246 research outputs found
Deriving Gauge Symmetry and Spontaneous Lorentz Violation
We consider a class of field theories with a four-vector field
in addition to other fields supplied with a global charge symmetry - theories
which have partial gauge symmetry in the sense of only imposing it on those
terms in the Lagrangian density which have derivatives as factors in them. We
suppose that spontaneous Lorentz invariance breaking occurs in such a theory
due to the four-vector field taking a non-zero vacuum expectation value. Under
some very mild assumptions, we show that this Lorentz violation is not
observable and the whole theory is practically gauge invariant. A very
important presupposition for this theorem is that an initial condition is
imposed on the no-derivative expressions corresponding to the early Universe
being essentially in a vacuum state. This condition then remains true forever
and can be interpreted as a gauge constraint. We formulate the conditions under
which the spontaneous Lorentz violation becomes observable. Spontaneously
broken Lorentz invariance could be seen by some primordially existing or
created "fossil" charges with the property of moving through the Universe with
a fixed velocity.Comment: Extended versio
Optical Activity From Extra Dimension
Optical activity, like Faraday effect, is a rotation of the plane of
polarization of propagating light in a medium and can be attributed to
different sources with distinct signatures. In this note we discuss the effect
of optical activity {\it{in vacuum}} due to Kaluza-Klein scalar field ,
in the presence of an external electro-magnetic field. The astrophysical
implication of this effect is indicated. We also point out the possibility of
observing the same in laboratory conditions.Comment: Four Page
Spontaneously Generated Tensor Field Gravity
An arbitrary local theory of a symmetric two-tensor field in
Minkowski spacetime is considered, in which the equations of motion are
required to be compatible with a nonlinear length-fixing constraint leading to spontaneous Lorentz invariance violation, SLIV
( is the proposed scale for SLIV). Allowing the parameters in the Lagrangian
to be adjusted so as to be consistent with this constraint, the theory turns
out to correspond to linearized general relativity in the weak field
approximation, while some of the massless tensor Goldstone modes appearing
through SLIV are naturally collected in the physical graviton. In essence the
underlying diffeomophism invariance emerges as a necessary condition for the
tensor field not to be superfluously restricted in degrees of
freedom, apart from the constraint due to which the true vacuum in the theory
is chosen by SLIV. The emergent theory appears essentially nonlinear, when
expressed in terms of the pure Goldstone tensor modes and contains a plethora
of new Lorentz and violating couplings. However, these couplings do not
lead to physical Lorentz violation once this tensor field gravity is properly
extended to conventional general relativity.Comment: 27 pages, published version, to appear in Nuclear Physics
Plasma scale length effects on protons generated in ultra-intense laserâplasmas
The energy spectra of protons generated by ultra-intense (1020 W cmâ2) laser interactions with a preformed plasma of scale length measured by shadowgraphy are presented. The effects of the preformed plasma on the proton beam temperature and the number of protons are evaluated. Two-dimensional EPOCH particle-in-cell code simulations of the proton spectra are found to be in agreement with measurements over a range of experimental parameter
Constrained Gauge Fields from Spontaneous Lorentz Violation
Spontaneous Lorentz violation realized through a nonlinear vector field
constraint of the type ( is the proposed scale for
Lorentz violation) is shown to generate massless vector Goldstone bosons,
gauging the starting global internal symmetries in arbitrary relativistically
invariant theories. The gauge invariance appears in essence as a necessary
condition for these bosons not to be superfluously restricted in degrees of
freedom, apart from the constraint due to which the true vacuum in a theory is
chosen by the Lorentz violation. In the Abelian symmetry case the only possible
theory proves to be QED with a massless vector Goldstone boson naturally
associated with the photon, while the non-Abelian symmetry case results in a
conventional Yang-Mills theory. These theories, both Abelian and non-Abelian,
look essentially nonlinear and contain particular Lorentz (and ) violating
couplings when expressed in terms of the pure Goldstone vector modes. However,
they do not lead to physical Lorentz violation due to the simultaneously
generated gauge invariance.Comment: 15 pages, minor corrections, version to be published in Nucl. Phys.
Dark Energy and Extending the Geodesic Equations of Motion: Its Construction and Experimental Constraints
With the discovery of Dark Energy, , there is now a universal
length scale, , associated with the
universe that allows for an extension of the geodesic equations of motion. In
this paper, we will study a specific class of such extensions, and show that
contrary to expectations, they are not automatically ruled out by either
theoretical considerations or experimental constraints. In particular, we show
that while these extensions affect the motion of massive particles, the motion
of massless particles are not changed; such phenomena as gravitational lensing
remain unchanged. We also show that these extensions do not violate the
equivalence principal, and that because Mpc, a
specific choice of this extension can be made so that effects of this extension
are not be measurable either from terrestrial experiments, or through
observations of the motion of solar system bodies. A lower bound for the only
parameter used in this extension is set.Comment: 19 pages. This is the published version of the first half of
arXiv:0711.3124v2 with corrections include
Modified Gravity via Spontaneous Symmetry Breaking
We construct effective field theories in which gravity is modified via
spontaneous breaking of local Lorentz invariance. This is a gravitational
analogue of the Higgs mechanism. These theories possess additional graviton
modes and modified dispersion relations. They are manifestly well-behaved in
the UV and free of discontinuities of the van Dam-Veltman-Zakharov type,
ensuring compatibility with standard tests of gravity. They may have important
phenomenological effects on large distance scales, offering an alternative to
dark energy. For the case in which the symmetry is broken by a vector field
with the wrong sign mass term, we identify four massless graviton modes (all
with positive-definite norm for a suitable choice of a parameter) and show the
absence of the discontinuity.Comment: 5 pages; revised versio
Collisions of particles in locally AdS spacetimes I. Local description and global examples
We investigate 3-dimensional globally hyperbolic AdS manifolds containing
"particles", i.e., cone singularities along a graph . We impose
physically relevant conditions on the cone singularities, e.g. positivity of
mass (angle less than on time-like singular segments). We construct
examples of such manifolds, describe the cone singularities that can arise and
the way they can interact (the local geometry near the vertices of ).
We then adapt to this setting some notions like global hyperbolicity which are
natural for Lorentz manifolds, and construct some examples of globally
hyperbolic AdS manifolds with interacting particles.Comment: This is a rewritten version of the first part of arxiv:0905.1823.
That preprint was too long and contained two types of results, so we sliced
it in two. This is the first part. Some sections have been completely
rewritten so as to be more readable, at the cost of slightly less general
statements. Others parts have been notably improved to increase readabilit
Time-Space Noncommutativity in Gravitational Quantum Well scenario
A novel approach to the analysis of the gravitational well problem from a
second quantised description has been discussed. The second quantised formalism
enables us to study the effect of time space noncommutativity in the
gravitational well scenario which is hitherto unavailable in the literature.
The corresponding first quantized theory reveals a leading order perturbation
term of noncommutative origin. Latest experimental findings are used to
estimate an upper bound on the time--space noncommutative parameter. Our
results are found to be consistent with the order of magnitude estimations of
other NC parameters reported earlier.Comment: 7 pages, revTe
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