86 research outputs found
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 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
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
On the dynamics of classicalization
We discuss the mechanism through which classicalization may occur during the
collapse of a spherical field configuration modelled as a wavepacket. We
demonstrate that the phenomenon is associated with the dynamical change of the
equation of motion from a second-order partial differential equation of
hyperbolic to one of elliptic type. Within this approach, we rederive the known
expression for the classicalization radius. We also find indications that
classicalization is associated with the absence of wave propagation at
distances below the classicalization radius and the generation of shock fronts.
The full quantitative picture can be obtained only through the numerical
integration of a partial differential equation of mixed type.Comment: enhanced version, 6 pages, 2 figure
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
Non-linear dark energy clustering
We consider a dark energy fluid with arbitrary sound speed and equation of
state and discuss the effect of its clustering on the cold dark matter
distribution at the non-linear level. We write the continuity, Euler and
Poisson equations for the system in the Newtonian approximation. Then, using
the time renormalization group method to resum perturbative corrections at all
orders, we compute the total clustering power spectrum and matter power
spectrum. At the linear level, a sound speed of dark energy different from that
of light modifies the power spectrum on observationally interesting scales,
such as those relevant for baryonic acoustic oscillations. We show that the
effect of varying the sound speed of dark energy on the non-linear corrections
to the matter power spectrum is below the per cent level, and therefore these
corrections can be well modelled by their counterpart in cosmological scenarios
with smooth dark energy. We also show that the non-linear effects on the matter
growth index can be as large as 10-15 per cent for small scales.Comment: 33 pages, 7 figures. Improved presentation. References added. Matches
published version in JCA
Direct Measurement of the Positive Acceleration of the Universe and Testing Inhomogeneous Models under Gravitational Wave Cosmology
One possibility for explaining the apparent accelerating expansion of the
universe is that we live in the center of a spherically inhomogeneous universe.
Although current observations cannot fully distinguish CDM and these
inhomogeneous models, direct measurement of the acceleration of the universe
can be a powerful tool in probing them. We have shown that, if CDM is
the correct model, DECIGO/BBO would be able to detect the positive redshift
drift (which is the time evolution of the source redshift ) in 3--5 year
gravitational wave (GW) observations from neutron-star binaries, which enables
us to rule out any Lema\^itre-Tolman-Bondi (LTB) void model with monotonically
increasing density profile. We may even be able to rule out any LTB model
unless we allow unrealistically steep density profile at . This test
can be performed with GW observations alone, without any reference to
electromagnetic observations, and is more powerful than the redshift drift
measurement using Lyman forest.Comment: 5 pages, 2 figure
CMB observations in LTB universes: Part I: Matching peak positions in the CMB spectrum
Acoustic peaks in the spectrum of the cosmic microwave background in
spherically symmetric inhomogeneous cosmological models are studied. At the
photon-baryon decoupling epoch, the universe may be assumed to be dominated by
non-relativistic matter, and thus we may treat radiation as a test field in the
universe filled with dust which is described by the Lema\^itre-Tolman-Bondi
(LTB) solution. First, we give an LTB model whose distance-redshift relation
agrees with that of the concordance CDM model in the whole redshift
domain and which is well approximated by the Einstein-de Sitter universe at and
before decoupling. We determine the decoupling epoch in this LTB universe by
Gamow's criterion and then calculate the positions of acoustic peaks. Thus
obtained results are not consistent with the WMAP data. However, we find that
one can fit the peak positions by appropriately modifying the LTB model,
namely, by allowing the deviation of the distance-redshift relation from that
of the concordance CDM model at where no observational data are
available at present. Thus there is still a possibility of explaining the
apparent accelerated expansion of the universe by inhomogeneity without
resorting to dark energy if we abandon the Copernican principle. Even if we do
not take this extreme attitude, it also suggests that local, isotropic
inhomogeneities around us may seriously affect the determination of the density
contents of the universe unless the possible existence of such inhomogeneities
is properly taken into account.Comment: 20 pages, 5 figure
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