arXiv subjects

Gianmassimo Tasinato

Publications and source records attributed to Gianmassimo Tasinato.

At least 127 records · Page 7Linked to original sources

arXiv2013

The role of vector fields in modified gravity scenarios

Gravitational vector degrees of freedom typically arise in many examples of modified gravity models. We start to systematically explore their role in these scenarios, studying the effects of coupling gravitational vector and scalar degrees of freedom. We focus on set-ups that enjoy a Galilean symmetry in the scalar sector and an Abelian gauge symmetry in the vector sector. These symmetries, together with the requirement that the equations of motion contain at most two space-time derivatives, only allow for a small number of operators in the Lagrangian for the gravitational fields. We investigate the role of gravitational vector fields for two broad classes of phenomena that characterize modified gravity scenarios. The first is self-acceleration: we analyze in general terms the behavior of vector fluctuations around self-accelerating solutions, and show that vanishing kinetic terms of vector fluctuations lead to instabilities on cosmological backgrounds. The second phenomenon is the screening of long range fifth forces by means of Vainshtein mechanism. We show that if gravitational vector fields are appropriately coupled to a spherically symmetric source, they can play an important role for defining the features of the background solution and the scale of the Vainshtein radius. Our general results can be applied to any concrete model of modified gravity, whose low-energy vector and scalar degrees of freedom satisfy the symmetry requirements that we impose.

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arXiv2013

Stability of the Self-accelerating Universe in Massive Gravity

We study linear perturbations around time dependent spherically symmetric solutions in the Lambda_3 massive gravity theory, which self-accelerate in the vacuum. We find that the dynamics of the scalar perturbations depend on the coordinate choice for the background solutions. For particular choices of coordinates there is a symmetry enhancement, leaving no propagating scalar degrees of freedom at linear order in perturbations. In contrast, any other coordinate choice propagates a single scalar mode. We find that the Hamiltonian of this scalar mode is unbounded from below for all self-accelerating solutions, signalling an instability.

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arXiv2013

Effective theory for the Vainshtein mechanism from the Horndeski action

Starting from the general Horndeski action, we derive the most general effective theory for scalar perturbations around flat space that allows us to screen fifth forces via the Vainshtein mechanism. The effective theory is described by a generalization of the Galileon Lagrangian, which we use to study the stability of spherically symmetric configurations exhibiting the Vainshtein effect. In particular, we discuss the phenomenological consequences of a scalar-tensor coupling that is absent in the standard Galileon Lagrangian. This coupling controls the superluminality and stability of fluctuations inside the Vainshtein radius in a way that depends on the density profile of a matter source. Particularly we find that the vacuum solution is unstable due to this coupling.

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arXiv2013

Implications of the Planck bispectrum constraints for the primordial trispectrum

The new Planck constraints on the local bispectrum parameter fnl are about 10^5 times tighter than the current constraints on the trispectrum parameter gnl, which means that the allowed numerical values of the second and third order terms in the perturbative expansion of the curvature perturbation are comparable. We show that a consequence of this is that if gnl is large enough to be detectable, then it will induce a large variation between the observable value of fnl and its value in a larger inflated volume. Even if there were only a few extra efoldings between the beginning of inflation and horizon crossing of our Hubble horizon, an observably large gnl means that fnl is unlikely to be as small as its current constraint, regardless of its true background value. This result is very general, it holds regardless of how many fields contributed to the curvature perturbation. We also generalise this result to other shapes of non-Gaussianity, beyond the local model. We show that the variance of the 3-point function in the squeezed limit is bounded from below by the square of the squeezed limit of the 4-point function.

astro-ph.CO↗

arXiv2013

Mixed non-Gaussianity in multiple-DBI inflation

We study a model of multiple-field DBI inflation leading to mixed form of primordial non-Gaussianity, including equilateral and local bispectrum shapes. We present a general formalism based on the Hamilton-Jacobi approach, allowing us to go beyond slow-roll, combining the three-point function for the fields at Hubble-exit with the non-linear evolution of super-Hubble scales. We are able to obtain analytic results by taking a separable Ansatz for the Hubble rate. We find general expressions for both the equilateral and local type non-Gaussianity parameter fNL. The equilateral non-Gaussianity includes the usual enhancement for small sound speeds, but multiplied by an analytic factor which can lead to a suppression. We illustrate our results with two scenarios. In the first model, previously found to have detectable local non-Gaussianity, we find that the equilateral signal is not sufficiently suppressed to evade current observational bounds. In our second scenario we construct a model which exhibits both a detectable equilateral fNL and a negative local fNL.

astro-ph.CO↗

arXiv2013

Exact Solutions in Massive Gravity

Massive gravity is a good theoretical laboratory to study modifications of General Relativity. The theory offers a concrete set-up to study models of dark energy, since it admits cosmological self-accelerating solutions in the vacuum, in which the size of the acceleration depends on the graviton mass. Moreover, non-linear gravitational self-interactions, in the proximity of a matter source, manage to mimic the predictions of linearised General Relativity, hence agreeing with solar-system precision measurements. In this article, we review our work in the subject, classifying, on one hand, static solutions, and on the other hand, self-accelerating backgrounds. For what respects static solutions we exhibit black hole configurations, together with other solutions that recover General Relativity near a source via the Vainshtein mechanism. For the self-accelerating solutions we describe a wide class of cosmological backgrounds, including an analysis of their stability.

