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19 pages, 846 KiB

Open AccessArticle

Scaling Invariance of Perturbations in k-Inflation Models

by Neven Bilić, Dragoljub D. Dimitrijević, Goran S. Djordjević, Milan Milošević and Marko Stojanović

Universe 2025, 11(4), 128; https://doi.org/10.3390/universe11040128 - 9 Apr 2025

Viewed by 153

Abstract

We study the background and perturbations in k-essence inflation models and show that a general k-essence exhibits a simple scaling property. In particular, we study two classes of k-inflation models with the potential characterized by an inflection point. We demonstrate [...] Read more.

We study the background and perturbations in k-essence inflation models and show that a general k-essence exhibits a simple scaling property. In particular, we study two classes of k-inflation models with the potential characterized by an inflection point. We demonstrate that these models enjoy scaling properties that could be used to redefine input parameters so that the perturbations spectra satisfy the correct normalization at the CMB pivot scale. The background and perturbation equations are integrated numerically for two specific models. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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15 pages, 412 KiB

Open AccessArticle

Gauss–Bonnet-Induced Symmetry Breaking/Restoration During Inflation

by Yermek Aldabergenov and Daulet Berkimbayev

Universe 2025, 11(3), 98; https://doi.org/10.3390/universe11030098 - 17 Mar 2025

Viewed by 280

Abstract

We propose a mechanism for symmetry breaking or restoration that can occur in the middle of inflation, due to the coupling of the Gauss–Bonnet term to a charged scalar. The Gauss–Bonnet coupling results in an inflaton-dependent effective squared mass of the charged scalar, [...] Read more.

We propose a mechanism for symmetry breaking or restoration that can occur in the middle of inflation, due to the coupling of the Gauss–Bonnet term to a charged scalar. The Gauss–Bonnet coupling results in an inflaton-dependent effective squared mass of the charged scalar, which can change its sign (around the symmetric point) during inflation. This can lead to spontaneous breaking of the symmetry, or to its restoration, if it is initially broken. We show the conditions under which the backreaction of the Gauss–Bonnet coupling on the inflationary background is negligible, such that the predictions of a given inflationary model are unaffected by the symmetry breaking/restoration process. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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40 pages, 1110 KiB

Open AccessArticle

Time Scales of Slow-Roll Inflation in Asymptotically Safe Cosmology

by József Nagy, Sándor Nagy and Kornél Sailer

Universe 2025, 11(3), 77; https://doi.org/10.3390/universe11030077 - 21 Feb 2025

Viewed by 308

Abstract

Making use of the well-known renormalization group (RG) scale dependences of the gravitational couplings in the framework of the two-parameter Einstein–Hilbert (EH) theory of gravity, the single scalar field-driven cosmological inflation is discussed in a spatially homogeneous, isotropic, and flat model universe. The [...] Read more.

Making use of the well-known renormalization group (RG) scale dependences of the gravitational couplings in the framework of the two-parameter Einstein–Hilbert (EH) theory of gravity, the single scalar field-driven cosmological inflation is discussed in a spatially homogeneous, isotropic, and flat model universe. The inflaton field is represented by a one-component real, non-self-interacting, massive scalar field minimally coupled to gravity. Cases without and with the incorporation of the RG scaling of the inflaton mass are compared with each other and with the corresponding classical case. It is shown that the quantum improvement drastically alters the timing of the slow-roll inflation with the desirable number

\underset{,}{N} \approx 60

e-foldings, as compared with the classical case. Furthermore, accounting for the RG flow of the inflaton mass has an enormous effect on the timing of the desirable slow roll, too. Although providing the desirable slow-roll inflation, none of the versions of the investigated quantum-improved toy models provide a realistic value of the amplitude of the scalar perturbations. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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43 pages, 1224 KiB

Open AccessArticle

Regularized Stress Tensor of Vector Fields in de Sitter Space

by Yang Zhang and Xuan Ye

Universe 2025, 11(2), 72; https://doi.org/10.3390/universe11020072 - 18 Feb 2025

Viewed by 352

Abstract

We study the Stueckelberg field in de Sitter space, which is a massive vector field with the gauge fixing (GF) term

\frac{1}{2 ζ} {({A^{μ}}_{; μ})}^{2}

. We obtain the vacuum stress tensor, which consists of the transverse, [...] Read more.

