Vector curvaton with varying kinetic function

Abstract

A new model realisation of the vector curvaton paradigm is presented and analysed. The model consists of a single massive Abelian vector field, with a Maxwell type kinetic term. By assuming that the kinetic function and the mass of the vector field are appropriately varying during inflation, it is shown that a scale invariant spectrum of superhorizon perturbations can be generated. These perturbations can contribute to the curvature perturbation of the Universe. If the vector field remains light at the end of inflation it is found that it can generate substantial statistical anisotropy in the spectrum and bispectrum of the curvature perturbation. In this case the non-Gaussianity in the curvature perturbation is predominantly anisotropic, which will be a testable prediction in the near future. If, on the other hand, the vector field is heavy at the end of inflation then it is demonstrated that particle production is approximately isotropic and the vector field alone can give rise to the curvature perturbation, without directly involving any fundamental scalar field. The parameter space for both possibilities is shown to be substantial. Finally, toy-models are presented which show that the desired variation of the mass and kinetic function of the vector field can be realistically obtained, without unnatural tunings, in the context of supergravity or superstrings.

Anisotropy in solid inflation

“…As we already showed, the solid that drives inflation needs to be extremely inefficient in responding to changes in the volume, and to the anisotropy. 9 We stress that the prolonged anisotropy is not consequence oḟ c T = O ( H). Even ifċ T = O (H), the exponent λ 2 = O ( ), which guarantees a slow isotropization.…”

Three-form cosmology

“…Only recently the possibility of significance of higher spin fields has begun to attract some modest interest. Vector inflation [3], using either time-like [4] or space-like [5] components, or featuring the vector as a curvaton [6] has been taken under consideration [7] and the stability of the vector models has been analyzed in details [8]. Vector dark energy [9] could allow alleviation of the coincidence problem [10] and even an electromagnetic origin of the cosmological term [11].…”

Anisotropic trispectrum of curvature perturbations induced by primordial non-Abelian vector fields

“…(5.18) through (5.23) containing 3 Notice that this procedure provides an approximate result, but it is certainly allowed since, when working in the equilateral configuration (as we do when we evaluate the amplitude of the trispectrum), it is easy to realize that the integrals " R dη ′ R dη ′′ " are independent of the specific set of operator contractions, which only differ by the T αβγδσ ijkl and by the Levi-Civita coefficients. 4 As another example, in models with varying kinetic function and mass [63,64], we have verified that n(x) ≫ 1 at late times and for a vector field that is light until the end of inflation.…”

“…The physical validity of this model was questioned in [65][66][67][68]. However these instabilities do not represent an issue in some models with a varying kinetic function where f (and eventually the effective mass) is instead not a constant [45,63,64].…”

“…Also, let us assume that n(x) is a smooth function: this is reasonable, based on requirement that the longitudinal mode asymptotically scales as the transverse mode at late times so as to produce a scale invariant spectrum. For example this happens in the models with the transverse mode given by (4.14) with m 0 ≃ 0 [13] , or in models with varying kinetic function and mass [63,64]. 2 It then appears to be a good approximation evaluating n(x) at the horizon crossing time x * and taking it out of the integrals in eq.…”