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arXiv2013

Loop corrections and a new test of inflation

Inflation is the leading paradigm for explaining the origin of primordial density perturbations and the observed temperature fluctuations of the cosmic microwave background. However many open questions remain, in particular whether one or more scalar fields were present during inflation and how they contributed to the primordial density perturbation. We propose a new observational test of whether multiple fields, or only one (not necessarily the inflaton) generated the perturbations. We show that our test, relating the bispectrum and trispectrum, is protected against loop corrections at all orders, unlike previous relations.

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arXiv2013

A non-Gaussian landscape

Primordial perturbations with wavelengths greater than the observable universe shift the effective background fields in our observable patch from their global averages over the inflating space. This leads to a landscape picture where the properties of our observable patch depend on its location and may significantly differ from the expectation values predicted by the underlying fundamental inflationary model. We show that if multiple fields are present during inflation, this may happen even if our horizon exit would be preceded by only a few e-foldings of inflation. Non-Gaussian statistics are especially affected: for example models of local non-Gaussianity predicting |f_NL|>> 10 over the entire inflating volume can have a probability up to a few tens of percent to generate a non-detectable bispectrum in our observable patch |fNL^{obs.}|<10. In this work we establish systematic connections between the observable local properties of primordial perturbations and the global properties of the inflating space which reflect the underlying high energy physics. We study in detail the implications of both a detection and non-detection of primordial non-Gaussianity by Planck, and discover novel ways of characterising the naturalness of different observational configurations.

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arXiv2012

Slow-walking inflation

We propose a new model of slow-roll inflation in string cosmology, based on warped throat supergravity solutions displaying `walking' dynamics, i.e. the coupling constant of the dual gauge theory slowly varies over a range of energy scales. The features of the throat geometry are sourced by a rich field content, given by the dilaton and RR and NS fluxes. By considering the motion of a D3-brane probe in this geometry, we are able to analytically calculate the brane potential in a physically interesting regime. This potential has an inflection point: in its proximity we realize a model of inflation lasting sixty e-foldings, and whose robust predictions are in agreement with current observations. We are also able to interpret some of the most interesting aspects of this scenario in terms of the properties of the QFT dual theory.

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arXiv2012

Vector instabilities and self-acceleration in the decoupling limit of massive gravity

We investigate vector contributions to the Lagrangian of $Λ_3-$massive gravity in the decoupling limit, the less explored sector of this theory. The main purpose is to understand the stability of maximally symmetric %self-accelerating vacuum solutions. Around self-accelerating configurations, vector degrees of freedom become strongly coupled since their kinetic terms vanish, so their dynamics is controlled by higher order interactions. Even in the decoupling limit, the vector Lagrangian contains an infinite number of terms. We develop a systematic method to covariantly determine the vector Lagrangian at each order in perturbations, fully manifesting the symmetries of the system. We show that, around self-accelerating solutions, the structure of higher order $p$-form Galileons arise, avoiding the emergence of a sixth BD ghost mode. However, a careful analysis shows that there are directions along which the Hamiltonian is unbounded from below. This instability can be interpreted as one of the available fifth physical modes behaving as a ghost. Therefore, we conclude that self-accelerating configurations, in the decoupling limit of $Λ_3$-massive gravity, are generically unstable.

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arXiv2012

Inhomogeneous non-Gaussianity

We propose a method to probe higher-order correlators of the primordial density field through the inhomogeneity of local non-Gaussian parameters, such as f_NL, measured within smaller patches of the sky. Correlators between n-point functions measured in one patch of the sky and k-point functions measured in another patch depend upon the (n+k)-point functions over the entire sky. The inhomogeneity of non-Gaussian parameters may be a feasible way to detect or constrain higher-order correlators in local models of non-Gaussianity, as well as to distinguish between single and multiple-source scenarios for generating the primordial density perturbation, and more generally to probe the details of inflationary physics.

astro-ph.CO↗

arXiv2012

Local non-Gaussianity from rapidly varying sound speeds

We study the effect of non-trivial sound speeds on local-type non-Gaussianity during multiple-field inflation. To this end, we consider a model of multiple-field DBI and use the deltaN formalism to track the super-horizon evolution of perturbations. By adopting a sum separable Hubble parameter we derive analytic expressions for the relevant quantities in the two-field case, valid beyond slow variation. We find that non-trivial sound speeds can, in principle, curve the trajectory in such a way that significant local-type non-Gaussianity is produced. Deviations from slow variation, such as rapidly varying sound speeds, enhance this effect. To illustrate our results we consider two-field inflation in the tip regions of two warped throats and find large local-type non-Gaussianity produced towards the end of the inflationary process.

