arXiv subjects

N. Tetradis

Publications and source records attributed to N. Tetradis.

At least 55 records · Page 3Linked to original sources

arXiv2003

Cosmological Acceleration from Energy Influx

We discuss the cosmological evolution of a 3-brane Universe in the presence of energy influx from the bulk. We show that this influx can lead to accelerated expansion on the brane, depending on the equations of state of the bulk and brane matter. The absorption of non-relativistic bulk matter by the brane at an increasing rate leads to a small positive acceleration parameter during the era of matter domination on the brane. On the other hand, the brane expansion remains decelerating during radiation domination.

hep-th↗

arXiv2003

The Quark-Meson Model and the Phase Diagram of Two-Flavour QCD

We study the qualitative features of the QCD phase diagram in the context of the linear quark-meson model with two flavours, using the exact renormalization group. We identify the universality classes of the second-order phase transitions and calculate critical exponents. In the absence of explicit chiral-symmetry breaking through the current quark masses, we discuss in detail the tricritical point and demonstrate that it is linked to the Gaussian fixed point. In its vicinity we study the universal crossover between the Gaussian and O(4) fixed points, and the weak first-order phase transitions. In the presence of explicit chiral-symmetry breaking, we study in detail the critical endpoint of the line of first-order phase transitions. We demonstrate the decoupling of pion fluctuations and identify the Ising universality class as the relevant one for the second-order phase transition.

hep-th↗

arXiv2002

Induced Gravity on RS Branes

It is shown that a localized four-dimensional Einstein term, induced by quantum corrections, modifies significantly the law of gravity in a Randall-Sundrum brane world. In particular, the short-distance behavior of gravity changes from five- to four-dimensional, while, depending on the values of parameters, there can be an intermediate range where gravity behaves as in five dimensions. The spectrum of graviton fluctuations around the brane, their relative importance for the gravitational force, and the relevance of their emission in the bulk for the brane cosmology are analysed. Finally, constraints on parameters are derived from energy loss in astrophysical and particle physics processes.

hep-th↗

arXiv2001

Induced brane gravity: realizations and limitations

The induced 4d gravity on a brane world is analyzed. The case of a thick brane is considered and the unexpected appearance of a new threshold scale, much larger than the thickness scale is found. In cases of phenomenological interest, this new length scale turns out to be in the submillimeter range. The effect of $R^2$ corrections, both in the bulk and on the brane, is also studied. It is shown that they introduce new threshold scales and may induce drastic modifications to the leading behavior. A concrete string/D-brane realization of the induced gravity scenario is also presented.

hep-th↗

arXiv2001

Large Gauged Q Balls

We study Q-balls associated with local U(1) symmetries. Such Q-balls are expected to become unstable for large values of their charge because of the repulsion mediated by the gauge force. We consider the possibility that the repulsion is eliminated through the presence in the interior of the Q-ball of fermions with charge opposite to that of the scalar condensate. Another possibility is that two scalar condensates of opposite charge form in the interior. We demonstrate that both these scenaria can lead to the existence of classically stable, large, gauged Q-balls. We present numerical solutions, as well as an analytical treatment of the ``thin-wall'' limit.

hep-ph↗

arXiv2001

On Brane Stabilization and the Cosmological Constant

We address the problem of the cosmological constant within the Randall-Sundrum scenario with a brane stabilization mechanism. We consider brane tensions of general form. We examine the conditions under which a small change of the positive tension of the first brane can be absorbed in a small modification of the two-brane configuration, instead of manifesting itself as a cosmological constant. We demonstrate that it is possible to have a cosmological constant in the range predicted by recent observational data, if there is an ultraviolet cutoff of order 10 TeV in the contributions to the brane tension from vacuum fluctuations.

