Depois de muito tempo, mais uma lista dos últimos pre-prints mais interessantes do arXiv. Essencialmente, só tenho tido tempo de ler o astro-ph, que é onde se situa minha principal área de pesquisa, a cosmologia.
The future of cosmology and the role of non-linear perturbations. (arXiv:1111.6853v1 [astro-ph.CO]):
Cosmological perturbation theory is a key tool to study the universe. The
linear or first order theory is well understood, however, developing and
applying the theory beyond linear order is at the cutting edge of current
research in theoretical cosmology. In this article, I will describe some
signatures of non-linear perturbation theory that do not exist at linear order,
focusing on vorticity generation at second order. In doing so, we discuss why
this, among other features such as induced gravitational waves and
non-Gaussianities, shows that cosmological perturbation theory is crucial for
testing models of the universe.
Cosmological Perturbations from a Group Theoretical Point of View. (arXiv:1111.7027v1 [gr-qc]):
We present a new approach to cosmological perturbations based on the theory
of Lie groups and their representations. After re-deriving the standard
covariant formalism from SO(3) considerations, we provide a new expansion of
the perturbed Friedmann-Lemaitre-Robertson-Walker (FLRW) metric in terms of
irreducible representations of the Lorentz group. The resulting decomposition
splits into (scalar, scalar), (scalar, vector) and (vector, vector) terms.
These equations directly correspond to the standard Lifshitz classification of
cosmological perturbations using scalar, vector and tensor modes which arise
from the irreducible SO(3) representation of the spatial part of the metric.
While the Lorentz group basis matches the underlying local symmetries of the
FLRW spacetime better than the SO(3), the new equations do not provide further
simplification compared to the standard cosmological perturbation theory. We
conjecture that this is due to the fact that the so(3,1) ~ su(2) x su(2)
Lorentz algebra has no pair of commuting generators commuting with any of the
translation group generators.
Cosmological models and misunderstandings about them. (arXiv:1110.1828v2 [gr-qc] CROSS LISTED):
Advantages of inhomogeneous cosmological models that are exact solutions of
Einstein’s equations over linearised perturbations of homogeneous models are
presented. Examples of effects that can be described in the inhomogeneous ones
are given: the non-repeatable light paths, the observed anisotropies in the
cosmic microwave background, the redshift drift and the maximum diameter
distance. Criticisms of inhomogeneous models that are based on
misunderstandings or fallacious reasonings are pointed out and corrected; these
include the “weak singularity”, the positivity of deceleration “theorem”, the
“pathology” of redshift behaviour at the “critical point” and the alleged
necessity of the bang time to be constant.
Quantifying the behaviour of curvature perturbations during inflation. (arXiv:1111.6940v1 [astro-ph.CO]):
How much does the curvature perturbation change after it leaves the horizon,
and when should one evaluate the power spectrum? To answer these questions we
study single field inflation models numerically, and compare the evolution of
different curvature perturbations from horizon crossing to the end of
inflation. In particular we calculate the number of efolds it takes for the
curvature perturbation at a given wavenumber to settle down to within a given
fraction of their value at the end of inflation.
We find that e.g. in chaotic inflation, the amplitude of the comoving and the
curvature perturbation on uniform density hypersurfaces differ by up to 180 %
at horizon crossing assuming the same amplitude at the end of inflation, and
that it takes approximately 3 efolds for the curvature perturbation to be
within 1 % of its value at the end of inflation.
Zero-point quantum fluctuations in cosmology. (arXiv:1111.5575v1 [astro-ph.CO]):
We re-examine the classic problem of the renormalization of zero-point
quantum fluctuations in a Friedmann-Robertson-Walker background. We discuss a
number of issues that arise when regularizing the theory with a momentum-space
cutoff, and show explicitly how introducing non-covariant counter-terms allows
to obtain covariant results for the renormalized vacuum energy-momentum tensor.
