Publications (17)71.88 Total impact
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ABSTRACT: latfield2 is a C++ library designed to simplify writing parallel codes for solving partial differen tial equations, developed for application to classical field theories in particle physics and cosmology. It is a significant rewrite of the latfield framework, moving from a slab domain decomposition to a rod decomposition, where the last two dimension of the lattice are scattered into a two dimensional process grid. Parallelism is implemented using the Message Passing Interface (MPI) standard, and hidden in the basic objects of gridbased simulations: Lattice, Site and Field. It comes with an integrated parallel fast Fourier transform, and I/O server class permitting computation to continue during the writing of large files to disk. latfield2 has been used for production runs on tens of thousands of processor elements, and is expected to be scalable to hundreds of thousands.  [Show abstract] [Hide abstract]
ABSTRACT: We suggest that LISA Pathfinder, a technology demonstrator for the future gravitational wave observatory LISA, could be used to carry out a direct experimental test of Modified Newtonian Dynamics (MOND). The LISA Pathfinder spacecraft is currently being built and the launch date is just a few years away. No modifications of the spacecraft are required, nor any interference with its nominal mission. The basic concept is to fly LISA Pathfinder through the region around the SunEarth saddle point, in an extended mission phase, once the original mission goals are achieved. We examine various strategies to reach the saddle point, and find that the preferred strategy, yielding relatively short transfer times of just over 1 year, probably involves a lunar flyby. LISA Pathfinder will be able to probe the intermediate MOND regime, i.e. the transition between deep MOND and Newtonian gravity. We present robust estimates of the anomalous gravity gradients that LISA Pathfinder should be exposed to, based on MONDian effects as derived from the TensorVectorScalar (TeVeS) theory. The spacecraft speed and spatial scale of the MOND signal combine in a way that the spectral signature of the signal falls precisely into LISA Pathfinder’s measurement bandwidth. We find that if the gravity gradiometer onboard the spacecraft achieves its currently predicted sensitivity, these anomalous gradients could not just be detected, but measured in some detail.  [Show abstract] [Hide abstract]
ABSTRACT: We present a significant update of the constraints on the Abelian Higgs cosmic string tension by cosmic microwave background (CMB) data, enabled both by the use of new highresolution CMB data from suborbital experiments as well as the latest results of the WMAP satellite, and by improved predictions for the impact of Abelian Higgs cosmic strings on the CMB power spectra. The new cosmic string spectra [1] were improved especially for small angular scales, through the use of larger Abelian Higgs string simulations and careful extrapolation. If Abelian Higgs strings are present then we find improved bounds on their contribution to the CMB anisotropies, AH < 0.095, and on their tension, G?AH < 0.57 × 10?6, both at 95% confidence level using WMAP7 data; and AH < 0.048 and G?AH < 0.42 × 10?6 using all the CMB data. We also find that using all the CMB data, a scale invariant initial perturbation spectrum, ns = 1, is now disfavoured at 2.4? even if strings are present. A Bayesian model selection analysis no longer indicates a preference for strings.  [Show abstract] [Hide abstract]
ABSTRACT: The proposed CMBPol mission will be able to detect the imprint of topological defects on the cosmic microwave background (CMB) provided the contribution is sufficiently strong. We quantify the detection threshold for cosmic strings and for textures, and analyse the satellite's ability to distinguish between these different types of defects. We also assess the level of danger of misidentification of a defect signature as from the wrong defect type or as an effect of primordial gravitational waves. A 0.002 fractional contribution of cosmic strings to the CMB temperature spectrum at multipole ten, and similarly a 0.001 fractional contribution of textures, can be detected and correctly identified at the 3{\sigma} level. We also confirm that a tensor contribution of r = 0.0018 can be detected at over 3{\sigma}, in agreement with the CMBpol mission concept study. These results are supported by a model selection analysis.  [Show abstract] [Hide abstract]
