228 research outputs found
Supersymmetric Large Extra Dimensions and the Cosmological Constant Problem
This article briefly summarizes and reviews the motivations for - and the
present status of - the proposal that the small size of the observed Dark
Energy density can be understood in terms of the dynamical relaxation of two
large extra dimensions within a supersymmetric higher-dimensional theory.Comment: Talk presented to Theory Canada I, Vancouver, June 2005. References
added in V
On the Initial Conditions for Brane Inflation
String theory gives rise to various mechanisms to generate primordial
inflation, of which ``brane inflation'' is one of the most widely considered.
In this scenario, inflation takes place while two branes are approaching each
other, and the modulus field representing the separation between the branes
plays the role of the inflaton field. We study the phase space of initial
conditions which can lead to a sufficiently long period of cosmological
inflation, and find that taking into account the possibility of nonvanishing
initial momentum can significantly change the degree of fine tuning of the
required initial conditions.Comment: 11 pages, 2 figure
Supersymmetric Large Extra Dimensions and the Cosmological Constant: An Update
This article critically reviews the proposal for addressing the cosmological
constant problem within the framework of supersymmetric large extra dimensions
(SLED), as recently proposed in hep-th/0304256. After a brief restatement of
the cosmological constant problem, a short summary of the proposed mechanism is
given. The emphasis is on the perspective of the low-energy effective theory in
order to see how it addresses the problem of why low-energy particles like the
electron do not contribute too large a vacuum energy. This is followed by a
discussion of the main objections, which are grouped into the following five
topics:
(1) Weinberg's No-Go Theorem.
(2) Are hidden tunings of the theory required, and a problem?
(3) Why should the mechanism not rule out earlier epochs of inflation?
(4) How big are quantum effects, and which are the most dangerous?
(5) Can the mechanism be consistent with cosmological constraints?
It is argued that there are plausible reasons why the mechanism can thread
the potential objections, but that a definitive proof that it does depends on
addressing well-defined technical points. These points include identifying what
fixes the size of the extra dimensions, checking how topological obstructions
renormalize and performing specific calculations of quantum corrections. More
detailed studies of these issues, which are well reach within our present
understanding of extra-dimensional theories, are currently underway. As such,
the jury remains out concerning the proposal, although the prospects for
acquittal still seem good.Comment: 21 pages; an extended version of the contribution to the proceedings
of SUSY 2003, University of Arizona, Tucson AZ, June 2003, which has also
been updated to include developments since the conference. (v2 includes some
updated references and corrects a minor error in the bulk loop section
A new heparan sulfate from the mollusk nodipecten nodosus inhibits merozoite invasion and disrupts rosetting and cytoadherence of plasmodium falciparum
Despite treatment with effective antimalarial drugs, the mortality rate is still high in severe cases of the disease, highlighting the need to find adjunct therapies that can inhibit the adhesion of Pf-iEs. In this context, we evaluated a new heparan sulfate (HS) from Nodipecten nodosus for antimalarial activity and inhibition of P. falciparum cytoadhesion and rosetting. Parasite inhibition was measured by SYBR green using a cytometer. HS was assessed in rosetting and cytoadhesion assays under static and flow conditions using CHO and HLEC cells expressing ICAM1 and CSA, respectively. This HS inhibited merozoite invasion similar to heparin. Moreover, mollusk HS decreased cytoadherence of P. falciparum to CSA (chondroitin sulfate A) and ICAM-1 (intercellular adhesion molecule-1) on the surface of endothelial cells under static and flow conditions. In addition, this glycan efficiently disrupted rosettes. These findings support a potential use for mollusk HS as adjunct therapy for severe malaria114CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTĂFICO E TECNOLĂGICO - CNPQFUNDAĂĂO CARLOS CHAGAS FILHO DE AMPARO Ă PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJFUNDAĂĂO DE AMPARO Ă PESQUISA DO ESTADO DE SĂO PAULO - FAPESPnĂŁo temnĂŁo tem2012/16525-2; 2017/18611-7; 2010/18571-6; 2015/20774-
Quantum Gravity Constraints on Inflation
We study quantum gravity constraints on inflationary model building. Our
approach is based on requiring the entropy associated to a given inflationary
model to be less than that of the de Sitter entropy. We give two prescriptions
for determining the inflationary entropy, based on either `bits per unit area'
or entanglement entropy. The existence of transPlanckian flat directions,
necessary for large tensor modes in the CMB, correlates with an inflationary
entropy greater than that allowed by de Sitter space. Independently these
techniques also constrain or exclude de Sitter models with large-rank gauge
groups and high UV cutoffs, such as racetrack inflation or the KKLT
construction.Comment: 22 pages; v2 references adde
Primeval Corrections to the CMB Anisotropies
We show that deviations of the quantum state of the inflaton from the thermal
vacuum of inflation may leave an imprint in the CMB anisotropies. The quantum
dynamics of the inflaton in such a state produces corrections to the
inflationary fluctuations, which may be observable. Because these effects
originate from IR physics below the Planck scale, they will dominate over any
trans-Planckian imprints in any theory which obeys decoupling. Inflation sweeps
away these initial deviations and forces its quantum state closer to the
thermal vacuum. We view this as the quantum version of the cosmic no-hair
theorem. Such imprints in the CMB may be a useful, independent test of the
duration of inflation, or of significant features in the inflaton potential
about 60 e-folds before inflation ended, instead of an unlikely discovery of
the signatures of quantum gravity. The absence of any such substructure would
suggest that inflation lasted uninterrupted much longer than
e-folds.Comment: 17 pages, latex, no figures; v3: added references and comments, final
version to appear in Phys. Rev.
