Ander Urio

We present a discussion on lattice techniques for the simulation of non-canonical field theory circumstances, complementing our previous monograph [1] on canonical cases. We begin by reviewing basic aspects of lattice field theory, including symplectic and non-symplectic evolution algorithms. We then introduce lattice implementations of non-canonical interactions, considering scalars with a non-minimal coupling to gravity, ϕ ² R , non-minimal scalar kinetic theories, 𝒢 _ab ({ ϕ _c }) ∂ _μ ϕ ^a ∂ ^μ ϕ ^b , and axion-like particle (ALP) interactions with Abelian gauge fields, ϕF _μν F̃ ^μν . Next, we discuss methods to set up special field configurations, including the creation of cosmic defect networks towards scaling (e.g. cosmic strings and domain walls), field configurations based on arbitrary power spectra or spatial profiles, and probabilistic methods as required e.g. for thermal configurations. We further extend the notion of non-canonical theories, discussing the discretization of scalar field dynamics in d + 1 dimensions, with d ≠ 3. Unrelated to non-canonical aspects, we also discuss implementation(s) of gravitational wave (GW) dynamics on the lattice. This document represents the theoretical basis for the non-canonical field theory aspects (interactions, initial conditions, dimensionality) and GW dynamics implemented in 𝒞 osmo ℒ attice v2.0 to be released in 2026.

Recent works have demonstrated the necessity of capturing the local inhomogeneous physics in axion inflation, and showed new genuine features, most notably the extension of the inflationary period dictated by an electromagnetic slow-roll phase. In this work, we further investigate the model by performing a systematic study of the effect of the inflationary potential in the dynamics during the strong backreaction regime. The results indicate that the novel features associated with the local backreaction are universal and intrinsic to the model, hence independent on the choice of inflationary potential. We find that the main quantitative differences between the different choices manifest in the lengthening of inflation. We discuss the possible observational impact of this. Finally, we assess the possible reconciliation of the homogeneous backreaction method with fully inhomogeneous lattice techniques, and obtain that the former fails to provide a correct description for the regime studied in this work.

Kinetic misalignment, one of the most compelling scenarios for the non-thermal generation of axion dark matter, is generally accompanied by axion fragmentation, a process in which the energy of the axion condensate is transferred to its perturbations. The dynamics of fragmentation, at least in the context of dark matter production, have so far been studied semi-analytically using perturbation theory. In this work, we present the first classical lattice simulation of kinetic axion fragmentation in the context of dark matter production, focusing on parameters relevant to the QCD axion. Our findings indicate that the non-perturbative dynamics captured by the lattice lead to a significantly broader spectrum of axion fluctuations, with a sustained transfer of energy to mildly relativistic modes and with smaller occupation numbers compared to the linear approximation. As a consequence, the final dark matter abundance is typically 𝒪(1) lower than in the linear approximation, which is itself 𝒪(1) lower than the zero-mode-only prediction. This broadening and suppression of the spectrum could have a significant impact on axion mini-halo formation, one of the main experimental handles on kinetic fragmentation.

Recent works have demonstrated the necessity of capturing the local inhomogeneous physics in axion inflation, and showed new genuine features, most notably the extension of the inflationary period dictated by an electromagnetic slow-roll phase. In this work, we further investigate the model by performing a systematic study of the effect of the inflationary potential in the dynamics during the strong backreaction regime. The results indicate that the novel features associated with the local backreaction are universal and intrinsic to the model, hence independent on the choice of inflationary potential. We find that the main quantitative differences between the different choices manifest in the lengthening of inflation. We discuss the possible observational impact of this. Finally, we assess the possible reconciliation of the homogeneous backreaction method with fully inhomogeneous lattice techniques, and obtain that the former fails to provide a correct description for the regime studied in this work.

We study the nonlinear 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 nontachyonic 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.

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.

Axioi Inflazioa produkzioaren informazio-fitxa

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.