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Wang, D.G.; Pimentel, G.; Achúcarro, A. 2023

Bootstrapping multi-field inflation: non-Gaussianities from light scalars revisited

Article / Letter to editor

open access

Achúcarro, A.; Céspedes, S.; Davis, A.C.; Palma, G.A. 2022

The hand-made tail: non-perturbative tails from multifield inflation

Article / Letter to editor

open access

It is becoming increasingly clear that large but rare fluctuations of the primordial curvature field, controlled by the tail of its probability distribution, could have dramatic effects on the... Show moreIt is becoming increasingly clear that large but rare fluctuations of the primordial curvature field, controlled by the tail of its probability distribution, could have dramatic effects on the current structure of the universe - e.g. via primordial black-holes. However, the use of standard perturbation theory to study the evolution of fluctuations during inflation fails in providing a reliable description of how non-linear interactions induce non-Gaussian tails. Here, we use the stochastic inflation formalism to study the non-perturbative effects from multi-field fluctuations on the statistical properties of the primordial curvature field. Starting from the effective action describing multi-field fluctuations, we compute the joint probability density function and show that enhanced non-Gaussian tails are a generic feature of slow-roll inflation with additional degrees of freedom. Show less

Cañas Herrera, G.; Torrado Cacho, J.; Achúcarro, A. 2021

Bayesian reconstruction of the inflaton's speed of sound using CMB data

Article / Letter to editor

open access

Achúcarro, A.; Palma, G.A.; Wang, D.; Welling, Y. 2020

Origin of ultra-light fields during inflation and their suppressed non-Gaussianity

Article / Letter to editor

open access

Achúcarro, A.; Copeland, E.J.; Iarygina, O.; Palma, G.A.; Wang, D.-G.; Welling, Y. 2020

Shift-symmetric orbital inflation: Single field or multifield?

Article / Letter to editor

open access

We present a new class of two-field inflationary attractor models, known as shift-symmetric orbital inflation, whose behavior is strongly multifield but whose predictions are remarkably close to those of single-field inflation. In these models, the field space metric and potential are such that the inflaton trajectory is along an “angular” isometry direction whose “radius” is constant but arbitrary. As a result, the radial (isocurvature) perturbations away from the trajectory are exactly massless and they freeze on superhorizon scales. These models are the first exact realization of the “ultra-light isocurvature” scenario, previously described in the literature, where a combined shift symmetry emerges between the curvature and isocurvature perturbations and results in primordial perturbation spectra that are entirely consistent with current observations. Due to the turning trajectory, the radial perturbation sources the tangential (curvature) perturbation and makes it grow linearly in time. As a result, only one degree of freedom (i.e., the one from isocurvature modes) is responsible for the primordial observables at the end of inflation, which yields the same phenomenology as in single-field inflation. In particular, isocurvature perturbations and local non-Gaussianity are highly suppressed here, even if the inflationary dynamics is truly multifield. We comment on the generalization to models with more than two fields.

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Achúcarro, A.; Avgoustidis, A.; Láopez-Eiguren, A.; Martins, C.J.A.P.; Urrestilla, J. 2019

Cosmological evolution of semilocal string networks

Article / Letter to editor

open access

Semilocal strings—a particular limit of electroweak strings—are an interesting example of a stable non-topological defect whose properties resemble those of their topological cousins, the Abrikosov... Show moreSemilocal strings—a particular limit of electroweak strings—are an interesting example of a stable non-topological defect whose properties resemble those of their topological cousins, the Abrikosov–Nielsen–Olesen vortices. There is, however, one important difference: a network of semilocal strings will contain segments. These are ‘dumbbells’ whose ends behave almost like global monopoles that are strongly attracted to one another. While closed loops of string will eventually shrink and disappear, the segments can either shrink or grow, and a cosmological network of semilocal strings will reach a scaling regime. We discuss attempts to find a ‘thermodynamic’ description of the cosmological evolution and scaling of a network of semilocal strings, by analogy with well-known descriptions for cosmic strings and for monopoles. We propose a model for the time evolution of an overall length scale and typical velocity for the network as well as for its segments, and some supporting (preliminary) numerical evidence.This article is part of a discussion meeting issue ‘Topological avatars of new physics’. Show less

