arXiv subjects

F. R. Klinkhamer

Publications and source records attributed to F. R. Klinkhamer.

At least 109 records · Page 6Linked to original sources

arXiv2009

Vacuum energy density kicked by the electroweak crossover

Using q-theory, we show that the electroweak crossover can generate a remnant vacuum energy density Λ\sim E_{ew}^8 / E_{planck}^4, with effective electroweak energy scale E_{ew} \sim 10^{3} GeV and reduced Planck-energy scale E_{planck} \sim 10^{18} GeV. The obtained expression for the effective cosmological constant Λmay be a crucial input for the suggested solution by Arkani-Hamed et al. of the triple cosmic coincidence puzzle (why the orders of magnitude of the energy densities of vacuum, matter, and radiation are approximately the same in the present Universe).

astro-ph.CO↗

arXiv2009

Lorentz violation and black-hole thermodynamics

We consider nonstandard photons from nonbirefringent modified Maxwell theory and discuss their propagation in a fixed Schwarzschild spacetime background. This particular modification of Maxwell theory is Lorentz-violating and allows for maximal photon velocities differing from the causal speed c of the asymptotic background spacetime. In the limit of geometrical optics, light rays from modified Maxwell theory are found to propagate along null geodesics in an effective metric. We observe that not every Lorentz-violating theory with multiple maximal velocities different from the causal speed c modifies the notion of the event horizon, contrary to naive expectations. This result implies that not every Lorentz-violating theory with multiple maximal velocities necessarily leads to a contradiction with the generalized second law of thermodynamics.

hep-th↗

arXiv2009

Gluonic vacuum, q-theory, and the cosmological constant

In previous work, q-theory was introduced to describe the gravitating macroscopic behavior of a conserved microscopic variable q. In this article, the gluon condensate of quantum chromodynamics is considered in terms of q-theory. The remnant vacuum energy density (i.e., cosmological constant) of an expanding universe is estimated as K_{QCD}^3 / E_{Planck}^2, with string tension K_{QCD} \approx (10^2 MeV)^2 and gravitational scale E_{Planck} \approx 10^{19} GeV. The only input for this estimate is general relativity, quantum chromodynamics, and the Hubble expansion of the present Universe.

gr-qc↗

arXiv2009

f(R) Cosmology from q-Theory

From a macroscopic theory of the quantum vacuum in terms of conserved relativistic charges (generically denoted by q^{(a)} with label a), we have obtained, in the low-energy limit, a particular type of f(R) model relevant to cosmology. The macroscopic quantum-vacuum theory allows us to distinguish between different phenomenological f(R) models on physical grounds.

gr-qc↗

arXiv2008

Maxwell-Chern-Simons theory for curved spacetime backgrounds

We consider a modified version of four-dimensional electrodynamics, which has a photonic Chern-Simons-like term with spacelike background vector in the action. Light propagation in curved spacetime backgrounds is discussed using the geometrical-optics approximation. The corresponding light path is modified, which allows for new effects. In a Schwarzschild background, for example, there now exist stable bounded orbits of light rays and the two polarization modes of light rays in unbounded orbits can have different gravitational redshifts. An Erratum with important corrections has been published, which appears as Appendix E in this arXiv version.

hep-th↗

arXiv2008

Lorentz invariance, vacuum energy, and cosmology

This contribution reviews recent work on a new approach to the cosmological constant problem, which starts from the macroscopic behavior of a conserved relativistic microscopic variable q. First, the statics of the vacuum energy density is discussed and, then, the dynamics in a cosmological context.

gr-qc↗

arXiv2008

New indirect bounds on Lorentz violation in the photon sector

Direct laboratory bounds on the 9 nonbirefringent Lorentz-violating dimensionless parameters of modified-Maxwell theory range from the 10^{-7} level to the 10^{-16} level. The detection of air showers initiated by charged primaries (ultrahigh-energy cosmic-rays) and neutral primaries (TeV gamma-rays) allows us to obtain new indirect bounds ranging from the 10^{-15} level to the 10^{-19} level. Possible physics implications are briefly discussed.

