16,309 research outputs found
Spectra of primordial fluctuations in two-perfect-fluid regular bounces
We introduce analytic solutions for a class of two components bouncing
models, where the bounce is triggered by a negative energy density perfect
fluid. The equation of state of the two components are constant in time, but
otherwise unrelated. By numerically integrating regular equations for scalar
cosmological perturbations, we find that the (would be) growing mode of the
Newtonian potential before the bounce never matches with the the growing mode
in the expanding stage. For the particular case of a negative energy density
component with a stiff equation of state we give a detailed analytic study,
which is in complete agreement with the numerical results. We also perform
analytic and numerical calculations for long wavelength tensor perturbations,
obtaining that, in most cases of interest, the tensor spectral index is
independent of the negative energy fluid and given by the spectral index of the
growing mode in the contracting stage. We compare our results with previous
investigations in the literature.Comment: 11 pages, 5 figure
Thermal and non-thermal emission from reconnecting twisted coronal loops
Twisted magnetic fields should be ubiquitous in flare-producing active
regions where the magnetic fields are strongly non-potential. It has been shown
that reconnection in helical magnetic coronal loops results in plasma heating
and particle acceleration distributed within a large volume, including the
lower coronal and chromospheric sections of the loops. This scenario can be an
alternative to the standard flare model, where particles are accelerated only
in a small volume located in the upper corona. We use a combination of MHD
simulations and test-particle methods, which describe the development of kink
instability and magnetic reconnection in twisted coronal loops using resistive
compressible MHD, and incorporate atmospheric stratification and large-scale
loop curvature. The resulting distributions of hot plasma let us estimate
thermal X-ray emission intensities. The electric and magnetic fields obtained
are used to calculate electron trajectories using the guiding-centre
approximation. These trajectories combined with the MHD plasma density
distributions let us deduce synthetic HXR bremsstrahlung intensities. Our
simulations emphasise that the geometry of the emission patterns produced by
hot plasma in flaring twisted coronal loops can differ from the actual geometry
of the underlying magnetic fields. The twist angles revealed by the emission
threads (SXR) are consistently lower than the field-line twist present at the
onset of the kink-instability. HXR emission due to the interaction of energetic
electrons with the stratified background are concentrated at the loop
foot-points in these simulations, even though the electrons are accelerated
everywhere within the coronal volume of the loop. The maximum of HXR emission
consistently precedes that of SXR emission, with the HXR light-curve being
approximately proportional to the temporal derivative of the SXR light-curve.Comment: (accepted for publication on A&A
Covariant Bardeen Perturbation Formalism
In a previous work we obtained a set of necessary conditions for the linear
approximation in cosmology. Here we discuss the relations of this approach with
the so called covariant perturbations. It is often argued in the literature
that one of the main advantages of the covariant approach to describe
cosmological perturbations is that the Bardeen formalism is coordinate
dependent. In this paper we will reformulate the Bardeen approach in a
completely covariant manner. For that, we introduce the notion of pure and
mixed tensors, which yields an adequate language to treat both perturbative
approaches in a common framework. We then stress that in the referred covariant
approach one necessarily introduces an additional hyper-surface choice to the
problem. Using our mixed and pure tensors approach, we were able to construct a
one-to-one map relating the usual gauge dependence of the Bardeen formalism
with the hyper-surface dependence inherent to the covariant approach. Finally,
through the use of this map, we define full non-linear tensors that at first
order correspond to the three known gauge invariant variables ,
and , which are simultaneously foliation and gauge invariant. We then
stress that the use of the proposed mixed tensors allows one to construct
simultaneously gauge and hyper-surface invariant variables at any order.Comment: 15 pages, no figures, revtex4-1, accepted for publication in PRD,
typos fixed, improved discussion about higher order gauge and foliation
invarianc
Measuring Baryon Acoustic Oscillations with Millions of Supernovae
Since type Ia Supernovae (SNe) explode in galaxies, they can, in principle,
be used as the same tracer of the large-scale structure as their hosts to
measure baryon acoustic oscillations (BAOs). To realize this, one must obtain a
dense integrated sampling of SNe over a large fraction of the sky, which may
only be achievable photometrically with future projects such as the Large
Synoptic Survey Telescope. The advantage of SN BAOs is that SNe have more
uniform luminosities and more accurate photometric redshifts than galaxies, but
the disadvantage is that they are transitory and hard to obtain in large number
at high redshift. We find that a half-sky photometric SN survey to redshift z =
0.8 is able to measure the baryon signature in the SN spatial power spectrum.
Although dark energy constraints from SN BAOs are weak, they can significantly
improve the results from SN luminosity distances of the same data, and the
combination of the two is no longer sensitive to cosmic microwave background
priors.Comment: 4 pages, 3 figures, ApJL accepte
Comments on the Quantum Potential Approach to a Class of Quantum Cosmological Models
In this comment we bring attention to the fact that when we apply the
ontological interpretation of quantum mechanics, we must be sure to use it in
the coordinate representation. This is particularly important when canonical
tranformations that mix momenta and coordinates are present. This implies that
some of the results obtained by A. B\l aut and J. Kowalski-Glikman are
incorrect.Comment: 7 pages, LaTe
Função Respiratória na Criança em Idade Pré-Escolar
A avaliação funcional respiratória é fundamental para a avaliação do doente com diversas patologias, nomeadamente respiratória, sendo corrente o seu uso em adultos e em crianças em idade escolar. Embora para o lactente tenham sido investigados vários métodos nas últimas décadas, a criança em idade pré-escolar tem permanecido como inacessível, pela necessidade de colaboração e pela impraticabilidade de sedação. Os autores descrevem os avanços em avaliação funcional
respiratória nos últimos anos, nesta faixa etária, nas técnicas de espirometria e pletismografia, explicitando a sua fundamentação teórica, metodologia e aplicabilidade
Earnings management and debt in private companies: The effect of economic adjustment programmes
Our study analyses the relationship between the level of earnings management and the debt of unlisted companies in two countries liable to economic adjustment programmes – Portugal and Ireland – between 2008 and 2016. We also analyse whether there are differences between both countries in what results are concerned. In a sample of 970 unlisted companies, and according the discretionary accruals, we find evidence of a positive relationship between the level of debt and the level of earnings management, thus concluding that the level of this relationship is different in each country. We also demonstrate that during the period of time in which the financial support occurred, there was mitigation in the relationship between the level of earnings management and indebtedness.info:eu-repo/semantics/acceptedVersio
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