arXiv subjects

Jon Urrestilla

Publications and source records attributed to Jon Urrestilla.

At least 37 records · Page 2Linked to original sources

arXiv2017

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 $\dotξ$ and of the mean square string velocity $\bar v^2$ 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 $\dotξ$ and $\bar v^2$ to values measured in simulations using the Nambu-Goto approximation, finding that the latter underestimate the mean string separation by about 25%, and overestimate $\bar v^2$ 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.

astro-ph.CO↗

arXiv2017

Cosmic Microwave Background constraints for global strings and global monopoles

We present the first CMB power spectra from numerical simulations of the global O(N) linear $σ$-model with N = 2,3, which have global strings and monopoles as topological defects. In order to compute the CMB power spectra we compute the unequal time correlators (UETCs) of the energy-momentum tensor, showing that they fall off at high wave number faster than naive estimates based on the geometry of the defects, indicating non-trivial (anti-)correlations between the defects and the surrounding Goldstone boson field. We obtain source functions for Einstein-Boltzmann solvers from the UETCs, using a recent method that improves the modelling at the radiation- matter transition. We show that the interpolation function that mimics the transition is similar to other defect models, but not identical, confirming the non-universality of the interpolation function. The CMB power spectra for global strings and monopoles have the same overall shape as those obtained using the non-linear $σ$-model approximation, which is well captured by a large-N calculation. However, the amplitudes are larger than the large-N calculation predict, and in the case of global strings much larger: a factor of 20 at the peak. Finally we compare the CMB power spectra with the latest CMB data to put limits on the allowed contribution to the temperature power spectrum at multipole $\ell$ = 10 of 1.7% for global strings and 2.4% for global monopoles. These limits correspond to symmetry-breaking scales of 2.9x1015 GeV (6.3x1014 GeV with the expected logarithmic scaling of the effective string tension between the simulation time and decoupling) and 6.4x1015 GeV respectively. The bound on global strings is a significant one for the ultra-light axion scenario with axion masses ma 10-28 eV. These upper limits indicate that gravitational wave from global topological defects will not be observable at the GW observatory LISA.

astro-ph.CO↗

arXiv2017

Measuring Global Monopole Velocities, one by one

We present an estimation of the average velocity of a network of global monopoles in a cosmological setting using large numerical simulations. In order to obtain the value of the velocity, we improve some already known methods, and present a new one. This new method estimates individual global monopole velocities in a network, by means of detecting each monopole position in the lattice and following the path described by each one of them. Using our new estimate we can settle an open question previously posed in the literature: velocity-dependent one-scale (VOS) models for global monopoles predict two branches of scaling solutions, one with monopoles moving at subluminal speeds and one with monopoles moving at luminal speeds. Previous attempts to estimate monopole velocities had large uncertainties and were not able to settle that question. Our simulations find no evidence of a luminal branch. We also estimate the values of the parameters of the VOS model. With our new method we can also study the microphysics of the complicated dynamics of individual monopoles. Finally we use our large simulation volume to compare the results from the different estimator methods, as well as to asses the validity of the numerical approximations made.

hep-ph↗

arXiv2017

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 $Λ$CDM model with the addition of strings and no primordial tensor perturbations, we find $Gμ< 2.0 \times 10^{-7}$ at 95% confidence, about 20% lower than the value obtained from previous simulations, which had 1/64 of the spatial volume. We investigate the source of the difference, showing that the main cause is an improved treatment of the string evolution across the radiation-matter transition. The increased computational volume also makes possible to simulate fully the physical equations of motion, in which the string cores shrink in comoving coordinates. This, and the larger dynamic range, changes the amplitude of the power spectra by only about 10%, demonstrating that field theory simulations of cosmic strings have now reached the required dynamic range for CMB calculations.

astro-ph.CO↗

arXiv2017

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 four-fold 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 behaviour of the correlators across cosmological transitions, and discover that the correlation functions evolve adiabatically, ie 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.

