63 research outputs found
Massive string modes and non-singular pre-big-bang cosmology
Perturbative corrections to the low energy string effective action
have been recently found to have a potentially regularizing effect on the
singularity of the lowest order pre-big-bang solutions. Whether they actually
regularize it, however, cannot be determined working at any finite order in a
perturbative expansion in powers of the string constant , because of
scheme dependence ambiguities. Physically, these corrections are dominated by
the integration over the first few massive string states. Very massive string
modes, instead, can have a regularizing effect which is non-perturbative in
and which basically comes from the fact that in a gravitational
field with Hubble constant they are produced with an effective Hawking
temperature , and an infinite production rate would occur if this
temperature exceeded the Hagedorn temperature. We discuss technical and
conceptual difficulties of this non-perturbative regularization mechanism.Comment: 25 pages, Latex, 3 figures. Conceptual revisions. To be published in
Nucl. Phys.
The graviton background at early times in string cosmology
We discuss some peculiar properties of the stochastic graviton background
predicted by string cosmology. At Planckian times, for the values of the
parameters of the model which are more interesting for the detection in
gravitational wave experiments, the number density of gravitons is
parametrically large compared to Planck density, and is peaked at small
energies. The large parameter is related to the duration of the string phase
and is a characteristic of string cosmology. The typical interaction time is
parametrically larger than the Planck or string time. Therefore the shape of
the graviton spectrum is not distorted by thermal effects in the Planck era and
can carry informations on the pre-big bang phase suggested by string cosmology.Comment: 4 pages, 1 figure, Late
Loop corrections and graceful exit in string cosmology
We examine the effect of perturbative string loops on the cosmological
pre-big-bang evolution. We study loop corrections derived from heterotic string
theory compactified on a orbifold and we consider the effect of the
all-order loop corrections to the Kahler potential and of the corrections to
gravitational couplings, including both threshold corrections and corrections
due to the mixed Kahler-gravitational anomaly. We find that string loops can
drive the evolution into the region of the parameter space where a graceful
exit is in principle possible, and we find solutions that, in the string frame,
connect smoothly the superinflationary pre-big-bang evolution to a phase where
the curvature and the derivative of the dilaton are decreasing. We also find
that at a critical coupling the loop corrections to the Kahler potential induce
a ghost-like instability, i.e. the kinetic term of the dilaton vanishes. This
is similar to what happens in Seiberg-Witten theory and signals the transition
to a new regime where the light modes in the effective action are different and
are related to the original ones by S-duality. In a string context, this means
that we enter a D-brane dominated phase.Comment: 39 pages, Latex, 17 eps figure
D-branes and Cosmology
D-branes, topological defects in string theory on which string endpoints can
live, may give new insight into the understanding of the cosmological evolution
of the Universe at early epochs. We analyze the dynamics of D-branes in curved
backgrounds and discuss the parameter space of M-theory as a function of the
coupling constant and of the curvature of the Universe. We show that D-branes
may be efficiently produced by gravitational effects. Furthermore, in curved
spacetimes the transverse fluctuations of the D-branes develop a tachyonic mode
and when the fluctuations grow larger than the horizon the branes become
tensionless and break up. This signals a transition to a new regime. We discuss
the implications of our findings for the singularity problem present in string
cosmology, suggesting the existence of a limiting value for the curvature which
is in agreement with the value suggested by the cosmological version of the
holography principle. We also comment on possible implications for the
so-called brane world scenario, where the Standard Model gauge and matter
fields live inside some branes while gravitons live in the bulk.Comment: LaTeX file, 30 pages, 2 eps figures, 1 reference adde
Ultrasound-assisted synthesis of WOx-decorated ZnO photocatalysts for NOx abatement
