arXiv subjects

Ilya L. Shapiro

Publications and source records attributed to Ilya L. Shapiro.

At least 127 records · Page 7Linked to original sources

arXiv2009

DM particles: how warm they can be?

One of important questions concerning particles which compose the Dark Matter (DM) is their average speed. We consider the model of relativistic weakly interacting massive particles and try to impose an upper bound on their actual and past warmness through the analysis of density perturbations and comparison with the LSS data. It is assumed that the DM can be described by the recently invented model of reduced relativistic gas (RRG). The equation of state of the RRG model is closely reproducing the one of the Maxwell distribution, while being much simpler. This advantage of the RRG model makes our analysis very efficient. As a result we arrive at the rigid and model-independent bound for the DM warmness without using the standard (much more sophisticated) approach based on the Einstein-Boltzmann system of equations.

astro-ph↗

arXiv2008

Polemic Notes On IR Perturbative Quantum Gravity

Quantum gravity is an important and to great extent unsolved problem. There are many different approaches to the quantization of the metric field, both perturbative and non-perturbative. The current situation in the perturbative quantum gravity is characterized by a number of different models, some of them well elaborated but no one perfect nor mathematically neither phenomenologically, mainly because there are no theoretically derived observables which can be experimentally measured. A very interesting one is an effective approach which separates the low-energy quantum effects from the UV sector. In this way one can calculate quantities which are potentially relevant for establishing certain universal features of quantum gravity. In this presentation we give a polemic consideration of the effective approach to the infrared quantum gravity. We question the validity of the recent results in this area and also discuss how one can check the alleged universality of the effective approach.

hep-th↗

arXiv2008

Can the cosmological "constant" run? - It may run

Using standard quantum field theory, we discuss several theoretical aspects of the possible running of the cosmological constant (CC) term in Einstein's equations. The basic motivation for the present work is to emphasize that this possibility should also be taken into account when considering dynamical models for the dark energy (DE), which are nowadays mainly focused on identifying the DE with the energy density associated to one or more ad hoc scalar fields. At the same time, we address some recent criticisms that have been published (or privately communicated to us) attempting to cast doubts on the fundamental possibility of such running. In this work, we argue that while there is no comprehensive proof of the CC running, there is no rigorous proof of the non-running either. In particular, some purported "non-running theorem" recently adduced in the literature is, in our opinion, completely insubstantial and formally incorrect. The way to the CC running is, therefore, still open and we take here the opportunity to present a pedagogical review of the present state of the art in this field, including a a brief historical account.

hep-th↗

arXiv2008

Effective Action of Vacuum: Semiclassical Approach

We present brief, to great extent pedagogical review on renormalization in curved space-time and of some recent results on the derivation and better understanding of quantum corrections to the action of gravity. The paper is mainly devoted to the semiclassical approach, but we also discuss its importance for quantum gravity and string theory.

gr-qc↗

arXiv2007

Exact Foldy-Wouthuysen transformation for gravitational waves and magnetic field background

We consider an exact Foldy-Wouthuysen transformation for the Dirac spinor field on the combined background of a gravitational wave and constant uniform magnetic field. By taking the classical limit of the spinor field Hamiltonian we arrive at the equations of motion for the non-relativistic spinning particle. Two different kinds of the gravitational fields are considered and in both cases the effect of the gravitational wave on the spinor field and on the corresponding spinning particle may be enforced by the sufficiently strong magnetic field. This result can be relevant for the astrophysical applications and, in principle, useful for creating the gravitational wave detectors based on atomic physics and precise interferometry.

gr-qc↗

arXiv2007

Density Perturbations for Running Cosmological Constant

The dynamics of density and metric perturbations is investigated for the previously developed model where the decay of the vacuum energy into matter (or vice versa) is due to the renormalization group (RG) running of the cosmological constant (CC) term. The evolution of the CC depends on the single parameter ν, which characterizes the running of the CC produced by the quantum effects of matter fields of the unknown high energy theory below the Planck scale. The sign of νindicates whether bosons or fermions dominate in the running. The spectrum of perturbations is computed assuming an adiabatic regime and an isotropic stress tensor. Moreover, the perturbations of the CC term are generated from the simplest covariant form suggested by the RG model under consideration. The corresponding numerical analysis shows that for ν>0 there is a depletion of the matter power spectrum at low scales (large wave numbers) as compared to the standard LCDM model, whereas for ν<0 there is an excess of power at low scales. We find that the LSS data rule out the range |ν|> 10^{-4} while the values |ν|< 10^{-6} look perfectly acceptable. For ν<0 the excess of power at low scales grows rapidly and the bound is more severe. From the particle physics viewpoint, the values |ν|\sim 10^{-6} correspond to the ``desert'' in the mass spectrum above the GUT scale M_X\sim 10^{16} GeV. Our results are consistent with those obtained in other dynamical models admitting an interaction between dark matter and dark energy. We find that the matter power spectrum analysis is a highly efficient method to discover a possible scale dependence of the vacuum energy.

