Search

We forecast the ability of bispectrum estimators to constrain primordial non-Gaussianity using future photometric galaxy redshift surveys. A full-sky survey with photometric redshift resolution of $\sigma_z/(1+z)=0.05$ in the redshift range $0.2

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational wave observations by LISA to probe the universe.

We investigate the sensitivity of the Laser Interferometer Space Antenna (LISA) to the anisotropies of the Stochastic Gravitational Wave Background (SGWB). We first discuss the main astrophysical and cosmological sources of SGWB which are characterized by anisotropies in the GW energy density, and we build a Signal-to-Noise estimator to quantify the sensitivity of LISA to different multipoles. We then perform a Fisher matrix analysis of the prospects of detectability of anisotropic features with LISA for individual multipoles, focusing on a SGWB with a power-law frequency profile. We compute the noise angular spectrum taking into account the specific scan strategy of the LISA detector. We analyze the case of the kinematic dipole and quadrupole generated by Doppler boosting an isotropic SGWB. We find that $\beta\, \Omega_{\rm GW}\sim 2\times 10^{-11}$ is required to observe a dipolar signal with LISA. The detector response to the quadrupole has a factor $\sim 10^3 \,\beta$ relative to that of the dipole. The characterization of the anisotropies, both from a theoretical perspective and from a map-making point of view, allows us to extract information that can be used to understand the origin of the SGWB, and to discriminate among distinct superimposed SGWB sources., Comment: 48 page + appendices, 13 figures

We consider the possibility of extra spinning particles during inflation, focussing on the spin-2 case. Our analysis relies on the well-known fully non-linear formulation of interacting spin-2 theories. We explore the parameter space of the corresponding inflationary Lagrangian and identify regions therein exhibiting signatures within reach of upcoming CMB probes. We provide a thorough study of the early and late-time dynamics ensuring that stability conditions are met throughout the cosmic evolution. We characterise in particular the gravitational wave spectrum and three-point function finding a local-type non-Gaussianity whose amplitude may be within the sensitivity range of both the LiteBIRD and CMB-S4 experiments., Comment: 30 pages, 6 figures; v2 -- references added, minor changes, accepted for publication in JCAP

Characterizing the physical properties of the stochastic gravitational waves background (SGWB) is a key step towards identifying the nature of its possible origin. We focus our analysis on SGWB anisotropies. The existence of a non-trivial primordial scalar-tensor-tensor (STT) correlation in the squeezed configuration may be inferred from the effect that a long wavelength scalar mode has on the gravitational wave power spectrum: an anisotropic contribution. Crucially, such contribution is correlated with temperature anisotropies in the cosmic microwave background (CMB). We show that, for inflationary models that generate suitably large STT non-Gaussianities, cross-correlating the CMB with the stochastic background of gravitational waves is a very effective probe of early universe physics. The resulting signal can be a smoking-gun for primordial SGWB anisotropies., Comment: 6 pages, 2 figures. Version matching the published manuscript

With an energy scale that can be as high as $10^{14}\,{\rm GeV}$, inflation may provide a unique probe of high-energy physics. Both scalar and tensor fluctuations generated during this early accelerated expansion contain crucial information about the particle content of the primordial universe. The advent of ground- and space-based interferometers enables us to probe primordial physics at length-scales much smaller than those corresponding to current CMB constraints. One key prediction of single-field slow-roll inflation is a red-tilted gravitational wave spectrum, currently inaccessible at interferometer scales. Interferometers probe directly inflationary physics that deviates from the minimal scenario and in particular additional particle content with sizeable couplings to the inflaton field. We adopt here an effective description for such fields and focus on the case of extra spin-2 fields. We find that a time-dependent sound speed for the helicity-2 modes can generate primordial gravitational waves with a blue-tilted spectrum, potentially detectable at interferometer scales., Comment: 15 pages, 6 figures; references added; version matching the one published in JCAP

