557 research outputs found
Dark energy: a quantum fossil from the inflationary Universe?
The discovery of dark energy (DE) as the physical cause for the accelerated
expansion of the Universe is the most remarkable experimental finding of modern
cosmology. However, it leads to insurmountable theoretical difficulties from
the point of view of fundamental physics. Inflation, on the other hand,
constitutes another crucial ingredient, which seems necessary to solve other
cosmological conundrums and provides the primeval quantum seeds for structure
formation. One may wonder if there is any deep relationship between these two
paradigms. In this work, we suggest that the existence of the DE in the present
Universe could be linked to the quantum field theoretical mechanism that may
have triggered primordial inflation in the early Universe. This mechanism,
based on quantum conformal symmetry, induces a logarithmic,
asymptotically-free, running of the gravitational coupling. If this evolution
persists in the present Universe, and if matter is conserved, the general
covariance of Einstein's equations demands the existence of dynamical DE in the
form of a running cosmological term whose variation follows a power law of the
redshift.Comment: LaTeX, 14 pages, extended discussion. References added. Accepted in
J. Phys. A: Mathematical and Theoretica
Cosmology with running parameters
The experimental evidence that the equation of state (EOS) of the dark energy
(DE) could be evolving with time/redshift (including the possibility that it
might behave phantom-like near our time) suggests that there might be dynamical
DE fields that could explain this behavior. We propose, instead, that a
variable cosmological term (including perhaps a variable Newton's gravitational
coupling too) may account in a natural way for all these features.Comment: Talk given at TAUP 2005, Zaragoza, Spain, 10-14 Sep 200
Outstanding intraindividual genetic diversity in fissiparous planarians (Dugesia, Platyhelminthes) with facultative sex.
Predicted genetic consequences of asexuality include high intraindividual genetic diversity (i.e., the Meselson effect) and accumulation of deleterious mutations (i.e., Muller’s Ratchet), among others. These consequences have been largely studied in parthenogenetic organisms, but studies on fissiparous species are scarce. Differing from parthenogens, fissiparous organisms inherit part of the soma of the progenitor, including somatic mutations. Thus, in the long term, fissiparous reproduction may also result in genetic mosaicism, besides the presence of the Meselson effect and Muller’s Ratchet. Dugesiidae planarians show outstanding regeneration capabilities, allowing them to naturally reproduce by fission, either strictly or combined with sex (facultative). Therefore, they are an ideal model to analyze the genetic footprint of fissiparous reproduction, both when it is alternated with sex and when it is the only mode of reproduction
Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem
Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure
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.
Generalizing the running vacuum energy model and comparing with the entropic-force models
We generalize the previously proposed running vacuum energy model by
including a term proportional to \dot{H}, in addition to the existing H^2 term.
We show that the added degree of freedom is very constrained if both low
redshift and high redshift data are taken into account. Best-fit models are
undistinguishable from LCDM at the present time, but could be distinguished in
the future with very accurate data at both low and high redshifts. We stress
the formal analogy at the phenomenological level of the running vacuum models
with recently proposed dark energy models based on the holographic or entropic
point of view, where a combination of \dot{H} and H^2 term is also present.
However those particular entropic formulations which do not have a constant
term in the Friedmann equations are not viable. The presence of this term is
necessary in order to allow for a transition from a decelerated to an
accelerated expansion. In contrast, the running vacuum models, both the
original and the generalized one introduced here contain this constant term in
a more natural way. Finally, important conceptual issues common to all these
models are emphasized.Comment: Version accepted in Phys. Rev. D. LaTeX, 24 pages and one figure.
Slightly extended discussio
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
On the scaling behavior of the cosmological constant and the possible existence of new forces and new light degrees of freedom
A large value of the cosmological constant (CC) is induced in the Standard
Model (SM) of Elementary Particle Physics because of Spontaneous Symmetry
Breaking. To provide a small value of the observable CC one has to introduce
the vacuum term which cancels the induced one at some point in the very far
infrared cosmic scale. Starting from this point we investigate whether the
cancellation is preserved at different energy scales. We find that the running
of the Higgs mass, couplings and the vacuum term inevitably result in a scaling
dependence of the observable CC value. As a consequence one meets a nonzero CC
at an energy scale comparable to the typical electron neutrino mass suggested
by some experiments, and the order of magnitude of this constant is roughly the
one derived from recent supernovae observations. However the sign of it is
negative -- opposite to what is suggested by these observations. This
discrepancy may be a hint of the existence of an extra very light scalar,
perhaps a Cosmon-like dilaton, which should essentially decouple from the SM
Lagrangian, but that it nevertheless could mediate new macroscopic forces in
the submillimeter range.Comment: LaTeX, 14 pages, no figures. Discussion of the new light scalar
extended, some new references adde
Cosmologies with a time dependent vacuum
The idea that the cosmological term, Lambda, should be a time dependent
quantity in cosmology is a most natural one. It is difficult to conceive an
expanding universe with a strictly constant vacuum energy density, namely one
that has remained immutable since the origin of time. A smoothly evolving
vacuum energy density that inherits its time-dependence from cosmological
functions, such as the Hubble rate or the scale factor, is not only a
qualitatively more plausible and intuitive idea, but is also suggested by
fundamental physics, in particular by quantum field theory (QFT) in curved
space-time. To implement this notion, is not strictly necessary to resort to ad
hoc scalar fields, as usually done in the literature (e.g. in quintessence
formulations and the like). A "running" Lambda term can be expected on very
similar grounds as one expects (and observes) the running of couplings and
masses with a physical energy scale in QFT. Furthermore, the experimental
evidence that the equation of state of the dark energy could be evolving with
time/redshift (including the possibility that it might currently behave
phantom-like) suggests that a time-variable Lambda term (possibly accompanied
by a variable Newton's gravitational coupling G=G(t)) could account in a
natural way for all these features. Remarkably enough, a class of these models
(the "new cosmon") could even be the clue for solving the old cosmological
constant problem, including the coincidence problem.Comment: LaTeX, 15 pages, 4 figure
Massive fields temper anomaly-induced inflation: the clue to graceful exit?
A method of calculating the vacuum effective action for massive quantum
fields in curved space-time is outlined. Our approach is based on the conformal
representation of the fields action and on the integration of the corresponding
conformal anomaly. As a relevant cosmological application, we find that if
taking the masses of the fields into account, then the anomaly-induced
inflation automatically slows down. The only relevant massive fields for this
purpose turn out to be the fermion fields. So in supersymmetric theories this
mechanism can be specially efficient, for it may naturally provide the graceful
exit from the inflationary to the FLRW phase. Taking the SUSY breaking into
account, the anomaly-induced inflation could be free of the well-known
difficulties with the initial data and also with the amplitude of the
gravitational waves.Comment: 12 pages, 2 figures. Extended version, in particular an important
discussion concerning the gravitational waves adde
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