2,556 research outputs found
Quantum Gravity effects near the null black hole singularity
The structure of the Cauchy Horizon singularity of a black hole formed in a
generic collapse is studied by means of a renormalization group equation for
quantum gravity. It is shown that during the early evolution of the Cauchy
Horizon the increase of the mass function is damped when quantum fluctuations
of the metric are taken into account.Comment: 15 Pages, one figure. Minor changes in the presentation, to appear on
Phys.Rev.
Cosmological Perturbations in Renormalization Group Derived Cosmologies
A linear cosmological perturbation theory of an almost homogeneous and
isotropic perfect fluid Universe with dynamically evolving Newton constant
and cosmological constant is presented. A gauge-invariant formalism
is developed by means of the covariant approach, and the acoustic propagation
equations governing the evolution of the comoving fractional spatial gradients
of the matter density, , and are thus obtained. Explicit solutions
are discussed in cosmologies where both and vary according to
renormalization group equations in the vicinity of a fixed point.Comment: 22 pages, revtex, subeqn.sty, to appear on IJMP
An effective action for asymptotically safe gravity
Asymptotically safe theories of gravitation have received great attention in
recent times. In this framework an effective action embodying the basic
features of the renormalized flow around the non-gaussian fixed point is
derived and its implications for the early universe are discussed. In
particular, a "landscape" of a countably infinite number of cosmological
inflationary solutions characterized by an unstable de Sitter phase lasting for
a large enough number of e-folds is found.Comment: 5 pages, to appear as a Rapid Communication in Physical Review
A Class of Renormalization Group Invariant Scalar Field Cosmologies
We present a class of scalar field cosmologies with a dynamically evolving
Newton parameter and cosmological term . In particular, we discuss
a class of solutions which are consistent with a renormalization group scaling
for and near a fixed point. Moreover, we propose a modified
action for gravity which includes the effective running of and
near the fixed point. A proper understanding of the associated variational
problem is obtained upon considering the four-dimensional gradient of the
Newton parameter.Comment: 10 pages, RevTex4, no figures, to appear on GR
Seismology of Procyon A: determination of mode frequencies, amplitudes, lifetimes, and granulation noise
The F5 IV-V star Procyon A (aCMi) was observed in January 2001 by means of
the high resolution spectrograph SARG operating with the TNG 3.5m Italian
telescope (Telescopio Nazionale Galileo) at Canary Islands, exploiting the
iodine cell technique. The time-series of about 950 spectra carried out during
6 observation nights and a preliminary data analysis were presented in Claudi
et al. 2005. These measurements showed a significant excess of power between
0.5 and 1.5 mHz, with ~ 1 m/s peak amplitude. Here we present a more detailed
analysis of the time-series, based on both radial velocity and line equivalent
width analyses. From the power spectrum we found a typical p-mode frequency
comb-like structure, identified with a good margin of certainty 11 frequencies
in the interval 0.5-1400 mHz of modes with l=0,1,2 and 7< n < 22, and
determined large and small frequency separations, Dn = 55.90 \pm 0.08 muHz and
dnu_02=7.1 \pm 1.3 muHz, respectively. The mean amplitude per mode (l=0,1) at
peak power results to be 0.45 \pm 0.07 m/s, twice larger than the solar one,
and the mode lifetime 2 \pm 0.4 d, that indicates a non-coherent, stochastic
source of mode excitation. Line equivalent width measurements do not show a
significant excess of power in the examined spectral region but allowed us to
infer an upper limit to the granulation noise.Comment: 10 pages, 15 figures, 4 tables. Accepted for publication in A&
The role of Background Independence for Asymptotic Safety in Quantum Einstein Gravity
We discuss various basic conceptual issues related to coarse graining flows
in quantum gravity. In particular the requirement of background independence is
shown to lead to renormalization group (RG) flows which are significantly
different from their analogs on a rigid background spacetime. The importance of
these findings for the asymptotic safety approach to Quantum Einstein Gravity
(QEG) is demonstrated in a simplified setting where only the conformal factor
is quantized. We identify background independence as a (the ?) key prerequisite
for the existence of a non-Gaussian RG fixed point and the renormalizability of
QEG.Comment: 2 figures. Talk given by M.R. at the WE-Heraeus-Seminar "Quantum
Gravity: Challenges and Perspectives", Bad Honnef, April 14-16, 2008; to
appear in General Relativity and Gravitatio
Networks of equities in financial markets
We review the recent approach of correlation based networks of financial
equities. We investigate portfolio of stocks at different time horizons,
financial indices and volatility time series and we show that meaningful
economic information can be extracted from noise dressed correlation matrices.
We show that the method can be used to falsify widespread market models by
directly comparing the topological properties of networks of real and
artificial markets.Comment: 9 pages, 8 figures. Accepted for publication in EPJ
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