We study the Stueckelberg field in de Sitter space, which is a massive vector field with the gauge fixing (GF) term

\frac{1}{2 ζ} {({A^{μ}}_{; μ})}^{2}

. We obtain the vacuum stress tensor, which consists of the transverse, longitudinal, temporal, and GF parts, and each contains various UV divergences. By the minimal subtraction rule, we regularize each part of the stress tensor to its pertinent adiabatic order. The transverse stress tensor is regularized to the 0th adiabatic order, while the longitudinal, temporal, and GF stress tensors are regularized to the 2nd adiabatic order. The resulting total regularized vacuum stress tensor is convergent and maximally symmetric, has a positive energy density, and respects the covariant conservation, and thus, it can be identified as the cosmological constant that drives the de Sitter inflation. Under the Lorenz condition

{A^{μ}}_{; μ} = 0

, the regularized Stueckelberg stress tensor reduces to the regularized Proca stress tensor that contains only the transverse and longitudinal modes. In the massless limit, the regularized Stueckelberg stress tensor becomes zero, and is the same as that of the Maxwell field with the GF term, and no trace anomaly exists. If the order of adiabatic regularization were lower than our prescription, some divergences would remain. If the order were higher, say, under the conventional 4th-order regularization, more terms than necessary would be subtracted off, leading to an unphysical negative energy density and the trace anomaly simultaneously. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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12 pages, 252 KiB

Open AccessArticle

The Geometric Proca–Weyl Field as a Candidate for Dark Matter

by Mauro Duarte, Fábio Dahia and Carlos Romero

Universe 2025, 11(2), 34; https://doi.org/10.3390/universe11020034 - 22 Jan 2025

Viewed by 625

Abstract

We consider the Weyl invariant theory of gravity as an alternative approach to the problem of the origin of dark matter. According to this theory, the geometric Weyl 1-form effectively behaves as a Proca field. In this work, our starting point is to [...] Read more.

We consider the Weyl invariant theory of gravity as an alternative approach to the problem of the origin of dark matter. According to this theory, the geometric Weyl 1-form effectively behaves as a Proca field. In this work, our starting point is to consider the existence of a gas of Weyl–Proca particles in a Bose–Einstein condensate and investigate its behavior in a cosmological context. The results obtained show that, for appropriate values of the free parameter of the model, the Weyl field behaves approximately as a dust fluid in the matter-dominated era as expected for a dark matter candidate. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

15 pages, 353 KiB

Open AccessArticle

Constraints on Metastable Dark Energy Decaying into Dark Matter

by Jônathas S. T. de Souza, Gustavo S. Vicente and Leila L. Graef

Universe 2024, 10(9), 371; https://doi.org/10.3390/universe10090371 - 18 Sep 2024

Cited by 3 | Viewed by 1109

Abstract

We revisit the proposal that an energy transfer from dark energy into dark matter can be described in field theory by a first order phase transition. We analyze a metastable dark energy model proposed in the literature, using updated constraints on the decay [...] Read more.