gr-qc↗

arXiv2007

Cosmological Constant Problems and Renormalization Group

The Cosmological Constant Problem emerges when Quantum Field Theory is applied to the gravitational theory, due to the enormous magnitude of the induced energy of the vacuum. The unique known solution of this problem involves an extremely precise fine-tuning of the vacuum counterpart. We review a few of the existing approaches to this problem based on the account of the quantum (loop) effects and pay special attention to the ones involving the renormalization group.

gr-qc↗

arXiv2006

Local conformal symmetry and its fate at quantum level

The purpose of this article is to present a short review of local conformal symmetry in curved 4d space-time. Furthermore we discuss the conformal anomaly and anomaly-induced effective actions. Despite the conformal symmetry is always broken at quantum level, it may be a basis of useful and interesting approximations for investigating quantum corrections.

hep-th↗

arXiv2006

Higher Derivative Quantum Gravity with Gauss-Bonnet Term

Higher derivative theory is one of the important models of quantum gravity, renormalizable and asymptotically free within the standard perturbative approach. We consider the $4-ε$ renormalization group for this theory, an approach which proved fruitful in $2-ε$ models. A consistent formulation in dimension $n=4-ε$ requires taking quantum effects of the topological term into account, hence we perform calculation which is more general than the ones done before. In the special $n=4$ case we confirm a known result by Fradkin-Tseytlin and Avramidi-Barvinsky, while contributions from topological term do cancel. In the more general case of $4-ε$ renormalization group equations there is an extensive ambiguity related to gauge-fixing dependence. As a result, physical interpretation of these equations is not universal unlike we treat $ε$ as a small parameter. In the sector of essential couplings one can find a number of new fixed points, some of them have no analogs in the $n=4$ case.

hep-th↗

arXiv2005

Simple Cosmological Model with Relativistic Gas

We construct simple and useful approximation for the relativistic gas of massive particles. The equation of state is given by an elementary function and admits analytic solution of the Friedmann equation, including more complex cases when the relativistic gas of massive particles is considered together with radiation or with dominating cosmological constant. The model of relativistic gas may be interesting for the description of primordial Universe, especially as a candidate for the role of a Dark Matter.

gr-qc↗

arXiv2005

Running G and Λat low energies from physics at M_X: possible cosmological and astrophysical implications

The renormalization group (RG) approach to cosmology is an efficient method to study the possible evolution of the cosmological parameters from the point of view of quantum field theory in curved space-time. In this work we continue our previous investigations of the RG method based on potential low-energy effects induced from physics at very high energy scales M_X near M_P. In the present instance we assume that both the Newton constant, G, and the cosmological term, Λ, can be functions of a scale parameter μ. It turns out that G(μ) evolves according to a logarithmic law which may lead to asymptotic freedom of gravity, similar to the gauge coupling in QCD. At the same time Λ(μ) evolves quadratically with μ. We study the consistency and cosmological consequences of these laws when μ=H. Furthermore, we propose to extend this method to the astrophysical domain after identifying the local RG scale at the galactic level. It turns out that Kepler's third law of celestial mechanics receives quantum corrections that may help to explain the flat rotation curves of the galaxies without introducing the dark matter hypothesis. The origin of these effects (cosmological and astrophysical) could be linked, in our framework, to physics at M_X= 10^{16-17} GeV.

hep-ph↗

arXiv2004

Effective field theory and fundamental interactions

There are many possible gravitational applications of an effective approach to Quantum Field Theory (QFT) in curved space. We present a brief review of effective approach and discuss its impact for such relevant issues as the cosmological constant (CC) problem and inflation driven by vacuum quantum effects. Furthermore it is shown how one can impose significant theoretical constraints on a non-metric gravity using only theoretical effective field theory framework.

