arXiv subjects

I. L. Shapiro

Publications and source records attributed to I. L. Shapiro.

At least 37 records · Page 2Linked to original sources

arXiv2001

On the Consistency of a Fermion-Torsion Effective Theory

We discuss the possibility to construct an effective quantum field theory for an axial vector coupled to a Dirac spinor field. A massive axial vector describes antisymmetric torsion. The consistency conditions include unitarity and renormalizability in the low-energy region. The investigation of the Ward identities and the one- and two-loop divergences indicate serious problems arising in the theory. The final conclusion is that torsion may exist as a string excitation, but there are very severe restrictions for the existence of a propagating torsion field, subject to the quantization procedure, at low energies.

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arXiv2001

On the high derivative fermionic operator and trace anomaly

We construct a new example of the high derivative four-dimensional conformal operator. This operator acts on fermions, and its contribution to the trace anomaly has opposite sign, as compared to conventional scalars, spinors and vectors. Possible generalizations and applications are discussed.

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arXiv2000

On the gravitational waves on the background of anomaly-induced inflation

In the very early Universe matter can be described as a conformal invariant ultra-relativistic perfect fluid, which does not contribute, on classical level, to the evolution of the isotropic and homogeneous metric. However, in this situation the vacuum effects of quantum matter fields become important. The vacuum effective action depends, essentially, on the particle content of the underlying gauge model. If we suppose that there is some desert in the particle spectrum, just below the Planck mass, then the effect of conformal trace anomaly is dominating at the corresponding energies. With some additional constraints on the gauge model (which favor extended or supersymmetric versions of the Standard Model rather than the minimal one), one arrives at the stable inflation. In this article we report about the calculation of the gravitational waves in this model. The result for the perturbation spectrum is close to the one for the conventional inflaton model, and is in agreement with the existing Cobe data.

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arXiv2000

Anomaly-Induced Effective Action and Inflation

In the early Universe matter can be described as a conformal invariant ultra-relativistic perfect fluid, which does not contribute, on classical level, to the evolution of the isotropic and homogeneous metric. If we suppose that there is some desert in the particle spectrum just below the Planck mass, then the effect of conformal trace anomaly is dominating at the corresponding energies. With some additional constraints on the particle content of the underlying gauge model (which favor extended or supersymmetric versions of the Standard Model rather than the minimal one), one arrives at the stable inflation. We review the model and report about the calculation of the gravitational waves on the background of the anomaly-induced inflation. The result for the perturbation spectrum is close to the one for the conventional inflaton model, and is in agreement with the existing Cobe data (see also [hep-th/0009197]).

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arXiv2000

Conformal symmetry, anomaly and effective action for metric-scalar gravity with torsion

We consider some aspects of conformal symmetry in a metric-scalar-torsion system. It is shown that, for some special choice of the action, torsion acts as a compensating field and the full theory is conformally equivalent to General Relativity on classical level. Due to the introduction of torsion, this equivalence can be provided for the positively-defined gravitational and scalar actions. One-loop divergences arising from the scalar loop are calculated and both the consequent anomaly and the anomaly-induced effective action are derived.

gr-qc↗

arXiv1999

Vacuum effective action and inflation

We consider vacuum quantum effects in the Early Universe, which may lead to inflation. The inflation is a direct consequence of the supposition that, at high energies, all the particles can be described by the weakly interacting, massless, conformally invariant fields. We discuss, from the effective field theory point of view, the stability of inflation, transition to the FRW solution, and also possibility to study metric and density perturbations.

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arXiv1999

Scalar QED $\hbar$-Corrections to the Coulomb Potential

The leading long-distance 1-loop quantum corrections to the Coulomb potential are derived for scalar QED and their gauge-independence is explicitly checked. The potential is obtained from the direct calculation of the 2-particle scattering amplitude, taking into account all relevant 1-loop diagrams. Our investigation should be regarded as a first step towards the same programme for effective Quantum Gravity. In particular, with our calculation in the framework of scalar QED, we are able to demonstrate the incompleteness of some previous studies concerning the Quantum Gravity counterpart.

