34 research outputs found
Partial rip scenario—a cosmology with a growing cosmological term
AbstractA cosmology with the growing cosmological term is considered. If there is no exchange of energy between vacuum and matter components, the requirement of general covariance implies the time dependence of the gravitational constant G. Irrespectively of the exact functional form of the cosmological term growth, the universe ends in a de Sitter regime with a constant asymptotic Λ, but vanishing G. Although there is no divergence of the scale factor in finite time, such as in the “Big Rip” scenario, gravitationally bound systems eventually become unbound. In the case of systems bound by non-gravitational forces, there is no unbounding effect, as the asymptotic Λ is insufficiently large to disturb these systems
Dark energy transition between quintessence and phantom regimes - an equation of state analysis
The dark energy transition between quintessence () and phantom ()
regimes (the crossing of the cosmological constant boundary) is studied using
the dark energy equation of state. Models characterized by this type of
transition are explicitly constructed and their equation of state is found to
be {\em implicitly} defined. The behavior of the more general models with the
implicitly defined equation of state, obtained by the generalization of the
explicitly constructed models, is studied to gain insight into the necessary
conditions for the occurrence of the transition, as well as to investigate the
mechanism behind the transition. It is found that the parameters of the
generalized models need to satisfy special conditions for the transition to
happen and that the mechanism behind the transition is the cancellation of the
contribution of the cosmological constant boundary. The aspects of the behavior
of the generalized models which are not related to the transition are briefly
discussed and the role of the implicitly defined dark energy equation of state
in the description of the dark energy evolution is emphasized.Comment: v1: 9 pages, 6 figures. v2: references added. v3: minor changes.
Version accepted for publication in Phys. Rev.
Cosmology with variable parameters and effective equation of state for Dark Energy
A cosmological constant, Lambda, is the most natural candidate to explain the
origin of the dark energy (DE) component in the Universe. However, due to
experimental evidence that the equation of state (EOS) of the DE could be
evolving with time/redshift (including the possibility that it might behave
phantom-like near our time) has led theorists to emphasize that there might be
a dynamical field (or some suitable combination of them) that could explain the
behavior of the DE. While this is of course one possibility, here we show that
there is no imperative need to invoke such dynamical fields and that a variable
cosmological constant (including perhaps a variable Newton's constant too) may
account in a natural way for all these features.Comment: LaTeX, 9 pages, 1 figure. Talk given at the 7th Intern. Workshop on
Quantum Field Theory Under the Influence of External Conditions (QFEXT 05
Enhancement of preasymptotic effects in inclusive beauty decays
We extend Voloshin's recent analysis of charmed and beauty hyperon decays
based on SU(3) symmetry and heavy quark effective theory, by introducing a
rather moderate model-dependence, in order to obtain more predictive power,
e.g. the values of lifetimes of the (\Lambda_{b},\Xi_{b}) hyperon triplet and
the lifetime of \Omega_{b}. In this way we obtain an improvement of the ratio
\tau(\Lambda_{b})/\tau(B_{d}^{0}) \sim 0.9 and the hierarchy of lifetimes
\tau(\Lambda_{b}) \simeq \tau(\Xi_{b}^{0}) < \tau(\Xi_{b}^{-}) <
\tau(\Omega_{b}) with lifetimes of \Xi_{b}^{-} and \Omega_{b} exceeding the
lifetime of \Lambda_{b} by 22% and 35%, respectively.Comment: Latex2e, 12 pages, 3 eps figures include
What is there in the black box of dark energy: variable cosmological parameters or multiple (interacting) components?
The coincidence problems and other dynamical features of dark energy are
studied in cosmological models with variable cosmological parameters and in
models with the composite dark energy. It is found that many of the problems
usually considered to be cosmological coincidences can be explained or
significantly alleviated in the aforementioned models.Comment: 6 pages, 1 figure, talk given at IRGAC2006 (Barcelona, July 11-15,
2006), to appear in J. Phys.
Renormalization-group running of the cosmological constant and its implication for the Higgs boson mass in the Standard Model
The renormalization-group equation for the zero-point energies associated
with vacuum fluctuations of massive fields from the Standard Model is examined.
