Neil Bevis

Fitting cosmic microwave background data with cosmic strings and inflation.

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 power-law tilt n, we find a 2-sigma detection of strings: f_10 = 0.11 +/- 0.05, where f_10 is the fractional contribution made by strings in the temperature power spectrum (at multipole l = 10). CMB data give moderate preference to the model n = 1 with cosmic strings over the standard zero-strings model with variable tilt. When additional non-CMB data are incorporated, the two models become on a par. With variable n and these extra data, we find that f_10<0.11, which corresponds to G mu<0.7x10^-6 (where mu is the string tension and G is the gravitational constant).

CMB power spectrum contribution from cosmic strings using field-evolution simulations of the Abelian Higgs model

We present the first field-theoretic 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 one-dimensional 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 energy-momentum tensor, the unequal-time 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 required to normalize to the WMAP 3-year data at multipole [script-l]=10 is G=[2.040.06(stat.)0.12(sys.)]10-6, where we have quoted statistical and systematic errors separately, and G is Newtons constant. This is a factor 23 higher than values in current circulation.

CMB power spectra from cosmic strings: predictions for the Planck satellite and beyond

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 sub-WMAP angular scales. These smaller scales are relevant for the now-operational 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 Einstein-Boltzmann calculations in order to yield accurate results over the multipole range 2 <= l <= 4000. Our results suggest that power-law behavior cuts in for l greater than or similar to 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 Silk-damped primary adiabatic contribution for l greater than or similar to 3500. Astrophysical contributions such as the Sunyaev-Zeldovich effect also become important at these scales and will reduce the sensitivity to strings, but these are potentially distinguishable by their frequency-dependence.

Abelian Higgs cosmic strings: Small-scale structure and loops

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 small-scale physics is captured correctly. The results confirm analytic predictions of Polchinski and Rocha [29] for the two-point correlation function of the string tangent vector, with a power law from length scales of order the string core width up to horizon scale. An analysis of the size distribution of string loops gives a very low number density, of order 1 per horizon volume, in contrast with Nambu-Goto simulations. Further, our loop distribution function does not support the detailed analytic predictions for loop production derived by Dubath et al. [30]. Better agreement to our data is found with a model based on loop fragmentation [32], 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 Nambu-Goto and field theory simulations.

CMB polarization power spectra contributions from a network of cosmic strings

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 B-mode polarization slightly greater. We explore a future ground-based 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 < 0.12x10(-6) (where G is the gravitational constant), above which it is likely that a detection would be possible.

Cosmic string parameter constraints and model analysis using small scale Cosmic Microwave Background data

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 high-resolution 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, GμAH < 0.57 × 10−6, both at 95% confidence level using WMAP7 data; and AH < 0.048 and GμAH < 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 2.4σ even if strings are present. A Bayesian model selection analysis no longer indicates a preference for strings.

The extragalactic submillimetre population: predictions for the SCUBA Half-Degree Extragalactic Survey (SHADES)

We present predictions for the angular correlation function and redshift distribution for SHADES, the SCUBA Half-Degree Extragalactic Survey, which will yield a sample of around 300 submillimetre sources in the 850-μm 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) guaranteed-time Spitzer data at mid-infrared wavelengths, and (iii) far-infrared maps to be produced by BLAST, the Balloon-borne Large-Aperture 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 sky-averaged angular correlation function to produce a more robust fit of a power-law 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.

WMAP constraints on inflationary models with global defects

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 first-year 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 high-redshift deuterium abundance limits the extent of the degeneracy and gives an upper bound on the defect fraction of 0.13 (95% confidence).

Cosmic string Y-junctions: A comparison between field theoretic and Nambu-Goto dynamics

We explore the formation of cosmic string Y-junctions 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 Nambu-Goto action, which is not strictly valid close to the junction. We find that the Nambu-Goto action performs only moderately well at predicting when the collision results in the formation of a pair of Y-junctions (with a composite string connecting them). However, we find that when they do form, the late time dynamics matches those of the Nambu-Goto approximation very closely. We also see little radiative emission from the Y-junction 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.

Evolution and stability of cosmic string loops with Y-junctions

Neil Bevis, ∗ Edmund J. Copeland, † Pierre-Yves Martin, 4, ‡ Gustavo Niz, § Alkistis Pourtsidou, ¶ Paul M. Saffin, ∗∗ and D. A. Steer †† Theoretical Physics, Blackett Laboratory, Imperial College, Prince Consort Road, London, SW7 2BZ, United Kingdom School of Physics & Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom APC, University Paris 7, 10, Rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France Glaizer Group, 32 rue Guy Moquet, 92240 Malakoff, France (Dated: October 9, 2018)