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Agullo, I., Navarro-Salas, J., & Parker, L. (2012). Enhanced local-type inflationary trispectrum from a non-vacuum initial state. J. Cosmol. Astropart. Phys., 05(5), 019–13pp.
Abstract: We compute the primordial trispectrum for curvature perturbations produced during cosmic inflation in models with standard kinetic terms, when the initial quantum state is not necessarily the vacuum state. The presence of initial perturbations enhances the trispectrum amplitude for configuration in which one of the momenta, say k(3), is much smaller than the others, k(3) << k(1,2,4). For those squeezed con figurations the trispectrum acquires the so-called local form, with a scale dependent amplitude that can get values of order epsilon(k(1)/k(3))(2). This amplitude could be larger than the prediction of the so-called Maldacena consistency relation by a factor as large as 10(6), and could reach the sensitivity of forthcoming observations, even for single-field inflationary models.
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Antusch, S., Marschall, K., & Torrenti, F. (2025). Equation of state during (p)reheating with trilinear. J. Cosmol. Astropart. Phys., 11(11), 002–35pp.
Abstract: We characterize the post-inflationary evolution of the equation of state of the universe from the end of inflation until the onset of radiation domination, when the inflaton is coupled to a daughter field through a trilinear interaction. We consider an inflaton potential that is quadratic near the minimum and flattens in the inflationary regime. By simulating the dynamics in 2+1-dimensional lattices, we have tracked the long-term evolution of the equation of state for about ten e-folds of expansion, for various coupling strengths. The trilinear interaction initially excites daughter field modes through a process of tachyonic resonance immediately after inflation and triggers a temporary deviation of the equation of state from (sic) = 0 to a maximum value (sic) = (sic)(max) < 1/3. However, at much later times, the inflaton homogeneous mode once again dominates the energy density, pushing the equation of state towards (sic) = 0 until the onset of perturbative reheating. By combining the lattice results with a Boltzmann approach, we characterize the entire post-inflationary expansion history, which allows to calculate precise predictions for the inflationary CMB observables. We also accurately compute the redshift of the stochastic gravitational wave background produced during preheating, and show that taking the temporary return of the equation of state towards (sic) = 0 into account can reduce the amplitude by many orders of magnitude relative to previous estimates.
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Baeza-Ballesteros, J., Figueroa, D. G., Florio, A., Lizarraga, J., Loayza, N., Marschall, K., et al. (2026). The art of simulating the early Universe. Part II. Non-canonical cases & gravitational waves. J. Cosmol. Astropart. Phys., 06(6), 087–119pp.
Abstract: 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 nonsymplectic evolution algorithms. We then introduce lattice implementations of non-canonical interactions, considering scalars with a non-minimal coupling to gravity, phi 2R, non-minimal scalar kinetic theories, Gab({phi c})partial derivative & micro;phi a partial derivative & micro;phi b, and axion-like particle (ALP) interactions with Abelian gauge fields, phi F & micro;nu F & micro;nu. 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 CosmoLattice v2.0 to be released in 2026.
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Barenboim, G. (2026). The noise of vacuum. J. Cosmol. Astropart. Phys., 06(6), 080–32pp.
Abstract: We investigate the evolution of primordial cosmological perturbations in a vacuum decay model where de Sitter space transitions to radiation domination through quantum-thermal decay processes. Unlike standard inflation, this framework generates curvature perturbations through stochastic noise from vacuum decay rather than quantum fluctuations of an inflaton field. We derive the stochastic differential equation governing the curvature perturbation R(t) and show that any horizon crossing is brief and does not constitute the primary mechanism for perturbation generation. Scale dependence emerges from spatial correlations in the noise rather than horizon crossing dynamics. The model naturally addresses the horizon and flatness problems through initial thermal equilibrium in de Sitter space and predicts zero tensor-to-scalar ratio. We demonstrate that spatially correlated noise can generate observationally viable spectral tilts while maintaining Gaussian statistics.
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Barenboim, G., & Park, W. I. (2016). New- vs. chaotic- inflations. J. Cosmol. Astropart. Phys., 02(2), 061–20pp.
Abstract: We show that “spiralized” models of new-inflation can be experimentally identified mostly by their positive spectral running in direct contrast with most chaotic-inflation models which have negative runnings typically in the range of O(10(-4)-10(-3)).
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