Rolling Tachyon in Brane World Cosmology from Superstring Field Theory
The pressureless tachyonic matter recently found in superstring field theory
has an over-abundance problem in cosmology. We argue that this problem is
naturally solved in the brane inflationary scenario if almost all of the
tachyon energy is drained (via its coupling to the inflaton and matter fields)
to heating the universe, while the rest of the tachyon energy goes to a network
of cosmic strings (lower-dimensional BPS D-branes) produced during the tachyon
rolling at the end of inflation.Comment: 4 pages, one figure. This version quantifies constraints on various
phenomenological models for tachyon deca
Loop-Generated Bounds on Changes to the Graviton Dispersion Relation
We identify the effective theory appropriate to the propagation of massless
bulk fields in brane-world scenarios, to show that the dominant low-energy
effect of asymmetric warping in the bulk is to modify the dispersion relation
of the effective 4-dimensional modes. We show how such changes to the graviton
dispersion relation may be bounded through the effects they imply, through
loops, for the propagation of standard model particles. We compute these bounds
and show that they provide, in some cases, the strongest constraints on
nonstandard gravitational dispersions. The bounds obtained in this way are the
strongest for the fewest extra dimensions and when the extra-dimensional Planck
mass is the smallest. Although the best bounds come for warped 5-D scenarios,
for which the 5D Planck Mass is O(TeV), even in 4 dimensions the graviton loop
can lead to a bound on the graviton speed which is comparable with other
constraints.Comment: 18 pages, LaTeX, 4 figures, uses revte
Towards a Naturally Small Cosmological Constant from Branes in 6D Supergravity
We investigate the possibility of self-tuning of the effective 4D
cosmological constant in 6D supergravity, to see whether it could naturally be
of order 1/r^4 when compactified on two dimensions having Kaluza-Klein masses
of order 1/r. In the models we examine supersymmetry is broken by the presence
of non-supersymmetric 3-branes (on one of which we live). If r were
sub-millimeter in size, such a cosmological constant could describe the
recently-discovered dark energy. A successful self-tuning mechanism would
therefore predict a connection between the observed size of the cosmological
constant, and potentially observable effects in sub-millimeter tests of gravity
and at the Large Hadron Collider. We do find self tuning inasmuch as 3-branes
can quite generically remain classically flat regardless of the size of their
tensions, due to an automatic cancellation with the curvature and dilaton of
the transverse two dimensions. We argue that in some circumstances
six-dimensional supersymmetry might help suppress quantum corrections to this
cancellation down to the bulk supersymmetry-breaking scale, which is of order
1/r. We finally examine an explicit realization of the mechanism, in which
3-branes are inserted into an anomaly-free version of Salam-Sezgin gauged 6D
supergravity compactified on a 2-sphere with nonzero magnetic flux. This
realization is only partially successful due to a topological constraint which
relates bulk couplings to the brane tension, although we give arguments why
these relations may be stable against quantum corrections.Comment: 31 pages, 1 figure. Uses JHEP class. Expanded discussions in
Introduction, Section 3.2 (Quantum Corrections) and Section 4.2 (Topological
Constraint). Note added on subsequent related articles. Results unchange
Spontaneous Creation of Inflationary Universes and the Cosmic Landscape
We study some gravitational instanton solutions that offer a natural
realization of the spontaneous creation of inflationary universes in the brane
world context in string theory. Decoherence due to couplings of higher
(perturbative) modes of the metric as well as matter fields modifies the
Hartle-Hawking wavefunction for de Sitter space. Generalizing this new
wavefunction to be used in string theory, we propose a principle in string
theory that hopefully will lead us to the particular vacuum we live in, thus
avoiding the anthropic principle. As an illustration of this idea, we give a
phenomenological analysis of the probability of quantum tunneling to various
stringy vacua. We find that the preferred tunneling is to an inflationary
universe (like our early universe), not to a universe with a very small
cosmological constant (i.e., like today's universe) and not to a 10-dimensional
uncompactified de Sitter universe. Such preferred solutions are interesting as
they offer a cosmological mechanism for the stabilization of extra dimensions
during the inflationary epoch.Comment: 52 pages, 7 figures, 1 table. Added discussion on supercritical
string vacua, added reference
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