ABSTRACT: We present a significant improvement over our previous calculations of the cosmic string contribution to cosmic microwave background (CMB) power spectra, with particular focus on subWMAP angular scales. These smaller scales are relevant for the nowoperational Planck satellite and additional suborbital CMB projects that have even finer resolutions. We employ larger Abelian Higgs string simulations than before and we additionally model and extrapolate the statistical measures from our simulations to smaller length scales. We then use an efficient means of including the extrapolations into our EinsteinBoltzmann calculations in order to yield accurate results over the multipole range 2 < l < 4000. Our results suggest that powerlaw behaviour cuts in for l > 3000 in the case of the temperature power spectrum, which then allows cautious extrapolation to even smaller scales. We find that a string contribution to the temperature power spectrum making up 10% of power at l=10 would be larger than the Silkdamped primary adiabatic contribution for l > 3500. Astrophysical contributions such as the SunyaevZeldovich effect also become important at these scales and will reduce the sensitivity to strings, but these are potentially distinguishable by their frequencydependence. Comment: 18 pages, 16 figures  [Show abstract] [Hide abstract]
ABSTRACT: We suggest that LISA Pathfinder could be used to subject TEVES, and in particular the nonrelativistic MOND phenomenology it incorporates, to a direct, controlled experimental test, in just a few years' time. The basic concept is to fly LISA Pathfinder through the region around the SunEarth saddle point, following its nominal mission, in order to look for anomalous gravity gradients. We examine various strategies to reach the saddle point, and conclude that the preferred strategy, resulting in relatively short transfer times of order one year, probably involves a lunar flyby. We present robust estimates of the MOND gravity gradients that LISA Pathfinder should be exposed to, and conclude that if the gradiometer onboard the spacecraft achieves its nominal performance, these gradients will not just be detected, but measured and characterised in some detail, should they exist. Conversely, given the large predicted signal based on standard assumptions, a null result would most likely spell the end of TEVES/MOND. Comment: Twin paper to arXiv:0912.0710  [Show abstract] [Hide abstract]
ABSTRACT: We study the evolution of a particular class of nonperiodic cosmic string loops containing Yjunctions, such as may form during the evolution of a network of (p,q) cosmic superstrings. We set up and solve the NambuGoto equations of motion for a loop with junctions, focusing attention on a specific initially static and planar loop configuration. After a given time, the junctions collide and the NambuGoto description breaks down. We also study the same loop configuration in a U(1)×U(1) gauge fieldtheory model that allows composite vortices with corresponding Yjunctions. We show that the fieldtheory and NambuGoto evolution are remarkably similar until the collision time. However, in the fieldtheory evolution a new phenomenon occurs: the composite vortices can unzip, producing in the process new Yjunctions, whose separation may grow significantly, destabilizing the configuration. In particular, an initial loop with two Yjunctions may evolve to a configuration with six Yjunctions (all distinct from each other). Setting up this new configuration as an initial condition for NambuGoto strings, we solve for its evolution and establish conditions under which it is stable to the decay mode seen in the fieldtheory case. Remarkably, the condition closely matches that seen in the fieldtheory simulations, and is expressed in terms of simple parameters of the NambuGoto system. This implies that there is an easy way to understand the instability in terms of which region of parameter space leads to stable or unstable unzippings.  [Show abstract] [Hide abstract]
ABSTRACT: In previous work it was shown that MOND theories predict anomalously strong tidal stresses near the saddle points of the Newtonian gravitational potential. An analytical examination of the saddle between two bodies revealed a linear and a nonlinear solution, valid for the outer and inner regions. Here we present a numerical algorithm for solving the MOND equations. We check the code against the twobody analytical solutions and explore the region transitioning between them. We then develop a a realistic model for the MONDian effects on the saddles of the SunEarthMoon system (including further sources is straightforward). For the SunEarth saddle we find that the twobody results are almost unchanged, with corrections increasing from full to new Moon. In contrast, the Moon saddle is an intrinsically threebody problem, but we numerically find a recipe for adapting the twobody solution to this case, by means of a suitable rescaling and axis reorientation. We explore possible experimental scenarios for LISA Pathfinder, and the prospect of a visit to the saddle(s) at the end of the mission. Given the chaotic nature of the orbits, awareness of the full range of the possibilities is crucial for a realistic prediction. We conclude that even with very conservative assumptions on the impact parameter, the accelerometers are abundantly sensitive to vindicate or rule out the theory. Comment: To be published in CQG  [Show abstract] [Hide abstract]