This paper discusses the science case for a sensitive spectro-polarimetric survey of the microwave sky. Such a survey would provide a tomographic and dynamic census of the three-dimensional distribution of hot gas, velocity flows, early metals, dust, and mass distribution in the entire Hubble volume, exploit CMB temperature and polarisation anisotropies down to fundamental limits, and track energy injection and absorption into the radiation background across cosmic times by measuring spectral distortions of the CMB blackbody emission. In addition to its exceptional capability for cosmology and fundamental physics, such a survey would provide an unprecedented view of microwave emissions at sub-arcminute to few-arcminute angular resolution in hundreds of frequency channels, a data set that would be of immense legacy value for many branches of astrophysics. We propose that this survey be carried-out with a large space mission featuring a broad-band polarised imager and a moderate resolution spectro-imager at the focus of a 3.5m aperture telescope actively cooled to about 8K, complemented with absolutely-calibrated Fourier Transform Spectrometer modules observing at degree-scale angular resolution in the 10-2000 GHz frequency range. We propose two observing modes: a survey mode to map the entire sky as well as a few selected wide fields, and an observatory mode for deeper observations of regions of specific interest., Comment: 20 pages, white paper submitted in answer to the "Voyage 2050" call to prepare the long term plan in the ESA science programme

Evidence for the presence of extra fields during inflation may be found in the anisotropies of the scalar and tensor spectra across a vast range of scales. Indeed, beyond the single-field slow-roll paradigm, a long tensor mode can modulate the power spectrum inducing a sizable quadrupolar anisotropy. We investigate how this dynamics plays out for the tensor two-point correlator. The resulting quadrupole stores information on squeezed tensor non-Gaussianities, specifically those sourced by the extra field content and responsible for the breaking of so-called consistency relations. We underscore the potential of anisotropies as a probe of new physics: testable at CMB scales through the detection of B-modes, they are accessible at smaller scales via interferometers and pulsar timing arrays., Comment: 7 pages, 2 figure

The next generation of instruments designed to measure the polarization of the cosmic microwave background (CMB) will provide a historic opportunity to open the gravitational wave window to the primordial Universe. Through high sensitivity searches for primordial gravitational waves, and tighter limits on the energy released in processes like phase transitions, the CMB polarization data of the next decade has the potential to transform our understanding of the laws of physics underlying the formation of the Universe., Comment: 5 pages + references; Submitted to the Astro2020 call for science white papers

The detection and characterization of primordial gravitational waves through their impact on the polarization anisotropies of the cosmic microwave background (CMB) is a primary science goal of current and future observations of the CMB. An ancillary dataset that will become accessible with the great leaps in sensitivity of CMB experiments is the polarized Sunyaev Zel'dovich (pSZ) effect, small-scale CMB polarization anisotropies induced by scattering from free electrons in the post-reionization Universe. The cross correlation of the pSZ effect with galaxy surveys, a technique known as pSZ tomography, can be used to reconstruct the remote quadrupole field: the CMB quadrupole observed from different locations in the Universe. Primordial gravitational waves leave a distinct imprint on the remote quadrupole field, making pSZ tomography a potential new method to characterize their properties. Building on previous work, we explore the utility of the full set of correlations between the primary CMB and the reconstructed remote quadrupole field to both provide exclusion limits on the amplitude of primordial gravitational waves, as well as to provide constraints on several phenomenological models of the tensor sector: axion gauge field inflation, general models with chiral tensors, and models with modified late-time decay of tensors. We find that relatively futuristic experimental requirements are necessary to provide competitive exclusion limits compared with the primary CMB. However, pSZ tomography can be a powerful probe of the late-time evolution of tensors and, through cross-correlations with the primary CMB, can provide mild improvements on parameter constraints in various models with chiral primordial gravitational waves., Comment: 16 pages, 6 figures

Tensor non-Gaussianity represents an important future probe of the physics of inflation. Inspired by recent works, we elaborate further on the possibility of significant primordial tensor non-Gaussianities sourced by extra fields during inflation. Unitarity constraints limit the impact of extra (spinning) particle content by means of a lower bound on the corresponding mass spectrum. For spin-2 particles, this takes the form of the well-known Higuchi bound. Massive ($m\gtrsim H$) particles will typically decay during inflation unless they are non-minimally coupled to the inflaton sector: the inflating field "lifts" the dynamics of the extra field(s), effectively getting around the limits imposed by unitarity. There exist several models that realize such a mechanism, but we focus here on the set-up of [1] where, through an EFT approach, one is able to capture the essential features common to an entire class of theories. In the presence of an extra massive spin-2 particle, the interactions in the tensor sector mimic very closely those in the scalar sector of quasi-single-field inflationary models. We calculate the tensor bispectrum in different configurations and extract its dependence on the extra tensor sound speed. We show in detail how one may obtain significant tensor non-Gaussianities whose shape-function interpolates between local and equilateral, depending on the mass of the extra field. We also estimate the LISA response functions to a tensor bispectrum supporting the intermediate-type shapes we find., Comment: 21 pages, 7 figures, References added

