77 research outputs found
Non-minimal couplings in two dimensional gravity: a quantum investigation
We investigate the quantum effects of the non-minimal matter-gravity
couplings derived by Cangemi and Jackiw in the realm of a specific fermionic
theory, namely the abelian Thirring model on a Riemann surface of genus zero
and one. The structure and the strength of the new interactions are seen to be
highly constrained, when the topology of the underlying manifold is taken into
account. As a matter of fact, by requiring to have a well-defined action, we
are led both to quantization rules for the coupling constants and to selection
rules for the correlation functions. Explicit quantum computations are carried
out in genus one (torus). In particular the two-point function and the chiral
condensate are carefully derived for this case. Finally the effective
gravitational action, coming from integrating out the fermionic degrees of
freedoom, is presented. It is different from the standard Liouville one: a new
non-local functional of the conformal factor arises and the central charge is
improved, depending also on the Thirring coupling constant. This last feature
opens the possibility of giving a new explicit representation of the minimal
series in terms of a fermionic interacting model.Comment: Latex, 41 Page
Stability of the GRS Model
We discuss the compatibility between the weaker energy condition and the
stability of Gregory, Rubakov and Sibiryakov (GRS) model. Because the GRS
spacetime violates the weak energy condition, it may cause the instability. In
the GRS model, the four dimensional gravity can be described by the massive KK
modes with the resonance. Hence, instead of considering the weaker energy
condition, we require for the stability of this model: no tachyon and no ghost
condition for graviton modes (). No tachyonic condition () is satisfied because the lowest state is supersymmetric vacuum
state. Further, no ghost state condition is achieved if one requires some
relations for the matter source: . It
turns out that, although the GRS spacetime does not satisfy the weaker energy
condition, it is stable against small perturbation.Comment: Revtex, 1 figure, 10 pages, a wrong citation correcte
Triviality Bounds in Two-Doublet Models
We examine perturbatively the two-Higgs-doublet extension of the \SM\ in the
context of the suspected triviality of theories with fundamental scalars.
Requiring the model to define a consistent effective theory for scales below a
cutoff of times the largest mass of the problem, as motivated by lattice
investigations of the one-Higgs-doublet model, we obtain combined bounds for
the parameters of the model. We find upper limits of 470 GeV for the mass of
the light --even neutral scalar and 650--700 GeV for the other scalar
masses.Comment: 15 pages (8 postscript figures in two files), BUHEP-93-
Generalised Penrose Limits and PP-Waves
In this paper we construct generalised Penrose limits for the solutions of
massive type IIA supergravity. We consider a Freund-Rubin type solution and
apply these {\it massive} Penrose limits and obtain supersymmetric pp-wave
which is a standard type IIA background. We point out that results in this
paper are easily generalised for the cases of gauged supergravities.Comment: 13 pages; v2: references added, v3: minor corrections and
modification
The signature of dark energy on the local Hubble flow
Using N-body simulations of flat, dark energy dominated cosmologies, we show
that galaxies around simulated binary systems resembling the Local Group (LG)
have low peculiar velocities, in good agreement with observational data. We
have compared results for LG-like systems selected from large, high resolution
simulations of three cosmologies: a LCDM model, a LWDM model with a 2 keV warm
dark matter candidate and a quintessence model (QCDM) with an equation of state
parameter w=-0.6.
The Hubble flow is significant colder around LGs selected in a flat, Lambda
dominated cosmology than around LGs in open or critical models, showing that a
dark energy component manifests itself on the scales of nearby galaxies,
cooling galaxy peculiar motions. Flows in the LWDM and QCDM models are
marginally colder than in the LCDM one.
The results of our simulations have been compared to existing data and a new
data set of 28 nearby galaxies with robust distance measures (Cepheids and
Surface Brightness Fluctuations). The measured line-of-sight velocity
dispersion is sigma = 88 +- 20 km/sec x (R/7 Mpc). The best agreement with
observations is found for LGs selected in the CDM cosmology in
environments with -0.1 <delta_rho/rho < 0.6 on scales of 7 Mpc, in agreement
with existing observational estimates on the local matter density.
These results provide new, independent evidence for the presence of dark
energy on scales of few Mpc, corroborating the evidence gathered from
observations of distant objects and the early Universe.Comment: 8 pages, 6 figures, minor changes to match the accepted version by
MNRA
Quantum Cryptography
Quantum cryptography could well be the first application of quantum mechanics
at the individual quanta level. The very fast progress in both theory and
experiments over the recent years are reviewed, with emphasis on open questions
and technological issues.Comment: 55 pages, 32 figures; to appear in Reviews of Modern Physic
Interacting Dark Energy and Dark Matter: observational Constraints from Cosmological Parameters
Several observational constraints are imposed on the interacting holographic
model of Dark Energy and Dark Matter. First we use the age parameter today, as
given by the WMAP results. Subsequently, we explained the reason why it is
possible, as recently observed, for an old quasar to be observed in early
stages of the universe. We discuss this question in terms of the evolution of
the age parameter as well as in terms of the structure formation. Finally, we
give a detailed discussion of the constraints implied by the observed CMB low
suppression. As a result, the interacting holographic model has been
proved to be robust and with reasonable bounds predicts a non vanishing
interaction of Dark Energy and Dark Matter.Comment: 20 pages, 9 figure
Monsters, Laws of Nature, and Teleology in Late Scholastic Textbooks
In the period of emergence of early modern science, âmonstersâ or individuals with physical congenital anomalies were considered as rare events which required special explanations entailing assumptions about the laws of nature. This concern with monsters was shared by representatives of the new science and Late Scholastic authors of university textbooks. This paper will reconstruct the main theses of the treatment of monsters in Late Scholastic textbooks, by focusing on the question as to how their accounts conceived natureâs regularity and teleology. It shows that they developed a naturalistic teratology in which, in contrast to the naturalistic explanations usually offered by the new science, finality was at central stage. This general point does not impede our noticing that some authors were closer to the views emerging in the Scientific Revolution insofar as they conceived nature as relatively autonomous from God and gave a relevant place to efficient secondary causation. In this connection, this paper suggests that the concept of the laws of nature developed by the new science âas exception-less regularitiesâtransferred to natureâs regularity the âstrongâ character that Late Scholasticism attributed to finality and that the decline of the Late Scholastic view of finality played as an important concomitant factor permitting the transformation of the concept of laws of nature
FACTS AND IDEAS IN MODERN COSMOLOGY
A review of the principles of observational testing of cosmological theories
is given with a special emphasis on the distinction between observational facts
and theoretical hypotheses. A classification of modern cosmological theories
and possible observational tests for these theories is presented. The main
rival cosmological models are analyzed from the point of view of observational
testing of their initial hypothesis. A comparison of modern observational data
with theoretical predictions is presented. In particular we discuss in detail
the validity of the two basic assumptions of modern cosmology that are the
Cosmological Principle and the Expanding Space Paradigm. It is found that
classical paradigms need to be reanalyzed and that it is necessary to develop
crucial cosmological tests to discriminate alternative theories.Comment: 84 pages, latex, figures are available to F.S.L
([email protected]). Accepted for publication in Vistas In astronomy,
Vol.38, Part.4, 199
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