arXiv subjects

Jon Urrestilla

Publications and source records attributed to Jon Urrestilla.

At least 19 recordsLinked to original sources

arXiv2025

The spectrum of axions in a scaling string network

Cosmic strings formed when the Peccei-Quinn symmetry breaks post-inflation are expected to emit axions throughout their lifetime. The details of the evolution of this network and the associated spectrum of axions are crucial for obtaining an accurate axion mass prediction, thus guiding searches at haloscopes. In a previous publication, we obtained evidence for the standard scaling of axion string networks, showing that the number of horizon lengths of string per horizon volume asymptotes to an $\mathcal{O}(1)$ constant. In this article, we turn our attention to the axion spectra, studying spectra of all components of the axion current and their unequal time correlators. With the new information we are better able to distinguish the contributions from propagating axions from the field carried by the strings, and show that previous measurements of the axion energy spectrum based only on the timelike component of the current are approximately 30\% derived from the string fields. We introduce a simple model based on an ensemble of string segments, which accounts for the general features of the spectra and time correlations. We conclude that axion emission from a scaling string network is close to scale-invariant ($q \approx 1$), and that the energy spectrum of sub-horizon modes behaves as $p_\text{ax}\ln( k \tau)$, where $k$ is the comoving wavenumber, $\tau$ the conformal time and $p_\text{ax} \simeq 10$. The number density spectrum evolves towards a single curve for $k\tau \lesssim 10^2$, with higher wavenumber deviations arising from initial conditions and resonant axion production at the string width scale. The total number density of axions produced from strings is $n_\text{ax}=1.66(17) f_\text{a}^2 H$, where $f_\text{a}$ is the axion decay constant and $H$ the Hubble rate. We report on axion production from the final collapse of the network in a future work.

hep-ph↗

arXiv2024

The non-linear dynamics of axion inflation: a detailed lattice study

We study in detail the fully inhomogeneous non-linear dynamics of axion inflation, identifying three regimes: weak-, mild-, and strong-backreaction, depending on the duration of inflation. We use lattice techniques that explicitly preserve gauge invariance and shift symmetry, and which we validate against other computational methods of the linear dynamics and of the homogeneous backreaction regime. Notably, we demonstrate that the latter fails to accurately describe the truly local dynamics of strong backreaction. We investigate the convergence of simulations of local backreaction, determining the requirements to achieve an accurate description of the dynamics, and providing useful parametrizations of the delay of the end of inflation. Additionally, we identify key features emerging from a proper local treatment of strong backreaction: the dominance of magnetic energy against the electric counterpart, the excitation of the longitudinal mode, and the generation of a scale-dependent chiral (im)balance. Our results underscore the necessity to accurately capture the local nature of the non-linear dynamics of the system, in order to correctly assess phenomenological predictions, such as e.g. the production of gravitational waves and primordial black holes.

astro-ph.CO↗

arXiv2024

Scaling density of axion strings in terasite simulations

We report on a study of axion string networks using fixed-grid simulations of up to $16384$ points per side. The length of string can be characterised in terms of standard dimensionless parameters $\zeta_\text{w}$ and $\zeta_\text{r}$, the length density measured in the cosmic rest frame and the string rest frame, scaled with the cosmic time. The motion of the string can be characterised by the root-mean-square (RMS) velocity of the string. Starting from a range of initial length densities and velocities, we analyse the string network in the standard scaling framework and find evolution towards a fixed point with estimated values $\hat{\zeta}_{\text{w},*} = 1.220(57)$ and $\hat{\zeta}_{\text{r},*} = 1.491(93)$. The two measures are related by the RMS velocity, which we estimate to be $\hat{v}_{*} = 0.5705(93)$. The length density is consistent with previous measurements, while the velocity is about 5% lower. For simulations starting from low enough density, the length density parameters $\zeta_\text{w}$ and $\zeta_\text{r}$ remain below their fixed point values throughout, while growing slowly, giving rise to an impression of approximately logarithmic increase with time. This has been proposed as the true long-term behaviour. We find that the growth tends to slow down as the values of $\zeta_\text{w}$ and $\zeta_\text{r}$ identified as fixed points are approached. In the case of $\zeta_\text{r}$, the growth stops for simulations which started close to the fixed point length density. The difference between $\zeta_\text{w}$ and $\zeta_\text{r}$ can be understood to result from the continuing velocity evolution. Our results indicate that the growth of $\zeta_\text{w}$ is a transient appearing at low densities and while the velocity is converging. This highlights the importance of studying the string density and the velocity together, and the preparation of initial conditions.

