1,125 research outputs found
Background Independence and Asymptotic Safety in Conformally Reduced Gravity
We analyze the conceptual role of background independence in the application
of the effective average action to quantum gravity. Insisting on a background
independent renormalization group (RG) flow the coarse graining operation must
be defined in terms of an unspecified variable metric since no rigid metric of
a fixed background spacetime is available. This leads to an extra field
dependence in the functional RG equation and a significantly different RG flow
in comparison to the standard flow equation with a rigid metric in the mode
cutoff. The background independent RG flow can possess a non-Gaussian fixed
point, for instance, even though the corresponding standard one does not. We
demonstrate the importance of this universal, essentially kinematical effect by
computing the RG flow of Quantum Einstein Gravity in the ``conformally
reduced'' Einstein--Hilbert approximation which discards all degrees of freedom
contained in the metric except the conformal one. Without the extra field
dependence the resulting RG flow is that of a simple -theory. Including
it one obtains a flow with exactly the same qualitative properties as in the
full Einstein--Hilbert truncation. In particular it possesses the non-Gaussian
fixed point which is necessary for asymptotic safety.Comment: 4 figures
Two-body Pion Absorption on at Threshold
It is shown that a satisfactory explanation of the ratio of the rates of the
reactions and for stopped pions is obtained
once the effect of the short range two-nucleon components of the axial charge
operator for the nuclear system is taken into account. By employing realistic
models for the nucleon-nucleon interaction in the construction of these
components of the axial charge operator, the predicted ratios agree with the
empirical value to within 10-20\%.Comment: 19, UHPHYDOR-94-
Atypical presentation of Crimean-Congo haemorrhagic fever: Lessons learned
An atypical case of Crimean-Congo haemorrhagic fever is presented. The diagnosis of the case in the presence of several comorbidities was complicated and illustrates the importance of maintaining a high index of suspicion for viral haemorrhagic fever in cases presenting with multisystem disease and an epidemiological history that could present opportunities for exposure to a haemorrhagic fever virus
Casimir effect: running Newton constant or cosmological term
We argue that the instability of Euclidean Einstein gravity is an indication
that the vacuum is non perturbative and contains a condensate of the metric
tensor in a manner reminiscent of Yang-Mills theories. As a simple step toward
the characterization of such a vacuum the value of the one-loop effective
action is computed for Euclidean de Sitter spaces as a function of the
curvature when the unstable conformal modes are held fixed. Two phases are
found, one where the curvature is large and gravitons should be confined and
another one which appears to be weakly coupled and tends to be flat. The
induced cosmological constant is positive or negative in the strongly or weakly
curved phase, respectively. The relevance of the Casimir effect in
understanding the UV sensitivity of gravity is pointed out.Comment: Final, slightly extended version, to appear in Classical and Quantum
Gravit
Gravitational Theory, Galaxy Rotation Curves and Cosmology without Dark Matter
Einstein gravity coupled to a massive skew symmetric field F_{\mu\nu\lambda}
leads to an acceleration law that modifies the Newtonian law of attraction
between particles. We use a framework of non-perturbative renormalization group
equations as well as observational input to characterize special
renormalization group trajectories to allow for the running of the effective
gravitational coupling G and the coupling of the skew field to matter. The
latter lead to an increase of Newton's constant at large galactic and
cosmological distances. For weak fields a fit to the flat rotation curves of
galaxies is obtained in terms of the mass (mass-to-light ratio M/L) of
galaxies. The fits assume that the galaxies are not dominated by exotic dark
matter and that the effective gravitational constant G runs with distance
scale. The equations of motion for test particles yield predictions for the
solar system and the binary pulsar PSR 1913+16 that agree with the
observations. The gravitational lensing of clusters of galaxies can be
explained without exotic dark matter. An FLRW cosmological model with an
effective G=G(t) running with time can lead to consistent fits to cosmological
data without assuming the existence of exotic cold dark matter.Comment: 33 pages, 20 figures, 1 table. Latex file. Additional text and
references. Corrections. To be published in Journal of Cosmology and
Astroparticle Physics (JCAP
Hints of (trans-Planckian) asymptotic freedom in semiclassical cosmology
We employ the semiclassical approximation to the Wheeler-DeWitt equation in
the spatially flat de Sitter Universe to investigate the dynamics of a
minimally coupled scalar field near the Planck scale. We find that, contrary to
naive intuition, the effects of quantum gravitational fluctuations become
negligible and the scalar field states asymptotically approach plane-waves at
very early times. These states can then be used as initial conditions for the
quantum states of matter to show that each mode essentially originated in the
minimum energy vacuum. Although the full quantum dynamics cannot be solved
exactly for the case at hand, our results can be considered as supporting the
general idea of asymptotic safety in quantum gravity.Comment: 11 pages, 2 figures; replaced to match content of published versio
Strategic responses to global challenges: The case of European banking, 1973–2000
In applying a strategy, structure, ownership and performance (SSOP) framework to three major clearing banks (ABN AMRO, UBS, Barclays), this article debates whether the conclusions generated by Whittington and Mayer about European manufacturing industry can be applied to the financial services sector. While European integration plays a key role in determining strategy, it is clear that global factors were far more important in determining management actions, leading to significant differences in structural adaptation. The article also debates whether this has led to improved performance, given the problems experienced with both geographical dispersion and diversification, bringing into question the quality of decision-making over the long term
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