1,721,092 research outputs found
Recent developments in string model-building and cosmology
No full textIn this talk I discuss recent developments in moduli stabilisation, SUSY breaking and chiral D-brane models together with several interesting features of cosmological models built in the framework of type IIB string compactifications. I show that a non-trivial pre-inflationary dynamics can give rise to a power loss at large angular scales for which there have been mounting observational hints from both WMAP and Planck. I then describe different stringy embeddings of inflationary models which yield large or small tensor modes. I finally argue that reheating is generically driven by the decay of the lightest modulus which can produce, together with Standard Model particles, also non-thermal dark matter and light hidden sector degrees of freedom that behave as dar
General analysis of dark radiation in sequestered string models
No full textAbstract: We perform a general analysis of axionic dark radiation produced from the decay of the lightest modulus in the sequestered LARGE Volume Scenario. We discuss several cases depending on the form of the Kähler metric for visible sector matter fields and the mechanism responsible for achieving a de Sitter vacuum. The leading decay channels which determine dark radiation predictions are to hidden sector axions, visible sector Higgses and SUSY scalars depending on their mass. We show that in most of the parameter space of split SUSY-like models squarks and sleptons are heavier than the lightest modulus. Hence dark radiation predictions previously obtained for MSSM-like cases hold more generally also for split SUSY-like cases since the decay channel to SUSY scalars is kinematically forbidden. However the inclusion of string loop corrections to the Kähler potential gives rise to a parameter space region where the decay channel to SUSY scalars opens up, leading to a significant reduction of dark radiation production. In this case, the simplest model with a shift-symmetric Higgs sector can suppress the excess of dark radiation ΔNeff to values as small as 0.14, in perfect agreement with current experimental bounds. Depending on the exact mass of the SUSY scalars all values in the range 0.14 ≲ ΔNeff ≲ 1.6 are allowed. Interestingly dark radiation overproduction can be avoided also in the absence of a Giudice-Masiero coupling
Chiral global embedding of Fibre Infation with D3 ̄ uplift
Full text linkWe carefully analyse the challenges posed by the construction of type IIB chiral global embeddings of Fibre Inflation with D3 ̄
uplift to a de Sitter vacuum. We present an explicit example involving an h1,1 = 4 Calabi-Yau manifold with a K3 fibration and a del Pezzo divisor supporting non-perturbative effects. The chosen orientifold involution induces O3-planes that can be placed on top of each other at the tip of the throat of a deformed conifold singularity. The D7-brane sector contains standard magnetised branes and a Whitney brane. The former induce chiral matter and generate quantum effects that stabilise the Kähler moduli, while the latter helps to increase the total D3-charge. We study in detail the constraints on the parameter space leading to an observationally viable inflationary dynamics, finding several regions where the effective field theory is under control
Primordial black holes from string inflation
Full text linkWe present a single-field string inflationary model which allows for the generation of primordial black holes in the low mass region where they can account for a significant fraction of the dark matter abundance. The potential is typical of type IIB fibre inflation models and features a plateau at CMB scales and a near inflection point at small distance scales where the power spectrum is enhanced due to a period of ultra slow-roll. The tunability of the underlying parameters may be guaranteed by scanning through the string landscape and their stability against quantum corrections is ensured by an effective shift symmetry
Robust Inflation from fibrous strings
Full text linkSuccessful inflationary models should (i) describe the data well; (ii) arise generically from sensible UV completions; (iii) be insensitive to detailed fine-tunings of parameters and (iv) make interesting new predictions. We argue that a class of models with these properties is characterized by relatively simple potentials with a constant term and negative exponentials. We here continue earlier work exploring UV completions for these models - including the key (though often ignored) issue of modulus stabilisation - to assess the robustness of their predictions. We show that string models where the inflaton is a fibration modulus seem to be robust due to an effective rescaling symmetry, and fairly generic since most known Calabi-Yau manifolds are fibrations. This class of models is characterized by a generic relation between the tensor-to-scalar ratio r and the spectral index ns of the form r propto; (ns-1)2 where the proportionality constant depends on the nature of the effects used to develop the inflationary potential and the topology of the internal space. In particular we find that the largest values of the tensor-to-scalar ratio that can be obtained by generalizing the original set-up are of order r lesssim; 0.01. We contrast this general picture with specific popular models, such as the Starobinsky scenario and α-attractors. Finally, we argue the self consistency of large-field inflationary models can strongly constrain non-supersymmetric inflationary mechanisms
A geometrical instability for ultra-light fields during inflation?
No full textWe study the behaviour of isocurvature perturbations in non-linear sigma models 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
Moduli Vacuum Misalignment and Precise Predictions in String Inflation
No full textThe predictions for all the cosmological observables of any inflationary model depend on the number of e-foldings which is sensitive to the post-inflationary history of the universe. In string models the generic presence of light moduli leads to a late-time period of matter domination which lowers the required number of e-foldings and, in turn, modifies the exact predictions of any inflationary model. In this paper we compute exactly the shift of the number of e-foldings in Kaehler moduli inflation which is determined by the magnitude of the moduli initial displacement caused by vacuum misalignment and the moduli decay rates. We find that the preferred number of e-foldings gets reduced from 50 to 45, causing a modification of the spectral index at the percent level. Our results illustrate the importance of understanding the full post-inflationary evolution of the universe in order to derive precise predictions in string inflation. To perform this task it is crucial to work in a setting where there is good control over modul
Global embedding of fibre inflation models
Full text linkWe present concrete embeddings of fibre inflation models in globally consistent type IIB Calabi-Yau orientifolds with closed string moduli stabilisation. After performing a systematic search through the existing list of toric Calabi-Yau manifolds, we find several examples that reproduce the minimal setup to embed fibre inflation models. This involves Calabi-Yau manifolds with h1,1 = 3 which are K3 fibrations over a P1 base with an additional shrinkable rigid divisor. We then provide different consistent choices of the underlying brane set-up which generate a non-perturbative superpotential suitable for moduli stabilisation and string loop corrections with the correct form to drive inflation. For each Calabi-Yau orientifold setting, we also compute the effect of higher derivative contributions and study their influence on the inflationary dynamics
Affleck-Dine baryogenesis in type IIB string models
Full text linkAbstract: We propose a possible string embedding of Affleck-Dine baryogenesis in type IIB sequestered models where the late-time decay of the lightest modulus reheats the universe to relatively low temperatures. We show that if inflation is driven by a blow-up Kähler modulus, the Affleck-Dine field can become tachyonic during inflation if the Kähler metric for matter fields has an appropriate inflaton-dependent contribution. We find that the Affleck-Dine mechanism can generate the observed baryon asymmetry for natural values of the underlying parameters which lead also to successful inflation and low-energy gaugino masses in a split supersymmetry scenario. The reheating temperature from the lightest modulus decay is high enough to allow thermal Higgsino-like dark matter
De Sitter from T-branes
Full text linkHidden sector D7-branes with non-zero gauge flux are a generic feature of type IIB compactifications. A non-vanishing Fayet-Iliopoulos term induced by non-zero gauge flux leads to a T-brane configuration. Expanding the D7-brane action around this T-brane background in the presence of three-form supersymmetry breaking fluxes, we obtain a positive definite contribution to the moduli scalar potential which can be used as an uplifting source for de Sitter vacua. In this way we provide a higher-dimensional understanding of known 4D mechanisms of de Sitter uplifting based on hidden sector F-terms which are non-zero because of D-term stabilisation
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