3,658 research outputs found

### UV physics and the speed of sound during inflation

We consider inflation as an effective field theory and study the effects of
the addition to the Lagrangian of irrelevant operators with higher powers of
first derivatives on its dynamics and observables. We find that significant
deviations from the two-derivative dynamics are possible within the regime of
validity of the effective field theory. Focusing on monomial potentials we show
that the main effect of the terms under consideration is to reduce the speed of
sound thereby reducing the tensor fraction, while having little impact on the
scalar tilt. Crucially, these effects can arise even when the UV cut-off is
well above the inflationary Hubble parameter

### Inflation with a graceful exit in a random landscape

We develop a stochastic description of small-field inflationary histories
with a graceful exit in a random potential whose Hessian is a Gaussian random
matrix as a model of the unstructured part of the string landscape. The
dynamical evolution in such a random potential from a small-field inflation
region towards a viable late-time de Sitter (dS) minimum maps to the dynamics
of Dyson Brownian motion describing the relaxation of non-equilibrium
eigenvalue spectra in random matrix theory. We analytically compute the
relaxation probability in a saddle point approximation of the partition
function of the eigenvalue distribution of the Wigner ensemble describing the
mass matrices of the critical points. When applied to small-field inflation in
the landscape, this leads to an exponentially strong bias against small-field
ranges and an upper bound $N\ll 10$ on the number of light fields $N$
participating during inflation from the non-observation of negative spatial
curvature.Comment: Published versio

### Towards Natural Inflation in String Theory

We provide type IIB string embeddings of two axion variants of natural
inflation. We use a combination of RR 2 form axions as the inflaton field and
have its potential generated by non perturbative effects in the superpotential.
Besides giving rise to inflation, the models developed take into account the
stabilization of the compact space, both in the KKLT and large volume scenario
regimes, an essential condition for any semi-realistic model of string
inflation.Comment: LaTeX, 1+26 pages, 2 figures; v2: added refs; v3: corrected a LaTeX
problem, where v2 refs were compiled with v1 tex fil

### A geometrical instability for ultra-light fields during inflation?

Westudythebehaviourofisocurvatureperturbationsinnon-linearsigmamodels which
naturally emerge in supergravity and string inflationary scenarios. We focus on
the case of negatively curved field manifolds which can potentially lead to a
geometrical destabilisation of isocurvature modes. We find however that heavy
fields are stable when their effective mass is computed on the attractor
background solution. On the other hand, we show that ultra-light fields can
potentially suffer from a geometrical instability when the background
trajectory is a geodesic in field space. In this case a full understanding of
the system is likely to require the use of non-perturbative methods.Comment: Matches published version, 5 pages, 2 figure

### Starobinsky-Type Inflation from $\alpha'$-Corrections

Working in the Large Volume Scenario (LVS) of IIB Calabi-Yau flux
compactifications, we construct inflationary models from recently computed
higher derivative $(\alpha')^3$-corrections. Inflation is driven by a Kaehler
modulus whose potential arises from the aforementioned corrections, while we
use the inclusion of string loop effects just to ensure the existence of a
graceful exit when necessary. The effective inflaton potential takes a
Starobinsky-type form $V=V_0(1-e^{-\nu\phi})^2$, where we obtain one set-up
with $\nu=-1/\sqrt{3}$ and one with $\nu=2/\sqrt{3}$ corresponding to inflation
occurring for increasing or decreasing $\phi$ respectively. The inflationary
observables are thus in perfect agreement with PLANCK, while the two scenarios
remain observationally distinguishable via slightly varying predictions for the
tensor-to-scalar ratio $r$. Both set-ups yield $r\simeq (2\ldots 7)\,\times
10^{-3}$. They hence realise inflation with moderately large fields
$\left(\Delta\phi\sim 6\thinspace M_{Pl}\right)$ without saturating the Lyth
bound. Control over higher corrections relies in part on tuning underlying
microscopic parameters, and in part on intrinsic suppressions. The intrinsic
part of control arises as a leftover from an approximate effective shift
symmetry at parametrically large volume.Comment: 29 pages, 6 figures; v2: clarifications and refs adde

### Just enough inflation: power spectrum modifications at large scales

We show that models of `just enough' inflation, where the slow-roll evolution
lasted only $50-60$ e-foldings, feature modifications of the CMB power spectrum
at large angular scales. We perform a systematic and model-independent analysis
of any possible non-slow-roll background evolution prior to the final stage of
slow-roll inflation. We find a high degree of universality since most common
backgrounds like fast-roll evolution, matter or radiation-dominance give rise
to a power loss at large angular scales and a peak together with an oscillatory
behaviour at scales around the value of the Hubble parameter at the beginning
of slow-roll inflation. Depending on the value of the equation of state
parameter, different pre-inflationary epochs lead instead to an enhancement of
power at low-$\ell$, and so seem disfavoured by recent observational hints for
a lack of CMB power at $\ell\lesssim 40$. We also comment on the importance of
initial conditions and the possibility to have multiple pre-inflationary
stages.Comment: 31 pages, 13 figure

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