such as finite size and amplitude effects, and strong dissipation, remain beyond our current understanding. Lacking comprehensive theoretical insight, we experimentally trace the formation of a spectral cascade under these conditions. Using an externally driven fluid-fluid interface, we successfully resolve individual wave modes and track their real-time evolution from one to few to many. This process culminates in a steady state whose power spectral density is fully characterised by a power-law scaling. We further quantify specific interactions through statistical correlations to reveal a hierarchy in the wave-mixing order, thus confirming a key assumption of weak-wave turbulence. We present a comprehensive time-evolution analysis that is crucial in identifying critical points where the interface undergoes significant changes. Our findings validate that the interfacial dynamics can be effectively modelled using a weakly nonlinear Lagrangian theory, enabling us to explore its applicability to other out-of-equilibrium systems. Notably, we uncover intriguing connections to reheating scenarios following cosmic inflation in the early universe.

We investigate cosmological models in which dynamical dark energy consists of a scalar field whose present-day value is controlled by a coupling to the neutrino sector. The behaviour of the scalar field depends on three functions: a kinetic function, the scalar field potential, and the scalar field-neutrino coupling function. We present an analytic treatment of the background evolution during radiation- and matter-domination for exponential and inverse power law potentials, and find a relaxation of constraints compared to previous work on the amount of early dark energy in the exponential case. We then carry out a numerical analysis of the background cosmology for both types of potential and various illustrative choices of the kinetic and coupling functions. By applying bounds from Planck on the amount of early dark energy, we are able to constrain the magnitude of the kinetic function at early times.

We present the first complete Markov chain Monte Carlo analysis of cosmological models with evolving cosmic (super)string networks, using the unconnected segment model in the unequal-time correlator formalism. For ordinary cosmic string networks, we derive joint constraints on Lambda cold dark matter (CDM) and string network parameters, namely the string tension Gmu, the loop-chopping efficiency c_r and the string wiggliness \\alpha. For cosmic superstrings, we obtain joint constraints on the fundamental string tension Gmu_F, the string coupling g_s, the self-interaction coefficient c_s, and the volume of compact extra dimensions w. This constitutes the most comprehensive CMB analysis of LambdaCDM cosmology + strings to date. For ordinary cosmic string networks our updated constraint on the string tension is, in relativistic units, Gmu<1.1x10^-7, while for cosmic superstrings our constraint on the fundamental string tension is Gmu_F<2.8x10^-8, both obtained using Planck2015 temperature and polarisation data.

For the past two hundred years, parametric instabilities have been studied in various physical systems, such as fluids, mechanical devices and even inflationary cosmology. It was not until a few decades ago that this subharmonic unstable response arose as a central mechanism for the thermalisation of the Early Universe, in a theory known as preheating. Here we study a parametrically driven two-fluid interface to simulate the key aspects of inflationary preheating dynamics through the onset of nonlinear Faraday waves. We present a detailed analysis of the effective field theory description for interfacial waves through the factorization properties of higher-order correlations. Despite the intricacies of a damped and highly interacting hydrodynamical system, we show that the scattering of large amplitude Faraday waves is connected to a broadening of primary resonance bands and the subsequent appearance of secondary instabilities as predicted in preheating dynamics.