We study the scalar-tensor-tensor non-Gaussian signal in an inflationary model comprising also an axion coupled with SU(2) gauge fields. In this set-up, metric fluctuations are sourced by the gauge fields already at the linear level providing an enhanced chiral gravitational waves spectrum. The same mechanism is at work in generating an amplitude for the three-point function that is parametrically larger than in standard single-field inflation., Comment: References added. 19 pages, 5 figures

We study how inflationary observables associated with primordial tensor modes are affected by coupling the minimal field content with an extra spin-2 particle during inflation. We work with a model that is ghost-free at the fully non-linear level and show how the new degrees of freedom modify standard consistency relations for the tensor bispectrum. The extra interacting spin-2 field is necessarily massive and unitarity dictates its mass be in the $m \gtrsim H$ range. Despite the fact that this bound selects a decaying solution for the corresponding tensor mode, cosmological correlators still carry the imprints of such "fossil" fields. Remarkably, fossil(s) of spin $\geq 1$ generate distinctive anisotropies in observables such as the tensor power spectrum. We show how this plays out in our set-up., Comment: 25 pages, 3 figures

We propose a new mechanism by which dark matter (DM) can affect the early universe. The hot interior of a macroscopic DM, or macro, can behave as a heat reservoir so that energetic photons are emitted from its surface. This results in spectral distortions (SDs) of the cosmic microwave background. The SDs depend on the density and the cooling processes of the interior, and the surface composition of the Macros. We use neutron stars as a model for nuclear-density Macros and find that the spectral distortions are mass-independent for fixed density. In our work, we find that, for Macros of this type that constitute 100$\%$ of the dark matter, the $\mu$ and $y$ distortions can be above detection threshold for typical proposed next-generation experiments such as PIXIE., Comment: 8 pages, 2 figures. Matches with PRD version

1I/'Oumuamua, formerly known as A/2017 U1, is a sizable body currently passing through the solar system. It is generally considered to be a rocky asteroid-like object that came from another planetary system in the Milky Way. We point out that 1I/'Oumuamua may instead be a chunk of dark matter, a "macro," possibly as massive as $10^{25}$g if it is of nuclear density. If so, then its passage will have caused measurable deviations in the orbits of Mercury, the Earth and Moon., Comment: 2 pages, 1 figure

We study the imprints of a massive spin-2 field on inflationary observables, and in particular on the breaking of consistency relations. In this setup, the minimal inflationary field content interacts with the massive spin-2 field through dRGT interactions, thus guaranteeing the absence of Boulware-Deser ghostly degrees of freedom. The unitarity requirement on spinning particles, known as Higuchi bound, plays a crucial role for the size of the observable signal., Comment: 24 pages, 6 figures

The kinetic Sunyaev Zel'dovich (kSZ) and polarized Sunyaev Zel'dovich (pSZ) effects are temperature and polarization anisotropies induced by the scattering of CMB photons from structure in the post-reionization Universe. In the case of the kSZ effect, small angular scale anisotropies in the optical depth are modulated by the cosmic microwave background (CMB) dipole field, i.e. the CMB dipole observed at each spacetime point, which is sourced by the primordial dipole and especially the local peculiar velocity. In the case of the pSZ effect, similar small-scale anisotropies are modulated by the CMB quadrupole field, which receives contributions from both scalar and tensor modes. Statistical anisotropies in the cross correlations of CMB temperature and polarization with tracers of the inhomogeneous distribution of electrons provide a means of isolating and reconstructing the dipole and quadrupole fields. In this paper, we present a set of unbiased minimum variance quadratic estimators for the reconstruction of the dipole and quadrupole fields, and forecast the ability of future CMB experiments and large scale structure surveys to perform this reconstruction. Consistent with previous work, we find that a high fidelity reconstruction of the dipole and quadrupole fields over a variety of scales is indeed possible, and demonstrate the sensitivity of the pSZ effect to primordial tensor modes. Using a principle component analysis, we estimate how many independent modes could be accessed in such a reconstruction. We also comment on a few first applications of a detection of the dipole and quadrupole fields, including a reconstruction of the primordial contribution to our locally observed CMB dipole, a test of statistical homogeneity on large scales from the first modes of the quadrupole field, and a reconstruction technique for the primordial potential on the largest scales., Comment: 24 pages, 8 figures, minor changes

