arXiv subjects

Daniel G. Figueroa

Publications and source records attributed to Daniel G. Figueroa.

At least 19 recordsLinked to original sources

arXiv2026

Cosmic string gravitational wave backgrounds at LISA: II. Reconstruction of conventional signals over astrophysical foregrounds

We study the reconstruction of conventional cosmic-string signals with LISA in the presence of all major known astrophysical foregrounds expected in the LISA band. These include stellar-origin black-hole binaries (SOBHBs), galactic (WDs) and extragalactic (ExWDs) white dwarfs, extreme-mass-ratio-inspirals (EMRIs), and massive black-hole binaries (MBHBs). Using the Simulation-based Inference package GWBackFinder, we perform a joint inference on the LISA noise, foregrounds, and signal, across a range of injected string tensions $Gμ$. We find that reconstructing tensions with an error $\lesssim 10\%$ requires values as large as $Gμ\gtrsim 10^{-11}$, i.e. a factor $\sim10^5$ larger than previous estimates with no foregrounds, and $\sim 10^2$ larger compared to estimates accounting only for SOBHB and WD foregrounds. This work is the second in a series initiated in Ref. arXiv:2508.05395, which aims to quantify LISA's ability to measure representative cosmic-string models.

astro-ph.CO↗

arXiv2026

CosmoLattice 2.0

This paper introduces $\tt {\mathcal C}osmo{\mathcal L}attice$ $\tt v2.0$, a major upgrade that substantially broadens the physical scope and computational capabilities of the code. It introduces lattice implementations of scalar fields non-minimally coupled to gravity through $ϕ^2R$, as well as axion-like fields coupled to Abelian gauge sectors as $ϕF_{μν}\widetilde F^{μν}$. It also provides new procedures for generating specialized initial conditions, including scaling networks of cosmic defects ($\it e.g.$ strings and domain walls), and fields with arbitrary power spectra. The release also incorporates low-storage Runge-Kutta integrators for non-symplectic systems (suitable $\it e.g.$ for non-minimal scalar kinetic terms as $\mathcal{G}_{ab}\partial_μϕ^a\partial^μϕ^b$), scalar-field simulations on reduced $(1+1)$- and $(2+1)$-dimensional lattices, new optimized gravitational-wave evolution, more flexible field and energy-density outputs, and GPU support that can accelerate simulations by a factor $\mathcal{O}(10)$ relative to CPU execution. Extensive documentation on the use of the code is provided on https://www.cosmolattice.com

astro-ph.CO↗

arXiv2026

The art of simulating the early Universe. Part III: Scalar-Gauge-Fluid Dynamics

We discuss lattice methods for the simulation of fluid dynamics in the early Universe. This review represents a third entry in the monographic series on lattice cosmology techniques~\cite{Figueroa:2020rrl,Baeza-Ballesteros:2025tme}, which previously covered canonical and non-canonical field theory dynamics. Here, we first review the continuum theory of fluid dynamics in flat spacetime, and then in an FLRW background. We consider conservation and non-conservation forms of the equations of motion for fluids in isolation or coupled to scalar and/or gauge fields, and either fully relativistic or subrelativistic regimes of fluid bulk motion. After reviewing basic lattice concepts, we introduce detailed discretization schemes for fluid dynamics in expanding backgrounds for: $i)$ isolated perfect fluids, $ii)$ isolated imperfect (viscous) fluids, $iii)$ fluids coupled to gauge fields, and $iv)$ fluids coupled to scalar fields. Our evolution algorithms accommodate self-consistent expansion sourced by all scalar, gauge, and fluid sectors, preserving gauge invariance to machine precision in some cases. We also review lattice methods to set up the initial conditions for fluids, and the implementation of gravitational wave dynamics sourced by all scalar, gauge, and fluid degrees of freedom. This document represents the theoretical basis for the scalar-gauge-fluid module that will be publicly released as part of ${\mathcal C}{\tt osmo}{\mathcal L}{\tt attice}~{\tt v3.0}$ after publication of this monograph, check http://www.cosmolattice.com for updates.

astro-ph.CO↗

arXiv2026

Cosmic string gravitational wave backgrounds at LISA: I. Signal survey, template reconstruction, and model comparison