hep-ph↗

arXiv2023

Nambu-Goto Dynamics of Field Theory Cosmic String Loops

We perform a detailed comparison of the dynamics of cosmic string loops obtained in cosmological field theory simulations with their expected motion according to the Nambu-Goto action. We demonstrate that these loops follow the trajectories predicted within the NG effective theory except in regions of high curvature where energy is emitted from the loop in the form of massive radiation. This energy loss continues for all the loops studied in this simulation until they self-intersect or become small enough that they annihilate and disappear well before they complete a single oscillation. We comment on the relevance of this investigation to the interpretation of the results from cosmological field theory simulations as well as their extrapolation to a cosmological context.

hep-th↗

arXiv2023

The dynamics of Domain Wall Strings

We study the dynamics of domain wall solitons in $(2+1)d$ field theories. These objects are extended along one of the spatial directions, so they also behave as strings; hence the name of domain wall strings. We show analytically and numerically that the amount of radiation from the propagation of wiggles on these objects is negligible except for regions of high curvature. Therefore, at low curvatures, the domain wall strings behave exactly as the Nambu-Goto action predicts. We show this explicitly with the use of several different numerical experiments of the evolution of these objects in a lattice. We then explore their dynamics in the presence of internal mode excitations. We do this again by performing field theory simulations and identify an effective action that captures the relevant interactions between the different degrees of freedom living on the string. We uncover a new parametric resonance instability that transfers energy from the internal mode to the position of the domain wall. We show that this instability accelerates the radiation of the internal mode energy. We also explore the possibility of exciting the internal mode of the soliton with the collision of wiggles on the domain wall. Our numerical experiments indicate that this does not happen unless the wiggles have already a wavelength of the order of the string thickness. Finally, we comment on the possible relevance of our findings to cosmological networks of defects. We argue that our results cast some doubts on the significance of the internal modes in cosmological applications beyond a brief transient period right after their formation. This, however, should be further investigated using cosmological simulations of our model.

hep-th↗

arXiv2023

The strong backreaction regime in axion inflation

We study the non-linear dynamics of axion inflation, capturing for the first time the inhomogeneity and full dynamical range during strong backreaction, till the end of inflation. Accounting for inhomogeneous effects leads to a number of new relevant results, compared to spatially homogeneous studies: i) the number of extra efoldings beyond slow roll inflation increases very rapidly with the coupling, ii) oscillations of the inflaton velocity are attenuated, iii) the tachyonic gauge field helicity spectrum is smoothed out (i.e.~the spectral oscillatory features disappear), broadened, and shifted to smaller scales, and iv) the non-tachyonic helicity is excited, reducing the chiral asymmetry, now scale dependent. Our results are expected to impact strongly on the phenomenology and observability of axion inflation, including gravitational wave generation and primordial black hole production.

astro-ph.CO↗

arXiv2023

Parametric Resonances in Axionic Cosmic Strings

In this letter we uncover a new parametric resonance of axionic cosmic strings. This process is triggered by the presence on the string of internal mode excitations that resonantly amplify the amplitude of transverse displacements of the string. We study this process by running numerical simulations that demonstrate the existence of this phenomenon in a $(3+1)$ dimensional lattice field theory and compare the results with the analytic expectations for the effective Lagrangian of the amplitude of these modes and their interactions. Finally, we also analyze the massless and massive radiation produced by these excited strings and comment on its relevance for the interpretation of the results of current numerical simulations of axionic cosmic string networks.

hep-th↗

arXiv2021

Comment on "More Axions from Strings''

We comment on a claim that axion strings show a long-term logarithmic increase in the number of Hubble lengths per Hubble volume [arXiv:2007.04990], thereby violating the standard "scaling" expectation of an O(1) constant. We demonstrate that the string density data presented in [arXiv:2007.04990] are consistent with standard scaling, at a string density consistent with that obtained by us [arXiv:1908.03522, arXiv:2102.07723] and other groups. A transient slow growth in Hubble lengths per Hubble volume towards its constant scaling value is explained by standard network modelling [arXiv:2102.07723].

