arXiv subjects

C. Wetterich

Publications and source records attributed to C. Wetterich.

At least 145 records · Page 8Linked to original sources

arXiv2002

Cosmon dark matter?

We investigate if the fluctuations of the scalar field mediating quintessence -- the cosmon -- can play an important role in cosmology. Small fluctuations with short wavelength behave similar to a relativistic gas. In contrast, the contribution to the energy density from horizon size fluctuations may decrease less rapidly than radiation. We discuss the possibility that the cosmon fluctuations grow nonlinearly, form lumps and constitute the clustering dark matter of the universe. Cosmon dark matter would lead to interesting consequences for the equation of state and the coupling between quintessence and dark matter.

hep-ph↗

arXiv2001

Are Galaxies Cosmon Lumps?

The scalar ``cosmon'' field mediating quintessence influences the dynamics of extended objects in the universe. We discuss cosmon lumps -- spherically symmetric solutions for the scalar field coupled to gravity. The two integration constants can be associated to the mass and the rotational velocity in a halo-like region with constant rotation curve. The presence of the scalar field also changes the singularity of the black hole solution. We ask if galaxies could be associated with cosmon lumps.

astro-ph↗

arXiv2001

Quintessence-the Dark Energy in the Universe?

Quintessence - the energy density of a slowly evolving scalar field - may constitute a dynamical form of the homogeneous dark energy in the universe. We review the basic idea and indicate observational tests which may distinguish quintessence from a cosmological constant.

astro-ph↗

arXiv2001

Effective average action in statistical physics and quantum field theory

An exact renormalization group equation describes the dependence of the free energy on an infrared cutoff for the quantum or thermal fluctuations. It interpolates between the microphysical laws and the complex macroscopic phenomena. We present a simple unified description of critical phenomena for O(N)-symmetric scalar models in two, three or four dimensions, including essential scaling for the Kosterlitz-Thouless transition.

hep-ph↗

arXiv2001

Unique Translation between Hamiltonian Operators and Functional Integrals

A careful treatment of the discretization errors in the path integral formulation of quantum mechanics leads to a unique prescription for the translation from the Hamiltonian to the action in the functional integral. An example is given by an interaction quadratic in the occupation number, characteristic for manybody bosonic systems. As a result, the term linear in the occupation number (chemical potential) receives a correction as compared to the usual formulation based on coherent states. A perturbative calculation supports the relevance of this correction.

cond-mat↗

arXiv2000

Higgs description of two-flavor QCD vacuum

A Higgs description for the QCD vacuum in the limit of two quark flavors is proposed. Complete ``spontaneous breaking'' of color symmetry is realized by condensation of an adjoint quark-antiquark and a quark-quark pair. The vacuum is invariant under isospin symmetry applying color and flavor transformations simultaneously. All elementary excitations acquire integral charges. The dressed microscopic quark and gluon degrees of freedom can be identified with the macroscopic baryon and vector meson degrees of freedom. In addition to the nucleons we find soliton--type fermions with integer isospin. Their presence provides an important test accessible to lattice QCD simulations.

hep-ph↗

arXiv2000

Rotation Symmetry Breaking Condensate in a Scalar Theory

Motivated by an analogy with the conformal factor problem in gravitational theories of the $R+R^2$-type we investigate a $d$-dimensional Euclidean field theory containing a complex scalar field with a quartic self interaction and with a nonstandard inverse propagator of the form $-p^2+p^4$. Nonconstant spin-wave configurations minimize the classical action and spontaneously break the rotation symmetry to a lower-dimensional one. In classical statistical physics this corresponds to a spontaneous formation of layers. Within the effective average action approach we determine the renormalization group flow of the dressed inverse propagator and of a family of generalized effective potentials for nonzero-momentum modes. Already in the leading order of the semiclassical expansion we find strong ``instability induced'' renormalization effects which are due to the fact that the naive vacuum (vanishing field) is unstable towards the condensation of modes with a nonzero momentum. We argue that the (quantum) ground state of our scalar model indeed leads to spontaneous breaking of rotation symmetry.

hep-th↗

arXiv2000

Instantons and color symmetry breaking in the vacuum

The instanton interaction in QCD generates an effective potential for scalar quark-antiquark condensates in the color singlet and octet channels. For three light quark flavors the cubic term in this potential induces an octet condensate and ``spontaneous breaking'' of color in the vacuum. Realistic masses of the rho- and eta'-mesons are compatible with renormalization-group-improved instanton perturbation theory.

hep-ph↗

arXiv2000

Gluon-Meson Duality in the Mean Field Approximation

In a gauge-fixed language gluon-meson duality can be described as the Higgs mechanism for ``spontaneous symmetry breaking'' of color. We present a mean field computation which suggests that this phenomenon is plausible in QCD. One obtains realistic masses of the light mesons and baryons.

hep-ph↗

arXiv2000

Natural Quintessence?

