69 research outputs found
Fading Gravity and Self-Inflation
We study the cosmology of a toy modified theory of gravity in which gravity
shuts off at short distances, as in the fat graviton scenario of Sundrum. In
the weak-field limit, the theory is perturbatively local, ghost-free and
unitary, although likely suffers from non-perturbative instabilities. We derive
novel self-inflationary solutions from the vacuum equations of the theory,
without invoking scalar fields or other forms of stress energy. The modified
perturbation equation expressed in terms of the Newtonian potential closely
resembles its counterpart for inflaton fluctuations. The resulting scalar
spectrum is therefore slightly red, akin to the simplest scalar-driven
inflationary models. A key difference, however, is that the gravitational wave
spectrum is generically not scale invariant. In particular the tensor spectrum
can have a blue tilt, a distinguishing feature from standard inflation.Comment: 35 pages, 4 figures. v3: version to appear in Phys. Rev.
Testing symmetries in effective models of higher derivative field theories
Higher derivative field theories with interactions raise serious doubts about
their validity due to severe energy instabilities. In many cases the
implementation of a direct perturbation treatment to excise the dangerous
negative-energies from a higher derivative field theory may lead to violations
of Lorentz and other symmetries. In this work we study a perturbative
formulation for higher derivative field theories that allows the construction
of a low-energy effective field theory being a genuine perturbations over the
ordinary-derivative theory and having a positive-defined Hamiltonian. We show
that some discrete symmetries are recovered in the low-energy effective theory
when the perturbative method to reduce the negative-energy degrees of freedom
from the higher derivative theory is applied. In particular, we focus on the
higher derivative Maxwell-Chern-Simons model which is a Lorentz invariant and
parity-odd theory in 2+1 dimensions. The parity violation arises in the
effective action of QED as a quantum correction from the massive fermionic
sector. We obtain the effective field theory which remains Lorentz invariant,
but parity invariant to the order considered in the perturbative expansion.Comment: 13 pages, Sec. III, additional references added, P symmetry revised,
accepted for publication in PR
Massive photons and Lorentz violation
All quadratic translation- and gauge-invariant photon operators for Lorentz
breakdown are included into the Stueckelberg Lagrangian for massive photons in
a generalized \xi-gauge. The corresponding dispersion relation and tree-level
propagator are determined exactly, and some leading-order results are derived.
The question of how to include such Lorentz-violating effects into a
perturbative quantum-field expansion is addressed. Applications of these
results within Lorentz-breaking quantum field theories include the
regularization of infrared divergences as well as the free propagation of
massive vector bosons.Comment: 12 pages, 1 figur
On the instability of classical dynamics in theories with higher derivatives
The development of instability in the dynamics of theories with higher
derivatives is traced in detail in the framework of the Pais-Uhlenbeck fourth
oder oscillator. For this aim the external friction force is introduced in the
model and the relevant solutions to equations of motion are investigated. As a
result, the physical implication of the energy unboundness from below in
theories under consideration is revealed.Comment: 9 pages, no figures and no tables, revtex4; a few misprints are
correcte
Effective Field Theory for Inflation
The methods of effective field theory are used to study generic theories of
inflation with a single inflaton field. For scalar modes, the leading
corrections to the correlation function are found to be purely of
the -inflation type. For tensor modes the leading corrections to the
correlation function arise from terms in the action that are quadratic in the
curvature, including a parity-violating term that makes the propagation of
these modes depend on their helicity. These methods are also briefly applied to
non-generic theories of inflation with an extra shift symmetry, as in so-called
ghost inflation.Comment: 14 pages, Latex, references added and minor additions and corrections
mad
Low-temperature thermodynamics of the classical frustrated ferromagnetic chain in magnetic field
Low-temperature magnetization curves of the classical frustrated
ferromagnetic chain in the external magnetic field near the transition point
between the ferromagnetic and the helical phases is studied. It is shown that
the calculation of the partition function in the scaling limit reduces to the
solution of the Schr\"{o}dinger equation of the special form for the quantum
particle. It is proposed that the magnetization of the classical model in the
ferromagnetic part of the phase diagram including the transition point defines
the universal scaling function which is valid for quantum model as well.