hep-th↗

arXiv2000

The Exact Renormalization Group and First-Order Phase Transitions

Studies of first-order phase transitions through the use of the exact renormalization group are reviewed. In the first part the emphasis is on universal aspects: We discuss the universal critical behaviour near weakly first-order phase transitions for a three-dimensional model of two coupled scalar fields -- the cubic anisotropy model. In the second part we review the application of the exact renormalization group to the calculation of bubble-nucleation rates. More specifically, we concentrate on the pre-exponential factor. We discuss the reliability of homogeneous nucleation theory that employs a saddle-point expansion around the critical bubble for the calculation of the nucleation rate.

hep-th↗

arXiv2000

Quantum-mechanical tunnelling and the renormalization group

We explore the applicability of the exact renormalization group to the study of tunnelling phenomena. We investigate quantum-mechanical systems whose energy eigenstates are affected significantly by tunnelling through a barrier in the potential. Within the approximation of the derivative expansion, we find that the exact renormalization group predicts the correct qualitative behaviour for the lowest energy eigenvalues. However, quantitative accuracy is achieved only for potentials with small barriers. For large barriers, the use of alternative methods, such as saddle-point expansions, can provide quantitative accuracy.

hep-th↗

arXiv2000

Non-Perturbative Renormalization Flow in Quantum Field Theory and Statistical Physics

We review the use of an exact renormalization group equation in quantum field theory and statistical physics. It describes the dependence of the free energy on an infrared cutoff for the quantum or thermal fluctuations. Non-perturbative solutions follow from approximations to the general form of the coarse-grained free energy or effective average action. They interpolate between the microphysical laws and the complex macroscopic phenomena. Our approach yields a simple unified description for O(N)-symmetric scalar models in two, three or four dimensions, covering in particular the critical phenomena for the second-order phase transitions, including the Kosterlitz-Thouless transition and the critical behavior of polymer chains. We compute the aspects of the critical equation of state which are universal for a large variety of physical systems and establish a direct connection between microphysical and critical quantities for a liquid-gas transition. Universal features of first-order phase transitions are studied in the context of scalar matrix models. We show that the quantitative treatment of coarse graining is essential for a detailed estimate of the nucleation rate. We discuss quantum statistics in thermal equilibrium or thermal quantum field theory with fermions and bosons and we describe the high temperature symmetry restoration in quantum field theories with spontaneous symmetry breaking. In particular, we explore chiral symmetry breaking and the high temperature or high density chiral phase transition in quantum chromodynamics using models with effective four-fermion interactions.

hep-ph↗

arXiv2000

The World as a Dual Josephson Junction

We examine some of the implications of the field-theoretical mechanism for the localization of gauge fields on hypersurfaces in higher-dimensional bulk space-time. This mechanism exploits the analogy between confinement and dual superconductivity. In the simplest case of a photon localized on a (2+1)-dimensional surface in a (3+1)-dimensional bulk, we argue that the system behaves like a dual Josephson junction. This implies that the effective gauge theory on the surface is not free, but displays weak confinement with a linear potential. We comment on the relevance of our results for the realistic case of a (3+1)-dimensional surface in a space-time with one or more extra dimensions.

hep-ph↗

arXiv2000

Comparison of two methods for calculating nucleation rates

First-order phase transitions that proceed via nucleation of bubbles are described by the homogeneous nucleation theory of Langer. The nucleation rate is one of the most interesting parameters of these transitions. In previous works we have computed nucleation rates with two different methods: (A) employing coarse-grained potentials; (B) by means of a saddle-point approximation, using dimensionally regularized field theory. In this article we compare the results of the two approaches in order to test their reliability and to determine the regions of applicability. We find a very good agreement.

cond-mat↗

arXiv1999

The Region of Validity of Homogeneous Nucleation Theory

We examine the region of validity of Langer's picture of homogeneous nucleation. Our approach is based on a coarse-grained free energy that incorporates the effect of fluctuations with momenta above a scale k. The nucleation rate I = A_k exp(-S_k) is exponentially suppressed by the action S_k of the saddle-point configuration that dominates tunnelling. The factor A_k includes a fluctuation determinant around this saddle point. Both S_k and A_k depend on the choice of k, but, for 1/k close to the characteristic length scale of the saddle point, this dependence cancels in the expression for the nucleation rate. For very weak first-order phase transitions or in the vicinity of the spinodal decomposition line, the pre-exponential factor A_k compensates the exponential suppression exp(-S_k). In these regions the standard nucleation picture breaks down. We give an approximate expression for A_k in terms of the saddle-point profile, which can be used for quantitative estimates and practical tests of the validity of homogeneous nucleation theory.