astro-ph.CO↗

arXiv2021

Internal Excitations of Global Vortices

We investigate the spectrum of linearized excitations of global vortices in $2+1$ dimensions. After identifying the existence of localized excitation modes, we compute the decay time scale of the first two and compare the results to the numerical evolution of the full non-linear equations. We show numerically how the interaction of vortices with an external source of radiation or other vortices can excite these modes dynamically. We then simulate the formation of vortices in a phase transition and their interaction with a thermal bath estimating the amplitudes of these modes in each case. These numerical experiments indicate that even though, in principle, vortices are capable of storing a large amount of energy in these internal excitations, this does not seem to happen dynamically. We then explore the evolution of a network of vortices in an expanding (2+1) dimensional background, in particular in a radiation dominated universe. We find that vortices are still excited after the course of the cosmological evolution but again the level of excitation is very small. The extra energy in the vortices in these cosmological simulations never exceeds the $1\%$ level of the total mass of the core of the vortex.

hep-th↗

arXiv2021

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 $ζ$, proportional to the number of Hubble lengths of string per Hubble volume. In simulations with initial conditions far from the fixed point $ζ$ 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 $ζ_*=1.20\pm0.09$ (calculated from the string length in the cosmic rest frame) and $v_* = 0.609\pm 0.014$.

astro-ph.CO↗

arXiv2021

Loop decay in Abelian-Higgs string networks

We study the decay of cosmic string loops in the Abelian-Higgs model. We confirm earlier results that loops formed by intersections of infinite strings formed from random-field initial conditions disappear quickly, with lifetime proportional to their initial rest-frame length $\ell_\text{init}$. We study a population with $\ell_\text{init}$ up to $6000$ inverse mass units, and measure the proportionality constant to be $0.14\pm0.04$, independently of the initial lengths. We propose a new method to construct oscillating non-self intersecting loops from initially stationary strings, and show that by contrast these loops have lifetimes scaling approximately as $\ell_\text{init}^2$, in line with previous works on artificially created string configurations. We show that the oscillating strings have mean-square velocity $\bar{v}^2 \simeq 0.500 \pm 0.004$, consistent with the Nambu-Goto value of $1/2$, while the network loops have $\bar{v}^2 \simeq 0.40 \pm 0.04$. We argue that whatever the mechanism behind the network loop decay is, it is non-linear, can only be suppressed by careful tuning of initial conditions, and is much stronger than gravitational radiation. An implication is that one cannot use the Nambu-Goto model to derive robust constraints on the tension of field theory strings. We advocate parametrising the uncertainty as the fraction $f_\text{NG}$ of Nambu-Goto-like loops surviving to radiate gravitationally. None of the 31 large network loops created survived longer than 0.25 of their initial length, so one can estimate that $f_\text{NG}<0.1$ at $95$% confidence level. If the recently reported NANOgrav signal is due to cosmic strings, $f_\text{NG}$ must be greater than $10^{-3}$ in order not to violate bounds from the Cosmic Microwave Background.

astro-ph.CO↗

arXiv2021

Exciting the Domain Wall Soliton

Many solitonic configurations in field theory have localized bound states in their spectrum of linear perturbations. This opens up the possibility of having long lived excitations of these solitons that could affect their dynamics. We start the study of these effects in the simplest configuration of a domain wall kink solution in the $λϕ^4$ theory in $1+1$ dimensions. We show that this solution has a single bound state and numerically study its slow decay rate in flat space. We then investigate the amplitude of this excitation by simulating a cosmological phase transition that leads to the formation of these kinks in an expanding universe. We find that kinks get formed with a $20\%$ excess of energy with respect to their lowest energy configuration. We also explore the kink solution interacting with a thermal bath and extract the amplitude of the localized excitation as a function of temperature. We note that this amplitude increases with temperature but again the extra energy in the kink never goes over the $20\%$ level. Finally, we argue that this extra energy may have important consequences in the subsequent evolution of defects in numerical simulations.