Gravitinos are a fundamental prediction of supergravity, their mass ($m_{G}$) is informative of the value of the SUSY breaking scale, and, if produced during reheating, their number density is a function of the reheating temperature ($T_{\text{rh}}$). As a result, constraining their parameter space provides in turn significant constraints on particles physics and cosmology. We have previously shown that for gravitinos decaying into photons or charged particles during the ($\mu$ and $y$) distortion eras, upcoming CMB spectral distortions bounds are highly effective in constraining the $T_{\text{rh}}-m_{G}$ space. For heavier gravitinos (with lifetimes shorter than a few $\times10^6$ sec), distortions are quickly thermalized and energy injections cause a temperature rise for the CMB bath. If the decay occurs after neutrino decoupling, its overall effect is a suppression of the effective number of relativistic degrees of freedom ($N_{\text{eff}}$). In this paper, we utilize the observational bounds on $N_{\text{eff}}$ to constrain gravitino decays, and hence provide new constaints on gravitinos and reheating. For gravitino masses less than $\approx 10^5$ GeV, current observations give an upper limit on the reheating scale in the range of $\approx 5 \times 10^{10}- 5 \times 10^{11}$GeV. For masses greater than $\approx 4 \times 10^3$ GeV they are more stringent than previous bounds from BBN constraints, coming from photodissociation of deuterium, by almost 2 orders of magnitude., Comment: 6 pages, 1 figure

Anisotropies in distortions to the frequency spectrum of the cosmic microwave background (CMB) can be created through spatially varying heating processes in the early Universe. For instance, the dissipation of small-scale acoustic modes does create distortion anisotropies, in particular for non-Gaussian primordial perturbations. In this work, we derive approximations that allow describing the associated distortion field. We provide a systematic formulation of the problem using Fourier-space window functions, clarifying and generalizing previous approximations. Our expressions highlight the fact that the amplitudes of the spectral-distortion fluctuations induced by non-Gaussianity depend also on the homogeneous value of those distortions. Absolute measurements are thus required to obtain model-independent distortion constraints on primordial non-Gaussianity. We also include a simple description for the evolution of distortions through photon diffusion, showing that these corrections can usually be neglected. Our formulation provides a systematic framework for computing higher order correlation functions of distortions with CMB temperature anisotropies and can be extended to describe correlations with polarization anisotropies., Comment: 11 pages, 3 figures, accepted by MNRAS

Inspired by the chromo-natural inflation model of Adshead&Wyman, we reshape its scalar content to relax the tension with current observational bounds. Besides an inflaton, the setup includes a spectator sector in which an axion and SU(2) gauge fields are coupled via a Chern-Simons-type term. The result is a viable theory endowed with an alternative production mechanism for gravitational waves during inflation. The gravitational wave signal sourced by the spectator fields can be much larger than the contribution from standard vacuum fluctuations, it is distinguishable from the latter on the basis of its chirality and, depending on the theory parameters values, also its tilt. This production process breaks the well-known relation between the tensor-to-scalar ratio and the energy scale of inflation. As a result, even if the Hubble rate is itself too small for the vacuum to generate a tensor amplitude detectable by upcoming experiments, this model still supports observable gravitational waves., Comment: 21 pages, 8 figures

Probing correlations among short and long-wavelength cosmological fluctuations is known to be decisive for deepening the current understanding of inflation at the microphysical level. Spectral distortions of the CMB can be caused by dissipation of cosmological perturbations when they re-enter Hubble after inflation. Correlating spectral distortions with temperature anisotropies will thus provide the opportunity to greatly enlarge the range of scales over which squeezed limits can be tested, opening up a new window on inflation complementing the ones currently probed with CMB and LSS. In this paper we discuss a variety of inflationary mechanisms that can be efficiently constrained with distortion-temperature correlations. For some of these realizations (representative of large classes of models) we derive quantitative predictions for the squeezed limit bispectra, finding that their amplitudes are above the sensitivity limits of an experiment such as the proposed PIXIE., Comment: 29 pages, 9 figures

If local supersymmetry is the correct extension of the standard model of particle physics, then following Inflation the early universe would have been populated by gravitinos produced from scatterings in the hot plasma during reheating. Their abundance is directly related to the magnitude of the reheating temperature. The gravitino lifetime is fixed as a function of its mass, and for gravitinos with lifetimes longer than the age of the universe at redshift $z\simeq 2\times 10^{6}$ (or roughly $6\times 10^6{\rm s}$), decay products can produce spectral distortion of the cosmic microwave background. Currently available COBE/FIRAS limits on spectral distortion can, in certain cases, already be competitive with respect to cosmological constraints from primordial nucleosynthesis for some gravitino decay scenarios. We show how the sensitivity limits on $\mu$ and $y$ distortions that can be reached with current technology would improve constraints and possibly rule out a significant portion of the parameter space for gravitino masses and Inflation reheating temperatures., Comment: submitted to Phys. Rev D., 8 pages, 4 figs (1 new figure added, references updated)

