Extending and testing observational signatures of Cosmic Strings

149 p.Soka kosmikoak unibertsoaren lehendabiziko garaietan gertatzen ziren prozesuen fisika ulertzeko oso erabilgarriak izan daitezke, hauek baitira kosmologiaren eredu estandarraren eta partikulen fisika duten energia altuko ereduen arteko zubi-lana egiteko hautagai interesgarrienetakoak. Hau dela eta, soka kosmikoek sor ditzaketen ondorio behagarriak ahalik eta modu zehatzenean ulertzeak eta hauek behaketa esperimentalekin konparatzeak, berebiziko garrantzia dute. Tesi honetan zehar soka kosmikoen froga esperimentalak eta ondorio behagarrien iragarpenak hobetu ditugu. Tesiaren lehenengo zatian sokakosmikoek sortutako seinaleen aztarnak neurketa esperimental zehatzetan aurkitzen saiatu gara, batetikkosmologia eredu estandarreko bestelako osagaiekin izan ditzaketen erlazioak aztertuz, eta bestetik,mikrouhinen hondo kosmikoaren BB polarizazioan izan dezaketen garrantzia neurtuz. Tesiaren bigarrenzatian berriz, eremu-teorien zenbakizko simulazioak erabili ditugu soka kosmikoen inguruko iragarpenak berritzeko, zehazki: sokek sor dezaketen CMB seinalearen deskribapen hobea lortzeko eta supersokateorietan ondorioztatutako soka-sareen propietateak hobeki ulertze

Approach to scaling in axion string networks

We study the approach to scaling in axion string networks in the radiation era, through measuring the root-mean-square velocity $v$ as well as the scaled mean string separation $x$. We find good evidence for a fixed point in the phase-space analysis in the variables $(x,v)$, providing a strong indication that standard scaling is taking place. We show that the approach to scaling can be well described by a two parameter velocity-one-scale (VOS) model, and show that the values of the parameters are insensitive to the initial state of the network. The string length has also been commonly expressed in terms of a dimensionless string length density $\zeta$, proportional to the number of Hubble lengths of string per Hubble volume. In simulations with initial conditions far from the fixed point $\zeta$ is still evolving after half a light-crossing time, which has been interpreted in the literature as a long-term logarithmic growth. We show that all our simulations, even those starting far from the fixed point, are accounted for by a VOS model with an asymptote of $\zeta_*=1.20\pm0.09$ (calculated from the string length in the cosmic rest frame) and $v_* = 0.609\pm 0.014$.Comment: 14 pages, 9 figures. v2: Minor changes, matches published versio

Scaling Density of Axion Strings

In the QCD axion dark matter scenario with postinflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time t, by convention written zeta/t(2). The expectation has been that the dimensionless parameter zeta tends to a constant zeta(0), a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations zeta shows a logarithmic increase with time, while theoretical modeling suggests an inverse logarithmic correction. Either case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings, we compare the standard scaling (constant-zeta) model to the logarithmic growth and inverse-logarithmic correction models. In the standard scaling model, by fitting to linear growth in the mean string separation xi = t/root zeta, we find zeta(0) = 1.19 +/- 0.20. We conclude that the apparent corrections to zeta are artifacts of the initial conditions, rather than a property of the scaling network. The residuals from the constant-zeta (linear xi) fit also show no evidence for logarithmic growth, restoring confidence that numerical simulations can be simply extrapolated from the Peccei-Quinn symmetry-breaking scale to the QCD scale. Reanalysis of previous work on the axion number density suggests that recent estimates of the axion dark matter mass in the postinflationary symmetry-breaking scenario we study should be increased by about 50%.Peer reviewe

Irreducible background of gravitational waves from a cosmic defect network : Update and comparison of numerical techniques

Cosmological phase transitions in the early Universe may produce relics in the form of a network of cosmic defects. Independently of the order of a phase transition, topology of the defects, and their global or gauge nature, the defects are expected to emit gravitational waves (GWs) as the network energy-momentum tensor adapts itself to maintaining scaling. We show that the evolution of any defect network (and for that matter any scaling source) emits a GW background with spectrum Omega(GW) proportional to f(3) for f > f(eq), where f(0) and f(eq) denote respectively the frequencies corresponding to the present and matter-radiation equality horizons. This background represents an irreducible emission of GWs from any scaling network of cosmic defects, with its amplitude characterized only by the symmetry-breaking scale and the nature of the defects. Using classical lattice simulations we calculate the GW signal emitted by defects created after the breaking of a global symmetry O(N) -> O(N - 1). We obtain the GW spectrum for N between 2 and 20 with two different techniques: integrating over unequal-time correlators of the energy-momentum tensor, updating our previous work on smaller lattices, and for the first time, comparing the result with the real-time evolution of the tensor perturbations sourced by the same defects. Our results validate the equivalence of the two techniques. Using cosmic microwave background upper bounds on the defects' energy scale, we discuss the difficulty of detecting this GW background in the case of global defects.Peer reviewe

