[en] Ultra-slow-roll (USR) inflation is a new mode of inflation which corresponds to the occasions when the inflaton field must traverse an extremely flat part of the scalar potential, when the usual slow-roll (SR) fails. We investigate USR and obtain an estimate for how long it lasts, given the initial kinetic density of the inflaton. We also find that, if the initial kinetic density is small enough, USR can be avoided and the usual SR treatment is valid. This has important implications for inflection-point inflation.

[en] We present supergravity realizations of chromo-natural inflationary models. We show that by using superpotentials with “imaginary” holomorphic functions of the inflaton one can obtain effective theories of inflation that are also consistent truncations of the original supergravity models.

[en] We revisit the renormalizable polynomial inflection point model of inflation, focusing on the small field scenario which can be treated fully analytically. In particular, the running of the spectral index is predicted to be α = -1.43 × 10^-3 +5.56 × 10^-5(N_CMB-65 ), which might be tested in future. We also analyze reheating through perturbative inflaton decays to either fermionic or bosonic final states via a trilinear coupling. The lower bound on the reheating temperature from successful Big Bang nucleosynthesis gives lower bounds for these couplings; on the other hand radiative stability of the inflaton potential leads to upper bounds. In combination this leads to a lower bound on the location ϕ₀ of the near inflection point, ϕ₀ > 3 · 10^-5 in Planckian units. The Hubble parameter during inflation can be as low as H_inf∼ 1 MeV, or as high as ∼ 10¹⁰ GeV. Similarly, the reheating temperature can lie between its lower bound of ∼ 4 MeV and about 4 · 10⁸ (10¹¹) GeV for fermionic (bosonic) inflaton decays. We finally speculate on the “prehistory” of the universe in this scenario, which might have included an epoch of eternal inflation. (paper)

[en] We address the problem of determining inflationary characteristics in a model independent way. We start from a recently proposed equation which allows to accurately calculate the value of the inflaton at horizon crossing ${\mathrm{\varphi <!--\; ??\; -->}}_k$. We then use an equivalent form of this equation to write a formula that relates the number of e-folds from horizon crossing to the pivot scale $N_{ke}+N_{ep}$ with the tensor-to-scalar index $r$, hence a general bound for $N_{ke}+N_{ep}$ follows. $N_{ke}$ is the number of e-folds from the scale factor $a_k$ during inflation to the end of inflation at $a_e$ and $N_{ep}$ is the number of e-folds from $a_e$ to the pivot scale factor $a_p$. In particular, at present $r<0.063$ implies $N_{ke}+N_{ep}<112.5$ e-folds at $k=k_p$ and 128.1 e-folds at the present scale with wavenumber mode $k_0$. We also give a lower bound to the size of the universe during the inflationary epoch that gave rise to the current observable universe. We also discussed the problem of degeneracy of inflationary models and argue that this degeneration can only be resolved by studying model predictions from the reheating epoch.

[en] We re-analyse current single-field inflationary models related to primordial black holes formation. We do so by taking into account recent developments on the estimations of their abundances and the influence of non-gaussianities. We show that, for all of them, the gaussian approximation, which is typically used to estimate the primordial black holes abundances, fails. However, in the case in which the inflaton potential has an inflection point, the contribution of non-gaussianities is only perturbative. Finally, we infer that only models featuring an inflection point in the inflationary potential, might predict, with a very good approximation, the desired abundances by the sole use of the gaussian statistics.

[en] A reformulation of inflationary model analyses appeared recently, in which inflationary observables are determined by the structure of a pole in the inflaton kinetic term rather than the shape of the inflaton potential. We comprehensively study this framework with an arbitrary order of the pole taking into account possible additional poles in the kinetic term or in the potential. Depending on the setup, the canonical potential becomes the form of hilltop or plateau models, variants of natural inflation, power-law inflation, or monomial/polynomial chaotic inflation. We demonstrate attractor behaviors of these models and compute corrections from the additional poles to the inflationary observables.

[en] If the spatial sections of the Universe are positively curved, extrapolating the inflationary stage backward in time inevitably leads to a classical bounce. This simple scenario, non-singular and free of exotic physics, deserves to be investigated in details. The background dynamics exhibits interesting features and is shown to be mostly insensitive to initial conditions as long as observational consequences are considered. The primordial scalar power spectrum is explicitly computed, for different inflaton potentials, and the subsequent CMB temperature anisotropies are calculated. The results are compatible with current measurements. Some deviations with respect to the standard paradigm can however appear at large scales and we carefully disentangle what is associated with the vacuum choice with what is more fundamentally due to the bounce itself.

[en] Recently a new inflationary scenario was proposed in [1] which can be applicable to an inflaton having multiple vacua. In this letter, we consider a more general situation where the inflaton potential has a (UV) saddle point around the Planck scale. This class of models can be regarded as a natural generalization of the hillclimbing Higgs inflation [2].

[en] We compute for general single-field inflation the intrinsic non-Gaussianity due to the self-interactions of the inflaton field in the squeezed limit. We recover the consistency relation in the context of the δN formalism, and argue that there is a particular field redefinition that makes the intrinsic non-Gaussianity vanishing, thus improving the estimate of the local non-Gaussianity using the δN formalism.

[en] In the model of the inflaton nonminimal coupling to the Gauss–Bonnet term, we discuss the constant-roll inflation with constant ϵ${}_1$, constant ϵ${}_2$ and constant η${}_H$, respectively, with the additional assumption that δ${}_1$ is a constant. Using the Bessel function approximation, we get the analytical expressions for the scalar and tensor power spectrum and derive the scalar spectral index n${}_R$ and the tensor to scalar ratio r to the first order of ϵ${}_1$. By using the Planck 2018 observations constraint on n${}_R$ and r, we obtain some feasible parameter space and show the result on the n${}_R$−r region. The scalar potential is also reconstructed in some special cases.