arXiv subjects

Ilya L. Shapiro

Publications and source records attributed to Ilya L. Shapiro.

At least 37 records · Page 2Linked to original sources

arXiv2024

Semiclassical bounce with strong minimal assumptions

We explore the possibility of avoiding cosmological singularity with a bounce solution in the early Universe. The main finding is that simple and well-known semiclassical correction, which describes the mixing of radiation and gravity in the effective action, may provide an analytic solution with a bounce. The solution requires a positive beta function for the total radiation term and the contraction of the Universe at the initial instant. The numerical estimate shows that the bounce may occur in an acceptable range of energies, but only under strong assumptions about the particle physics beyond the Standard Model.

gr-qc↗

arXiv2024

On the renormalization of massive vector field theory coupled to scalar in curved space-time

We consider the renormalization of massive vector field interacting with charged scalar field in curved spacetime. Starting with the theory minimally coupled to external gravity and using the formulations with and without Stückelberg fields, we show that the longitudinal mode of vector field is completely decoupled and the remaining theory of transverse vector field is renormalizable by power counting. The formal arguments based on the covariance and power counting indicate that multiplicative renormalizability of the interacting theory may require introducing two non-minimal terms linear in Ricci tensor in the vector sector. Nevertheless, a more detailed analysis shows that these non-minimal terms violate the decoupling of the longitudinal mode and are prohibited. As a verification of general arguments, we derive the one-loop divergences in the minimal massive scalar QED, using Stückelberg procedure and the heat-kernel technique. The theory without non-minimal terms proves one-loop renormalizable and admits the renormalization group equations for all the running parameters in the scalar and vector sectors. One-loop beta functions do not depend on the gauge fixing and can be used to derive the effective potential.

hep-th↗

arXiv2024

Scale-dependent cosmology from effective quantum gravity in the invariant framework

We explore the possibility of a consistent cosmology based on the gauge-fixing independent running of the gravitational and cosmological constants ($G$ and $Λ$) in the framework of effective quantum gravity. In particular, their running in this framework was found to satisfy $G \propto Λ^4$. In the cosmological setting, the covariance of the theory provides energy conservation relations, which are impossible to satisfy with the unique scale parameter. However, by introducing the second sub-dominant scale corresponding to the higher-loop corrections and higher-derivative terms, one can close the system of equations for the running of parameters and arrive at the consistent cosmological solutions. This approach yields a change in the cosmological expansion history that affects the ratio of the Hubble parameter today to the Hubble parameter at high redshift.

gr-qc↗

arXiv2023

Kantoswki-Sachs model with a running cosmological constant and radiation

The simplest anisotropic model of the early Universe is the one with two conformal factors, which can be identified as Kantowski-Sachs metric, or the reduced version of the Bianchi-I metric. To fit the existing observational data, it is important that the anisotropy is washed out in the early stage of the evolution. We explore the possible effect of the running cosmological constant (RCC) on the dynamics of isotropy, in the case of the space filled by radiation.

gr-qc↗

arXiv2023

On the local term in the anomaly-induced action of Weyl quantum gravity

The finite local conformally non-invariant $R^2$-term emerges in the one-loop effective action of the model of quantum gravity based on the Weyl-squared classical action. This term is related to the $\Box R$ contribution to the conformal anomaly, which in a wide class of regularization schemes is determined by the second Schwinger-DeWitt (or Gilkey-Seeley) coefficient of the heat kernel expansion for inverse propagators of the theory. The calculation of this term requires evaluating the contributions of the fourth-order derivative minimal and of the second-order nonminimal operators in the tensor and vector sectors of the theory, corresponding to metric, ghost and gauge-fixing operators. To ensure the correctness of existing formulas, we derived (and confirmed) the result using a special technique of calculations, based on the heat-kernel representation of the Euclidean Green's function and the method of universal functional traces.

hep-th↗

arXiv2023

Effective approach to the Antoniadis-Mottola model: quantum decoupling of the higher derivative terms