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arXiv2012

Lifshitz black holes in IIA supergravity

We compute string theoretic black hole solutions having Lifshitz asymptotics with a general dynamical exponent z>1. We start by constructing solutions in a flux compactification of six dimensional supergravity, then uplift them to massive type IIA supergravity. Alongside the Lifshitz black holes we study the simpler anti-de Sitter solutions, of which there are a 1-parameter family in this supergravity, and compare and contrast their properties. The black holes are characterized by a two-form and scalar charge, and we numerically explore their configuration space and thermodynamical aspects.

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arXiv2012

New symmetries in Fierz-Pauli massive gravity

We expose a new symmetry for linear perturbations around a solution of non-linear Fierz-Pauli massive gravity plus a bare cosmological constant. The cosmological constant is chosen such that the background metric is flat while the Stuckelberg fields have a non-trivial profile. Around this background, at linear order the new symmetry reduces the propagating degrees of freedom to those of General Relativity, namely the massless helicity 2 modes only. We discuss the physical consequences and possible applications of these findings.

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arXiv2012

Natural Quintessence in String Theory

We introduce a natural model of quintessence in string theory where the light rolling scalar is radiatively stable and couples to Standard Model matter with weaker-than- Planckian strength. The model is embedded in an anisotropic type IIB compactification with two exponentially large extra dimensions and TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the scale of the gravitino mass is of the order of the observed value of the cosmological constant. The quintessence field is a modulus parameterising the size of an internal four-cycle which naturally develops a potential of the order (gravitino mass)^4, leading to a small dark energy scale without tunings. The mass of the quintessence field is also radiatively stable since it is protected by supersymmetry in the bulk. Moreover, this light scalar couples to ordinary matter via its mixing with the volume mode. Due to the fact that the quintessence field is a flat direction at leading order, this mixing is very small, resulting in a suppressed coupling to Standard Model particles which avoids stringent fifth-force constraints. On the other hand, if dark matter is realised in terms of Kaluza-Klein states, unsuppressed couplings between dark energy and dark matter can emerge, leading to a scenario of coupled quintessence within string theory. We study the dynamics of quintessence in our set-up, showing that its main features make it compatible with observations.

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arXiv2011

Non-Gaussianity beyond slow roll in multi-field inflation

We study the non-Gaussianity generated during multiple-field inflation. We provide an exact expression for the bispectrum parameter f_NL which is valid beyond the slow-roll regime, valid for certain classes of inflationary models. We then study a new, exact multi-field inflationary model considering a case where the bispectrum grows to observable values at the end of inflation. We show that in this case the trispectrum is also large and may even provide the dominant signal of non-Gaussianity.

astro-ph.CO↗

arXiv2011

Strong interactions and exact solutions in non-linear massive gravity

We investigate strong coupling effects in a covariant massive gravity model, which is a candidate for a ghost free non-linear completion of Fierz-Pauli. We analyse the conditions to recover general relativity via Vainshtein mechanism in the weak field limit, and find three main cases depending on the choice of parameters. In the first case, the potential is such that all non-linearities disappear and the vDVZ discontinuity cannot be avoided. In the second case, the Vainshtein mechanism allows to recover general relativity within a macroscopic radius from a source. In the last case, the strong coupling of the scalar graviton completely shields the massless graviton, and weakens gravity when approaching the source. In the second part of the paper, we explore new exact vacuum solutions, that asymptote de Sitter or anti de Sitter space depending on the parameter choice. The curvature of the space is proportional to the mass of the graviton, thus providing a cosmological background which may explain the present day acceleration in terms of the graviton mass. Moreover, by expressing the potential for non-linear massive gravity in a convenient form, we also suggest possible connections with a higher dimensional framework.

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arXiv2011

Poly-instanton Inflation

We propose a new inflationary scenario in type IIB Calabi-Yau compactifications, where the inflaton is a Kähler modulus parameterising the volume of an internal four-cycle. The inflaton potential is generated via poly-instanton corrections to the superpotential which give rise to a naturally flat direction due to their double exponential suppression. Given that the volume mode is kept stable during inflation, all the inflaton-dependent higher dimensional operators are suppressed. Moreover, string loop effects can be shown to be negligible throughout all the inflationary dynamics for natural values of the underlying parameters. The model is characterised by a reheating temperature of the order $T_{\rm rh}\simeq 10^6$ GeV which requires $N_e \simeq 54$ e-foldings of inflation. All the inflationary observables are compatible with current observations since the spectral index is $n_s \simeq 0.96$, while the tensor-to-scalar ratio is $r\simeq 10^{-5}$. The volume of the Calabi-Yau is of order $10^3$ in string units, corresponding to an inflationary scale around $10^{15}$ GeV

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