Attention has focussed recently on models of inflation that involve a second or more fields with a mass near the inflationary Hubble parameter $H$, as may occur in supersymmetric theories if the supersymmetry-breaking scale is not far from $H$. Quasi-single-field (QsF) inflation is a relatively simple family of phenomenological models that serve as a proxy for theories with additional fields with masses $m\sim H$. Since QsF inflation involves fields in addition to the inflaton, the consistency conditions (ccs) between correlations that arise in single-clock inflation are not necessarily satisfied. As a result, correlation functions in the squeezed limit may be larger than in single-field inflation. Scalar non-Gaussianities mediated by the massive isocurvature field in QsF have been shown to be potentially observable. These are especially interesting since they would convey information about the mass of the isocurvature field. Here we consider non-Gaussian correlators involving tensor modes and their observational signatures. A physical correlation between a (long-wavelength) tensor mode and two scalar modes (tss), for instance, may give rise to local departures from statistical isotropy or, in other words, a non-trivial four-point function. The presence of the tensor mode may moreover be inferred geometrically from the shape dependence of the four-point function. We compute tss and stt (one soft curvature mode and two hard tensors) bispectra in QsF inflation, identifying the conditions necessary for these to "violate" the ccs. We find that while ccs are violated by stt correlations, they are preserved by the tss in the minimal QsF model. Our study of primordial correlators which include gravitons in seeking imprints of additional fields with masses $m\sim H$ during inflation can be seen as complementary to the recent "cosmological collider physics" proposal., Comment: 20 pages, 6 figures, references added and discussion in Section 4 extended

Many inflation models predict that primordial density perturbations have a nonzero three-point correlation function, or bispectrum in Fourier space. Of the several possibilities for this bispectrum, the most commmon is the local-model bispectrum, which can be described as a spatial modulation of the small-scale (large-wavenumber) power spectrum by long-wavelength density fluctuations. While the local model predicts this spatial modulation to be scale-independent, many variants have some scale-dependence. Here we note that this scale dependence can be probed with measurements of frequency-spectrum distortions in the cosmic microwave background (CMB), in particular highlighting Compton-$y$ distortions. Dissipation of primordial perturbations with wavenumbers $50\,{\rm Mpc}^{-1} \lesssim k \lesssim 10^4\,{\rm Mpc}^{-1}$ give rise to chemical-potential ($\mu$) distortions, while those with wavenumbers $1\,{\rm Mpc}^{-1} \lesssim k \lesssim 50\,{\rm Mpc}^{-1}$ give rise to Compton-$y$ distortions. With local-model non-Gaussianity, the distortions induced by this dissipation can be distinguished from those due to other sources via their cross-correlation with the CMB temperature $T$. We show that the relative strengths of the $\mu T$ and $yT$ correlations thus probe the scale-dependence of non-Gaussianity and estimate the magnitude of possible signals relative to sensitivities of future experiments. We discuss the complementarity of these measurements with other probes of squeezed-limit non-Gaussianity., Comment: 5 pages, 1 figure

Reheating is a transition era after the end of inflation, during which the inflaton is converted into the particles that populate the Universe at later times. No direct cosmological observables are normally traceable to this period of reheating. Indirect bounds can however be derived. One possibility is to consider cosmological evolution for observable CMB scales from the time of Hubble crossing to the present time. Depending upon the model, the duration and final temperature after reheating, as well as its equation of state, may be directly linked to inflationary observables. For single-field inflationary models, if we approximate reheating by a constant equation of state, one can derive relations between the reheating duration (or final temperature), its equation of state parameter, and the scalar power spectrum amplitude and spectral index. While this is a simple approximation, by restricting the equation of state to lie within a broad physically allowed range, one can in turn bracket an allowed range of $n_s$ and $r$ for these models. The added constraints can help break degeneracies between inflation models that otherwise overlap in their predictions for $n_s$ and $r$., Comment: 32 pages, 15 figures. Revised in response to comments on the original version, and in preparation for submission for publication. More references and a new figure were added