We explore the decoupling of massive ghost mode in the $4D$ (four-dimensional) theory of the conformal factor of the metric. The model was introduced by Antoniadis and Mottola in [1] and can be regarded as a close analog of the fourth-derivative quantum gravity. The analysis of the derived one-loop nonlocal form factors includes their asymptotic behavior in the UV and IR limits. In the UV (high energy) domain, our results reproduce the Minimal Subtraction scheme-based beta functions of [1]. In the IR (i.e., at low energies), the diagrams with massive ghost internal lines collapse into tadpole-type graphs without nonlocal contributions and become irrelevant. On the other hand, those structures that contribute to the running of parameters of the action and survive in the IR, are well-correlated with the divergent part (or the leading in UV contributions to the form factors), coming from the effective low-energy theory of the conformal factor. This effective theory describes only the light propagating mode. Finally, we discuss whether these results may shed light on the possible running of the cosmological constant at low energies.

hep-th↗

arXiv2023

Trace anomaly and induced action for a metric-scalar background

The conformal anomaly and anomaly-induced effective action represent useful and economic ways to describe semiclassical contributions to the action of gravity. We discuss the anomaly in the case when the background is formed by metric and scalar fields and formulate the induced action in two standard covariant forms. The analysis of induced action at low energies reveals existing connection to the renormalization group and effective potential. The classification of anomalous terms is extended to the scalar background and ambiguities in the total derivative terms in the anomaly are considered using Pauli-Villars regularization.

hep-th↗

arXiv2022

Covariant derivative of fermions and all that

We present detailed pedagogical derivation of covariant derivative of fermions and some related expressions, including commutator of covariant derivatives and energy-momentum tensor of a free Dirac field. On top of that, local conformal transformations for a Dirac fermion in curved spacetime are considered and we obtain the expression for the energy-momentum tensor on the cosmological background.

gr-qc↗

arXiv2022

The background information about perturbative quantum gravity

The purpose of this Chapter is to give a general introduction and status review on the perturbative approach to quantum gravity (QG). This text is a modified version of the corresponding chapters of Part II of the recent textbook on quantum field theory (QFT) and QG, co-authored with I.L. Buchbinder and published in Oxford University Press. We discuss the choice of the starting action in the QG models, degrees of freedom and propagator of metric perturbations, power counting and renormalizability of these models, the problems related to higher derivative theories and ghosts, such as quantum unitarity and the stability of classical solutions in general relativity; and the perspective to overcome these problems. The gauge fixing and parametrization dependencies are discussed in detail using the corresponding general QFT theorems developed in gauge theories. On top of that, we present a basic example of deriving the one-loop divergences and discuss an important example of the renormalization group in QG. The gauge invariant renormalizability of QG is considered in another Chapter of the Handbook, written together with P.M. Lavrov.

hep-th↗

arXiv2022

Nonlocality of quantum matter corrections and cosmological constant running

Semiclassical contributions to the gravitational action include various terms with zero, two, and four derivatives of the metric as well as nonlocal form factors for these terms. Contributions to some of these terms could be confused with others on a specific metric background or for a particular gauge fixing. We present a critical analysis of the recent works where the tensor structure and the number of derivatives in the action of gravity were not properly taken into account. Taking these relevant aspects into account, we show that although some contributions owing to the quantum fluctuations of massive or massless scalar as well as fermion and vector fields may be attributed to the "running" of the cosmological constant, in reality they correspond to the fourth derivative terms of the action.

gr-qc↗

arXiv2022

Effective quantum gravity, cosmological constant and the Standard Model of particle physics

The renormalization group in effective quantum gravity can be consistently formulated using the Vilkovisky and DeWitt version of effective action and assuming a non-zero cosmological constant. Taking into account that the vacuum counterpart of the cosmological constant is dramatically different from the observed energy density of vacuum, the running of the last quantity in the late cosmology indicates strong constraints on the physics beyond the minimal Standard Model of particle physics.

hep-th↗

arXiv2022

Anomaly-induced vacuum effective action with torsion: covariant solution and ambiguities

The conformal anomaly in curved spacetime with antisymmetric torsion is reconsidered, taking into account new important details. We formulate, for the first time, the covariant solution of the anomaly-induced effective action. The covariant effective action includes local terms corresponding to total derivatives in the conformal anomaly. The contribution of massless fermions to these terms is characterized by multiplicative anomaly, coming from two different choices of doubling for the spinor operator. On the other hand, the nonlocal part of anomaly-induced action is free of ambiguities and admits a low-energy limit, similar to the effective potential in the metric-scalar theory.