ABSTRACT: Classical lattice simulations of the Abelian Higgs model are used to investigate small scale structure and loop distributions in cosmic string networks. Use of the field theory ensures that the smallscale physics is captured correctly. The results confirm analytic predictions of Polchinski & Rocha [1] for the twopoint correlation function of the string tangent vector, with a power law from length scales of order the string core width up to horizon scale with evidence to suggest that the small scale structure builds up from small scales. An analysis of the size distribution of string loops gives a very low number density, of order 1 per horizon volume, in contrast with NambuGoto simulations. Further, our loop distribution function does not support the detailed analytic predictions for loop production derived by Dubath et al. [2]. Better agreement to our data is found with a model based on loop fragmentation [3], coupled with a constant rate of energy loss into massive radiation. Our results show a strong energy loss mechanism which allows the string network to scale without gravitational radiation, but which is not due to the production of string width loops. From evidence of small scale structure we argue a partial explanation for the scale separation problem of how energy in the very low frequency modes of the string network is transformed into the very high frequency modes of gauge and Higgs radiation. We propose a picture of string network evolution which reconciles the apparent differences between NambuGoto and field theory simulations.  [Show abstract] [Hide abstract]
ABSTRACT: We explore the formation of cosmic string Yjunctions when strings of two different types collide, which has recently become important since string theory can yield cosmic strings of distinct types. Using a model containing two types of local U(1) string and stable composites, we simulate the collision of two straight strings and investigate whether the dynamics matches that previously obtained using the NambuGoto action, which is not strictly valid close to the junction. We find that the NambuGoto action performs only moderately well at predicting when the collision results in the formation of a pair of Yjunctions (with a composite string connecting them). However, we find that when they do form, the late time dynamics matches those of the NambuGoto approximation very closely. We also see little radiative emission from the Yjunction system, which suggests that radiative decay due to bridge formation does not appear to be a means via which a cosmological network of such string would rapidly lose energy. 
Article: On the degeneracy between primordial tensor modes and cosmic strings in future CMB data from Planck
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ABSTRACT: While observations indicate that the predominant source of cosmic inhomogeneities are adiabatic perturbations, there are a variety of candidates to provide auxiliary trace effects, including inflationgenerated primordial tensors and cosmic defects which both produce Bmode cosmic microwave background (CMB) polarization. We investigate whether future experiments may suffer confusion as to the true origin of such effects, focusing on the ability of Planck to distinguish tensors from cosmic strings, and show that there is no significant degeneracy. Comment: 4 pages, 3 figures. Minor changes to match published version  [Show abstract] [Hide abstract]
ABSTRACT: We perform a multiparameter likelihood analysis to compare measurements of the cosmic microwave background (CMB) power spectra with predictions from models involving cosmic strings. Adding strings to the standard case of a primordial spectrum with powerlaw tilt ns, we find a 2sigma detection of strings: f10=0.11+/0.05, where f10 is the fractional contribution made by strings in the temperature power spectrum (at l=10). CMB data give moderate preference to the model ns=1 with cosmic strings over the standard zerostrings model with variable tilt. When additional nonCMB data are incorporated, the two models become on a par. With variable ns and these extra data, we find that f10<0.11, which corresponds to Gmicro<0.7x10(6) (where micro is the string tension and G is the gravitational constant).  [Show abstract] [Hide abstract]
ABSTRACT: We present the first ever calculation of cosmic microwave background CMB anisotropy power spectra from semilocal cosmic strings, obtained via simulations of a classical field theory. Semilocal strings are a type of nontopological defect arising in some models of inflation motivated by fundamental physics, and are thought to relax the constraints on the symmetry breaking scale as compared to models with (topological) cosmic strings. We derive constraints on the model parameters, including the string tension parameter mu, from fits to cosmological data, and find that in this regard BPS semilocal strings resemble global textures more than topological strings. The observed microwave anisotropy at l = 10 is reproduced if Gmu = 5.3x10^{6} (G is Newton's constant). However as with other defects the spectral shape does not match observations, and in models with inflationary perturbations plus semilocal strings the 95% confidence level upper bound is Gmu<2.0x10^{6} when CMB data, Hubble Key Project and Big Bang Nucleosynthesis data are used (c.f. Gmu<0.9x10^{6} for cosmic strings). We additionally carry out a Bayesian model comparison of several models with and without defects, showing models with defects are neither conclusively favoured nor disfavoured at present. Comment: 15 pages, 13 figures. Minor correction of numerical results, matches published version  [Show abstract] [Hide abstract]