The most conventional mechanism for gravitational waves (gw) production during inflation is the amplification of vacuum metric fluctuations. In this case the gw production can be uniquely related to the inflationary expansion rate $H$. For example, a gw detection close to the present experimental limit (tensor-to-scalar ratio $r \sim 0.1$) would indicate an inflationary expansion rate close to $10^{14} \, {\rm GeV}$. This conclusion, however, would be invalid if the observed gw originated from a different source. We construct and study one of the possible covariant formulations of the mechanism suggested in [43], where a spectator field $\sigma$ with a sound speed $c_{s} \ll 1$ acts as a source for gw during inflation. In our formulation $\sigma$ is described by a so-called $P(X)$ Lagrangian and a non-minimal coupling to gravity. This field interacts only gravitationally with the inflaton, which has a standard action. We compute the amount of scalar and tensor density fluctuations produced by $\sigma$ and find that, in our realization, $r$ is not enhanced with respect to the standard result but it is strongly sensitive to $c_s$, thus breaking the direct $r \leftrightarrow H$ connection., Comment: 22 pages

Inflation models make specific predictions for a tensor-scalar-scalar three-point correlation, or bispectrum, between one gravitational-wave (tensor) mode and two density-perturbation (scalar) modes. This tensor-scalar-scalar correlation leads to a local power quadrupole, an apparent departure from statistical isotropy in our Universe, as well as characteristic four-point correlations in the current mass distribution in the Universe. So far, the predictions for these observables have been worked out only for single-clock models in which certain consistency conditions between the tensor-scalar-scalar correlation and tensor and scalar power spectra are satisfied. Here we review the requirements on inflation models for these consistency conditions to be satisfied. We then consider several examples of inflation models, such as non-attractor and solid inflation models, in which these conditions are put to the test. In solid inflation the simplest consistency conditions are already violated whilst in the non-attractor model we find that, contrary to the standard scenario, the tensor-scalar-scalar correlator probes directly relevant model-dependent information. We work out the predictions for observables in these models. For non-attractor inflation we find an apparent local quadrupolar departure from statistical isotropy in large-scale structure but that this power quadrupole decreases very rapidly at smaller scales. The consistency of the CMB quadrupole with statistical isotropy then constrains the distance scale that corresponds to the transition from the non-attractor to attractor phase of inflation to be larger than the currently observable horizon. Solid inflation predicts clustering fossils signatures in the current galaxy distribution that may be large enough to be detectable with forthcoming, and possibly even current, galaxy surveys., Comment: 28 pages, 3 figures, version matching the one published in JCAP plus additional typos fixed

Gauge-flation is a recently proposed model in which inflation is driven solely by a non-Abelian gauge field thanks to a specific higher order derivative operator. The nature of the operator is such that it does not introduce ghosts. We compute the cosmological scalar and tensor perturbations for this model, improving over an existing computation. We then confront these results with the Planck data. The model is characterized by the quantity \gamma = (g^2 Q^2)/H^2 (where g is the gauge coupling constant, Q the vector vev, and H the Hubble rate). For \gamma < 2, the scalar perturbations show a strong tachyonic instability. In the stable region, the scalar power spectrum n_s is too low at small \gamma, while the tensor-to-scalar ratio r is too high at large \gamma. No value of \gamma leads to acceptable values for n_s and r, and so the model is ruled out by the CMB data. The same behavior with \gamma was obtained in Chromo-natural inflation, a model in which inflation is driven by a pseudo-scalar coupled to a non-Abelian gauge field. When the pseudo-scalar can be integrated out, one recovers the model of Gauge-flation plus corrections. It was shown that this identification is very accurate at the background level, but differences emerged in the literature concerning the perturbations of the two models. On the contrary, our results show that the analogy between the two models continues to be accurate also at the perturbative level., Comment: 31 pages, 6 figures

We present a detailed study of the trispectrum of the curvature perturbation generated within a stable, well defined and predictive theory which comprises an inflationary phase. In this model the usual shift symmetry is enhanced up to the so-called Galileon symmetry. The appeal of this type of theories rests on being unitary and stable under quantum corrections. Furthermore, in the specific model under consideration here, these properties have been shown to approximately hold in realistic scenarios which account for curved spacetime and the coupling with gravity. In the literature, the analysis of the bispectrum of the curvature perturbation for this theory revealed non-Gaussian features which are shared by a number of inflationary models, including stable ones. It is therefore both timely and useful to investigate further and turn to observables such as the trispectrum. We find that, in a number of specific momenta configurations, the trispectrum shape-functions present strikingly different features as compared to, for example, the entire class of the so-called $P(X,\phi)$ inflationary models., Comment: 39 pages, 14 figures