We clarify some confusion in the literature concerning the equation of state of
vacuum fluctuations. Further, we point out that the general structure of the
effective action becomes richer if the theory contains a scalar field phi with
mass m smaller than the Hubble parameter H(t). Such an ultra-light particle
cannot be integrated out completely to get the effective action. Apart from the
volume term and the Einstein-Hilbert term, that are reabsorbed into
renormalizations of the cosmological constant and Newton’s constant, the
effective action in general also has a term proportional to F(phi)R, for some
function F(phi). As a result, vacuum fluctuations of ultra-light scalar fields
naturally lead to models where the dark energy density has the form
rho_{DE}(t)=rho_X(t)+rho_Z(t), where rho_X is the component that accelerates
the Hubble expansion at late times and rho_Z(t) is an extra contribution
proportional to H^2(t). We perform a detailed comparison of such models with
CMB, SNIa and BAO data.
Slow Roll Inflation: A Somehow Different Perspective. (arXiv:1112.1083v1 [astro-ph.CO]):
In this note we point out that, contrary to the standard point of view, slow
roll inflation is due to high gravitational friction. We show that the
requirement of slow roll coincides with the requirement of a flat scalar field
potential in the case of minimally coupled scalar field. In this sense, the
search for a successful inflationary theory may be more fruitful by shifting
the focus on models with high gravitational friction. We review then a
gravitational mechanism, the so called “Gravitationally Enhanced Friction”
mechanism, such that high gravitational friction is dynamically generated
during inflation allowing even steep (i.e. non-flat) scalar potential to
inflate.
Stochastic Quantization and Casimir Forces. (arXiv:1110.2308v1 [quant-ph]):
In this paper we show how the stochastic quantization method developed by
Parisi and Wu can be used to obtain Casimir forces. Both quantum and thermal
fluctuations are taken into account by a Langevin equation for the field. The
method allows the Casimir force to be obtained directly, derived from the
stress tensor instead of the free energy. It only requires the spectral
decomposition of the Laplacian operator in the given geometry. The formalism
provides also an expression for the fluctuations of the force. As an
application we compute the Casimir force on the plates of a finite piston of
arbitrary cross section. Fluctuations of the force are also directly obtained,
and it is shown that, in the piston case, the variance of the force is twice
the force squared.
Cosmological constraints on non-standard inflationary quantum collapse models. (arXiv:1112.1830v1 [astro-ph.CO]):
We briefly review an important shortcoming –unearthed in previous works– of
the standard version of the inflationary model for the emergence of the seeds
of cosmic structure. We consider here some consequences emerging from a
proposal inspired on ideas of Penrose and Di\’osi about a quantum-gravity
induced reduction of the wave function, which has been put forward to address
the shortcomings, arguing that its effect on the inflaton field is what can
lead to the emergence of the seeds of cosmic structure. The proposal leads to a
deviation of the primordial spectrum from the scale-invariant
Harrison-Zel’dovich one, and consequently, to a different CMB power spectrum.
We perform statistical analyses to test two quantum collapse schemes with
recent data from the CMB, including the 7-yr release of WMAP and the matter
power spectrum measured using LRGs by the Sloan Digital Sky Survey. Results
from the statistical analyses indicate that several collapse models are
compatible with CMB and LRG data, and establish constraints on the free
parameters of the models. The data put no restriction on the timescale for the
collapse of the scalar field modes.
Is there a flatness problem in classical cosmology?. (arXiv:1112.1666v1 [astro-ph.CO]):
I briefly review the flatness problem within the context of classical
cosmology and examine some of the debate in the literature with regard to its
definition and even the question whether it exists. I then present some new
calculations for cosmological models which will collapse in the future;
together with previous work by others for models which will expand forever,
this allows one to examine the flatness problem quantitatively for all
cosmological models. This leads to the conclusion that the flatness problem
does not exist, not only for the cosmological models corresponding to the
currently popular values of lambda_0 and Omega_0 but indeed for all
Friedmann-Lema\^itre models.
Black-Scholes model under subordination. (arXiv:1111.3263v1 [q-fin.PR]):
In this paper we consider a new mathematical extension of the Black-Scholes
model in which the stochastic time and stock share price evolution is described
by two independent random processes. The parent process is Brownian, and the
directing process is inverse to the totally skewed, strictly \alpha-stable
process. The subordinated process represents the Brownian motion indexed by an
independent, continuous and increasing process. This allows us to introduce the
long-term memory effects in the classical Black-Scholes model.
