Tomohiro Matsuda

Nontachyonic brane inflation

We consider non-tachyonic hybrid inflation in the context of the braneworld cosmology. When one considers models for brane inflation, hybrid inflation is a natural consequence of the tachyon condensation if it appears at the end of inflation. In this case, however, reheating is a difficult problem due to the peculiar properties of the tachyon. In this paper we show some models for brane inflation where a new type of hybrid inflation is realized due to the localized matter fields. The obvious advantage of our scenario is successful reheating, which is due to the potential that is localized on the brane. The serious problem of the loop correction is also avoided.

Curvaton paradigm can accommodate multiple low inflation scales

Recent arguments show that some curvaton field may generate the cosmological curvature perturbation. As the curvaton is independent of the inflaton field, there is a hope that the fine tunings of inflation models can be cured by the curvaton scenario. More recently, however, Lyth discussed that there is a strong bound for the Hubble parameter during inflation even if one assumes the curvaton scenario. Although the most serious constraint was evaded, the bound seems rather crucial for many models of a low inflation scale. In this paper we try to remove the constraint. We show that the bound is drastically modified if there were multiple stages of inflation.

F-term, D-term and hybrid brane inflation

We study inflation and reheating in models for the brane Universe, considering hybrid brane inflation without tachyon condensation. We expect that some fields that are localized on different branes interact with O(1) coupling when branes are on top of each other, while the interaction vanishes when branes are separated at a distance. If the interaction is needed to avoid spontaneous breaking of supersymmetry on the brane, our idea for hybrid brane inflation works. In our model, when branes are far apart, supersymmetry is spontaneously broken by the fields on a brane, which induces inflation. The inflaton field is the modulus for the brane distance. At the end of inflation, when branes come close, supersymmetry is restored by the interaction between fields on the branes, then the oscillation starts to reheat the Universe. In this paper we construct explicit models for F-term and D-term inflation. There are at least two major advantages. The most attractive point is that reheating is natural in our model, because the trigger field is not the tachyon but a conventional field on the brane. The serious constraint from the loop correction, which always appears in conventional models for hybrid inflation, is also avoided.

Affleck-Dine baryogenesis after thermal brane inflation

We propose a new scenario of Affleck-dine baryogenesis in the context of theories with large extra dimensions. In this paper we consider baryogenesis after thermal brane inflation and show how our mechanism works. We specifically consider models in which supersymmetry is broken at the distant brane.

Cosmological perturbations from an inhomogeneous phase transition

A mechanism for generating metric perturbations in inflationary models is considered. Long-wavelength inhomogeneities of light scalar fields in a decoupled sector may give rise to superhorizon fluctuations of couplings and masses in the low-energy effective action. Cosmological phase transitions may then occur that are not simultaneous in space, but occur with time lags in different Hubble patches that arise from the long-wavelength inhomogeneities. Here an interesting model in which cosmological perturbations may be created at the electroweak phase transition is considered. The results show that phase transitions may be a generic source of non-Gaussianity.

Baryon number violation, baryogenesis, and defects with extra dimensions

In generic models for grand unified theories (GUT), various types of baryon-number-violating processes are expected when quarks and leptons propagate in the background of GUT strings. On the other hand, in models with large extra dimensions, the baryon number violation in the background of a string is not trivial because it must depend on the mechanism of the proton stabilization. In this paper, we argue that cosmic strings in models with extra dimensions can enhance the baryon number violation to a phenomenologically interesting level, if the proton decay is suppressed by the mechanism of localized wave functions. We also make some comments on baryogenesis mediated by cosmological defects. We show that at least two scenarios will be successful in this direction. One is the scenario of leptogenesis where the required lepton number conversion is mediated by cosmic strings, and the other is the baryogenesis from the decaying cosmological domain wall. Both scenarios are new and have not been discussed in the past.

Elliptic inflation: generating the curvature perturbation without slow-roll

There are many inflationary models in which the inflaton field does not satisfy the slow-roll condition. However, in such models, it is always difficult to generate the curvature perturbation during inflation. Thus, to generate the curvature perturbation, one must introduce another component into the theory. To cite a case, curvatons may generate the dominant part of the curvature perturbation after inflation. However, we question whether it is realistic to consider the generation of the curvature perturbation during inflation without slow-roll. Assuming multifield inflation, we encounter the generation of curvature perturbation during inflation without slow-roll. The potential along the equipotential surface is flat by definition and thus we do not have to worry about symmetry. We also discuss KKLT (Kachru–Kallosh–Linde–Trivedi) models, in which corrections lifting the inflationary direction may not become a serious problem if there is a symmetry enhancement at the tip (not at the moving brane) of the inflationary throat.

Kaluza-Klein modes in hybrid inflation

When one constructs specific models with a fundamental scale as low as the TeV scale, there arise many difficulties. In this Brief Report we examine hybrid inflation due to bulk scalar fields, which has been proposed to solve the problem of fine-tuning in producing density perturbations. We find that the Kaluza-Klein modes play significant roles, which enhance the speed of the phase transition and alter the reheating process. We also argue that a lower bound must be put to the fundamental scale in order to construct successful hybrid inflation due to bulk scalar fields.

Brane inflation without slow-roll

The scenario of brane inflation without using the conventional slow-roll approximations has been investigated. Based on the mechanism of generating the curvature perturbations at the end of inflation, a new brane inflation paradigm was developed. The conditions for making a sufficiently large enough number of e-foldings and for generating the curvature perturbations without producing dangerous relics were also examined. Benefits of our scenario are subsequently discussed in detail.

Cosmological perturbations from inhomogeneous preheating and multi-field trapping

We consider inhomogeneous preheating in a multi-field trapping model. The curvature perturbation is generated by inhomogeneous preheating which induces multi-field trapping at the enhanced symmetric point (ESP), and results in fluctuation in the number of e-foldings. Instead of considering simple reheating after preheating, we consider a scenario of shoulder inflation induced by the trapping. The fluctuation in the number of e-foldings is generated during this weak inflationary period, when the additional light scalar field is trapped at the local maximum of its potential. The situation may look similar to locked or thermal inflation or even to hybrid inflation, but we will show that the present mechanism of generating the curvature perturbation is very different from these others. Unlike the conventional trapped inflationary scenario, we do not make the assumption that an ESP appears at some unstable point on the inflaton potential. This assumption is crucial in the original scenario, but it is not important in the multi-field model. We also discuss inhomogeneous preheating at late-time oscillation, in which the magnitude of the curvature fluctuation can be enhanced to accommodate low inflationary scale.