We calculate the trispectrum of curvature perturbations for a model of inflation endowed with Galilean symmetry at the level of the fluctuations around an FRW background. Such a model has been shown to posses desirable properties such as unitarity (up to a certain scale) and non-renormalization of the leading operators, all of which point towards the reasonable assumption that a full theory whose fluctuations reproduce the one here might exist as well as be stable and predictive. The cubic curvature fluctuations of this model produce quite distinct signatures at the level of the bispectrum. Our analysis shows how this holds true at higher order in perturbations. We provide a detailed study of the trispectrum shape-functions in different configurations and a comparison with existent literature. Most notably, predictions markedly differ from their P(X,\phi) counterpart in the so called equilateral trispectrum configuration. The zoo of inflationary models characterized by somewhat distinctive predictions for higher order correlators is already quite populated; what makes this model more compelling resides in the above mentioned stability properties., Comment: 24 pages, 10 figures

We perform the complete stability study of the model of chromo-natural inflation (Adshead and Wyman '12), where, due to its coupling to a SU(2) vector, a pseudo-scalar inflaton chi slowly rolls on a steep potential. As a typical example, one can consider an axion with a sub-Planckian decay constant f. The phenomenology of the model was recently studied (Dimastrogiovanni, Fasiello, and Tolley '12) in the m_g >> H limit, where m_g is the mass of the fluctuations of the vector field, and H the Hubble rate. We show that the inflationary solution is stable for m_g > 2 H, while it otherwise experiences a strong instability due to scalar perturbations in the sub-horizon regime. The tensor perturbations are instead standard, and the vector ones remain perturbatively small. Depending on the parameters, this model can give a gravity wave signal that can be detected in ongoing or forthcoming CMB experiments. This detection can occur even if, during inflation, the inflaton spans an interval of size Delta chi = O (f) which is some orders of magnitude below the Planck scale, evading a well known bound that holds for a free inflaton (Lyth '97)., Comment: 15 pages, 4 figures. Revised study of tensor modes

Chromo-natural inflation is a novel model of inflation which relies on the existence of non-abelian gauge fields interacting with an axion. In its simplest realization, an SU(2) gauge field is assumed to begin inflation in a rotationally invariant VEV. The dynamics of the gauge fields significantly modifies the equations of motion for the axion, providing an additional damping term that supports slow-roll inflation, without the need to fine tune the axion decay constant. We demonstrate that in an appropriate slow-roll limit it is possible to integrate out the massive gauge field fluctuations whilst still maintaining the nontrivial modifications of the gauge field to the axion. In this slow-roll limit, chromo-natural inflation is exactly equivalent to a single scalar field effective theory with a non-minimal kinetic term, i.e. a P(X,\chi) model. This occurs through a precise analogue of the gelaton mechanism, whereby heavy fields can have unsuppressed effects on the light field dynamics without contradicting decoupling. The additional damping effect of the gauge fields can be completely captured by the non-minimal kinetic term of the single scalar field effective theory. We utilize the single scalar field effective theory to infer the power spectrum and non-gaussianities in chromo-natural inflation and confirm that the mass squared of all the gauge field fluctuations is sufficiently large and positive that they completely decouple during inflation. These results confirm that chromo-natural inflation is a viable, stable and compelling model for the generation of inflationary perturbations., Comment: 26 pages, references added, improved discussion of stability

We perform a thorough computation of the one-loop corrections from both scalar and tensor degrees of freedom to the power spectrum of curvature fluctuations for non-canonical Lagrangians in single-field inflation. We consider models characterized by a small sound speed c_{s}, which produce large non-Gaussianities. As expected, the corrections turn out to be inversely proportional to powers of c_{s}; evaluating their amplitudes it is then possible to derive some theoretical bounds on the sound speed by requesting the conditions necessary for perturbation theory to hold., Comment: 39 pages, 5 figures, version accepted for publication in JCAP, added references for sections 2 and 3

