12,241 research outputs found
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
The Accelerated expansion of the Universe as a crossover phenomenon
We show that the accelerated expansion of the Universe can be viewed as a
crossover phenomenon where the Newton constant and the Cosmological constant
are actually scaling operators, dynamically evolving in the attraction basin of
a non-Gaussian infrared fixed point, whose existence has been recently
discussed. By linearization of the renormalized flow it is possible to evaluate
the critical exponents, and it turns out that the approach to the fixed point
is ruled by a marginal and a relevant direction. A smooth transition between
the standard Friedmann--Lemaitre--Robertson--Walker (FLRW) cosmology and the
observed accelerated expansion is then obtained, so that at late times.Comment: 12 pages, latex, use bibtex. In the final version, the presentation
has been improved, and new references have been adde
Scalar-Tensor gravity with system-dependent potential and its relation with Renormalization Group extended General Relativity
We show that Renormalization Group extensions of the Einstein-Hilbert action
for large scale physics are not, in general, a particular case of standard
Scalar-Tensor (ST) gravity. We present a new class of ST actions, in which the
potential is not necessarily fixed at the action level, and show that this
extended ST theory formally contains the Renormalization Group case. We also
propose here a Renormalization Group scale setting identification that is
explicitly covariant and valid for arbitrary relativistic fluids.Comment: 29 pages, 2 figs. v2: small changes in text and ref's. v3: further
details on the relation between this work and others on the Renormalization
Group. Version to appear in JCA
Asymptotically safe cosmology - a status report
Asymptotic Safety, based on a non-Gaussian fixed point of the gravitational
renormalization group flow, provides an elegant mechanism for completing the
gravitational force at sub-Planckian scales. At high energies the fixed point
controls the scaling of couplings such that unphysical divergences are absent
while the emergence of classical low-energy physics is linked to a crossover
between two renormalization group fixed points. These features make Asymptotic
Safety an attractive framework for cosmological model building. The resulting
scenarios may naturally give rise to a quantum gravity driven inflationary
phase in the very early universe and an almost scale-free fluctuation spectrum.
Moreover, effective descriptions arising from an renormalization group
improvement permit a direct comparison to cosmological observations as, e.g.
Planck data.Comment: Invited review for the special issue "Testing quantum gravity with
cosmology" to appear in Compte Rendus Physique
Non-Equilibrium Large N Yukawa Dynamics: marching through the Landau pole
The non-equilibrium dynamics of a Yukawa theory with N fermions coupled to a
scalar field is studied in the large N limit with the goal of comparing the
dynamics predicted from the renormalization group improved effective potential
to that obtained including the fermionic backreaction. The effective potential
is of the Coleman-Weinberg type. Its renormalization group improvement is
unbounded from below and features a Landau pole. When viewed self-consistently,
the initial time singularity does not arise. The different regimes of the
dynamics of the fully renormalized theory are studied both analytically and
numerically. Despite the existence of a Landau pole in the model, the dynamics
of the mean field is smooth as it passes the location of the pole. This is a
consequence of a remarkable cancellation between the effective potential and
the dynamical chiral condensate. The asymptotic evolution is effectively
described by a quartic upright effective potential. In all regimes, profuse
particle production results in the formation of a dense fermionic plasma with
occupation numbers nearly saturated up to a scale of the order of the mean
field. This can be interpreted as a chemical potential. We discuss the
implications of these results for cosmological preheating.Comment: 36 pages, 14 figures, LaTeX, submitted to Physical Review
Entropy Production during Asymptotically Safe Inflation
The Asymptotic Safety scenario predicts that the deep ultraviolet of Quantum
Einstein Gravity is governed by a nontrivial renormalization group fixed point.
Analyzing its implications for cosmology using renormalization group improved
Einstein equations we find that it can give rise to a phase of inflationary
expansion in the early Universe. Inflation is a pure quantum effect here and
requires no inflaton field. It is driven by the cosmological constant and ends
automatically when the renormalization group evolution has reduced the vacuum
energy to the level of the matter energy density. The quantum gravity effects
also provide a natural mechanism for the generation of entropy. It could easily
account for the entire entropy of the present Universe in the massless sector.Comment: 17 pages, 4 figures, Invited contribution to the special issue of
Entropy on "Entropy in Quantum Gravity
Cosmology with Self-Adjusting Vacuum Energy Density from a Renormalization Group Fixed Point
Cosmologies with a time dependent Newton constant and cosmological constant
are investigated. The scale dependence of and is governed by a
set of renormalization group equations which is coupled to Einstein's equation
in a consistent way. The existence of an infrared attractive renormalization
group fixed point is postulated, and the cosmological implications of this
assumption are explored. It turns out that in the late Universe the vacuum
energy density is automatically adjusted so as to equal precisely the matter
energy density, and that the deceleration parameter approaches . This
scenario might explain the data from recent observations of high redshift type
Ia Supernovae and the cosmic microwave background radiation without introducing
a quintessence field.Comment: v2: published version, two references update
Dynamical renormalization of black-hole spacetimes
We construct a black-hole spacetime which includes the running of the
gravitational coupling in a self-consistent way. Starting from a classical
Schwarzschild black hole, the backreaction effects produced by the running
Newton's coupling are taken into account iteratively. The sequence, described
by a simple recurrence relation, flows towards a self-consistent solution that
can be derived analytically. As a key result, if the gravitational
renormalization group flow attains a non-trivial fixed point at high energies,
the sequence converges to a "renormalized" black-hole spacetime of the
Dymnikova-type, which is free of singularities.Comment: 7 pages, 1 figur
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