3,118 research outputs found
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
Modelling of atmospheric boundary layer: Generation of shear.
Roughness length, z0 and friction velocity, u* are the defining parameters of wind log profile that must be matched in wind tunnel simulation. To fully understand the role of these parameters, the basics and review from the primitive equations and its relation to the logarithmic profile obtained for wind tunnel conditions were discussed. The problem of roughness, although well known, still needs to be addressed more rigorously especially when determining values of z0 and u* from wind tunnel data and their relation to the roughness element geometry. A review of classic literature and new published material were carried out, focusing on the applicability to wind tunnel modelling
Effective growth of matter density fluctuations in the running LCDM and LXCDM models
We investigate the matter density fluctuations \delta\rho/\rho for two dark
energy (DE) models in the literature in which the cosmological term \Lambda is
a running parameter. In the first model, the running LCDM model, matter and DE
exchange energy, whereas in the second model, the LXCDM model, the total DE and
matter components are conserved separately. The LXCDM model was proposed as an
interesting solution to the cosmic coincidence problem. It includes an extra
dynamical component, the "cosmon" X, which interacts with the running \Lambda,
but not with matter. In our analysis we make use of the current value of the
linear bias parameter, b^2(0)= P_{GG}/P_{MM}, where P_{MM} ~
(\delta\rho/\rho)^2 is the present matter power spectrum and P_{GG} is the
galaxy fluctuation power spectrum. The former can be computed within a given
model, and the latter is found from the observed LSS data (at small z) obtained
by the 2dF galaxy redshift survey. It is found that b^2(0)=1 within a 10%
accuracy for the standard LCDM model. Adopting this limit for any DE model and
using a method based on the effective equation of state for the DE, we can set
a limit on the growth of matter density perturbations for the running LCDM
model, the solution of which is known. This provides a good test of the
procedure, which we then apply to the LXCDM model in order to determine the
physical region of parameter space, compatible with the LSS data. In this
region, the LXCDM model is consistent with known observations and provides at
the same time a viable solution to the cosmic coincidence problem.Comment: LaTeX, 38 pages, 8 figures. Version accepted in JCA
Perturbations in the relaxation mechanism for a large cosmological constant
Recently, a mechanism for relaxing a large cosmological constant (CC) has
been proposed [arxiv:0902.2215], which permits solutions with low Hubble rates
at late times without fine-tuning. The setup is implemented in the LXCDM
framework, and we found a reasonable cosmological background evolution similar
to the LCDM model with a fine-tuned CC. In this work we analyse analytically
the perturbations in this relaxation model, and we show that their evolution is
also similar to the LCDM model, especially in the matter era. Some tracking
properties of the vacuum energy are discussed, too.Comment: 18 pages, LaTeX; discussion improved, accepted by CQ
Impact of a hospice rapid response service on preferred place of death, and costs
Background: Many people with a terminal illness would prefer to die at home. A new palliative rapid response service (RRS) provided by a large hospice provider in South East England was evaluated (2010) to provide evidence of impact on achieving preferred place of death and costs. The RRS was delivered by a team of trained health care assistants and available 24/7. The purpose of this study was to (i) compare the characteristics of RRS users and non-users, (ii) explore differences in the proportions of users and non-users dying in the place of their choice, (iii) monitor the whole system service utilisation of users and non-users, and compare costs. Methods: All hospice patients who died with a preferred place of death recorded during an 18 month period were included. Data (demographic, preferences for place of death) were obtained from hospice records. Dying in preferred place was modelled using stepwise logistic regression analysis. Service use data (period between referral to hospice and death) were obtained from general practitioners, community providers, hospitals, social services, hospice, and costs calculated using validated national tariffs. Results: Of 688 patients referred to the hospice when the RRS was operational, 247 (35.9 %) used it. Higher proportions of RRS users than non-users lived in their own homes with a co-resident carer (40.3 % vs. 23.7 %); more non-users lived alone or in residential care (58.8 % vs. 76.3 %). Chances of dying in the preferred place were enhanced 2.1 times by being a RRS user, compared to a non-user, and 1.5 times by having a co-resident carer, compared to living at home alone or in a care home. Total service costs did not differ between users and non-users, except when referred to hospice very close to death (users had higher costs). Conclusions: Use of the RRS was associated with increased likelihood of dying in the preferred place. The RRS is cost neutral