We formulate conditions for the naturalness of cosmological quintessence scenarios. The quintessence lagrangian is taken to be the sum of a simple exponential potential and a non-canonical kinetic term. This parameterization covers most variants of quintessence and makes the naturalness conditions particularly transparent. Several ``natural'' scalar models lead, for the present cosmological era, to a large fraction of homogeneous dark energy density and an acceleration of the scale factor as suggested by observation.

hep-ph↗

arXiv2000

Spontaneously broken color

The vacuum of QCD is characterized by the Higgs mechanism. Color is ``spontaneously broken'' by a quark-antiquark condensate in the octet representation. The massive gluons carry integer electric charges and are identified with the vector mesons. The fermionic excitations consist of the low mass baryon octet and a singlet. The interactions between these particles and the light pseudoscalar octet are largely determined by chiral symmetry and a nonlinear local symmetry. A consistent phenomenological picture of strong interactions at long distances arises from a simple effective action.

hep-ph↗

arXiv2000

Nonperturbative Renormalization Flow and Essential Scaling for the Kosterlitz-Thouless Transition

The Kosterlitz-Thouless phase transition is described by the nonperturbative renormalization flow of the two dimensional $ϕ^4$-model. The observation of essential scaling demonstrates that the flow equation incorporates nonperturbative effects which have previously found an alternative description in terms of vortices. The duality between the linear and nonlinear $σ$-model gives a unified description of the long distance behaviour for O(N)-models in arbitrary dimension $d$. We compute critical exponents in first order in the derivative expansion.

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

Quintessential Adjustment of the Cosmological Constant

We construct a time dependent adjustment mechanism for the cosmological "constant" which could be at work in a late Friedmann-Robertson-Walker universe dominated by quintessence and matter. It makes use of a Brans-Dicke field that couples to the evolving standard-model vacuum energy density. Our explicit model possesses a stable late-time solution with a fixed ratio of matter and field energy densities. No fine tuning of model parameters or initial conditions is required.

hep-ph↗

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

Flow Equations for Phase Transitions in Statistical Physics and QCD

We review the formalism of the effective average action in quantum field theory which corresponds to a coarse grained free energy in statistical mechanics. The associated exact renormalization group equation and possible nonperturbative approximations for its solution are discussed. We describe in detail O(N)-symmetric scalar theories in two and three dimensions. These ideas are also applied to QCD where one observes the consecutive emergence of mesonic bound states and spontaneous chiral symmetry breaking as the coarse graining scale is lowered. We finally present a study of the chiral phase transition in two flavor QCD. A precision estimate of the universal critical equation of state for the three-dimensional O(4) Heisenberg model is presented. We explicitly connect the O(4) universal behavior near the critical temperature and zero quark mass with the physics at zero temperature and a realistic pion mass. For realistic quark masses the pion correlation length near T_c turns out to be smaller than its zero temperature value.

hep-ph↗

arXiv1998

Natural Maximal ny_my - ny_tau Mixing

The naturalness of maximal mixing between myon- and tau-neutrinos is investigated. A spontaneously broken nonabelian generation symmetry can explain a small parameter which governs the deviation from maximal mixing. In many cases all three neutrino masses are almost degenerate. Maximal ny_my-ny_tau-mixing would indicate that the leading contribution to the light neutrino masses arises from the expectation value of a heavy weak triplet rather than from the seesaw mechanism. In this scenario the deviation from maximal mixing is predicted to be less than about 1%.

hep-ph↗

arXiv1998

The Chiral Phase Transition at High Baryon Density from Nonperturbative Flow Equations

We investigate the QCD chiral phase transition at high baryon number density within the linear quark meson model for two flavors. The method we employ is based on an exact renormalization group equation for the free energy. Truncated nonperturbative flow equations are derived at nonzero chemical potential and temperature. Whereas the renormalization group flow leads to spontaneous chiral symmetry breaking in vacuum, we find a chiral symmetry restoring first order transition at high density. Combined with previous investigations at nonzero temperature, the result implies the presence of a tricritical point with long--range correlations in the phase diagram.

hep-ph↗