256 research outputs found
Effects of inflationary bubbles on the polarization and temperature anisotropies of the cosmic microwave background
We predict the imprint of linear bubbly perturbations on the polarization and
temperature anisotropies of the cosmic microwave background (CMB).
We analytically model a bubbly density perturbation at the beginning of the
radiation dominated era and we apply the linear theory of cosmological
perturbations to compute its time evolution. At decoupling, it uniquely signs
the CMB polarization and temperature anisotropy sky. During evolution the
perturbation propagates beyond the size of the bubble and reaches the CMB sound
horizon at the time considered. Therefore, its signal appears as a series of
concentric rings, each characterized by its own amplitude and sign, on the
scale of 1^{o} on the sky, even if the real seed size is much smaller.
Polarization and temperature rings are strictly correlated. As expected for
linear perturbations with size L and density contrast \delta at decoupling,
\delta T/T is roughly \delta (L/H^{-1})^{2}; the polarization is about 10% of
the temperature anisotropy.
We predict the impact of a distribution of bubbles on the CMB polarization
and temperature power spectra. Considering models containing both CDM Gaussian
and bubbly non-Gaussian fluctuations, we simulate and analyze 10^{o} x 10^{o}
sky patches with angular resolution of about 3.5^{'}. The CMB power associated
with the bubbles is entirely on sub-degree angular scales (200<= l<=1000), that
will be explored by the forthcoming high resolution CMB experiments with the
percent precision. Depending on the parameters of the bubbly distribution we
find extra-power with respect to the ordinary CDM Gaussian fluctuations; we
infer simple analytical scalings of the power induced by bubbly perturbations
and we constrain our parameters with the existing data.Comment: 12 pages, 9 figures (two with nice colors), accepted for publication
by MNRA
The CMB as a dark energy probe
We give a brief review of the known effects of a dynamical vacuum
cosmological component, the dark energy, on the anisotropies of the cosmic
microwave background (CMB). We distinguish between a "classic" class of
observables, used so far to constrain the average of the dark energy abundance
in the redshift interval in which it is relevant for acceleration, and a
"modern" class, aiming at the measurement of its differential redshift
behavior. We show that the gravitationally lensed CMB belongs to the second
class, as it can give a measure of the dark energy abundance at the time of
equality with matter, occurring at about redshift 0.5. Indeed, the dark energy
abundance at that epoch influences directly the lensing strength, which is
injected at about the same time, if the source is the CMB. We illustrate this
effect focusing on the curl (BB) component of CMB polarization, which is
dominated by lensing on arcminute angular scales. An increasing dark energy
abundance at the time of equality with matter, parameterized by a rising first
order redshift derivative of its equation of state today, makes the BB power
dropping with respect to a pure LambdaCDM cosmology, keeping the other
cosmological parameters and primordial amplitude fixed. We briefly comment on
the forthcoming probes which might measure the lensing power on CMB.Comment: 12 pages, 9 figures, proceedings of the invited talk at the CMB and
Physics of the Early Universe Conference, Ischia, Italy, April 20-22, 200
Early time perturbations behaviour in scalar field cosmologies
We consider the problem of the initial conditions and behaviour of the
perturbations in scalar field cosmology with general potential.
We use the general definition of adiabatic and isocurvature conditions to set
the appropriate initial values for the perturbation in the scalar field and in
the ordinary matter and radiation components. In both the cases of initial
adiabaticity and isocurvature, we solve the Einstein and fluid equation at
early times and on superhorizon scales to find the initial behaviour of the
relevant quantities. In particular, in the isocurvature case, we consider
models in which the initial perturbation arises from the matter as well as from
the scalar field itself, provided that the initial value of the gauge invariant
curvature is zero.
We extend the standard code to include all these cases, and we show some
results concerning the power spectrum of the cosmic microwave background
temperature and polarization anisotropies. In particular, it turns out that the
acoustic peaks follow opposite behaviours in the adiabatic and isocurvature
regimes: in the first case their amplitude is higher than in the corresponding
pure cold dark matter model, while they make the opposite thing for pure
isocurvature initial perturbations.Comment: 21 pages, 8 figures, accepted for publication in Phys.Rev.