astro-ph.CO↗

arXiv2016

Bubbles of Nothing and Supersymmetric Compactifications

We investigate the non-perturbative stability of supersymmetric compactifications with respect to decay via a bubble of nothing. We show examples where this kind of instability is not prohibited by the spin structure, i.e., periodicity of fermions about the extra dimension. However, such "topologically unobstructed" cases do exhibit an extra-dimensional analog of the well-known Coleman-De Luccia suppression mechanism, which prohibits the decay of supersymmetric vacua. We demonstrate this explicitly in a four dimensional Abelian-Higgs toy model coupled to supergravity. The compactification of this model to $M_3 \times S_1$ presents the possibility of vacua with different windings for the scalar field. Away from the supersymmetric limit, these states decay by the formation of a bubble of nothing, dressed with an Abelian-Higgs vortex. We show how, as one approaches the supersymmetric limit, the circumference of the topologically unobstructed bubble becomes infinite, thereby preventing the realization of this decay. This demonstrates the dynamical origin of the decay suppression, as opposed to the more familiar argument based on the spin structure. We conjecture that this is a generic mechanism that enforces stability of any topologically unobstructed supersymmetric compactification.

hep-th↗

arXiv2016

Survival of pq-superstrings in field theory simulations

We perform large-scale field theoretical simulations in expanding universe to characterize a network of strings that can form composed bound states. The network consists of two copies of Abelian Higgs strings (which we label p and q, respectively) coupled via a potential term to give pq bound states. The simulations are performed using two different kinds of initial conditions: the first one with a network of p- and q-strings, and the second one with a network of q- and pq-strings. This way, we start from two opposite situations: one with no initial pq-strings, and one with a large initial number of pq-strings. We find that in both cases the system scales, and in both cases the system prefers to have a low fraction of pq-strings. This is somewhat surprising in the case for the second type of conditions, showing that the unzipping mechanism is very efficient. We also find hints that both initial conditions tend to asymptote to a common configuration, though we would need a larger dynamical range to confirm it. The average velocities of the different types of strings in the network have also been explored for the first time.

astro-ph.CO↗

arXiv2014

Fitting BICEP2 with defects, primordial gravitational waves and dust

In this work we discuss the possibility of cosmic defects being responsible for the B-mode signal measured by the BICEP2 collaboration. We also allow for the presence of other cosmological sources of B-modes such as inflationary gravitational waves and polarized dust foregrounds, which might contribute to or dominate the signal. On the one hand, we find that defects alone give a poor fit to the data points. On the other, we find that defects help to improve the fit at higher multipoles when they are considered alongside inflationary gravitational waves or polarized dust. Finally, we derive new defect constraints from models combining defects and dust. This proceeding is based on previous works [1,2].

astro-ph.CO↗

arXiv2014

Constraining topological defects with temperature and polarization anisotropies

We analyse the possible contribution of topological defects to cosmic microwave anisotropies, both temperature and polarisation. We allow for the presence of both inflationary scalars and tensors, and of polarised dust foregrounds that may contribute to or dominate the B-mode polarisation signal. We confirm and quantify our previous statements that topological defects on their own are a poor fit to the B-mode signal. However, adding topological defects to a models with a tensor component or a dust component improves the fit around $\ell=200$. Fitting simultaneously to both temperature and polarisation data, we find that textures fit almost as well as tensors ($Δχ^2 = 2.0$), while Abelian Higgs strings are ruled out as the sole source of the B-mode signal at low $\ell$. The 95% confidence upper limits on models combining defects and dust are $Gμ< 2.7\times 10^{-7}$ (Abelian Higgs strings), $Gμ< 9.8\times 10^{-7}$ (semilocal strings) and $Gμ< 7.3\times 10^{-7}$ (textures), a small reduction on the $Planck$ bounds. The most economical fit overall is obtained by the standard $Λ$CDM model with a polarised dust component.

astro-ph.CO↗

arXiv2014

Can topological defects mimic the BICEP2 B-mode signal?