We present a catalog of gravitational wave background (GWB) signal templates from cosmic-string networks, based on relevant models proposed in the literature. We classify templates as conventional, based on standard cosmology and Nambu-Goto results (VOS and BOS), and beyond conventional, based on modifications of a) the loop number density (LRS, super, metastable, current-carrying strings), b) the expansion history (non-standard cosmologies, extra degrees of freedom, either thermal or secluded), or c) the loop properties (birth length, power emission). Using the SBI package $\texttt{GWBackFinder}$, we quantify the reconstruction precision of each signal by LISA, scanning over their parameter space, and performing model comparisons. For conventional signals, LISA reconstructs the tension $Gμ$ with an error $\lesssim 10\%$ for $Gμ\gtrsim 5\cdot 10^{-15}$, which decreases down to $2-3\%$ for $Gμ\gtrsim 10^{-12}$. BOS and VOS modelings become distinguishable confidently for $Gμ\gtrsim 5\cdot 10^{-13}$. For beyond-conventional signals, we identify SNR and error-threshold intervals for each parameter, and determine (for few examples) the regions where they can be distinguished from conventional signals. Analogous quality reconstruction studies of cosmic-string GWBs, superimposed over leading astrophysical foregrounds in the LISA window, will be presented in a series of upcoming papers.

astro-ph.CO↗

arXiv2026

Nanohertz Gravitational Waves

Evidence of a gravitational wave (GW) signal has emerged in pulsar timing array (PTA) data, opening a new window into the nanoHz GW Universe. We explore the physics of GW signals potentially explaining the data, with a primary focus on GW backgrounds (GWBs), considering both astrophysical and cosmological origins. We describe how: (i) An astrophysical nanoHz GWB emerges as the superposition of individual signals from inspiralling massive black-hole binaries (MBHBs); (ii) Environment coupling, eccentricity, and sparse sampling, affect the MBHB signal spectrum and statistical properties, causing great uncertainty in theoretical predictions, but simultaneously offering a handle to discriminate a potential astrophysical origin; (iii) PTA data offers unprecedented opportunities to constrain high-energy physics beyond the standard model, by probing early Universe GWBs, originated during or after inflation; (iv) Different early Universe GWBs, typically created by non-linear and out-of-equilibrium dynamics, can explain the PTA data, as e.g. from inflation scenarios, first order phase transitions, or topological defects; (v) The PTA detection of GWs opens a new window to explore the Universe, with profound implications for astrophysics and particle physics, probing e.g. the equation of state of the early Universe, the origin of the cosmological perturbations, the nature of the dark matter, or whether exotic objects like primordial black holes or cosmic strings exist.

astro-ph.CO↗

arXiv2026

Challenges and Opportunities of Gravitational Wave Searches above 10 kHz

The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines the challenges and gains expected in gravitational-wave searches at frequencies above the LIGO/Virgo band. The scarcity of possible astrophysical sources in most of this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising of these sources. We review several detector concepts that have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of a series of workshops on the topic of high-frequency gravitational wave detection, held in 2019 (ICTP, Trieste, Italy), 2021 (online) and 2023 (CERN, Geneva, Switzerland).

gr-qc↗

arXiv2026

The art of simulating the early Universe. Part II. Non-canonical cases & gravitational waves

We present a discussion on lattice techniques for the simulation of non-canonical field theory circumstances, complementing our previous monograph (arXiv:2006.15122) 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, $ϕ^2R$, non-minimal scalar kinetic theories, $\mathcal{G}_{ab}(\lbraceϕ_c\rbrace)\partial_μϕ^a\partial^μϕ^b$, and axion-like particle (ALP) interactions with Abelian gauge fields, $ϕF_{μν}\tilde F^{μν}$. 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 $d \neq 3$. 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 ${\mathcal C}$osmo${\mathcal L}$attice v2.0, to be released in 2026.

astro-ph.CO↗

arXiv2026

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↗

arXiv2025

Implications of stochastic effects for primordial black hole production in ultra-slow-roll inflation

We study the impact of stochastic noise on the generation of primordial black hole (PBH) seeds in ultra-slow-roll (USR) inflation with numerical simulations. We consider the non-linearity of the system by consistently taking into account the noise dependence on the inflaton perturbations, while evolving the perturbations on the coarse-grained background affected by the noise. We capture in this way the non-Markovian nature of the dynamics, and demonstrate that non-Markovian effects are subleading. Using the $ΔN$ formalism, we find the probability distribution $P(\mathcal{R})$ of the comoving curvature perturbation $\mathcal{R}$. We consider inflationary potentials that fit the CMB and lead to PBH dark matter with $i)$ asteroid, $ii)$ solar, or $iii)$ Planck mass, as well as $iv)$ PBHs that form the seeds of supermassive black holes. We find that stochastic effects enhance the PBH abundance by a factor of $\mathcal{O}(10)-\mathcal{O}(10^8)$, depending on the PBH mass. We also show that the usual approximation, where stochastic kicks depend only on the Hubble rate, either underestimates or overestimates the abundance by orders of magnitude, depending on the potential. We evaluate the gauge dependence of the results, discuss the quantum-to-classical transition, and highlight open issues of the application of the stochastic formalism to USR inflation.

astro-ph.CO↗

arXiv2025

Non-Gaussian tail of the curvature perturbation in stochastic ultra-slow-roll inflation: implications for primordial black hole production