hep-th↗

arXiv2004

Testing the running of the cosmological constant with Type Ia Supernovae at high z

Within the Quantum Field Theory context the idea of a "cosmological constant" (CC) evolving with time looks quite natural as it just reflects the change of the vacuum energy with the typical energy of the universe. In the particular frame of Ref.[30], a "running CC" at low energies may arise from generic quantum effects near the Planck scale, M_P, provided there is a smooth decoupling of all massive particles below M_P. In this work we further develop the cosmological consequences of a "running CC" by addressing the accelerated evolution of the universe within that model. The rate of change of the CC stays slow, without fine-tuning, and is comparable to H^2 M_P^2. It can be described by a single parameter, ν, that can be determined from already planned experiments using SNe Ia at high z. The range of allowed values for νfollow mainly from nucleosynthesis restrictions. Present samples of SNe Ia can not yet distinguish between a "constant" CC or a "running" one. The numerical simulations presented in this work show that SNAP can probe the predicted variation of the CC either ruling out this idea or confirming the evolution hereafter expected.

hep-ph↗

arXiv2004

A Friedmann-Lemaitre-Robertson-Walker cosmological model with running Lambda

The idea of a variable dark energy has been entertained many times in the literature and from many different points of view. Quintessence is just a popular way to implement this idea in recent times, but so far with little success. Another possibility is to think of the cosmological term, Lambda, as a ``running quantity'' much in the same way as the electromagnetic coupling constant. However, the fact that Lambda is a dimension-four parameter implies that it may obey a peculiar renormalization group equation, which at low energies could be dominated by ``soft decoupling'' contributions of the form Lambda ~ H^2 M_P^2 stemming from physics near the Planck scale. This value lies in the ballpark of the measurements from CMB and high-z supernovae. A ``renormalized'' FLRW cosmology of this kind may reveal itself as a sound, and testable, proposal for a variable Lambda model within quantum field theory in curved space time.

astro-ph↗

arXiv2003

An Overview of the Anomaly-Induced Inflation

The anomaly-induced inflation (modified Starobinsky model) is based on the application of the effective quantum field theory approach to the Early Universe. We present a brief general review of this model with a special attention to the existing difficulties and unsolved problems.

hep-ph↗

arXiv2003

On the renormalization group for the interacting massive scalar field theory in curved space

The effective action for the interacting massive scalar field in curved space-time is derived using the heat-kernel method. Starting from this effective action, we establish a smooth quadratic form of the low-energy decoupling for the four-scalar coupling constant and for the nonminimal interaction parameter. The evolution of this parameter from the conformal value 1/6 at high energies down to the IR regime is investigated within the two toy models with negative and positive four-scalar coupling constants.

hep-th↗

arXiv2003

Renormalization Group and Decoupling in Curved Space: III. The Case of Spontaneous Symmetry Breaking

We continue investigation of the renormalization group and decoupling of the quantized massive fields in curved space. In the present work we analyze a theory, where fields gain their masses due to the Spontaneous Symmetry Breaking (SSB), that is the case providing a remarkable exception from the Appelquist-Carazzone theorem in the matter fields sector. In the vacuum sector, already at the classical level, the theory with SSB includes, in the general case an infinite number of the non-local terms in the induced vacuum action. Despite this surprising property, we show that the theory is renormalizable and moreover the low-energy decoupling in the higher-derivative gravitational sector performs similar to the AC theorem.

hep-ph↗

arXiv2003

Variable Cosmological Constant as a Planck Scale Effect

We construct a semiclassical FLRW cosmological model assuming a running cosmological constant (CC). It turns out that the CC becomes variable at arbitrarily low energies due to the remnant quantum effects of the heaviest particles, e.g. the Planck scale physics. These effects are universal in the sense that they lead to a low-energy structure common to a large class of high-energy theories. Remarkably, the uncertainty concerning the unknown high-energy dynamics is accumulated into a single parameter ν, such that the model has an essential predictive power. Future Type Ia supernovae experiments (like SNAP) can verify whether this framework is correct. For the flat FLRW case and a moderate value ν~0.01, we predict an increase of 10-20% in the value of Omega_{Lambda} at redshifts z=1-1.5 perfectly reachable by SNAP.

astro-ph↗