hep-ph↗

arXiv1999

A consistent approach to bubble-nucleation theory

We summarize recent work on the consistent calculation of bubble-nucleation rates. Our approach is based on the notion of a real coarse-grained potential. The bubble-nucleation rate is calculated through an expansion around the semiclassical saddle point associated with tunnelling. We resolve outstanding problems related to the convexity of the potential, the double-counting of the effect of fluctuations and the inherent ultraviolet divergences. We determine the region of validity of the expansion around the saddle point. We find that this expansion fails near the spinodal line, and for weak or radiatively-induced first-order phase transitions. We apply our method to the bound on the Higgs-boson mass from vacuum metastability and the electroweak phase transition.

hep-ph↗

arXiv1998

A Consistent Calculation of Bubble-Nucleation Rates

We present a consistent picture of tunnelling in field theory. Our results apply both to high-temperature field theories in four dimensions and to zero-temperature three-dimensional ones. Our approach is based on the notion of a coarse-grained potential U_k that incorporates the effect of fluctuations with characteristic momenta above a given scale k. U_k is non-convex and becomes equal to the convex effective potential for k --> 0. We demonstrate that a consistent calculation of the nucleation rate must be performed at non-zero values of k, larger than the typical scale of the saddle-point configuration that dominates tunnelling. The nucleation rate is exponentially suppressed by the action S_k of this saddle point. The pre-exponential factor A_k, which includes the fluctuation determinant around the saddle-point configuration, is well-defined and finite. Both S_k and A_k are k-dependent, but this dependence cancels in the expression for the nucleation rate. This picture breaks down in the limit of very weakly first-order phase transitions, for which the pre-exponential factor compensates the exponential suppression.

hep-ph↗

arXiv1998

Two-Stage Inflation as a Solution of the Initial Condition Problem of Hybrid Inflation

We address the issue of fine-tuning of the initial field configuration that can lead to hybrid inflation in the context of global supersymmetry. This problem is generated by the difference between the energy scale at which the Universe emerges from the Planck era and the inflationary scale implied by the COBE observations: V^{1/4} \sim 10^{-3} m_{Pl}. We propose a scenario with two stages of inflation. The first stage, with a typical scale not far from m_{Pl}, occurs ``naturally'' and provides the necessary homogeneity for the second stage. The latter generates the density perturbations that result in the cosmic microwave background anisotropy observed by COBE.

hep-ph↗

arXiv1998

Fine tuning of the initial conditions for hybrid inflation

We study the evolution of regions of space with various initial field values for a simple theory that can support hybrid inflation. Only very narrow domains within the range of initial field values below the Planck scale lead to the onset of inflation. This implies a severe fine tuning for the initial configuration that will produce inflation.

astro-ph↗

arXiv1998

Renormalization-group study of weakly first-order phase transitions

We study the universal critical behaviour near weakly first-order phase transitions for a three-dimensional model of two coupled scalar fields -- the cubic anisotropy model. Renormalization-group techniques are employed within the formalism of the effective average action. We calculate the universal form of the coarse-grained free energy and deduce the ratio of susceptibilities on either side of the phase transition. We compare our results with those obtained through Monte Carlo simulations and the epsilon-expansion.

hep-th↗

arXiv1998

Two-Stage Inflation in Supergravity

We investigate the viability of a two-stage inflationary scenario in the context of supergravity, so as to resolve the problem of initial conditions for hybrid inflation. We allow for non-renormalizable terms in the superpotential and consider the most general form of the Kahler potential and the gauge kinetic function. We construct a model with two stages of inflation, the first driven by D-term and the second by F-term energy density. The viability of this scenario depends on the non-minimal terms in the Kahler potential, for which we derive the necessary constraints.

hep-ph↗