We consider quantum diffusion in ultra-slow-roll (USR) inflation. Using the $ΔN$ formalism, we present the first stochastic calculation of the probability distribution $P(\mathcal{R})$ of the curvature perturbation during USR. We capture the non-linearity of the system, solving the coupled evolution of the coarse-grained background with random kicks from the short wavelength modes, simultaneously with the mode evolution around the stochastic background. This leads to a non-Markovian process from which we determine the highly non-Gaussian tail of $P(\mathcal{R})$. Studying the production of primordial black holes in a viable model, we find that stochastic effects during USR increase their abundance by a factor $\sim 10^5$ compared to the Gaussian approximation.

astro-ph.CO↗

arXiv2025

Gravitational wave and particle emission from a cosmic string loop: local case

Using lattice field simulations of the Abelian-Higgs model, we characterize the simultaneous emission of (scalar and gauge) particles and gravitational waves (GWs) by local string loops. We use {\it network} loops created in a phase transition, and {\it artificial} loops formed by either crossing straight-boosted or curved-static infinite strings. Loops decay via both particle and GW emission, on time scales $Δt_{\rm dec} \propto L^p$, where $L$ is the loop length. For particle production, we find $p \simeq 2$ for artificial loops and $p \simeq 1$ for network loops, whilst for GW emission, we find $p \simeq 1$ for all loops. We find that below a critical length, artificial loops decay primarily through particle production, whilst for larger loops GW emission dominates. However, for network loops, which represent more realistic configurations, particle emission always dominates, as supported by our data with length-to-core ratios up to $L/r_\text{c} \lesssim 6000$. Our results indicate that the GW background from a local string network should be greatly suppressed compared to estimations that ignore particle emission.

astro-ph.CO↗

arXiv2024

Gravitational Wave Emission from a Cosmic String Loop, I: Global Case

We study the simultaneous decay of global string loops into scalar particles (massless and massive modes) and gravitational waves (GWs). Using field theory simulations in flat space-time of isolated loops with initial length $\sim 80-1700$ times their core width, we determine the power emitted into scalar particles, $P_φ$, and GWs, $P_{\rm GW}$, and characterize the loop decay timescale as a function of its initial length, energy and angular momentum. We quantify infrared and ultraviolet lattice dependencies of our results. For all type of loops and initial conditions considered, GW emission is always suppressed compared to particles as $P_{\rm GW}/P_φ \approx \mathcal{O}(10)(v/m_\text{p})^2\ll 1$, where $v$ is the vacuum expectation value associated with string formation. These conclusions are robust for the length-to-width ratios considered, with no indication they should change if the ratio is increased. The results suggest that the GW background from a global string network, such as in dark matter axion scenarios, will be suppressed compared to previous expectations.

astro-ph.CO↗

arXiv2024

Fast Likelihood-free Reconstruction of Gravitational Wave Backgrounds

We apply state-of-the-art, likelihood-free statistical inference (machine-learning-based) techniques for reconstructing the spectral shape of a gravitational wave background (GWB). We focus on the reconstruction of an arbitrarily shaped signal by the LISA detector, but the method can be easily extended to either template-dependent signals, or to other detectors, as long as a characterisation of the instrumental noise is available. As proof of the technique, we quantify the ability of LISA to reconstruct signals of arbitrary spectral shape (${\it blind}$ reconstruction), considering a diversity of frequency profiles, and including astrophysical backgrounds in some cases. As a teaser of how the method can reconstruct signals characterised by a parameter-dependent template (${\it template}$ reconstruction), we present a dedicated study for power-law signals. While our technique has several advantages with respect to traditional MCMC methods, we validate it with the latter for concrete cases. This work opens the door for both fast and accurate Bayesian parameter estimation of GWBs, with essentially no computational overhead during the inference step. Our set of tools are integrated into the package ${\tt GWBackFinder}$, which is publicly available in https://github.com/AndronikiDimitriou/GWBackFinder.

astro-ph.CO↗

arXiv2024

Present and future of CosmoLattice

We discuss the present state and planned updates of CosmoLattice, a cutting-edge code for lattice simulations of non-linear dynamics of scalar-gauge field theories in an expanding background. We first review current capabilities of the code, including the simulation of interacting singlet scalars and of Abelian and non-Abelian scalar-gauge theories. We also comment on new features recently implemented, such as the simulation of gravitational waves from scalar and gauge fields. Secondly, we discuss new extensions of CosmoLattice that we plan to release publicly. On the one hand, we comment on new physics modules, which include axion-gauge interactions $ϕF \widetilde{F}$, non-minimal gravitational couplings $ϕ^2 R$, creation and evolution of cosmic defect networks, and magneto-hydro-dynamics (MHD). On the other hand, we discuss new technical features, including evolvers for non-canonical interactions, arbitrary initial conditions, simulations in 2+1 dimensions, and higher accuracy spatial derivatives.