Energy-momentum correlations for Abelian Higgs cosmic strings

We report on the energy-momentum correlators obtained with recent numerical simulations of the Abelian Higgs model, essential for the computation of cosmic microwave background and matter perturbations of cosmic strings. Due to significant improvements both in raw computing power and in our parallel simulation framework, the dynamical range of the simulations has increased fourfold both in space and time, and for the first time we are able to simulate strings with a constant physical width in both the radiation and matter eras. The new simulations improve the accuracy of the measurements of the correlation functions at the horizon scale and confirm the shape around the peak. The normalization is slightly higher in the high wave-number tails, due to a small increase in the string density. We study, for the first time, the behavior of the correlators across cosmological transitions and discover that the correlation functions evolve adiabatically; i.e., the network adapts quickly to changes in the expansion rate. We propose a new method for constructing source functions for Einstein-Boltzmann integrators, comparing it with two other methods previously used. The new method is more consistent, easier to implement, and significantly more accurate.Peer reviewe

Scaling from gauge and scalar radiation in Abelian-Higgs string networks

We investigate cosmic string networks in the Abelian Higgs model using data from a campaign of large-scale numerical simulations on lattices of up to 4096(3) grid points. We observe scaling or self-similarity of the networks over a wide range of scales, and estimate the asymptotic values of the mean string separation in horizon length units xi and of the mean square string velocity v(-2) in the continuum and large time limits. The scaling occurs because the strings lose energy into classical radiation of the scalar and gauge fields of the Abelian Higgs model. We quantify the energy loss with a dimensionless radiative efficiency parameter, and show that it does not vary significantly with lattice spacing or string separation. This implies that the radiative energy loss underlying the scaling behaviour is not a lattice artefact, and justifies the extrapolation of measured network properties to large times for computations of cosmological perturbations. We also show that the core growth method, which increases the defect core width with time to extend the dynamic range of simulations, does not introduce significant systematic error. We compare xi and v(-2) to values measured in simulations using the Nambu-Goto approximation, finding that the latter underestimate the mean string separation by about 25%, and overestimate v(-2) by about 10%. The scaling of the string separation implies that string loops decay by the emission of massive radiation within a Hubble time in field theory simulations, in contrast to the Nambu-Goto scenario which neglects this energy loss mechanism. String loops surviving for only one Hubble time emit much less gravitational radiation than in the Nambu-Goto scenario and are consequently subject to much weaker gravitational wave constraints on their tension.Peer reviewe

New CMB constraints for Abelian Higgs cosmic strings

We present cosmic microwave background (CMB) power spectra from recent numerical simulations of cosmic strings in the Abelian Higgs model and compare them to CMB power spectra measured by Planck. We obtain revised constraints on the cosmic string tension parameter Gμ. For example, in the ΛCDM model with the addition of strings and no primordial tensor perturbations, we find Gμ < 2.0 × 10−7 at 95% confidence, about 20% lower than the value obtained from previous simulations, which had 1/64 of the spatial volume. The increased computational volume also makes it possible to simulate fully the physical equations of motion, in which the string cores shrink in comoving coordinates. We find however that this, and the larger dynamic range, changes the amplitude of the power spectra by only about 10%. The main cause of the stronger constraints on Gμ is instead an improved treatment of the string evolution across the radiation-matter transition

Type I Abelian Higgs strings : Evolution and cosmic microwave background constraints

We present results from the first simulations of networks of Type 1 Abelian Higgs cosmic strings to include both matter and radiation eras and cosmic microwave background (CMB) constraints. In Type I strings, the string tension is a slowly decreasing function of the ratio of the scalar and gauge mass-squared, beta. We find that the mean string separation shows no dependence on beta, and that the energy-momentum tensor correlators decrease approximately in proportion to the square of the string tension, with additional 0(1) correction factors which asymptote to constants below beta less than or similar to 0.01. Strings in models with low self-couplings can therefore satisfy current CMB bounds at higher symmetry-breaking scales. This is particularly relevant for models where the gauge symmetry is broken in a supersymmetric flat direction, for which the effective self-coupling can be extremely small. If our results can be extrapolated to beta similar or equal to 10(-15), even strings formed at 10(16) GeV (approximately the grand unification scale in supersymmetric extensions of the Standard Model) can be compatible with CMB constraints.Peer reviewe