This thesis is centered on three main subjects within the theory of inflation and cosmological perturbations: loop corrections to the power spectrum of curvature fluctuations generated during inflation; evolution of cosmological fluctuations in anisotropic pre-inflationary cosmologies; statistical anisotropy and non-Gaussianity predictions of models of inflation populated with vector fields. Currently, what makes even more interesting the study of 2-nd and higher order corrections to cosmological correlation functions as well as the computation of higher-than-two order correlators, is the almost unprecedented chance to confront theories with new and increasingly accurate experimental data that will shed more light in the physics of the early Universe. In the context of loop calculations, we have computed the corrections arising from scalar-tensor interactions in models of single-field inflation (both for the standard slow-roll model and for models described by Lagrangians with non-canonical kinetic terms). In the context of anisotropic cosmologies, also motivated by the observation of "anomalies" in the Cosmic Microwave Background (CMB) fluctuations, we have computed the bispectrum and the trispectrum of the curvature fluctuations in inflationary models with SU(2) vector fields, analyzing the statistical anisotropy features of the correlators in these models; finally, we have studied cosmological perturbations for a Universe with a Bianchi type-I background metric, with an energy density dominated by a pressureless fluid and in the presence of a cosmological constant., Comment: Ph.D thesis

We present a review of vector field models of inflation and, in particular, of the statistical anisotropy and non-Gaussianity predictions of models with SU(2) vector multiplets. Non-Abelian gauge groups introduce a richer amount of predictions compared to the Abelian ones, mostly because of the presence of vector fields self-interactions. Primordial vector fields can violate isotropy leaving their imprint in the comoving curvature fluctuations zeta at late times. We provide the analytic expressions of the correlation functions of zeta up to fourth order and an analysis of their amplitudes and shapes. The statistical anisotropy signatures expected in these models are important and, potentially, the anisotropic contributions to the bispectrum and the trispectrum can overcome the isotropic parts., Comment: 31 pages,9 figures; Invited Review for the special issue "Testing the Gaussianity and Statistical Isotropy of the Universe" for Advances in Astronomy

Motivated by the interest in models of the early universe where statistical isotropy is broken and can be revealed in cosmological observations, we consider an SU(2) theory of gauge interactions in a single scalar field inflationary scenario. We calculate the trispectrum of curvature perturbations, as a natural follow up to a previous paper of ours, where we studied the bispectrum in the same kind of models. The choice of a non-Abelian set-up turns out to be very convenient: on one hand, gauge boson self-interactions can be very interesting being responsible for extra non-trivial terms (naturally absent in the Abelian case) appearing in the cosmological correlation functions; on the other hand, its results can be easily reduced to the U(1) case. As expected from the presence of the vector bosons, preferred spatial directions arise and the trispectrum reveals anisotropic signatures. We evaluate its amplitude tau_{NL}, which receives contributions both from scalar and vector fields, and verify that, in a large subset of its parameter space, the latter contributions can be larger than the former. We carry out a shape analysis of the trispectrum; in particular we discuss, with some examples, how the anisotropy parameters appearing in the analytic expression of the trispectrum can modulate its profile and we show that the amplitude of the anisotropic part of the trispectrum can be of the same order of magnitude as the isotropic part., Comment: 36 pages, 8 figures

We consider a primordial SU(2) vector multiplet during inflation in models where quantum fluctuations of vector fields are involved in producing the curvature perturbation. Recently, a lot of attention has been paid to models populated by vector fields, given the interesting possibility of generating some level of statistical anisotropy in the cosmological perturbations. The scenario we propose is strongly motivated by the fact that, for non-Abelian gauge fields, self-interactions are responsible for generating extra terms in the cosmological correlation functions, which are naturally absent in the Abelian case. We compute these extra contributions to the bispectrum of the curvature perturbation, using the delta N formula and the Schwinger-Keldysh formalism. The primordial violation of rotational invariance (due to the introduction of the SU(2) gauge multiplet) leaves its imprint on the correlation functions introducing, as expected, some degree of statistical anisotropy in our results. We calculate the non-Gaussianity parameter f_{NL}, proving that the new contributions derived from gauge bosons self-interactions can be important, and in some cases the dominat ones. We study the shape of the bispectrum and we find that it turns out to peak in the local configuration, with an amplitude that is modulated by the preferred directions that break statistical isotropy., Comment: 31 pages, 2 figures. This version matches the one in press by JCAP

We compute one-loop corrections to the power spectrum of the curvature perturbation in single-field slow-roll inflation arising from gravitons and inflaton interactions. The quantum corrections due to gravitons to the power spectrum of the inflaton field are computed around the time of horizon crossing and their effect on the curvature perturbation is obtained on superhorizon scales through the delta-N formalism. We point out that one-loop corrections from the tensor modes are of the same magnitude as those coming from scalar self-interactions, therefore they cannot be neglected in a self-consistent calculation., Comment: LateX file; 29 pages; typo corrected; comments added to Sec. 3 and 4. Matches published version in JCAP