Iarygina, O.; Sfakianakis, E.; Wang, D.-G.; Achúcarro, A. 2019

Universality and scaling in multi-field alpha-attractor preheating

Article / Letter to editor

open access

We explore preheating in multi-field models of inflation in which the field-space metric is a highly curved hyperbolic manifold. One broad family of such models is called alpha-attractors, whose... Show moreWe explore preheating in multi-field models of inflation in which the field-space metric is a highly curved hyperbolic manifold. One broad family of such models is called alpha-attractors, whose single-field regimes have been extensively studied in the context of inflation and supergravity. We focus on a simple two-field generalization of the T-model, which has received renewed attention in the literature. Krajewski et al. concluded, using lattice simulations, that multi-field effects can dramatically speed-up preheating. We recover their results and further demonstrate that significant analytical progress can be made for preheating in these models using the WKB approximation and Floquet analysis. We find a simple scaling behavior of the Floquet exponents for large values of the field-space curvature, that enables a quick estimation of the T-model reheating efficiency for any large value of the field-space curvature. In this regime we further observe and explain universal preheating features that arise for different values of the potential steepness. In general preheating is faster for larger negative values of the field-space curvature and steeper potentials. For very highly curved field-space manifolds preheating is essentially instantaneous. Show less

Achúcarro, A.; Céspedes, S.; Davis, A.C.; Palma, G.A. 2019

Constraints on holographic multifield inflation and models based on the Hamilton-Jacobi formalism

Article / Letter to editor

open access

In holographic inflation, the 4D cosmological dynamics is postulated to be dual to the renormalization group flow of a 3D Euclidean conformal field theory with marginally relevant operators. The... Show moreIn holographic inflation, the 4D cosmological dynamics is postulated to be dual to the renormalization group flow of a 3D Euclidean conformal field theory with marginally relevant operators. The scalar potential of the 4D theory—in which inflation is realized—is highly constrained, with use of the Hamilton–Jacobi equations. In multifield holographic realizations of inflation, fields additional to the inflaton cannot display underdamped oscillations (that is, their wave functions contain no oscillatory phases independent of the momenta). We show that this result is exact, independent of the number of fields, the field space geometry, and the shape of the inflationary trajectory followed in multifield space. In the specific case where the multifield trajectory is a straight line or confined to a plane, it can be understood as the existence of an upper bound on the dynamical masses m of extra fields of the form m≤3H/2 up to slow roll corrections. This bound corresponds to the analytic continuation of the well-known Breitenlohner–Freedman bound found in anti–de Sitter spacetimes in the case when the masses are approximately constant. The absence of underdamped oscillations implies that a detection of “cosmological collider” oscillatory patterns in the non-Gaussian bispectrum would not only rule out single-field inflation, but also holographic inflation or any inflationary model based on the Hamilton–Jacobi equations. Hence, future observations have the potential to exclude, at once, an entire class of inflationary theories, regardless of the details involved in their model building. Show less

Achúcarro, A.; Palma, G.A. 2019

The string swampland constraints require multi-field inflation

Article / Letter to editor

open access

An important unsolved problem that affects practically all attempts to connect string theory to cosmology and phenomenology is how to distinguish effective field theories belonging to the string... Show moreAn important unsolved problem that affects practically all attempts to connect string theory to cosmology and phenomenology is how to distinguish effective field theories belonging to the string landscape from those that are not consistent with a quantum theory of gravity at high energies (the ``string swampland{''}). It was recently proposed that potentials of the string landscape must satisfy at least two conditions, the ``swampland criteria{''}, that severely restrict the types of cosmological dynamics they can sustain. The first criterion states that the (multi-field) effective field theory description is only valid over a field displacement Delta phi <= Delta similar to O (1) (in units where the Planck mass is 1), measured as a distance in the target space geometry. A second, more recent, criterion asserts that, whenever the potential V is positive, its slope must be bounded from below, and suggests vertical bar del V vertical bar/V >= c similar to O (1). A recent analysis concluded that these two conditions taken together practically rule out slow-roll models of inflation. In this note we show that the two conditions rule out inflationary backgrounds that follow geodesic trajectories in field space, but not those following curved, non-geodesic, trajectories (which are parametrized by a non-vanishing bending rate Omega of the multi-field trajectory). We derive a universal lower bound on Omega (relative to the Hubble parameter H) as a function of Delta,c and the number of efolds N-e, assumed to be at least of order 60. If later studies confirm c and Delta to be strictly O (1), the bound implies strong turns with Omega/H >= 3 N-e similar to 180. Slow-roll inflation in the landscape is not ruled out, but it is strongly multi-field.}} Show less