Ido Ben-Dayan

Cosmic microwave background observables of small field models of inflation

We construct a class of single small field models of inflation that can predict, contrary to popular wisdom, an observable gravitational wave signal in the cosmic microwave background anisotropies. The spectral index, its running, the tensor to scalar ratio and the number of e-folds can cover all the parameter space currently allowed by cosmological observations. A unique feature of models in this class is their ability to predict a negative spectral index running in accordance with recent cosmic microwave background observations. We discuss the new class of models from an effective field theory perspective and show that if the dimensionless trilinear coupling is small, as required for consistency, then the observed spectral index running implies a high scale of inflation and hence an observable gravitational wave signal. All the models share a distinct prediction of higher power at smaller scales, making them easy targets for detection.

Hierarchical axion inflation.

We propose a new field theory mechanism for generating an effective trans-Planckian decay constant from sub-Planckian ones. Using the minimal two axions and a hierarchy between two axion decay constants is sufficient for realizing inflation through nonperturbative effects only and with minimal tuning. The inflationary motion is kept entirely within a sub-Planckian domain. We outline possible strategies of embedding the model in a string theory setup.

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.

Models of modular inflation and their phenomenological consequences

We study models of modular inflation of the form expected to arise from low energy effective actions of superstring theories. We argue on general grounds that the most likely models are small field models in which the inflaton moves about a Planck distance from an extremum of the potential. We then explain the generic difficulties in designing small field models of supergravity modular inflation. We show that if the Kahler potential of the inflaton is logarithmic as in perturbative string theories, then it is not possible to satisfy the slow-roll conditions for any superpotential. We find that if the corrections to the Kahler potential are large enough that it can be approximated by a canonical Kahler potential in the vicinity of the extremum, then viable slow-roll inflation is possible and we give a prescription for designing such models. In this case, several parameters have to be tuned to a fraction of a per cent. Generic models of this class predict a red spectrum of scalar perturbations and negligible spectral index running. They also predict a characteristic suppression of tensor perturbations despite the high scale of inflation. Consequently, a detection of primordial tensor anisotropies or spectral index running in cosmic microwave background observations in the foreseeable future will rule out this entire class of modular inflation models.

R{sup 2}log R quantum corrections and the inflationary observables

We study a model of inflation with terms quadratic and logarithmic in the Ricci scalar, where the gravitational action is f(R)=R+α R2+β R2 ln R. These terms are expected to arise from one loop corrections involving matter fields in curved space-time. The spectral index ns and the tensor to scalar ratio yield 4 × 10-4 r0.03 and 0.94 ns 0.99. i.e. r is an order of magnitude bigger or smaller than the original Starobinsky model which predicted r~ 10-3. Further enhancement of r gives a scale invariant ns~ 1 or higher. Other inflationary observables are d ns/dln k -5.2 × 10-4, μ 2.1 × 10-8 , y 2.6 × 10-9. Despite the enhancement in r, if the recent BICEP2 measurement stands, this model is disfavoured.

Supergravity Higgs Inflation and Shift Symmetry in Electroweak Theory

We present a model of inflation in a supergravity framework in the Einstein frame where the Higgs field of the next to minimal supersymmetric standard model (NMSSM) plays the role of the inflaton. Previous attempts which assumed non-minimal coupling to gravity failed due to a tachyonic instability of the singlet field during inflation. A canonical Kahler potential with minimal coupling to gravity can resolve the tachyonic instability but runs into the η-problem. We suggest a model which is free of the η-problem due to an additional coupling in the Kahler potential which is allowed by the Standard Model gauge group. This induces directions in the potential which we call K-flat. For a certain value of the new coupling in the (N)MSSM, the Kahler potential is special, because it can be associated with a certain shift symmetry for the Higgs doublets, a generalization of the shift symmetry for singlets in earlier models. We find that K-flat direction has Hu0 = −Hd0*. This shift symmetry is broken by interactions coming from the superpotential and gauge fields. This flat direction fails to produce successful inflation in the MSSM but yields a more interesting model in the NMSSM, even though it does not pass existing cosmological constraints. We point out that, in building more sophisticated models of this type, one may also need to take into account their implications for axion searches or other elementary particle constraints.

Constraining the primordial power spectrum from SNIa lensing dispersion

The (absence of detecting) lensing dispersion of Supernovae type Ia (SNIa) can be used as a novel and extremely efficient probe of cosmology. In this preliminary example we analyze its consequences for the primordial power spectrum. The main setback is the knowledge of the power spectrum in the non-linear regime, 1 Mpc^{-1} < k < 10^2-10^3 Mpc^{-1} up to redshift of about unity. By using the lensing dispersion and conservative estimates in this regime of wavenumbers, we show how the current upper bound sigma_mu(z=1) < 0.12on existing data gives strong indirect constraints on the primordial power spectrum. The probe extends our handle on the spectrum to a total of 12-15 inflation e-folds. These constraints are so strong that they are already ruling out a large portion of the parameter space allowed by PLANCK for running alpha = d n_s / d ln k and running of running beta = d^2 n_s / d ln k^2. The bounds follow a linear relation to a very good accuracy. A conservative bound disfavors any enhancement above the line beta(k_0)=0.036-0.42 alpha(k_0) and a realistic estimate disfavors any enhancement above the line beta(k_0)=0.022-0.44 alpha(k_0).

Gravitational Waves in Bouncing Cosmologies from Gauge Field Production

We calculate the gravitational waves (GW) spectrum produced in various Early Universe scenarios from gauge field sources, thus generalizing earlier inflationary calculations to bouncing cosmologies. We consider generic couplings between the gauge fields and the scalar field dominating the energy density of the Universe. We analyze the requirements needed to avoid a backreaction that will spoil the background evolution. When the scalar is coupled only to

Vacuum energy sequestering and conformal symmetry

F \tilde F

Random and correlated roughening in slow fracture by damage nucleation.

term, the sourced GW spectrum is exponentially enhanced and parametrically the square of the vacuum fluctuations spectrum,