Our main observation is that at any scale the running is necessarily dominated
by the heaviest degrees of freedom, in clear contradistinction with the
Appelquist & Carazzone decoupling theorem. Such an enhanced running would
represent a disaster for cosmology, unless a fine-tuned relation among the
masses of heavy particles is imposed. In this way, we obtain for the Higgs mass, a value safely within the unitarity bound, but far
above the more stringent triviality bound for the case when the validity of the
Standard Model is pushed up to the grand unification (or Planck) scale.Comment: 11 pages, LaTex2
Renormalization-group running cosmologies - a scale-setting procedure
For cosmologies including scale dependence of both the cosmological and the
gravitational constant, an additional consistency condition dictated by the
Bianchi identities emerges, even if the energy-momentum tensor of ordinary
matter stays individually conserved. For renormalization-group (RG) approaches
it is shown that such a consistency relation ineluctably fixes the RG scale
(which may have an explicit as well as an implicit time dependence), provided
that the solutions of the RG equation for both quantities are known. Hence,
contrary to the procedures employed in the recent literature, we argue that
there is no more freedom in identification of the RG scale in terms of the
cosmic time in such cosmologies. We carefully set the RG scale for the RG
evolution phrased in a quantum gravity framework based on the hypothetical
existence of an infrared (IR) fixed point, for the perturbative regime within
the same framework, as well as for an evolution within quantum field theory
(QFT) in a curved background. In the latter case, the implications of the scale
setting for the particle spectrum are also briefly discussed.Comment: v1:15 pages, 1 figure. v2: references added. v3: discussion of the
physical interpretation of the scale-setting procedure added. v4: discussions
added. Version to appear in Phys. Rev.
Cabibbo suppressed decays and the lifetime
The problem of the lifetime is considered in the framework of
{\em Heavy-Quark Expansion} and symmetry. The lifetime of
is expressed in terms of measurable inclusive quantities of the
other two charmed baryons belonging to the same multiplet in a
model-independent way. In such a treatment, inclusive decay rates of singly
Cabibbo suppressed decay modes have a prominent role. An analogous approach is
applied to the multiplet of charmed mesons yielding interesting predictions on
properties. The results obtained indicate that a more precise
measurement of inclusive decay quantities of some charmed hadrons (such as
) that are more amenable to experiment can contribute
significantly to our understanding of decay properties of other charmed hadrons
(such as ) where discrepancies or ambiguities exist.Comment: 9 pages, revtex4; certain improvements in text as suggested by the
referee, acknowledgement changed; version to appear in Phys. Rev.
Renormalization group scale-setting from the action - a road to modified gravity theories
The renormalization group (RG) corrected gravitational action in
Einstein-Hilbert and other truncations is considered. The running scale of the
renormalization group is treated as a scalar field at the level of the action
and determined in a scale-setting procedure recently introduced by Koch and
Ramirez for the Einstein-Hilbert truncation. The scale-setting procedure is
elaborated for other truncations of the gravitational action and applied to
several phenomenologically interesting cases. It is shown how the logarithmic
dependence of the Newton's coupling on the RG scale leads to exponentially
suppressed effective cosmological constant and how the scale-setting in
particular RG corrected gravitational theories yields the effective
modified gravity theories with negative powers of the Ricci scalar . The
scale-setting at the level of the action at the non-gaussian fixed point in
Einstein-Hilbert and more general truncations is shown to lead to universal
effective action quadratic in Ricci tensor.Comment: v1: 15 pages; v2: shortened to 10 pages, main results unchanged,
published in Class. Quant. Gra
New Isotropic and Anisotropic Sudden Singularities
We show the existence of an infinite family of finite-time singularities in
isotropically expanding universes which obey the weak, strong, and dominant
energy conditions. We show what new type of energy condition is needed to
exclude them ab initio. We also determine the conditions under which
finite-time future singularities can arise in a wide class of anisotropic
cosmological models. New types of finite-time singularity are possible which
are characterised by divergences in the time-rate of change of the
anisotropic-pressure tensor. We investigate the conditions for the formation of
finite-time singularities in a Bianchi type universe with anisotropic
pressures and construct specific examples of anisotropic sudden singularities
in these universes.Comment: Typos corrected. Published versio