1,721,059 research outputs found
Isocurvature perturbations in the ekpyrotic universe
No full textThe Ekpyrotic scenario assumes that our visible Universe is a boundary brane in a five-dimensional bulk and that the hot Big Bang occurs when a nearly supersymmetric five-brane travelling along the fifth dimension collides with our visible brane. We show that the generation of isocurvature perturbations is a generic prediction of the Ekpyrotic Universe. This is due to the interactions in the kinetic terms between the brane modulus parametrizing the position of the five-brane in the bulk and the dilaton and volume moduli. We show how to separate explicitly the adiabatic and isocurvature modes by performing a rotation in field space. Our results indicate that adiabatic and isocurvature perturbations might be cross-correlated and that curvature perturbations might be entirely seeded by isocurvature perturbations
ON ANOMALIES IN ORBIFOLD THEORIES
No full textWe study the issue of gauge invariance in five-dimensional theories compactified on an orbifold S1/(Z2 ×Z′2) in the presence of an external U(1) gauge field. From the four-dimensional point of view the theory contains a tower of Kaluza–Klein Dirac fermions with chiral couplings and it looks anomalous at the quantum level. We show that this “anomaly” is cancelled by a topological Chern–Simons term which is generated in the effective action when the gauge theory is regularized introducing a Pauli–Villars fermion with an odd mass term. In the presence of a classical background gauge field, the fermionic current acquires a vacuum expectation value, thus generating the suitable Chern–Simons term and a gauge invariant theory
On the non-Gaussianity from Recombination
No full textThe non-linear effects operating at the recombination epoch generate a non-Gaussian signal in the CMB anisotropies. Such a contribution is relevant because it represents a major part of the second-order radiation transfer function which must be determined in order to have a complete control of both the primordial and non-primordial part of non-Gaussianity in the CMB anisotropies. We provide an estimate of the level of non-Gaussianity in the CMB arising from the recombination epoch which shows up mainly in the equilateral configuration. We find that it causes a contamination to the possible measurement of the equilateral primordial bispectrum shifting the minimum detectable value of the non-Gaussian parameter f^equil_NL by Delta f^equil_NL= O(10) for an experiment like Planck
Evolution of Second-Order Cosmological Perturbations and Non-Gaussianity
No full textWe present a second-order gauge-invariant formalism to study the evolution of curvature perturbations in a Friedmann–Robertson–Walker universe filled by multiple interacting fluids. We apply such a general formalism to describe the evolution of the second-order curvature perturbations in the standard one-single-field inflation, in the curvaton and in the inhomogeneous reheating scenarios for the generation of the cosmological perturbations. Moreover, we provide the exact expression for the second-order temperature anisotropies on large scales, including second-order gravitational effects and extend the well known formula for the Sachs–Wolfe effect at linear order. Our findings clarify what is the exact non-linearity parameter fNL entering in the determination of higher-order statistics such as the bispectrum of Cosmic Microwave Background temperature anisotropies. Finally, we compute the level of non-Gaussianity in each scenario for the creation of cosmological perturbations
Possibility of observable amount of gravitational waves from inflation
No full textThe curvaton and inhomogeneous reheating scenarios for the generation of the cosmological curvature perturbation on large scales represent an alternative to the standard slow-roll scenario. The basic assumption of these mechanisms is that the initial curvature perturbation due to the inflaton field is negligible. This is usually attained by lowering the energy scale of inflation, thereby concluding that the amount of gravitational waves produced during inflation is highly suppressed. We show that the curvaton and inhomogeneous reheating scenarios are compatible with a level of gravity-wave fluctuations which may well be observed in future satellite experiments
Non-Gaussianity in the curvaton scenario
No full textSince a positive future detection of nonlinearity in the cosmic microwave background anisotropy pattern might allow us to descriminate among different mechanisms giving rise to cosmological adiabatic perturbations, we study the evolution of the second-order cosmological curvature perturbation on superhorizon scales in the curvaton scenario. We provide the exact expression for the non-Gaussianity in the primordial perturbations including gravitational second-order corrections which are particularly relevant in the case in which the curvaton dominates the energy density before it decays. As a by-product, we show that in the standard scenario where cosmological curvature perturbations are induced by the inflaton field, the second-order curvature perturbation is conserved even during the reheating stage after inflation
Non-Gaussianity from Inflation
No full textCorrelated adiabatic and isocurvature perturbation modes are produced during inflation through an oscillation mechanism when extra scalar degrees of freedom, other than the inflaton field, are present. We show that this correlation generically leads to sizable non-Gaussian features both in the adiabatic and isocurvature perturbations. The non-Gaussianity is first generated by large nonlinearities in some scalar sector and then efficiently transferred to the inflaton sector by the oscillation process. We compute the cosmic microwave background angular bispectrum, providing a characteristic feature of such inflationary non-Gaussianity, which might be detected by upcoming satellite experiments
OLD INFLATION IN STRING THEORY
No full textWe propose a stringy version of the old inflation scenario which does not require any slow-roll inflaton potential and is based on a specific example of string compactification with warped metric. Our set-up admits the presence of anti-D3-branes in the deep infrared region of the metric and a false vacuum state with positive vacuum energy density. The latter is responsible for the accelerated period of inflation. The false vacuum exists only if the number of anti-D3-branes is smaller than a critical number and the graceful exit from inflation is attained if a number of anti-D3-branes travels from the ultraviolet towards the infrared region. The cosmological curvature perturbation is generated through the curvaton mechanism
Gauge-Invariant Temperature Anisotropies and Primordial Non-Gaussianity
Full text linkWe provide the gauge-invariant expression for large-scale cosmic microwave background temperature fluctuations at second-order perturbation theory. This enables us to define unambiguously the nonlinearity parameter fNL, which is used by experimental collaborations to pin down the level of non-Gaussianity in the temperature fluctuations. Furthermore, it contains a primordial term encoding all the information about the non-Gaussianity generated at primordial epochs and about the mechanism which gave rise to cosmological perturbations, thus neatly disentangling the primordial contribution to non-Gaussianity from the one caused by the postinflationary evolution
Primordial non-Gaussianity from different cosmological scenarios
No full textWe review recent results about the level of non-Gaussianity in the primordial density perturbations generated in various cosmological scenarios, spanning from the standard single-field models of slow-roll inflation to new alternative scenarios. Through a full calculation at second order in perturbation theory we provide all the tools necessary to determine the level of the primordial non-Gaussianity in the large-scale Cosmic Microwave Background (CMB) temperature anisotropies in any model of cosmological perturbations. In light of present and future CMB observations we stress the importance of non-Gaussianity as a key discriminator among competing cosmological scenarios
- …
