3,740 research outputs found

### Inflation in asymptotically safe f(R) theory

We discuss the existence of inflationary solutions in a class of
renormalization group improved polynomial f(R) theories, which have been
studied recently in the context of the asymptotic safety scenario for quantum
gravity. These theories seem to possess a nontrivial ultraviolet fixed point,
where the dimensionful couplings scale according to their canonical
dimensionality. Assuming that the cutoff is proportional to the Hubble
parameter, we obtain modified Friedmann equations which admit both power law
and exponential solutions. We establish that for sufficiently high order
polynomial the solutions are reliable, in the sense that considering still
higher order polynomials is very unlikely to change the solution.Comment: Presented at 14th Conference on Recent Developments in Gravity: NEB
14, Ioannina, Greece, 8-11 Jun 201

### Dynamical System Analysis of Cosmologies with Running Cosmological Constant from Quantum Einstein Gravity

We discuss a mechanism that induces a time-dependent vacuum energy on
cosmological scales. It is based on the instability induced renormalization
triggered by the low energy quantum fluctuations in a Universe with a positive
cosmological constant. We employ the dynamical systems approach to study the
qualitative behavior of Friedmann-Robertson-Walker cosmologies where the
cosmological constant is dynamically evolving according with this
nonperturbative scaling at low energies. It will be shown that it is possible
to realize a "two regimes" dark energy phases, where an unstable early phase of
power-law evolution of the scale factor is followed by an accelerated expansion
era at late times.Comment: 26 pages, 4 figures. To appear in New Journal of Physic

### 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.

### Improved Action Functionals in Non-Perturbative Quantum Gravity

Models of gravity with variable G and Lambda have acquired greater relevance
after the recent evidence in favour of the Einstein theory being
non-perturbatively renormalizable in the Weinberg sense. The present paper
builds a modified Arnowitt-Deser-Misner (ADM) action functional for such models
which leads to a power-law growth of the scale factor for pure gravity and for
a massless phi**4 theory in a Universe with Robertson-Walker symmetry, in
agreement with the recently developed fixed-point cosmology. Interestingly, the
renormalization-group flow at the fixed point is found to be compatible with a
Lagrangian description of the running quantities G and Lambda.Comment: Latex file. Record without file already exists on SLAC-SPIRES, and
hence that record and the one for the present arxiv submission should become
one record onl

### Chiral symmetry breaking in the Wegner-Houghton approach

The Wegner-Houghton formulation of the exact renormalization group evolution
equation is used in order to study the chiral symmetry breaking of the linear
sigma model coupled to an isospin doublet of quarks. A numerical investigation
for a particular truncation of the equation which includes the scalar field
renormalization function is presented.Comment: 4 pages,4 eps figures. Contribution to CRIS2000, Catania, May 200

### 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 $G$
and cosmological constant $\Lambda$ 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, $G$, and $\Lambda$ are thus obtained. Explicit solutions
are discussed in cosmologies where both $G$ and $\Lambda$ vary according to
renormalization group equations in the vicinity of a fixed point.Comment: 22 pages, revtex, subeqn.sty, to appear on IJMP

### Proper Time Flow Equation for Gravity

We analyze a proper time renormalization group equation for Quantum Einstein
Gravity in the Einstein-Hilbert truncation and compare its predictions to those
of the conceptually different exact renormalization group equation of the
effective average action. We employ a smooth infrared regulator of a special
type which is known to give rise to extremely precise critical exponents in
scalar theories. We find perfect consistency between the proper time and the
average action renormalization group equations. In particular the proper time
equation, too, predicts the existence of a non-Gaussian fixed point as it is
necessary for the conjectured nonperturbative renormalizability of Quantum
Einstein Gravity.Comment: 11 pages, revtex4, no figures, bibte

### Relativistic effects in the solar EOS

We study the sensitivity of the sound speed to relativistic corrections of
the equation of state (EOS) in the standard solar model by means of a
helioseismic forward analysis. We use the latest GOLF/SOHO data for $\ell =
0,1,2,3$ modes to confirm that the inclusion of the relativistic corrections to
the adiabatic exponent $\Gamma_1$ computed from both OPAL and MHD EOS leads to
a more reliable theoretical modelling of the innermost layers of the Sun.Comment: 3 pages, 3 figures, aa.cls, to appear on Astronomy and Astrophysic

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