arXiv subjects

Y. Ageeva

Publications and source records attributed to Y. Ageeva.

9 recordsLinked to original sources

arXiv2024

The primordial non-Gaussianities for non-singular Horndeski cosmologies

We consider a novel Bounce Universe model constructed within the framework of Horndeski gravity. We have analyzed the bispectrum of primordial scalar perturbations and evaluated the corresponding non-Gaussianities for this specific model. The non-linear parameter $f_{\text{NL}}$ was computed for various wave vector configurations, including: (i) the local configuration, (ii) the equilateral configuration, and (iii) the enfolded configuration, which is a linear combination of the equilateral and orthogonal shapes. We demonstrate that the observational constraints on the scalar spectrum index tilt and the scalar-to-tensor ratio, together with the unitarity bounds, ensure that the bounds from non-Gaussianities are trivially satisfied. Therefore, this particular Bounce Universe scenario is fully viable in light of modern observational data.

hep-th↗

arXiv2024

Non-renormalizable theories and finite formulation of QFT

In this paper, we show how the finite formulation of QFT based on Callan-Symanzik equations can be generalised to the case of non-renormalizable theories. We derive an equation for effective action for an arbitrary single scalar field theory, allowing us to perform computations without running in intermediate divergencies. We illustrate the method with the use of $\lambda\phi^4 + \phi^6/M^2$ theory by the explicit (and fully finite) calculations of the effective potential as well as two-, four- and six-point correlation functions at one loop level and demonstrate that no quantum corrections to scalar mass $m^2$, depending on $M^2$-scale, are generated.

hep-th↗

arXiv2023

K-inflation: the legitimacy of classical treatment

In this paper we consider a general theory of k-inlation and find out, that it may be in strong coupling regime. We derive accurate conditions of classical description validity using unitarity bounds for this model. Next, we choose simple toy model of k-inflation and obtain the explicit condition, which guarantees that the generation of perturbations is performed in a controllable way, i.e the exit from the effective horizon occurs in the weak coupling regime. However, for the same toy model the corresponding experimental bounds on a non-linear parameter $f^{\text{equil}}_{\text{NL}}$ associated with non-Gaussianities of the curvature perturbation provide much stronger constraint than strong coupling absence condition. Nevertheless, for other known models of inflation this may not be the case. Generally, one should always check if classical description is legitimate for chosen models of inflation.

hep-th↗

arXiv2022

Generating cosmological perturbations in non-singular Horndeski cosmologies

We construct a concrete model of Horndeski bounce with strong gravity in the past. Within this model we show that the correct spectra of cosmological perturbations may be generated at early contracting epoch, with mild fine-tuning ensuring that the scalar spectral tilt $n_S$ and tensor-to-scalar ratio $r$ are consistent with observations. The smallness of $r$ is governed by the smallness of the scalar sound speed. Arbitrarily small values of $r$ are forbidden in our setup because of the strong coupling in the past. Nevertheless, we show that it is possible to generate perturbations in a controllable way, i.e. in the regime where the background evolution and perturbations are legitimately described within classical field theory and weakly coupled quantum theory.

hep-th↗

arXiv2022

On the strong coupling problem in cosmologies with "strong gravity in the past"

We examine the potential strong coupling problem at early times in a bouncing cosmological model with "strong gravity in the past" (Jordan frame), which is conformally related to inflation (Einstein frame). From naive dimensional analysis in the Jordan frame one would conclude that the quantum strong coupling energy scale can be lower than the classical energy scale. However, from the Einstein frame prospective this should not be the case. We illustrate this point by calculation in the Jordan frame which shows cancellations of the dangerous contributions in the tree level amplitude.

gr-qc↗

arXiv2022

Unitarity relation and unitarity bounds for scalars with different sound speeds

Motivated by scalar-tensor gravities, we consider a theory which contains massless scalar fields with different sound speeds. We derive unitarity relations for partial wave amplitudes of $2 \to 2$ scattering, with explicit formulas for contributions of two-particle intermediate states. Making use of these relations, we obtain unitarity bounds both in the most general case and in the case considered in literature for unit sound speed. These bounds can be used for estimating the strong coupling scale of a pertinent EFT. We illustrate our unitarity relations by explicit calculation to the first non-trivial order in couplings in a simple model of two scalar fields with different sound speeds.

hep-th↗

arXiv2021

Nonsingular cosmological models with strong gravity in the past

In scalar-tensor Horndeski theories, nonsingular cosmological models - bounce and genesis - are problematic because of potential ghost and/or gradient instabilities. One way to get around this obstacle is to send the effective Planck mass to zero in the asymptotic past ("strong gravity in the past"). One may suspect that this feature is a signal of a strong coupling problem at early times. However, the classical treatment of the cosmological background is legitimate, provided that the strong coupling energy scale remains at all times much higher than the scale associated with the classical evolution. We construct various models of this sort, namely (i) bouncing Universe which proceeds through inflationary epoch to kination (expansion within general relativity, driven by massless scalar field); (ii) bouncing Universe with kination stage immediately after bounce; (iii) combination of genesis and bounce, with the Universe starting from flat space-time, then contracting and bouncing to the expansion epoch; (iv) "standard" genesis evading the strong coupling problem in the past. All these models are stable, and perturbations about the backgrounds are not superluminal.

hep-th↗

arXiv2020

Horndeski genesis: consistency of classical theory

Genesis within the Horndeski theory is one of possible scenarios for the start of the Universe. In this model, the absence of instabilities is obtained at the expense of the property that coefficients, serving as effective Planck masses, vanish in the asymptotics $t\rightarrow -\infty$, which signalizes the danger of strong coupling and inconsistency of the classical treatment. We investigate this problem in a specific model and extend the analysis of cubic action for perturbations (arXiv:2003.01202) to arbitrary order. Our study is based on power counting and dimensional analysis of the higher order terms. We derive the latter, find characteristic strong coupling energy scales and obtain the conditions for the validity of the classical description. Curiously, we find that the strongest condition is the same as that obtained in already examined cubic case.

hep-th↗

arXiv2020

Toward evading the strong coupling problem in Horndeski genesis

It is of interest to understand whether or not one can construct a classical field theory description of early cosmology which would be free of the initial singularity and stable throughout the whole evolution. One of the known possibilities is genesis within the Horndeski theory, which is thought to be an alternative to or a possible completion of the inflationary scenario. In this model, the strong coupling energy scale tends to zero in the asymptotic past, $t \to - \infty$, making the model potentially intractable. We point out that despite the latter property, the classical setup may be trustworthy since the energy scale of the classical evolution (the inverse of its timescale) also vanishes as $t \to -\infty$. In the framework of a concrete model belonging to the Horndeski class, we show that the strong coupling energy scale of the cubic interactions vastly exceeds the classical energy scale in a certain range of parameters, indicating that the classical description is possible.

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