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arXiv1998

Anomaly-induced effective action for gravity and inflation

In the very early Universe the matter may be described by the free radiation, that is by the set of massless fields with negligible interactions between them. Then the dominating quantum effect is the trace anomaly which comes from the renormalization of the conformal invariant part of the vacuum action. The anomaly-induced effective action can be found with accuracy to an arbitrary conformal functional which vanishes for the special case of the conformally flat metric. This gives the solid basis for the study of the conformally-flat cosmological solutions, first of which was discovered by Mamaev and Mostepanenko and by Starobinski in 1980. Treating the anomaly-induced action as quantum correction to the Einstein-Hilbert term we explore the possibility to have inflationary solutions, investigate their dependence on the initial data and discuss the restrictions in considering the density perturbations. The shape of inflationary solutions strongly depends on the underlying gauge model of the elementary particles physics. Two special cases are considered: Minimal Standard Model and the matter sector of N=8, D=4 supergravity. It turns out that inflation is almost inevitable consequence of the great difference between Planck mass and the mass of the heaviest massive particle.

gr-qc↗

arXiv1998

Quantum gravity correction, evolution of scalar field and inflation

We take the first nontrivial coefficient of the Schwinger-DeWitt expansion as a leading correction to the action of the second-derivative metric-dilaton gravity. To fix the ambiguities related with an arbitrary choice of the gauge fixing condition and the parametrization for the quantum field, one has to use the classical equations of motion. As a result, the only corrections are the ones to the potential of the scalar field. It turns out that the parameters of the initial classical action may be chosen in such a way that the potential satisfies most of the conditions for successful inflation.

gr-qc↗

arXiv1998

Torsion action and its possible observables

The methods of effective field theory are used to explore the theoretical and phenomenological aspects of the torsion field. Spinor action coupled to electromagnetic field and torsion possesses an additional softly broken gauge symmetry. This symmetry enables one to derive the unique form of the torsion action compatible with unitarity and renormalizability. It turns out that the antisymmetric torsion field is equivalent to an massive axial vector field. The introduction of scalars leads to serious problems which are revealed after the calculation of the leading two-loop divergences. Thus the phenomenological aspects of torsion may be studied only for the fermion-torsion systems. In this part of the paper we obtain an upper bounds for the torsion parameters using present experimental data on forward-backward Z-pole asymmetries, data on the experimental limits on four-fermion contact interaction (LEP, HERA, SLAC, SLD, CCFR) and also TEVATRON limits on the cross section of new gauge boson, which could be produced as a resonance at high energy $p\bar{p}$ collisions. The present experimental data enable one to put the limits on torsion parameters for the various range of the torsion mass. We emphasize that for the torsion mass of the order of the Planck mass no any independent theory for torsion is possible, and one must directly use string theory.

hep-ph↗

arXiv1997

On the inflationary solutions in higher-derivative gravity with dilaton field

We discuss the existence of de Sitter inflationary solutions for the string-inspired fourth-derivative gravity theories with dilaton field. We consider a space-time of arbitrary dimension D and an arbitrary parametrization of the target space metric. The specific features of the theory in dimension D=4 and those of the special ghost-free parametrization of the metric are found. We also consider similar string-inspired theories with torsion and construct an inflationary solution with torsion and dilaton for D=4. The stability of the inflationary solutions is also investigated.

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arXiv1996

On the conformal transformation and duality in gravity

The theory described by the sum of the Einstein-Hilbert action and the action of conformal scalar field possesses the duality symmetry which includes some special conformal transformation of the metric, and also inversion of scalar field and of the gravitational constant. In the present paper the conformal duality is generalized for arbitrary space-time dimension $n \neq 2$ and for the general sigma-model type conformal scalar theory. We also consider to apply the conformal duality for the investigation of quantum gravity in the strong curvature regime. The trace of the first coefficient of the Schwinger-DeWitt expansion is derived and it's dependence on the gauge fixing condition is considered. After that we discuss the way to extract the gauge-fixing independent result and also it's possible physical applications.

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arXiv1996

Some remarks on high derivative quantum gravity

We analyze the perturbative implications of the most general high derivative approach to quantum gravity based on a diffeomorphism invariant local action. In particular, we consider the super-renormalizable case with a large number of metric derivatives in the action. The structure of ultraviolet divergences is analyzed in some detail. We show that they are independent of the gauge fixing condition and the choice of field reparametrization. The cosmological counterterm is shown to vanish under certain parameter conditions. We elaborate on the unitarity problem of high derivative approaches and the distribution of masses of unphysical ghosts. We also discuss the properties of the low energy regime and explore the possibility of having a multi-scale gravity with different scaling regimes compatible with Einstein gravity at low energies. Finally, we show that the ultraviolet scaling of matter theories is not affected by the quantum corrections of high derivative gravity. As a consequence, asymptotic freedom is stable under those quantum gravity corrections.