astro-ph.CO↗

arXiv2024

Dynamics of Non-minimally Coupled Scalar Fields in the Jordan Frame

The presence of scalar fields with non-minimal gravitational interactions of the form $ξ|ϕ|^2 R$ may have important implications for the physics of the early universe. While many studies solve the dynamics of non-minimally coupled scalars in the Einstein frame, where gravity is simply described by the Einstein-Hilbert action, we instead propose a procedure to solve the dynamics directly in the original Jordan frame where the non-minimal couplings are maintained explicitly. Our algorithm can be applied to scenarios that include minimally coupled fields and an arbitrary number of non-minimally coupled scalars, with the expansion of the universe sourced by all fields present. This includes situations when the dynamics become fully inhomogeneous, fully non-linear (due to e.g.~backreaction or mode rescattering effects), and/or when the expansion of the universe is dominated by non-minimally coupled species. As an example, we study geometric preheating with a non-minimally coupled scalar spectator field when the inflaton oscillates following the end of inflation. In the future, our technique may be used to shed light on aspects of the equivalence of the Jordan and Einstein frames at the quantum level.

astro-ph.CO↗

arXiv2024

Geometric reheating of the Universe

We study the post-inflationary energy transfer from the inflaton ($ϕ$) into a scalar field ($χ$) non-minimally coupled to gravity through $ξR|χ|^2$, considering models with inflaton potential $V_{\rm inf} \propto |ϕ|^{\,p}$ around $ϕ= 0$. This corresponds to the paradigm of {\it geometric preheating}, which we extend to its non-linear regime via lattice simulations. Considering $α$-attractor T-model potentials as a proxy, we study the viability of proper {\it reheating} for $p=2, 4, 6$, determining whether radiation domination (RD) due to energetic dominance of $χ$ over $ϕ$, can be achieved. For large inflationary scales $Λ$, reheating is frustrated for $p = 2$, it can be partially achieved for $p = 4$, and it becomes very efficient for $p = 6$. Efficient reheating can be however blocked if $χ$ sustains self-interactions (unless these are extremely feeble), or if $Λ$ is low enough, so that inflaton fragmentation brings the universe rapidly into RD. Whenever RD is achieved, either due to reheating or to inflaton fragmentation, we characterize the energy and time scales of the problem, as a function of $Λ$ and $ξ$.

astro-ph.CO↗

arXiv2024

Ricci Reheating on the Lattice

We study the dynamics of a non-minimally coupled (NMC) scalar spectator field in non-oscillatory inflationary scenarios, where there is a transition from inflation to kination domination (KD). Engineering a realistic finite-duration transition through a CMB-compatible inflaton potential, we calculate the initial tachyonic growth of the NMC field during KD and perform lattice simulations of the subsequent non-linear dynamics. We characterize the regularization effect on the tachyonic growth, either due to self-interactions, or via gravitational backreaction when the NMC field grows to dominate the energy of the universe. Our study provides the first realistic treatment of the dynamics, with significant improvements compared to previous work, where one or more of the following aspects were assumed: ($i$) the background expansion can be neglected during the tachyonic growth, ($ii$) coherence of the NMC field, ($iii$) coherence of the inflaton, ($iv$) instantaneous transition, and ($v$) a KD equation of state of exactly $w = 1$. Using our methodology, which requires none of the above assumptions, we determine the conditions to achieve proper reheating, i.e. energetic dominance of the NMC field over the inflaton. We characterize the time and energy scales of the problem, either for backreaction due to self-interactions, or (as a novelty of this work) due to gravitational effects. Finally, we calculate $\mathcal{O}(1)$ lattice correction factors to analytic scaling relations derived by some of us in previous work. This enables simple future studies without the need to run lattice simulations.

astro-ph.CO↗

arXiv2024

Cosmological Background Interpretation of Pulsar Timing Array Data

We discuss the interpretation of the detected signal by Pulsar Timing Array (PTA) observations as a gravitational wave background (GWB) of cosmological origin. We combine NANOGrav 15-years and EPTA-DR2new data sets and confront them against backgrounds from supermassive black hole binaries (SMBHBs), and cosmological signals from inflation, cosmic (super)strings, first-order phase transitions, Gaussian and non-Gaussian large scalar fluctuations, and audible axions. We find that scalar-induced, and to a lesser extent audible axion and cosmic superstring signals, provide a better fit than SMBHBs. These results depend, however, on modeling assumptions, so further data and analysis are needed to reach robust conclusions. Independently of the signal origin, the data strongly constrain the parameter space of cosmological signals, for example, setting an upper bound on primordial non-Gaussianity at PTA scales as $|f_{nl}| \lesssim 2.34$ at 95% CL.

astro-ph.CO↗