32,682 research outputs found
On the growth of structure in theories with a dynamical preferred frame
We study the cosmological stability of a class of theories with a dynamical
preferred frame. For a range of actions, we find cosmological solutions which
are compatible with observations of the recent history of the Universe: a
matter dominated era followed by accelerated expansion. We then study the
evolution of linear perturbations on these backgrounds and find conditions on
the parameters of the theory which allow for the growth of structure sourced by
the new degrees of freedom
Discrete and continuous symmetries in multi-Higgs-doublet models
We consider the Higgs sector of multi-Higgs-doublet models in the presence of
simple symmetries relating the various fields. We construct basis invariant
observables which may in principle be used to detect these symmetries for any
number of doublets. A categorization of the symmetries into classes is
required, which we perform in detail for the case of two and three Higgs
doublets.Comment: 13 pages, RevTex, references adde
Tensor Microwave Anisotropies from a Stochastic Magnetic Field
We derive an expression for the angular power spectrum of cosmic microwave
background anisotropies due to gravity waves generated by a stochastic magnetic
field and compare the result with current observations; we take into account
the non-linear nature of the stress energy tensor of the magnetic field.
For almost scale invariant spectra, the amplitude of the magnetic field at
galactic scales is constrained to be of order 10^{-9} Gauss. If we assume that
the magnetic field is damped below the Alfven damping scale, we find that its
amplitude at
0.1 h^{-1}Mpc, B_\lambda, is constrained to be B_\lambda<7.9 x10^{-6} e^{3n}
Gauss, for n-3/2, where
n is the spectral index of the magnetic field and H_0=100h km s^{-1}Mpc^{-1} is
the Hubble constant today.Comment: 6 pages, 1 figure, accepted for publication in Phys. Rev.
The Vector-Tensor nature of Bekenstein's relativistic theory of Modified Gravity
Bekenstein's theory of relativistic gravity is conventionally written as a
bi-metric theory. The two metrics are related by a disformal transformation
defined by a dynamical vector field and a scalar field. In this comment we show
that the theory can be re-written as Vector-Tensor theory akin to
Einstein-Aether theories with non-canonical kinetic terms. We discuss some of
the implications of this equivalence.Comment: Updated version: Notation cleaned up and some typos corrected-TG
Mass for Plasma Photons from Gauge Symmetry Breaking
We derive the effective masses for photons in unmagnetized plasma waves using
a quantum field theory with two vector fields (gauge fields). In order to
properly define the quantum field degrees of freedom we re-derive the classical
wave equations on light-front gauge. This is needed because the usual scalar
potential of electromagnetism is, in quantum field theory, not a physical
degree of freedom that renders negative energy eigenstates. We also consider a
background local fluid metric that allows for a covariant treatment of the
problem. The different masses for the longitudinal (plasmon) and transverse
photons are in our framework due to the local fluid metric. We apply the
mechanism of mass generation by gauge symmetry breaking recently proposed by
the authors by giving a non-trivial vacuum-expectation-value to the second
vector field (gauge field). The Debye length is interpreted as an
effective compactification length and we compute an explicit solution for the
large gauge transformations that correspond to the specific mass eigenvalues
derived here. Using an usual quantum field theory canonical quantization we
obtain the usual results in the literature. Although none of these ingredients
are new to physicist, as far as the authors are aware it is the first time that
such constructions are applied to Plasma Physics. Also we give a physical
interpretation (and realization) for the second vector field in terms of the
plasma background in terms of known physical phenomena.
Addendum: It is given a short proof that equation (10) is wrong, therefore
equations (12-17) are meaningless. The remaining results are correct being
generic derivations for nonmagnetized plasmas derived in a covariant QFT
framework.Comment: v1: 1+6 pages v2: Several discussions rewritten; Abstract rewritten;
References added; v3: includes Addendu
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