1,722,825 research outputs found
Effects of inflationary bubbles on the sub-degree polarization and temperature anisotropies of the cosmic microwave background
No full textWe 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
Sub-horizon perturbation behavior in extended quintessence
No full textIn the general context of scalar-tensor theories, we consider a model in which a scalar field coupled to the Ricci scalar in the gravitational sector of the Lagrangian, is also playing the role of an "Extended Quintessence" field, dominating the energy content of the Universe at the present time. In this framework, we study the linear evolution of the perturbations in the Quintessence energy density, showing that a new phenomenon, named here "gravitational dragging", can enhance the scalar field density perturbations as much as they reach the non-linear regime. The possibility of dark energy clumps formation is thus discussed
Early time perturbations behavior in scalar field cosmologies
Full text linkWe consider the problem of the initial conditions and behavior of the perturbations in scalar field cosmology with a 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 equations at early times and on superhorizon scales to find the initial behavior 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 behavior in the adiabatic and isocurvature regimes: in the first case their amplitude is higher than in the corresponding pure cold dark matter model, while it is the opposite for pure isocurvature initial perturbations
Tracking Extended Quintessence
No full textWe investigate the cosmological role of a tracking field φ in extended quintessence scenarios, where the dynamical vacuum energy driving the acceleration of the universe today possesses an explicit coupling with the Ricci scalar R of the form F(φ)R/2, where F(φ) mimics general relativity today, F(φ0)=1/8πG. We analyze explicit nonminimally coupled (NMC) models where F(φ)=1/8πG+ξ(φ2-φ20), with ξ is the coupling constant and φ0 is the Q value today. Tracker solutions for these NMC models, with inverse power-law potentials, possess an initial enhancement of the scalar field dynamics, named the R-boost, caused by the effective potential generated by the Ricci scalar in the Klein-Gordon equation. During this phase the field performs a ``gravitational'' slow rolling until the true potential becomes important. We give accurate analytic formulas describing the R-boost, showing that the quintessence energy in this phase scales with the redshift z as (1+z)2. When the R-boost ends, the field trajectory matches the tracker solution in minimally coupled theories. We compute perturbations in these tracking extended quintessence models, by integrating the full set of equations for the evolution of linear fluctuations in scalar-tensor theories of gravity, and assuming Gaussian scale-invariant initial perturbations. The integrated Sachs-Wolfe (ISW) effect on the cosmic microwave background (CMB) angular spectrum causes a change δCl/Cl~=6[1-8πGF(φdec)] at l<~10, where ``dec'' stands for decoupling. Similarly, the CMB acoustic peak multipoles shift compared to ordinary tracking quintessence models by roughly an amount δl/l~=[8πGF(φdec)-1]/8. The turnover wave number kturn in the matter power spectrum shifts by an amount δkturn/kturn~=[1-8πGF(φeq)]/2, where ``eq'' stands for matter-radiation equivalence. All these corrections may assume positive as well as negative values, depending on the sign of the NMC parameter ξ. We show that the above effects can be as large as 10-30 % with respect to equivalent cosmological constant and ordinary tracking quintessence models, respecting all the existing experimental constraints on scalar-tensor theories of gravity. These results demonstrate that the playground where the data of the next decade will have their impact includes the nature of the dark energy in the Universe, as well as the structure of the theory of gravity
A BUBBLY UNIVERSE
No full textWe propose to explain the present large scale structure of the universe in terms of a first order phase transition in a two field inflation: the seeds of structure are assumed to be the ensuing strong, non-Gaussian, bubblelike inhomogeneities generated by the tunneling field. Along with this, of course, the ordinary zero-point fluctuations of the slow rolling inflaton are also present: they are seen as Gaussian and small perturbations of the microwave background on the large angular scales. We describe a biparametric model of bubbles in the matter dominated era (MDE) in which caustics form at a redshift z* in the surrounding shells and we assume that the caustics themselves are the loci of galaxy formation, i.e., the places where light is turned on. (Most likely z* will then define also the epoch of reionization.) The two parameters are then determined by the bubble’s two main features, present depth and z*. The caustics will evolve into the shells of galaxies observed today around the nearly empty and spherical voids. Among the possible scenarios we focus on two that yield late or early caustic formation. In the MDE the shells born with the caustics experience a strong overcomoving growth (the larger the deeper is the central cavity): this phenomenon may turn bubbles substantially subdominant at decoupling (i.e., filling then only a small fraction of the available space) into the dominant features by the present time, as the observations require. For compensated voids, from the Sachs-Wolfe, adiabatic, and Doppler effects, we find that the largest present radii compatible with COBE amount to ≈100h−1 Mpc in either scenario. Thus, if the large scale structure were generated by bubbles, the present luminous universe could look bubbly up to scales of the order of one hundred Mpc mimicking a fractal with dimension D≈2 without conflicting with the isotropy of the microwave background, because homogeneity is restored thereabove