Heterojunctions based on ZnO have numerous applications, such as water splitting, sensing and energy storage [1]. Recently, ZnO/WO3 composites have shown promising results in the sonocatalytic and photocatalytic degradation of aqueous and gas pollutants [2]. Several synthetic approaches have been reported, including chemical vapor deposition, magnetron sputtering, hydrothermal methods and high temperature annealing. Ultrasound-assisted synthesis can provide a scalable and cost-effective strategy to tailor the catalyst structural and morphological properties [3]. In the present work, pristine ZnO and ZnO/WOx composites were synthesized via a sonochemical method, studying the role of the ultrasound amplitude and mode (continuous/pulsed), metal precursor, WOx content and post-synthetic annealing. The resulting materials were extensively characterized, investigating their structural, morphological, optical, and surface properties. Samples were tested towards the photocatalytic removal of NOx under both UV and visible light irradiation in a batch reactor. A good degree of crystallinity is appreciable even before calcination and better morphological features are observed with respect to reference samples prepared without ultrasounds. The morphological properties can be further tuned by changing the metal precursor and adding a post-synthetic annealing step. Photocatalytic activity is promoted with respect to both benchmark samples (Figure 1)
Anisotropic String Cosmology at Large Curvatures
We study the effect of the antisymmetric tensor field on the
large curvature phase of string cosmology. It is well-known that a
non-vanishing value of leads to an anisotropic expansion of the spatial
dimensions. Correspondingly, in the string phase of the model, including
corrections, we find anisotropic fixed points of the evolution,
which act as regularizing attractors of the lowest order solutions. The
attraction basin can also include isotropic initial conditions for the scale
factors. We present explicit examples at order for different values
of the number of spatial dimensions and for different ans\"{a}tze for .Comment: 16 pages, Latex, 2 figure
Adjunctive treatment and BoNT-A for post-stroke spasticity: are we really focusing on the patient-centered goals?
[no abstract available
Gravitational Wave Experiments and Early Universe Cosmology
Gravitational-wave experiments with interferometers and with resonant masses
can search for stochastic backgrounds of gravitational waves of cosmological
origin. We review both experimental and theoretical aspects of the search for
these backgrounds. We give a pedagogical derivation of the various relations
that characterize the response of a detector to a stochastic background. We
discuss the sensitivities of the large interferometers under constructions
(LIGO, VIRGO, GEO600, TAMA300, AIGO) or planned (Avdanced LIGO, LISA) and of
the presently operating resonant bars, and we give the sensitivities for
various two-detectors correlations. We examine the existing limits on the
energy density in gravitational waves from nucleosynthesis, COBE and pulsars,
and their effects on theoretical predictions. We discuss general theoretical
principles for order-of-magnitude estimates of cosmological production
mechanisms, and then we turn to specific theoretical predictions from
inflation, string cosmology, phase transitions, cosmic strings and other
mechanisms. We finally compare with the stochastic backgrounds of astrophysical
origin.Comment: 99 pages, Latex, 17 figures. To appear in Physics Report. v4:
conceptual changes in sect. 7.
MxA mRNA quantification and disability progression in interferon beta-treated Multiple Sclerosis patients
Even though anti-interferon beta (IFNβ) antibodies are the main determinants of IFNβ bioactivity loss and Myxovirus-resistance protein A (MxA) is the most established marker of IFNβ biological activity in IFNβ-treated multiple sclerosis patients, their usefulness in the routine clinical practice is still debated. Therefore, 118 multiple sclerosis patients naïve for treatment were enrolled for a 3-year longitudinal observational study mimicking the conditions of a real-world setting. In order to evaluate the kinetics of bioactivity loss in blood samples obtained every 6 months after therapy initiation, MxA and interferon receptor isoform/subunit mRNA were quantified by real-time PCR, anti-IFNβ binding antibodies were detected by radioimmunoprecipitation, and neutralizing antibodies by cytopathic effect inhibition assay. Clinical measures of disease activity and disability progression were also obtained at all time points. We found that, at the individual-patient level, the response to IFNβ therapy was extremely heterogeneous, including patients with stable or transitory, early or late loss of IFNβ bioactivity, and patients with samples lacking MxA mRNA induction in spite of absence of antibodies. No interferon receptor isoform alterations that could explain these findings were found. At the group level, none of these biological features correlated with the measures of clinical disease activity or progression. However, when MxA mRNA was evaluated not at the single time point as a dichotomic marker (induced vs. non-induced), but as the mean of its values measured over the 6-to-24 month period, the increasing average MxA predicted a decreasing risk of short-term disability progression, independently from the presence of relapses. Therefore, a more bioactive treatment, even if unable to suppress relapses, reduces their severity by an amount that is proportional to MxA levels. Together with its feasibility in the routine laboratory setting, these data warrant the quantification of MxA mRNA as a primary tool for a routine monitoring of IFNβ therapy
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