Explicit analytical formulae for the magnetization are given in the limiting
cases of low and high magnetic fields. The influence of the easy-axis
anisotropy on the magnetic properties of the model is studied. It is shown that
even small anisotropy essentially changes the behavior of the susceptibility in
the vicinity of the transition point.Comment: 14 pages, 5 figure
Supersymmetric Galileons
Galileon theories are of considerable interest since they allow for stable
violations of the null energy condition. Since such violations could have
occurred during a high-energy regime in the history of our universe, we are
motivated to study supersymmetric extensions of these theories. This is carried
out in this paper, where we construct generic classes of N=1 supersymmetric
Galileon Lagrangians. They are shown to admit non-equivalent stress-energy
tensors and, hence, vacua manifesting differing conditions for violating the
null energy condition. The temporal and spatial fluctuations of all component
fields of the supermultiplet are analyzed and shown to be stable on a large
number of such backgrounds. In the process, we uncover a surprising connection
between conformal Galileon and ghost condensate theories, allowing for a deeper
understanding of both types of theories.Comment: 41 pages, v2: added a referenc
Consistent Construction of Perturbation Theory on Noncommutative Spaces
We examine the effect of non-local deformations on the applicability of
interaction point time ordered perturbation theory (IPTOPT) based on the free
Hamiltonian of local theories. The usual argument for the case of quantum field
theory (QFT) on a noncommutative (NC) space (based on the fact that the
introduction of star products in bilinear terms does not alter the action) is
not applicable to IPTOPT due to several discrepancies compared to the naive
path integral approach when noncommutativity involves time. These discrepancies
are explained in detail. Besides scalar models, gauge fields are also studied.
For both cases, we discuss the free Hamiltonian with respect to non-local
deformations.Comment: 22 pages; major changes in Section 3; minor changes in the
Introduction and Conclusio
Infinite spin particles
We show that Wigner's infinite spin particle classically is described by a
reparametrization invariant higher order geometrical Lagrangian. The model
exhibit unconventional features like tachyonic behaviour and momenta
proportional to light-like accelerations. A simple higher order superversion
for half-odd integer particles is also derived. Interaction with external
vector fields and curved spacetimes are analyzed with negative results except
for (anti)de Sitter spacetimes. We quantize the free theories covariantly and
show that the resulting wave functions are fields containing arbitrary large
spins. Closely related infinite spin particle models are also analyzed.Comment: 43 pages, Late
Electrodynamics with Lorentz-violating operators of arbitrary dimension
The behavior of photons in the presence of Lorentz and CPT violation is
studied. Allowing for operators of arbitrary mass dimension, we classify all
gauge-invariant Lorentz- and CPT-violating terms in the quadratic Lagrange
density associated with the effective photon propagator. The covariant
dispersion relation is obtained, and conditions for birefringence are
discussed. We provide a complete characterization of the coefficients for
Lorentz violation for all mass dimensions via a decomposition using
spin-weighted spherical harmonics. The resulting nine independent sets of
spherical coefficients control birefringence, dispersion, and anisotropy. We
discuss the restriction of the general theory to various special models,
including among others the minimal Standard-Model Extension, the isotropic
limit, the case of vacuum propagation, the nonbirefringent limit, and the
vacuum-orthogonal model. The transformation of the spherical coefficients for
Lorentz violation between the laboratory frame and the standard Sun-centered
frame is provided. We apply the results to various astrophysical observations
and laboratory experiments. Astrophysical searches of relevance include studies
of birefringence and of dispersion. We use polarimetric and dispersive data
from gamma-ray bursts to set constraints on coefficients for Lorentz violation
involving operators of dimensions four through nine, and we describe the mixing
of polarizations induced by Lorentz and CPT violation in the cosmic-microwave
background. Laboratory searches of interest include cavity experiments. We
present the theory for searches with cavities, derive the experiment-dependent
factors for coefficients in the vacuum-orthogonal model, and predict the
corresponding frequency shift for a circular-cylindrical cavity.Comment: 58 pages two-column REVTeX, accepted in Physical Review
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