hep-ph↗

arXiv2008

Equilibrium boundary conditions, dynamic vacuum energy, and the Big Bang

The near-zero value of the cosmological constant Λin an equilibrium context may be due to the existence of a self-tuning relativistic vacuum variable q. Here, a cosmological nonequilibrium context is considered with a corresponding time-dependent cosmological parameter Λ(t) or vacuum energy density ρ_V(t). A specific model of a closed Friedmann-Robertson-Walker universe is presented, which is determined by equilibrium boundary conditions at one instant of time (t=t_{eq}) and a particular form of vacuum-energy dynamics (dρ_V/dt \propto ρ_M). This homogeneous and isotropic model has a standard Big Bang phase at early times (t << t_{eq}) and reproduces the main characteristics of the present universe (t=t_0 < t_{eq}).

gr-qc↗

arXiv2008

New two-sided bound on the isotropic Lorentz-violating parameter of modified Maxwell theory

There is a unique Lorentz-violating modification of the Maxwell theory of photons, which maintains gauge invariance, CPT, and renormalizability. Restricting the modified-Maxwell theory to the isotropic sector and adding a standard spin-one-half Dirac particle p^\pm with minimal coupling to the nonstandard photon \widetildeγ, the resulting modified-quantum-electrodynamics model involves a single dimensionless "deformation parameter," \widetildeκ_{tr}. The exact tree-level decay rates for two processes have been calculated: vacuum Cherenkov radiation p^\pm \to p^\pm \widetildeγ for the case of positive \widetildeκ_{tr} and photon decay \widetildeγ \to p^+ p^- for the case of negative \widetildeκ_{tr}. From the inferred absence of these decays for a particular high-quality ultrahigh-energy-cosmic-ray event detected at the Pierre Auger Observatory and an excess of TeV gamma-ray events observed by the High Energy Stereoscopic System telescopes, a two-sided bound on \widetildeκ_{tr} is obtained, which improves by eight orders of magnitude upon the best direct laboratory bound. The implications of this result are briefly discussed.

hep-ph↗

arXiv2008

UHECR bounds on Lorentz violation in the photon sector

The aim of this brief review is to present a case study of how astrophysics data can be used to get bounds on Lorentz-violating parameters. For this purpose, a particularly simple Lorentz-violating modification of the Maxwell theory of photons is considered, which maintains gauge invariance, CPT, and renormalization. With a standard spin-one-half Dirac particle minimally coupled to this nonstandard photon, the resulting modified-quantum-electrodynamics model involves nineteen dimensionless "deformation parameters." Ten of these parameters lead to birefringence and are already tightly constrained by astrophysics. New bounds on the remaining nine nonbirefringent parameters have been obtained from the inferred absence of vacuum Cherenkov radiation in ultrahigh-energy-cosmic-ray (UHECR) events. The resulting astrophysics bounds improve considerably upon current laboratory bounds and the implications of this "null experiment" may be profound, both for elementary particle physics and cosmology.

hep-ph↗

arXiv2008

Dynamic vacuum variable and equilibrium approach in cosmology

A modified-gravity theory is considered with a four-form field strength F, a variable gravitational coupling parameter G(F), and a standard matter action. This theory provides a concrete realization of the general vacuum variable q as the four-form amplitude F and allows for a study of its dynamics. The theory gives a flat Friedmann-Robertson-Walker universe with rapid oscillations of the effective vacuum energy density (cosmological "constant"), whose amplitude drops to zero asymptotically. Extrapolating to the present age of the Universe, the order of magnitude of the average vacuum energy density agrees with the observed near-critical vacuum energy density of the present universe. It may even be that this type of oscillating vacuum energy density constitutes a significant part of the so-called cold dark matter in the standard Friedmann-Robertson-Walker framework.

