CMB anisotropies from pre-big bang cosmology

We present an alternative scenario for cosmic structure formation where initial fluctuations are due to Kalb-Ramond axions produced during a pre-big bang phase of inflation. We investigate whether this scenario, where the fluctuations are induced by seeds and therefore are of isocurvature nature, can be brought in agreement with present observations by a suitable choice of cosmological parameters. We also discuss several observational signatures which can distinguish axion seeds from standard inflationary models. We finally discuss the gravitational wave background induced in this model and we show that it may be well within the range of future observations.Comment: 33 pages, 18 figures, corrected some typo

Nonlinear perturbations of cosmological scalar fields

We present a covariant formalism to study nonlinear perturbations of scalar fields. In particular, we consider the case of two scalar fields and introduce the notion of adiabatic and isocurvature covectors. We obtain differential equations governing the evolution of these two covectors, as well as the evolution equation for the covector associated to the curvature perturbation. The form of these equations is very close to the analogous equations obtained in the linear theory but our equations are fully nonlinear and exact. As an application of our formalism, we expand these equations at second order in the perturbations. On large scales, we obtain a closed system of coupled scalar equations giving the evolution of the second-order adiabatic and entropy perturbations in terms of the first-order perturbations. These equations in general contain a nonlocal term which, however, rapidly decays in an expanding universe

Cosmic Microwave Background anisotropies and extra dimensions in String Cosmology

A recently proposed mechanism for large-scale structure in string cosmology --based on massless axionic seeds-- is further analyzed and extended to the acoustic-peak region. Existence, structure, and normalization of the peaks turn out to depend crucially on the overall evolution of extra dimensions during the pre-big bang phase: conversely, precise cosmic microwave background anisotropy data in the acoustic-peak region will provide, within the next decade, a window on string-theory's extra dimensions before their eventual compactification

Dark-energy instabilities induced by gravitational waves

We point out that dark-energy perturbations may become unstable in the presence of a gravitational wave of sufficiently large amplitude. We study this effect for the cubic Horndeski operator (braiding), proportional to \u3b1B. The scalar that describes dark-energy fluctuations features ghost and/or gradient instabilities for gravitational-wave amplitudes that are produced by typical binary systems. Taking into account the populations of binary systems, we conclude that the instability is triggered in the whole Universe for |\u3b1B | 73 10-2, i.e. when the modification of gravity is sizeable. The instability is triggered by massive black-hole binaries down to frequencies corresponding to 1010 km: the instability is thus robust, unless new physics enters on even longer wavelengths. The fate of the instability and the subsequent time-evolution of the system depend on the UV completion, so that the theory may end up in a state very different from the original one. The same kind of instability is present in beyond-Horndeski theories for |\u3b1H| 73 10-20. In conclusion, the only dark-energy theories with sizeable cosmological effects that avoid these problems are k-essence models, with a possible conformal coupling with matter

Through the lens of Sgr A*: identifying strongly lensed Continuous Gravitational Waves beyond the Einstein radius

Once detected, lensed gravitational waves will afford new means to probe thematter distribution in the universe, complementary to electromagnetic signals.Sources of continuous gravitational waves (CWs) are long-lived and stable,making their lensing signatures synergic to short mergers of compact binaries.CWs emitted by isolated neutron stars and lensed by Sgr A$^*$, thesuper-massive black hole at the center of our galaxy, might be observable bythe next generation of gravitational wave detectors. However, it is unknownunder which circumstances these sources can be identified as lensed. Here weshow that future detectors can distinguish lensed CWs and measure allparameters with precision $\sim 1-10\%$ for sources within $2-4$ Einstein radiiof Sgr A$^*$, depending on the source's distance. Such a detection, whichrelies on the relative motion of the observer-lens-source system, can beobserved for transverse velocities above 3 km/s. Therefore, the chances ofobserving strongly lensed neutron stars increase by one order of magnitude withrespect to previous estimates. Observing strongly lensed CWs will enable novelprobes of the galactic center and fundamental physics.<br

From heaviness to lightness during inflation

We study the quantum fluctuations of scalar fields with a variable effective mass during an inflationary phase. We consider the situation where the effective mass depends on a background scalar field, which evolves during inflation from being frozen into a damped oscillatory phase when the Hubble parameter decreases below its mass. We find power spectra with suppressed amplitude on large scales, similar to the standard massless spectrum on small scales, and affected by modulations on intermediate scales. We stress the analogies and differences with the parametric resonance in the preheating scenario. We also discuss some potentially observable consequences when the scalar field behaves like a curvaton.Comment: 23 pages; 8 figures; published versio

Covariant generalization of cosmological perturbation theory

We present an approach to cosmological perturbations based on a covariant perturbative expansion between two worldlines in the real inhomogeneous universe. As an application, at an arbitrary order we define an exact scalar quantity which describes the inhomogeneities in the number of e-folds on uniform density hypersurfaces and which is conserved on all scales for a barotropic ideal fluid. We derive a compact form for its conservation equation at all orders and assign it a simple physical interpretation. To make a comparison with the standard perturbation theory, we develop a method to construct gauge-invariant quantities in a coordinate system at arbitrary order, which we apply to derive the form of the n-th order perturbation in the number of e-folds on uniform density hypersurfaces and its exact evolution equation. On large scales, this provides the gauge-invariant expression for the curvature perturbation on uniform density hypersurfaces and its evolution equation at any order.Comment: Minor changes to match the version published in PRD. RevTex, 22 pages, 1 figur