270 research outputs found
On Modified Gravity
We consider some aspects of nonlocal modified gravity, where nonlocality is
of the type . In particular, using ansatz of the form
we find a few solutions for the spatially flat
FLRW metric. There are singular and nonsingular bounce solutions. For late
cosmic time, scalar curvature R(t) is in low regime and scale factor a(t) is
decelerated. R (t) = 0 satisfies all equations when k = -1.Comment: added references; made some clarifications; 8 page
Weyl-Conformally-Invariant Lightlike p-Brane Theories: New Aspects in Black Hole Physics and Kaluza-Klein Dynamics
We introduce and study in some detail the properties of a novel class of
Weyl-conformally invariant p-brane theories which describe intrinsically
lightlike branes for any odd world-volume dimension. Their dynamics
significantly differs from that of the ordinary (conformally non-invariant)
Nambu-Goto p-branes. We present explicit solutions of the Weyl-invariant
lightlike brane- (WILL-brane) equations of motion in various gravitational
models of physical relevance exhibiting various new phenomena. In D=4 the
WILL-membrane serves as a material and charged source for gravity and
electromagnetism in the coupled Einstein-Maxwell-WILL-membrane system; it
automatically positions itself on (``straddles'') the common event horizon of
the corresponding matching black hole solutions, thus providing an explicit
dynamical realization of the membrane paradigm in black hole physics. In
product spaces of interest in Kaluza-Klein theories the WILL-brane wraps
non-trivially around the compact (internal)dimensions and still describes
massless mode dynamics in the non-compact (space-time) dimensions. Due to
nontrivial variable size of the internal compact dimensions we find new types
of physically interesting solutions describing massless brane modes trapped on
bounded planar circular orbits with non-trivial angular momentum, and with
linear dependence between energy and angular momentum.Comment: 28 pages, published versio
Vacuum String Field Theory ancestors of the GMS solitons
We define a sequence of VSFT D-branes whose low energy limit leads exactly to
a corresponding sequence of GMS solitons. The D-branes are defined by acting on
a fixed VSFT lump with operators defined by means of Laguerre polynomials whose
argument is quadratic in the string creation operators. The states obtained in
this way form an algebra under the SFT star product, which is isomorphic to a
corresponding algebra of GMS solitons under the Moyal product. In order to
obtain a regularized field theory limit we embed the theory in a constant
background B field.Comment: 1+16 pages; v2: typos corrected; v3: two appendices added, final
versio
Dynamics with Infinitely Many Derivatives: The Initial Value Problem
Differential equations of infinite order are an increasingly important class
of equations in theoretical physics. Such equations are ubiquitous in string
field theory and have recently attracted considerable interest also from
cosmologists. Though these equations have been studied in the classical
mathematical literature, it appears that the physics community is largely
unaware of the relevant formalism. Of particular importance is the fate of the
initial value problem. Under what circumstances do infinite order differential
equations possess a well-defined initial value problem and how many initial
data are required? In this paper we study the initial value problem for
infinite order differential equations in the mathematical framework of the
formal operator calculus, with analytic initial data. This formalism allows us
to handle simultaneously a wide array of different nonlocal equations within a
single framework and also admits a transparent physical interpretation. We show
that differential equations of infinite order do not generically admit
infinitely many initial data. Rather, each pole of the propagator contributes
two initial data to the final solution. Though it is possible to find
differential equations of infinite order which admit well-defined initial value
problem with only two initial data, neither the dynamical equations of p-adic
string theory nor string field theory seem to belong to this class. However,
both theories can be rendered ghost-free by suitable definition of the action
of the formal pseudo-differential operator. This prescription restricts the
theory to frequencies within some contour in the complex plane and hence may be
thought of as a sort of ultra-violet cut-off.Comment: 40 pages, no figures. Added comments concerning fractional operators
and the implications of restricting the contour of integration. Typos
correcte
Exact noncommutative solitons in p-adic strings and BSFT
The tachyon field of p-adic string theory is made noncommutative by replacing
ordinary products with noncommutative products in its exact effective action.