gr-qc↗

arXiv2008

Ultrahigh-energy cosmic-ray bounds on nonbirefringent modified-Maxwell theory

A particularly simple Lorentz-violating modification of the Maxwell theory of photons maintains gauge invariance, CPT, and renormalization. This modified-Maxwell theory, coupled to standard Dirac particles, involves nineteen dimensionless "deformation parameters." Ten of these parameters lead to birefringence and are already tightly constrained by astrophysics. New bounds on the remaining nine nonbirefringent parameters can be obtained from the absence of vacuum Cherenkov radiation for ultrahigh-energy cosmic rays (UHECRs). Using selected UHECR events recorded at the Pierre Auger Observatory and assigning pseudo-random directions (i.e., assuming large-scale isotropy), Cherenkov bounds are found at the 10^{-18} level, which improve considerably upon current laboratory bounds. Future UHECR observations may reduce these Cherenkov bounds to the 10^{-23} level. An Addendum with two-sided bounds has been published separately [Phys. Rev. D 77, 117901 (2008), arXiv:0806.4351].

hep-ph↗

arXiv2008

Addendum: Ultrahigh-energy cosmic-ray bounds on nonbirefringent modified-Maxwell theory

Nonbirefringent modified-Maxwell theory, coupled to standard Dirac particles, involves nine dimensionless parameters, which can be bounded by the inferred absence of vacuum Cherenkov radiation for ultrahigh-energy cosmic rays (UHECRs). With selected UHECR events, two-sided bounds on the eight nonisotropic parameters are obtained at the 10^{-18} level, together with an improved one-sided bound on the single isotropic parameter at the 10^{-19} level.

hep-ph↗

arXiv2008

Self-tuning vacuum variable and cosmological constant

A spacetime-independent variable is introduced which characterizes a Lorentz-invariant self-sustained quantum vacuum. For a perfect (Lorentz-invariant) quantum vacuum, the self-tuning of this variable nullifies the effective energy density which enters the low-energy gravitational field equations. The observed small but nonzero value of the cosmological constant may then be explained as corresponding to the effective energy density of an imperfect quantum vacuum (perturbed by, e.g., the presence of thermal matter).

gr-qc↗

arXiv2007

Small-Scale Structure of Spacetime: Bounds and Conjectures

This review consists of two parts. The first part establishes certain astrophysical bounds on the smoothness of classical spacetime. Some of the best bounds to date are based on the absence of vacuum Cherenkov radiation in ultrahigh-energy cosmic rays. The second part discusses possible implications of these bounds for the quantum structure of spacetime. One conjecture is that the fundamental length scale of quantum spacetime may be different from the Planck length.

hep-ph↗

arXiv2007

Fundamental length scale of quantum spacetime foam

It is argued that the fundamental length scale for the quantum dynamics of spacetime need not be equal to the Planck length. Possibly, this new length scale is related to a nonvanishing cosmological constant or vacuum energy density.

gr-qc↗

arXiv2007

Vacuum Cherenkov radiation in spacelike Maxwell-Chern-Simons theory

A detailed analysis of vacuum Cherenkov radiation in spacelike Maxwell-Chern-Simons (MCS) theory is presented. A semiclassical treatment reproduces the leading terms of the tree-level result from quantum field theory. Moreover, certain quantum corrections turn out to be suppressed for large energies of the charged particle, for example, the quantum corrections to the classical MCS Cherenkov angle. It is argued that MCS-theory Cherenkov radiation may, in principle, lead to anisotropy effects for ultra-high-energy cosmic rays (UHECRs). In addition, a qualitative discussion of vacuum Cherenkov radiation from a modified-Maxwell term in the action is given, together with UHECR bounds on some of its dimensionless "coupling constants."

hep-th↗

arXiv2007

Bounds on length scales of classical spacetime foam models

Simple models of a classical spacetime foam are considered, which consist of identical static defects embedded in Minkowski spacetime. Plane-wave solutions of the vacuum Maxwell equations with appropriate boundary conditions at the defect surfaces are obtained in the long-wavelength limit. The corresponding dispersion relations ω^2=ω^2(\vec{k}) are calculated, in particular, the coefficients of the quadratic and quartic terms in \vec{k}. Astronomical observations of gamma-ray bursts and ultra-high-energy cosmic rays then place bounds on the coefficients of the dispersion relations and, thereby, on particular combinations of the fundamental length scales of the static spacetime-foam models considered. Spacetime foam models with a single length scale are excluded, even models with a length scale close to the Planck length (as long as a classical spacetime remains relevant).

hep-ph↗