hep-th↗

arXiv2020

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 $Ω_{\rm GW} \propto f^3$ for $f \ll f_0$, $Ω_{\rm GW} \propto 1/f^2$ for $f_0 \lesssim f \lesssim f_{\rm eq}$, and $Ω_{\rm GW} \propto~const$ (i.e.~exactly scale-invariant) for $f \gg f_{\rm eq}$, where $f_0$ and $ f_{\rm 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 $O(N) \rightarrow O(N-1)$. 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 CMB upper bounds on the defects' energy scale, we discuss the difficulty of detecting this GW background in the case of global defects.

astro-ph.CO↗

arXiv2020

Scalarization of asymptotically Anti-de Sitter black holes with applications to holographic phase transitions

We study the spontaneous scalarization of spherically symmetric, static and asymptotically Anti-de Sitter (aAdS) black holes in a scalar-tensor gravity model with non-mininal coupling of the form $ϕ^2\left(α{\cal R} + γ{\cal G}\right)$, where $α$ and $γ$ are constants, while ${\cal R}$ and ${\cal G}$ are the Ricci scalar and Gauss-Bonnet term, respectively. Since these terms act as an effective ``mass'' for the scalar field, non-trivial values of the scalar field in the black hole space-time are possible for {\it a priori} vanishing scalar field mass. In particular, we demonstrate that the scalarization of an aAdS black hole requires the curvature invariant $-\left(α{\cal R} + γ{\cal G}\right)$ to drop below the Breitenlohner-Freedman bound close to the black hole horizon, while it asymptotes to a value well above the bound. The dimension of the dual operator on the AdS boundary depends on the parameters $α$ and $γ$ and we demonstrate that -- for fixed operator dimension -- the expectation value of this dual operator increases with decreasing temperature of the black hole, i.e. of the dual field theory. When taking backreaction of the space-time into account, we find that the scalarization of the black hole is the dual description of a phase transition in a strongly coupled quantum system, i.e. corresponds to a holographic phase transition. A possible application are liquid-gas quantum phase transitions, e.g. in $^4$He. Finally, we demonstrate that extremal black holes with $AdS_2\times S^2$ near-horizon geometry {\it cannot support regular scalar fields on the horizon} in the scalar-tensor model studied here.

gr-qc↗

arXiv2020

The scaling density of axion strings

In the QCD axion dark matter scenario with post-inflationary 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 $ζ/t^2$. The expectation has been that the dimensionless parameter $ζ$ tends to a constant $ζ_0$, a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations $ζ$ shows a logarithmic increase with time, while theoretical modelling 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-$ζ$) 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 $ξ= t/\sqrtζ$, we find $ζ_0 = 1.19 \pm 0.20$. We conclude that the apparent corrections to $ζ$ are artefacts of the initial conditions, rather than a property of the scaling network. The residuals from the constant-$ζ$ (linear $ξ$) 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. Re-analysis of previous work on the axion number density suggests that recent estimates of the axion dark matter mass in the post-inflationary symmetry-breaking scenario we study should be increased by about 50%.

astro-ph.CO↗

arXiv2019

Type I Abelian Higgs strings: evolution and Cosmic Microwave Background constraints

We present results from the first simulations of networks of Type I 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, $β$. We find that the mean string separation shows no dependence on $β$, and that the energy-momentum tensor correlators decrease approximately in proportion to the square of the string tension, with additional O(1) correction factors which asymptote to constants below $β\lesssim 0.01$. 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 $β\simeq 10^{-15}$, 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.

astro-ph.CO↗

arXiv2017

Solitons and black hole in shift symmetric scalar-tensor gravity with cosmological constant

We demonstrate the existence of static, spherically symmetric globally regular, i.e. solitonic solutions of a shift-symmetric scalar-tensor gravity model with negative cosmological constant. The norm of the Noether current associated to the shift symmetry is finite in the full space-time. We also discuss the corresponding black hole solutions and demonstrate that the interplay between the scalar-tensor coupling and the cosmological constant leads to the existence of new branches of solutions. To linear order in the scalar-tensor coupling, the asymptotic space-time corresponds to an Anti-de Sitter space-time with a non-trivial scalar field on its conformal boundary. This allows the interpretation of our solutions in the context of the AdS/CFT correspondence. Finally, we demonstrate that - for physically relevant, small values of the scalar-tensor coupling - solutions with positive cosmological constant do not exist in our model.

gr-qc↗

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↗