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arXiv1995

One loop renormalization of the four-dimensional theory for quantum dilaton gravity.

We study the one loop renormalization in the most general metric-dilaton theory with the second derivative terms only. The general theory can be divided into two classes, models of one are equivalent to conformally coupled with gravity scalar field and also to general relativity with cosmological term. The models of second class have one extra degree of freedom which corresponds to dilaton. We calculate the one loop divergences for the models of second class and find that the arbitrary functions of dilaton in the starting action can be fine-tuned in such a manner that all the higher derivative counterterms disappear on shell. The only structures in both classical action and counterterms, which survive on shell, are the potential (cosmological) ones. They can be removed by renormalization of the dilaton field which acquire the nontrivial anomalous dimension, that leads to the effective running of the cosmological constant. For some of the renormalizable solutions of the theory the observable low energy value of the cosmological constant is small as compared with the Newtonian constant. We also discuss another application of our result.

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arXiv1995

On the renormalization of higher derivative two dimensional gravity.

The fourth derivative models for two dimensional gravity are shown to be equivalent to the special version of the nonlinear sigma models coupled to 2d quantum gravity. The reduction consists in the introduction of the auxiliary scalar fields and can be performed in an explicit way for both metric and general metric-dilaton cases. In view of this we can evaluate the structure of possible counterterms and show that they contains second derivative structures only. The calculations in the theory with an auxiliary fields require some special procedure to be applied. We perform the explicit calculations in a different gauges and explore the features of the auxiliary fields.

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arXiv1994

A Four-Dimensional Theory for Quantum Gravity with Conformal and Nonconformal Explicit Solutions

The most general version of a renormalizable $d=4$ theory corresponding to a dimensionless higher-derivative scalar field model in curved spacetime is explored. The classical action of the theory contains $12$ independent functions, which are the generalized coupling constants of the theory. We calculate the one-loop beta functions and then consider the conditions for finiteness. The set of exact solutions of power type is proven to consist of precisely three conformal and three nonconformal solutions, given by remarkably simple (albeit nontrivial) functions that we obtain explicitly. The finiteness of the conformal theory indicates the absence of a conformal anomaly in the finite sector. The stability of the finite solutions is investigated and the possibility of renormalization group flows is discussed as well as several physical applications.

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arXiv1994

Induced Gravity from the Consistent String Theory Coupled with Two - Dimensional Quantum Gravity

The unified theory of string and two-dimensional quantum gravity is considered. The action for two-dimensional gravity is choosen in a well-known induced form and thus gravity posesses it's oun nontrivial dynamics even on the classical level. Three classically equivalent forms of the action of the unified theory are described, that is $D$, $D+1$ and $D+2$ formulations where the last one corresponds to the special kind of nonlinear sigma model on the background of classical two-dimensional metric. In all cases we find the action of the theory to be invariant under generalized symmetry transformations, and point out the arbitrariness of renormalization which reflects this symmetry. The one - loop counterterms are calculated in a general covariant gauge and also in conformal gauge. The difference between the counterterms for all classically equivalent variants of the theory is trivial on mass-shell. Since the different versions of the theory lead to the one-loop contributions which coincide on mass shell, the corresponding arbitrariness is related with the only generalized reparametrizations of the theory. Hence one can formulate the conditions of Weyl symmetry for any equivalent form of the action. Such a conditions are written for the $D+2$ formulation and then are reduced to other ones. The equations for the background interaction fields differ from the ones in standard approach and look much more complicated.

hep-th↗

arXiv1994

On the possible experimental manifestations of the torsion field at low energies

We construct the theoretical base for the search of the possible experimental manifestations of the torsion field at low energies. The weakrelativistic approximation to the Dirac equation in an external torsion field is considered. For the sake of generality we introduce the external electromagnetic field in parallel. The generalized (due to torsion dependent terms) Pauly equation contains new terms which have the different structure if compared with standard electromagnetic ones. Just the same takes place for the weakrelativistic equations for spin $\frac{1}{2}$ particle in an external torsion and electromagnetic field. It is given the brief description of the possible experiments.

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