Dark Energy and CMB Bispectrum
Get PDFWe consider the CMB bispectrum signal induced by structure formation through the correlation between the Integrated Sachs-Wolfe and the weak lensing effect. We investigate how the bispectrum knowledge can improve our knowledge of the most important cosmological parameters, focusing on the dark energy ones. The bispectrum signal arises at intermediate redshifts, being null at present and infinity, and is characterized by a large scale regime (dominated by linear dynamics of cosmological perturbation) and a small scale one (dominated by density perturbations in a non-linear regime); on the other hand, the effect induced by dark energy on the power spectrum is mostly geometrical and imprinted at redshift close to the present. Because of this, the knowledge of power spectrum and bispectrum yield two complementary informations at very different cosmological epochs, particularly suitable to extract informations about the onset of the cosmic acceleration and dark energy properties that provide it. In order to quantify how much the bispectrum can help the power spectrum in constraining the dark energy parameters, we choose a fiducial model on a three-dimensional space including the following dark energy parameters: dark energy density ΩV; dark energy equation of state today w0 and dark energy equation of state in the past w∞ (w∞ - w0 is related to the first derivative of equation of state). Then we simulate a likelihood analysis showing how contour levels become narrower when bispectrum is included. Preliminary results suggest a consistent improvement on the estimation of dark energy abundance and on dynamical properties of the equation of state. This indicates that the knowledge of the bispectrum in future high resolution and high sensitivity CMB observations could yield a substantial improvement with respect to the traditional analysis based on the power spectrum only
Low-complexity voltage and current sources for large-scale quench detection of high-temperature superconducting cables
No full textIn this work, the design of precision voltage and current sources for a new quench detection system for theprotection of superconducting power cables is proposed. The key strength of the design resides in its lowcomplexity, which allows to obtain a low-cost and reliable hardware implementation for the new detector. Thevoltage source output goes from 200 mV to 800 mV, while the current source output is adjustable between50 mA and 1 A. Experimental results from the sources characterization campaign validate the design: 24-hourstime stability is demonstrated to be about 20 ppm for the voltage source, and below 1000 ppm for the minimumcurrent of 50 mA of the current source. Furthermore, the measured superposed noise is less than−80 dBVand the Mean Time Between Failures (MTBF) is estimated to be about 20 million hours for both sources. Theimprovement of the quench detector features is also demonstrated through further experiments. The obtainedresults demonstrate the effectiveness of the proposed detection mechanism and pave the way for further researchon large-scale use of high-temperature superconducting cables
Principal component analysis of the primordial tensor power spectrum
No full textWe study how the shape of the spectrum of primordial gravitational waves can be constrained by future experiments looking at the B-mode of the Cosmic Microwave Background (CMB) polarization. We implement a Principal Component Analysis (PCA) including the effects of diffuse foreground residuals, following component separation, in the uncertainty of CMB angular power spectra, and taking into account the gravitational lensing by Large Scale Structure. We perform our study by considering the capabilities of future B-mode CMB experiments such as LiteBIRD, the Simons Observatory (SO) and Stage-IV (CMB-S4), in particular in detecting deviations of the primordial tensor spectrum from the scale-invariant behavior. We find that diffuse foreground residuals impact substantially both the derivation of the PCA basis and the corresponding constraining power, in all cases. In particular, depending on which experimental specifications and which value r of tensor-to-scalar ratio for cosmological perturbations are considered, adding foregrounds residuals can determine an increase as large as a factor ∼ 4 both on the uncertainty on r and on the recovery of the PCA modes. We study the limitations of the methodology, including the effect of physicality priors on the PCA, which we quantify via a Monte Carlo Markov chain (MCMC) analysis of the combined cosmological and PCA power spectrum parameter space
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