ABSTRACT: We present the first calculation of the possible (local) cosmic string contribution to the cosmic microwave background polarization spectra from simulations of a string network (rather than a stochastic collection of unconnected string segments). We use field theory simulations of the Abelian Higgs model to represent local U(1) strings, including their radiative decay and microphysics. Relative to previous estimates, our calculations show a shift in power to larger angular scales, making the chance of a future cosmic string detection from the Bmode polarization slightly greater. We explore a future groundbased polarization detector, taking the CLOVER project as our example. In the null hypothesis (that cosmic strings make a zero contribution) we find that CLOVER should limit the string tension \mu to G\mu<0.12e6 (where G is the gravitational constant), above which it is likely that a detection would be possible.  [Show abstract] [Hide abstract]
ABSTRACT: We present the first fieldtheoretic calculations of the contribution made by cosmic strings to the temperature power spectrum of the cosmic microwave background (CMB). Unlike previous work, in which strings were modeled as idealized onedimensional objects, we evolve the simplest example of an underlying field theory containing local U(1) strings, the Abelian Higgs model. Limitations imposed by finite computational volumes are overcome using the scaling property of string networks and a further extrapolation related to the lessening of the string width in comoving coordinates. The strings and their decay products, which are automatically included in the field theory approach, source metric perturbations via their energymomentum tensor, the unequaltime correlation functions of which are used as input into the CMB calculation phase. These calculations involve the use of a modified version of CMBEASY, with results provided over the full range of relevant scales. We find that the string tension $\mu$ required to normalize to the WMAP 3year data at multipole $\ell = 10$ is $G\mu = [2.04\pm0.06\textrm{(stat.)}\pm0.12\textrm{(sys.)}] \times 10^{6}$, where we have quoted statistical and systematic errors separately, and $G$ is Newton's constant. This is a factor 23 higher than values in current circulation.  [Show abstract] [Hide abstract]
ABSTRACT: We present predictions for the angular correlation function and redshift distribution for SHADES, the SCUBA HalfDegree Extragalactic Survey, which will yield a sample of around 300 submillimetre sources in the 850μm waveband in two separate fields. Complete and unbiased photometric redshift information on these submillimetre sources will be derived by combining the SCUBA data with (i) deep radio imaging already obtained with the Very Large Array, (ii) guaranteedtime Spitzer data at midinfrared wavelengths, and (iii) farinfrared maps to be produced by BLAST, the Balloonborne LargeAperture Submillimeter Telescope. Predictions for the redshift distribution and clustering properties of the final anticipated SHADES sample have been computed for a wide variety of models, each constrained to fit the observed number counts. As we are dealing with around 150 sources per field, we use the skyaveraged angular correlation function to produce a more robust fit of a powerlaw shape w(θ) = (θ/A)−δ to the model data. Comparing the predicted distributions of redshift and of the clustering amplitude A and slope δ, we find that models can be constrained from the combined SHADES data with the expected photometric redshift information.  [Show abstract] [Hide abstract]
ABSTRACT: We use the cosmic microwave background angular power spectra to place upper limits on the degree to which global defects may have aided cosmic structure formation. We explore this under the inflationary paradigm, but with the addition of textures resulting from the breaking of a global O(4) symmetry during the early stages of the Universe. As a measure of their contribution, we use the fraction of the temperature power spectrum that is attributed to the defects at a multipole of 10. However, we find a parameter degeneracy enabling a fit to the firstyear WMAP data to be made even with a significant defect fraction. This degeneracy involves the baryon fraction and the Hubble constant, plus the normalization and tilt of the primordial power spectrum. Hence, constraints on these cosmological parameters are weakened. Combining the WMAP data with a constraint on the physical baryon fraction from big bang nucleosynthesis calculations and highredshift deuterium abundance, limits the extent of the degeneracy and gives an upper bound on the defect fraction of 0.13 (95% confidence). Comment: 10pp LaTeX/RevTeX, 6 eps figs; matches accepted version
Publication Stats
773  Citations  
71.88  Total Impact Points  
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Institutions

20072012

Imperial College London
 Department of Physics
Londinium, England, United Kingdom


20052011

University of Sussex
 • Department of Physics and Astronomy
 • Astronomy Centre
Brighton, England, United Kingdom