hep-th↗

arXiv2022

Inflation with sterile scalar coupled to massive fermions and to gravity

In the recent paper [3] it was shown that the consistency of the quantum theory of a sterile scalar coupled to massive fermions requires the inclusion of odd-power terms in the potential of scalar self-interaction. One of the most important examples of a sterile scalar is the inflaton, that is typically a real scalar field which does not belong to representations of particle physics gauge groups, such as $SU(2)$. Here we explore the effects of the odd-power terms in the inflaton potential on main observables, such as the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$, in the case of a strong, non-minimal coupling of the inflaton to gravity.

hep-th↗

arXiv2022

Universal leading quantum correction to the Newton potential

The derivation of effective quantum gravity corrections to Newton's potential is an important step in the whole effective quantum field theory approach. We hereby add new strong arguments in favor of omitting all the diagrams with internal lines of the massive sources, and we also recalculate the corrections to the Newtonian potential using functional methods in an arbitrary parametrization of the quantum fluctuations of the metric. The general proof of the gauge- and parametrization-independence within this approach is also explicitly given. On top of that, we argue that the universality of the result holds regardless of the details of the ultraviolet completion of quantum gravity theory. Indeed, it turns out that the logarithm quantum correction depends only on the low energy spectrum of the theory that is responsible for the analytic properties of loop's amplitudes.

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arXiv2021

The running vacuum in effective quantum gravity

We briefly review the previous works on the renormalization group in quantum general relativity with the cosmological constant, based on the Vilkovisky and DeWitt version of effective action. On top of that, we discuss the prospects of the applications of this version of renormalization group to the cosmological models with a running Newton constant and vacuum energy density.

gr-qc↗

arXiv2021

Renormalization Group in Six-derivative Quantum Gravity

The exact one-loop beta functions for the four-derivative terms (Weyl tensor squared, Ricci scalar squared and the Gauss-Bonnet) are derived for the minimal six-derivative quantum gravity (QG) theory in four spacetime dimensions. The calculation is performed by means of the Barvinsky and Vilkovisky generalized Schwinger-DeWitt technique. With this result we gain, for the first time, the full set of the relevant beta functions in a super-renormalizable model of QG. The complete set of renormalization group (RG) equations, including also those for the Newton and the cosmological constant, is solved explicitly in the general case and for the six-derivative Lee-Wick (LW) quantum gravity proposed in a previous paper by two of the authors. In the ultraviolet regime, the minimal theory is shown to be asymptotically free and describes free gravitons in Minkowski or (anti-) de Sitter ((A)dS) backgrounds, depending on the initial conditions for the RG equations. The ghostlike states appear in complex conjugate pairs at any energy scale consistently with the LW prescription. However, owing to the running, these ghosts may become tachyons. We argue that an extension of the theory that involves operators cubic in Riemann tensor may change the beta functions and hence be capable of overcoming this problem.

hep-th↗

arXiv2021

On the vector conformal models in an arbitrary dimension

The conventional model of the gauge vector field is invariant under the local conformal symmetry only in the four-dimensional space ($4d$). Conformal generalization to an arbitrary dimension $d$ is impossible even for the free theory, differently from scalar and fermion fields. We discuss how to overcome this restriction and eventually construct four vector conformal actions. One of these models is the particular case of the previously known conformal theory of $n$-forms and others are new, up to our knowledge. In some of these models the gauge invariance is preserved, two of the new models are described by local actions with auxiliary compensating scalar fields, and the extended version of one of these models is on shell equivalent to the last, non-analytic, purely metric version.

hep-th↗

arXiv2020

Bounce and stability in the early cosmology with anomaly-induced corrections

An extremely fast exponential expansion of the Universe is typical for the stable version of the inflationary model, based on the anomaly-induced action of gravity. The total amount of exponential $e$-folds could be very large, before the transition to the unstable version and the beginning of the Starobinsky inflation. Thus, the stable exponential expansion can be seen as a pre-inflationary semiclassical cosmological solution. We explore whether this stable phase could follow after the bounce, subsequent to the contraction of the Universe. Extending the previous consideration of the bounce, we explore both stable expansion and the bounce solutions in the models with non-zero cosmological constant and the presence of background radiation. The critical part of the analysis concerns stability for small perturbations of the Hubble parameter. It is shown that the stability is possible for the variations in the bounce region, but not in the sufficiently distant past in the contraction phase.

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