The same is done for the boundary string field theory, treated as the p -> 1
limit of the p-adic string. Solitonic lumps corresponding to D-branes are
obtained for all values of the noncommutative parameter theta. This is in
contrast to usual scalar field theories in which the noncommutative solitons do
not persist below a critical value of theta. As theta varies from zero to
infinity, the solution interpolates smoothly between the soliton of the p-adic
theory (respectively BSFT) to the noncommutative soliton.Comment: 1+14 pages (harvmac b), 1 eps figure, v2: references added, typos
correcte
Towards a Resolution of the Cosmological Singularity in Non-local Higher Derivative Theories of Gravity
One of the greatest problems of standard cosmology is the Big Bang
singularity. Previously it has been shown that non-local ghostfree
higher-derivative modifications of Einstein gravity in the ultra-violet regime
can admit non-singular bouncing solutions. In this paper we study in more
details the dynamical properties of the equations of motion for these theories
of gravity in presence of positive and negative cosmological constants and
radiation. We find stable inflationary attractor solutions in the presence of a
positive cosmological constant which renders inflation {\it geodesically
complete}, while in the presence of a negative cosmological constant a cyclic
universe emerges. We also provide an algorithm for tracking the super-Hubble
perturbations during the bounce and show that the bouncing solutions are free
from any perturbative instability.Comment: 38 pages, 6 figures. V2: Added: a word to the title, clarifications,
an appendix, many references. To appear in JCA
Analysis of scalar perturbations in cosmological models with a non-local scalar field
We develop the cosmological perturbations formalism in models with a single
non-local scalar field originating from the string field theory description of
the rolling tachyon dynamics. We construct the equation for the energy density
perturbations of the non-local scalar field in the presence of the arbitrary
potential and formulate the local system of equations for perturbations in the
linearized model when both simple and double roots of the characteristic
equation are present. We carry out the general analysis related to the
curvature and entropy perturbations and consider the most specific example of
perturbations when important quantities in the model become complex.Comment: LaTeX, 25 pages, 1 figure, v2: Subsection 3.2 and Section 5 added,
references added, accepted for publication in Class. Quant. Grav. arXiv admin
note: text overlap with arXiv:0903.517
Large Nongaussianity from Nonlocal Inflation
We study the possibility of obtaining large nongaussian signatures in the
Cosmic Microwave Background in a general class of single-field nonlocal
hill-top inflation models. We estimate the nonlinearity parameter f_{NL} which
characterizes nongaussianity in such models and show that large nongaussianity
is possible. For the recently proposed p-adic inflation model we find that
f_{NL} ~ 120 when the string coupling is order unity. We show that large
nongaussianity is also possible in a toy model with an action similar to those
which arise in string field theory.Comment: 27 pages, no figures. Added references and some clarifying remark
Epidermal growth factor receptor structural alterations in gastric cancer
<p>Abstract</p> <p>Background</p> <p>EGFR overexpression has been described in many human tumours including gastric cancer. In NSCLC patients somatic EGFR mutations, within the kinase domain of the protein, as well as gene amplification were associated with a good clinical response to EGFR inhibitors. In gastric tumours data concerning structural alterations of EGFR remains controversial. Given its possible therapeutic relevance, we aimed to determine the frequency and type of structural alterations of the <it>EGFR </it>gene in a series of primary gastric carcinomas.</p> <p>Methods</p> <p>Direct sequencing of the kinase domain of the <it>EGFR </it>gene was performed in a series of 77 primary gastric carcinomas. FISH analysis was performed in 30 cases. Association studies between <it>EGFR </it>alterations and the clinical pathological features of the tumours were performed.</p> <p>Results</p> <p>Within the 77 primary gastric carcinomas we found two <it>EGFR </it>somatic mutations and several <it>EGFR </it>polymorphisms in exon 20. Six different intronic sequence variants of <it>EGFR </it>were also found. Four gastric carcinomas showed balanced polysomy or <it>EGFR </it>gene amplification. We verified that gastric carcinoma with alterations of <it>EGFR </it>(somatic mutations or copy number variation) showed a significant increase of tumour size (<it>p </it>= 0.0094) in comparison to wild-type <it>EGFR </it>carcinomas.</p> <p>Conclusion</p> <p>We demonstrate that <it>EGFR </it>structural alterations are rare in gastric carcinoma, but whenever present, it leads to tumour growth. We considered that searching for <it>EGFR </it>alterations in gastric cancer is likely to be clinically important in order to identify patients susceptible to respond to tyrosine kinase inhibitors.</p
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