123 research outputs found
Inhomogeneities and cosmological expansion
I review work on the influence of inhomogeneities in the matter distribution
on the determination of the luminosity distance of faraway sources, and the
connection to the perceived cosmological acceleration.Comment: 4 pages, 2 figures, talk at the 9th Hellenic School and Workshops
(Corfu): Standard Model and Beyond -- Standard Cosmolog
Suppression of Quantum Corrections by Classical Backgrounds
We use heat-kernel techniques in order to compute the one-loop effective
action in the cubic Galileon theory for a background that realizes the
Vainshtein mechanism. We find that the UV divergences are suppressed relative
to the predictions of standard perturbation theory at length scales below the
Vainshtein radius.Comment: 10 pages, 1 figure, major revision, inclusion of higher-order terms,
version to appear in PR
Neutrino Lumps in Quintessence Cosmology
Neutrinos interacting with the quintessence field can trigger the accelerated
expansion of the Universe. In such models with a growing neutrino mass the
homogeneous cosmological solution is often unstable to perturbations. We
present static, spherically symmetric solutions of the Einstein equations in
the same models. They describe astophysical objects composed of neutrinos, held
together by gravity and the attractive force mediated by the quintessence
field. We discuss their characteristics as a function of the present neutrino
mass. We suggest that these objects are the likely outcome of the growth of
cosmological perturbations.Comment: 9 pages, 4 figures, references and discussion of formation adde
Quantum corrections in Galileon theories
We calculate the one-loop quantum corrections in the cubic Galileon theory,
using cutoff regularization. We confirm the expected form of the one-loop
effective action and that the couplings of the Galileon theory do not get
renormalized. However, new terms, not included in the tree-level action, are
induced by quantum corrections. We also consider the one-loop corrections in an
effective brane theory, which belongs to the Horndeski or generalized Galileon
class. We find that new terms are generated by quantum corrections, while the
tree-level couplings are also renormalized. We conclude that the structure of
the generalized Galileon theories is altered by quantum corrections more
radically than that of the Galileon theory.Comment: 8 pages; v2 minor typos corrected, references added; v3 minor
clarifications; v4 version published in PR
Nonlinear evolution of density and flow perturbations on a Bjorken background
Density perturbations and their dynamic evolution from early to late times
can be used for an improved understanding of interesting physical phenomena
both in cosmology and in the context of heavy-ion collisions. We discuss the
spectrum and bispectrum of these perturbations around a longitudinally
expanding fireball after a heavy-ion collision. The time-evolution equations
couple the spectrum and bispectrum to each other, as well as to higher-order
correlation functions through nonlinear terms. A non-trivial bispectrum is thus
always generated, even if absent initially. For initial conditions
corresponding to a model of independent sources, we discuss the linear and
nonlinear evolution is detail. We show that, if the initial conditions are
sufficiently smooth for fluid dynamics to be applicable, the nonlinear effects
are relatively small.Comment: 32 pages, 17 figures, published versio
Nonlinear matter spectra in growing neutrino quintessence
We investigate the nonlinear power spectra of density perturbations and
acoustic oscillations in growing neutrino quintessence. In this scenario, the
neutrino mass has a strong dependence on the quintessence field. The induced
coupling stops the evolution of the field when the neutrinos become
nonrelativistic, and triggers the transition to the accelerating phase of the
cosmological expansion. At redshifts around five, the neutrino fluctuations are
still linear and acoustic oscillations are present in the neutrino power
spectrum, induced by the acoustic oscillations in the baryonic and dark-matter
sectors. The neutrino perturbations become nonlinear at redshifts around three.
The mode coupling generated by the nonlinearities erases the oscillations in
the neutrino spectrum at some redshift above two. There is a potential danger
that at later times the influence of the gravitational potentials induced by
the neutrino inhomogeneities could erase the oscillations from the baryonic and
dark-matter spectra, making the scenario incompatible with observations. For
the scenario to be viable, the neutrino-induced gravitational potentials in the
range of baryonic acoustic oscillations should not grow to average values much
larger than 10^{-4}. The magnitude of the expected potentials is still not
known reliably, as the process of structure formation is poorly understood in
growing neutrino quintessence.Comment: 11 pages, 3 figures, typo correcte
Modified brane cosmologies with induced gravity, arbitrary matter content and a Gauss-Bonnet term in the bulk
We extend the covariant analysis of the brane cosmological evolution in order
to take into account, apart from a general matter content and an
induced-gravity term on the brane, a Gauss-Bonnet term in the bulk. The
gravitational effect of the bulk matter on the brane evolution can be described
in terms of the total bulk mass as measured by a bulk observer at the location
of the brane. This mass appears in the effective Friedmann equation through a
term characterized as generalized dark radiation that induces mirage effects in
the evolution. We discuss the normal and self-accelerating branches of the
combined system. We also derive the Raychaudhuri equation that can be used in
order to determine if the cosmological evolution is accelerating.Comment: 12 pages, no figures, RevTex 4.0; (v2) new references are added;
(v3,v4) minor changes, acknowledgment is included; to appear in Phys. Rev.
Dynamical classicalization
We integrate numerically the nonlinear equation of motion for a collapsing
spherical wavepacket in the context of theories that are expected to display
behavior characteristic of classicalization. The classicalization radius sets
the scale for the onset of significant deformations of the collapsing
configuration, which result in the formation of shock fronts. A characteristic
observable feature of the classicalization process is the creation of an
outgoing field configuration that extends far beyond the classicalization
radius. This feature develops before the deformed wavepacket reaches distances
of the order of the fundamental scale. We find that in some models the
scattering problem may not have real solutions over the whole space at late
times. We determine the origin of this behavior and discuss the consistency of
the underlying models.Comment: 16 pages, 6 figures, published versio
Analytical Estimate of the Effect of Spherical Inhomogeneities on Luminosity Distance and Redshift
We provide an analytical estimate of the effect of a spherical inhomogeneity
on light beams that travel through it. We model the interior of the
inhomogeneity in terms of the Lemaitre-Tolman-Bondi metric. We assume that the
beam source is located outside the inhomogeneity. We study the relative
deviations of travelling time, redshift, beam area and luminosity distance from
their values in a homogeneous cosmology. They depend on the ratio Hb=H r_0 of
the radius r_0 of the inhomogeneity to the horizon distance 1/H. For an
observer located at the center, the deviations are of order Hb^2. For an
observer outside the inhomogeneity, the deviations of crossing time and
redshift are of order Hb^3. The deviations of beam area and luminosity distance
are of order Hb^2. However, when averaged over all possible locations of the
observer outside the inhomogeneity, they also become of order Hb^3. We discuss
the implications for the possibility of attributing the observed cosmological
acceleration to the emergence of large-scale structure.Comment: 11 pages, references added, discussion expande
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