We revisit the proposal that an energy transfer from dark energy into dark matter can be described in field theory by a first order phase transition. We analyze a metastable dark energy model proposed in the literature, using updated constraints on the decay time of a metastable dark energy from recent data. The results of our analysis show no prospects for potentially observable signals that could distinguish this scenario from the

Λ CDM

. We analyze, for the first time, the process of bubble nucleation in this model, showing that such model would not drive a complete transition to a dark matter dominated phase even in a distant future. Nevertheless, the model is not excluded by the latest data and we confirm that the mass of the dark matter particle that would result from such a process corresponds to the mass of an axion-like particle, which is currently one of the best motivated dark matter candidates. We argue that extensions to this model, possibly with additional couplings, still deserve further attention as it could provide an interesting and viable description for an interacting dark sector scenario based in a single scalar field. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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6 pages, 226 KiB

Open AccessCommunication

The de Sitter Swampland Conjectures in the Context of Chaplygin-Inspired Inflation

by Orfeu Bertolami, Robertus Potting and Paulo M. Sá

Universe 2024, 10(7), 271; https://doi.org/10.3390/universe10070271 - 23 Jun 2024

Cited by 2 | Viewed by 819

Abstract

In this work, we discuss the de Sitter swampland conjectures in the context of the generalized Chaplygin-inspired inflationary model. We demonstrate that these conjectures can be satisfied, but only in the region of the parameter space far away from the General Relativity limit. [...] Read more.

In this work, we discuss the de Sitter swampland conjectures in the context of the generalized Chaplygin-inspired inflationary model. We demonstrate that these conjectures can be satisfied, but only in the region of the parameter space far away from the General Relativity limit. The cosmic microwave background data had already been found to restrict the allowed inflationary potentials of this model. Our results impose a further limitation on the possible potentials. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

10 pages, 263 KiB

Open AccessArticle

Cosmic Strings from Thermal Inflation

by Robert Brandenberger and Aline Favero

Universe 2024, 10(6), 253; https://doi.org/10.3390/universe10060253 - 4 Jun 2024

Viewed by 928

Abstract

Thermal inflation was proposed as a mechanism to dilute the density of cosmological moduli. Thermal inflation is driven by a complex scalar field possessing a large vacuum expectation value and a very flat potential, called a “flaton”. Such a model admits cosmic string [...] Read more.

Thermal inflation was proposed as a mechanism to dilute the density of cosmological moduli. Thermal inflation is driven by a complex scalar field possessing a large vacuum expectation value and a very flat potential, called a “flaton”. Such a model admits cosmic string solutions, and a network of such strings will inevitably form in the symmetry breaking phase transition at the end of the period of thermal inflation. We discuss the differences of these strings compared to the strings which form in the Abelian Higgs model. Specifically, we find that the upper bound on the symmetry breaking scale is parametrically lower than in the case of Abelian Higgs strings, and that the lower cutoff on the string loop distribution is determined by cusp annihilation rather than by gravitational radiation (for the value of the transition temperature proposed in the original work on thermal inflation). Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

21 pages, 370 KiB

Open AccessArticle

The Equation of State of Novel Double-Field Pure K-Essence for Inflation, Dark Matter and Dark Energy

by Changjun Gao

Universe 2024, 10(6), 235; https://doi.org/10.3390/universe10060235 - 24 May 2024

Cited by 2 | Viewed by 952

Abstract

K-essence theories are usually studied in the framework of a single scalar field

ϕ

. Namely, the Lagrangian of K-essence is the function of the single scalar field

ϕ

and its covariant derivative. However, in this paper, we explore a double-field pure K-essence, [...] Read more.

K-essence theories are usually studied in the framework of a single scalar field

ϕ

. Namely, the Lagrangian of K-essence is the function of the single scalar field

ϕ

and its covariant derivative. However, in this paper, we explore a double-field pure K-essence, i.e., the corresponding Lagrangian is the function of covariant derivatives of double scalar fields without a dependency on scalar fields themselves. This is why we call it double-field pure K-essence. The novelty of this K-essence is that its Lagrangian contains the quotient term of the kinetic energies from the two scalar fields. This results in the presence of many interesting features; for example, the equation of state can be arbitrarily small and arbitrarily large. In comparison, the range of the equation of state for quintessence is

- 1

to

+ 1

. Interestingly, this novel K-essence can play the role of an inflation field, dark matter, or dark energy by appropriately selecting the expressions of Lagrangian. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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