We show that the B-mode polarization signal detected at low multipoles by BICEP2 cannot be entirely due to topological defects. This would be incompatible with the high-multipole B-mode polarization data and also with existing temperature anisotropy data. Adding cosmic strings to a model with tensors, we find that B-modes on their own provide a comparable limit on the defects to that already coming from Planck satellite temperature data. We note that strings at this limit give a modest improvement to the best-fit of the B-mode data, at a somewhat lower tensor-to-scalar ratio of $r \simeq 0.15$.

astro-ph.CO↗

arXiv2013

Supermassive Cosmic String Compactifications

The space-time dimensions transverse to a static straight cosmic string with a sufficiently large tension (supermassive cosmic strings) are compact and typically have a singularity at a finite distance form the core. In this paper, we discuss how the presence of multiple supermassive cosmic strings in the 4D Abelian-Higgs model can induce the spontaneous compactification of the transverse space and explicitly construct solutions where the gravitational background becomes regular everywhere. We discuss the embedding of this model in N=1 supergravity and show that some of these solutions are half-BPS, in the sense that they leave unbroken half of the supersymmetries of the model.

hep-th↗

arXiv2013

Exact Scale-Invariant Background of Gravitational Waves from Cosmic Defects

We demonstrate that any scaling source in the radiation era produces a background of gravitational waves with an exact scale-invariant power spectrum. Cosmic defects, created after a phase transition in the early Universe, are such a scaling source. We emphasise that the result is independent of the topology of the cosmic defects, the order of phase transition, and the nature of the symmetry broken, global or gauged. As an example, using large-scale numerical simulations, we calculate the scale invariant gravitational wave power spectrum generated by the dynamics of a global O(N) scalar theory. The result approaches the large N theoretical prediction as N^(-2), albeit with a large coefficient. The signal from global cosmic strings is O(100) times larger than the large N prediction.

astro-ph.CO↗

arXiv2013

Gravitating cosmic strings with flat directions

We study field theoretical models for cosmic strings with flat directions in curved space-time. More precisely, we consider minimal models with semilocal, axionic and tachyonic strings, respectively. In flat space-time, the string solutions of these models have a flat direction, i.e., a uniparametric family of configurations with the same energy exists which is associated to a zero mode. We prove that the zero mode survives coupling to gravity, and study the role of the flat direction when coupling the strings to gravity. Even though the total energy of the solution is the same, and thus the global properties of the family of solutions remains unchanged, the energy density, and therefore the gravitational properties, are different. The local structure of the solutions depends strongly on the value of the parameter describing the flat direction; for example, for supermassive strings, the value of the free parameter can determine the size of the universe.

hep-th↗

arXiv2013

CMB Anisotropies by Collapsing Textures

CMB photons passing through a collapsing texture knot receive an energy shift, creating characteristic cold and hot spots on the sky. We calculate the anisotropy pattern produced by collapsing texture knots of arbitrary shape. The texture dynamics are solved numerically on a Minkowski background.

astro-ph.CO↗

arXiv2012

Correlations between cosmic strings and extra relativistic species

The recent observation that the Cosmic Microwave Background (CMB) may prefer a neutrino excess has triggered a number of works studying this possibility. The effect obtained by the non-interacting massless neutrino excess could be mimicked by some extra radiation component in the early universe, such as a cosmological gravitational wave background. Prompted by the fact that a possible candidate to source those gravitational waves would be cosmic strings, we perform a parameter fitting study with models which considers both cosmic strings and the effective number of neutrinos as free parameters, using CMB and non-CMB data. We find that there is a correlation between cosmic strings and the number of extra relativistic species, and that strings can account for all the extra radiation necessary. In fact, CMB data prefer strings at a 2sigma level, paying the price of a higher extra radiation component. CMB data also give a moderate preference for a model with ns=1. The inclusion of non-CMB data lowers both the preference for strings and for the extra relativistic species.

astro-ph.CO↗

arXiv2010

Detecting and distinguishing topological defects in future data from the CMBPol satellite

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 3σ level. We also confirm that a tensor contribution of r = 0.0018 can be detected at over 3σ, in agreement with the CMBpol mission concept study. These results are supported by a model selection analysis.

astro-ph.CO↗

arXiv2010

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 sub-orbital 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 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 Silk-damped primary adiabatic contribution for l > 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.

astro-ph.CO↗

arXiv2009

Gravitating Semilocal strings

We discuss the properties of semilocal strings minimally coupled to gravity. Semilocal strings are solutions of the bosonic sector of the Standard Model in the limit $\sin^2θ_W=1$ (where $θ_W$ is the Weinberg angle) and correspond to embedded Abelian-Higgs strings for a particular choice of the scalar doublet. We focus on the limit where the gauge boson mass is equal to the Higgs boson mass such that the solutions fulfill the Bogomolnyi-Prasad-Sommerfield (BPS) bound.

gr-qc↗