Hubble expansion and structure formation in the "running FLRW model" of the cosmic evolution
A new class of FLRW cosmological models with time-evolving fundamental
parameters should emerge naturally from a description of the expansion of the
universe based on the first principles of quantum field theory and string
theory. Within this general paradigm, one expects that both the gravitational
Newton's coupling, G, and the cosmological term, Lambda, should not be strictly
constant but appear rather as smooth functions of the Hubble rate. This
scenario ("running FLRW model") predicts, in a natural way, the existence of
dynamical dark energy without invoking the participation of extraneous scalar
fields. In this paper, we perform a detailed study of these models in the light
of the latest cosmological data, which serves to illustrate the
phenomenological viability of the new dark energy paradigm as a serious
alternative to the traditional scalar field approaches. By performing a joint
likelihood analysis of the recent SNIa data, the CMB shift parameter, and the
BAOs traced by the Sloan Digital Sky Survey, we put tight constraints on the
main cosmological parameters. Furthermore, we derive the theoretically
predicted dark-matter halo mass function and the corresponding redshift
distribution of cluster-size halos for the "running" models studied. Despite
the fact that these models closely reproduce the standard LCDM Hubble
expansion, their normalization of the perturbation's power-spectrum varies,
imposing, in many cases, a significantly different cluster-size halo redshift
distribution. This fact indicates that it should be relatively easy to
distinguish between the "running" models and the LCDM cosmology using realistic
future X-ray and Sunyaev-Zeldovich cluster surveys.Comment: Version published in JCAP 08 (2011) 007: 1+41 pages, 6 Figures, 1
Table. Typos corrected. Extended discussion on the computation of the
linearly extrapolated density threshold above which structures collapse in
time-varying vacuum models. One appendix, a few references and one figure
adde
The J_1-J_2 antiferromagnet with Dzyaloshinskii-Moriya interaction on the square lattice: An exact diagonalization study
We examine the influence of an anisotropic interaction term of
Dzyaloshinskii-Moriya (DM) type on the groundstate ordering of the J_1-J_2
spin-1/2-Heisenberg antiferromagnet on the square lattice. For the DM term we
consider several symmetries corresponding to different crystal structures. For
the pure J_1-J_2 model there are strong indications for a quantum spin liquid
in the region of 0.4 < J_2/J_1 < 0.65. We find that a DM interaction influences
the breakdown of the conventional antiferromagnetic order by i) shifting the
spin liquid region, ii) changing the isotropic character of the groundstate
towards anisotropic correlations and iii) creating for certain symmetries a net
ferromagnetic moment.Comment: 7 pages, RevTeX, 6 ps-figures, to appear in J. Phys.: Cond. Ma
Cosmological models with interacting components and mass-varying neutrinos
A model for a homogeneous and isotropic spatially flat Universe, composed of
baryons, radiation, neutrinos, dark matter and dark energy is analyzed. We
infer that dark energy (considered to behave as a scalar field) interacts with
dark matter (either by the Wetterich model, or by the Anderson and Carroll
model) and with neutrinos by a model proposed by Brookfield et al.. The latter
is understood to have a mass-varying behavior. We show that for a very-softly
varying field, both interacting models for dark matter give the same results.
The models reproduce the expected red-shift performances of the present
behavior of the Universe.Comment: 8 pages, 5 figures, to be published in Gravitation and Cosmolog
Initial results from the C1XS X-ray spectrometer on Chandrayaan-1
This article does not have an abstract
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
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