Extended Quintessence
We study Quintessence cosmologies in the context of scalar-tensor theories of
gravity, where a scalar field , assumed to provide most of the cosmic
energy density today, is non-minimally coupled to the Ricci curvature scalar
. Such `Extended Quintessence' cosmologies have the appealing feature that
the same field causing the time (and space) variation of the cosmological
constant is the source of a varying Newton's constant \`a la
Jordan-Brans-Dicke. We investigate here two classes of models, where the
gravitational sector of the Lagrangian is with (Induced Gravity, IG) and (Non-Minimal
Coupling, NMC). As a first application of this idea we consider a specific
model, where the Quintessence field, , obeying the simplest inverse power
potential, has today, in the context of the Cold Dark
Matter scenario, with scale-invariant adiabatic initial perturbations. We find
that, if for IG and for NMC ( is the present Quintessence
value) our Quintessence field satisfies the existing solar system experimental
constraints. Using linear perturbation theory we then obtain the polarization
and temperature anisotropy spectra of the Cosmic Microwave Background (CMB) as
well as the matter power-spectrum. The perturbation behavior possesses
distinctive features, that we name `QR-effects', regarding acoustic peak
location and height, late time integrated Sachs-Wolfe effect, as well as
turnover and amplitude in the matter power spectrum. These features could be
detected in the upcoming observations on CMB and large-scale structure.Comment: 19 pages including 10 figures, final version to be published in
Phys.Rev.
Cosmic microwave background constraints on dark energy dynamics: analysis beyond the power spectrum
We consider the distribution of the non-Gaussian signal induced by weak
lensing on the primary total intensity cosmic microwave background (CMB)
anisotropies. Our study focuses on the three point statistics exploiting an
harmonic analysis based on the CMB bispectrum. By considering the three
multipoles as independent variables, we reveal a complex structure of peaks and
valleys determined by the re-projection of the primordial acoustic oscillations
through the lensing mechanism. We study the dependence of this system on the
expansion rate at the epoch in which the weak lensing power injection is
relevant, probing the dark energy equation of state at redshift corresponding
to the equivalence with matter or higher (). We evaluate the impact
of the bispectrum observable on the CMB capability of constraining the dark
energy dynamics. We perform a maximum likelihood analysis by varying the dark
energy abundance, the present equation of state and . We show
that the projection degeneracy affecting a pure power spectrum analysis in
total intensity is broken if the bispectrum is taken into account. For a
Planck-like experiment, assuming nominal performance, no foregrounds or
systematics, and fixing all the parameters except , and the
dark energy abundance, a percent and ten percent precision measure of and
is achievable from CMB data only. These results indicate that the
detection of the weak lensing signal by the forthcoming CMB probes may be
relevant to gain insight into the dark energy dynamics at the onset of cosmic
acceleration.Comment: 14 pages, 9 figures. Matching version accepted by Physical Review D.
High resolution figures available upon request to the author
Extended Quintessence: imprints on the cosmic microwave background spectra
We describe the observable features of the recently proposed Extended
Quintessence scenarios on the Cosmic Microwave Background (CMB) anisotropy
spectra. In this class of models a scalar field , assumed to provide most
of the cosmic energy density today, is non-minimally coupled to the Ricci
curvature scalar . We implement the linear theory of cosmological
perturbations in scalar tensor gravitational theories to compute CMB
temperature and polarization spectra. All the interesting spectral features are
affected: on sub-degree angular scales, the acoustic peaks change both in
amplitude and position; on larger scales the low redshift dynamics enhances the
Integrated Sachs Wolfe effect. These results show how the future CMB
experiments could give information on the vacuum energy as well as on the
structure of the gravitational Lagrangian term.Comment: 4 pages including 1 figure, to be published in the proceedings of the
COSMO99 meeting, held in Trieste, September 199
Stochastic gravitational background from inflationary phase transitions
We consider true vacuum bubbles generated in a first order phase transition
occurring during the slow rolling era of a two field inflation: it is known
that gravitational waves are produced by the collision of such bubbles. We find
that the epoch of the phase transition strongly affects the characteristic peak
frequency of the gravitational waves, causing an observationally interesting
redshift in addition to the post-inflationary expansion. In particular it is
found that a phase transition occurring typically 1020 foldings
before the reheating at GeV may be detected by the next Ligo
gravity waves interferometers. Moreover, for recently proposed models capable
of generating the observed large scale voids as remnants of the primordial
bubbles (for which the characteristic wave lengths are several tens of Mpc), it
is found that the level of anisotropy of the cosmic microwave background
provides a deep insight upon the physical parameters of the effective
Lagrangian.Comment: 12 pages, 3 figures. Phys.Rev.D in pres
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