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Tomohiro Matsuda

Publications and source records attributed to Tomohiro Matsuda.

At least 73 records · Page 4Linked to original sources
arXiv2006

Elliptic Inflation: Generating the curvature perturbation without slow-roll

There are many inflationary models in which 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 to the theory. To cite a case, curvatons may generate dominant part of the curvature perturbation after inflation. However, we have a question whether it is unrealistic to consider the generation of the curvature perturbation during inflation without slow-roll. Assuming multi-field inflation, we encounter the generation of the curvature perturbation during inflation without slow-roll. The potential along equipotential surface is flat by definition and thus we do not have to worry about symmetry. We also discuss about KKLT 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.

hep-ph↗
arXiv2006

Vacuum energy reduction through destabilization: a unification of quintessence and a dynamical approach?

Although quintessence cosmologies seem to explain the amount of cosmological constant today, the required conditions are severe. For example, an extremely slowly varying and light scalar field that rolls toward the vanishing vacuum energy state is needed, which must satisfy the required initial conditions so that the energy density in matter is comparable to the energy density stored in quintessence today. Our first question is whether it is possible to explain why quintessence ``knows'' that the Universe is going toward the vanishing energy state. We show from a new perspective that the conventional $O(H)$ corrections may reproduce the required potential. Our second question is whether it is possible to obtain plausible amount of the potential energy density today without tuning initial conditions. Since our resultant potentials mimic quintessence, one may solve the coincidence problem using the conventional techniques for solving the coincidence problem in quintessence cosmology. However, in our scenario we have another option, which is qualitatively different from the one that have been discussed in quintessence cosmology. Besides quintessence cosmology, we show that there is a relation between a dynamical approach, which suggests that we can solve the coincidence problem in a dynamical approach. As far as we know, this is the first attempt to discuss about the unified picture and the important relations between these distinctive scenarios.

hep-ph↗
arXiv2006

Primordial black holes from cosmic necklaces

Cosmic necklaces are hybrid topological defects consisting of monopoles and strings. We argue that primordial black holes(PBHs) may have formed from loops of the necklaces, if there exist stable winding states, such as coils and cycloops. Unlike the standard scenario of PBH formation from string loops, in which the kinetic energy plays important role when strings collapse into black holes, the PBH formation may occur in our scenario after necklaces have dissipated their kinetic energy. Then, the significant difference appears in the production ratio. In the standard scenario, the production ratio $f$ becomes a tiny fraction $f\sim 10^{-20}$, however it becomes $f \sim 1$ in our case. On the other hand, the typical mass of the PBHs is much smaller than the standard scenario, if they are produced in the same epoch. As the two mechanisms may work at the same time, the necklaces may have more than one channel of the gravitational collapse. Although the result obtained in this paper depends on the evolution of the dimensionless parameter $r$, the existence of the winding state could be a serious problem in some cases. Since the existence of the winding state in brane models is due to the existence of a non-tivial circle in the compactified space, the PBH formation can be used to probe the structure of the compactified space. Black holes produced by this mechanism may have peculiar properties.

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arXiv2006

Dark matter production from cosmic necklaces

Cosmic strings have gained a great interest, since they are formed in a large class of brane inflationary models. The most interesting story is that cosmic strings in brane models are distinguished in future cosmological observations. If the strings in brane models are branes or superstrings that can move along compactified space, and also if there are degenerated vacua along the compactified space, kinks interpolate between degenerated vacua become ``beads'' on the strings. In this case, strings turn into necklaces. In the case that the compact manifold in not simply connected, a string loop that winds around a nontrivial circle is stable due to the topological reason. Since the existence of the (quasi-)degenerated vacua and the nontrivial circle is a common feature of the brane models, it is important to study cosmological constraints on the cosmic necklaces and the stable winding states. In this paper, we consider dark matter production from loops of the cosmic necklaces. Our result suggests that necklaces can put stringent bound on certain kinds of brane models.

hep-ph↗
arXiv2006

PBH and DM from cosmic necklaces

Cosmic strings in the brane Universe have recently gained a great interest. I think the most interesting story is that future cosmological observations distinguish them from the conventional cosmic strings. If the strings are the higher-dimensional objects that can (at least initially) move along the compactified space, and finally settle down to (quasi-)degenerated vacua in the compactified space, then kinks should appear on the strings, which interpolate between the degenerated vacua. These kinks look like ``beads'' on the strings, which means that the strings turn into necklaces. Moreover, in the case that the compact manifold is not simply connected, the string loop that winds around a non-trivial circle is stable due to the topological reason. Since the existence of degenerated vacua and a non-trivial circle is the common feature of the brane models, it is important to study cosmological constraints on the cosmic necklaces and their stable winding states in the brane Universe.

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arXiv2005

Topological Curvatons

Topological defects may play the role of the curvatons. We propose a new mechanism of generating density perturbations from cosmological defects in inflationary models. We show several examples in which defects play crucial role in generating density perturbations.

hep-ph↗
arXiv2005

Monopolonium in confining phase

This paper has been withdrawn by the author(s), due the following reasons. A similar system for a different purpose has already been studied in Phys.Rev.D60:083001,1999 (astro-ph/9904315). Moreover, our analysis was wrong because we have missed the large frictional forces that will rapidly damp out the energy in the oscillating monopolonium system.

hep-ph↗
arXiv2005

Brane necklaces and brane coils

We investigate the evolution of the networks of the cosmic strings in angled brane inflation. We show how they can be distinguished from the conventional ones. The cosmic strings in angled inflation are the daughter $D_{p-2}$ branes that are extended between the mother $D_p$ branes. In the effective action, the strings should have a moduli, since the endpoints of the $D_{p-2}$ branes can move freely on the $D_p$ branes. Then naturally the position of the $D_{p-2}$ branes, which corresponds to the moduli of the $(1+1)$-dimensional effective action, can vary along the cosmic strings. The variation of the moduli results in the peculiar $(1+1)$-dimensional kink configurations. The kinks are the monopoles on the strings. Therefore, the cosmic strings in angled inflation become necklaces. The loops of the necklaces can shrink to produce stable winding states, which look like coils. We show why the cosmological implications of the brane necklaces are important. We point out that the cosmic strings in generic models of brane inflation should become necklaces, depending on the structure of the compactified space and the effective potential of the model.

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arXiv2004

Formation of cosmological brane defects

We study cosmological formation of D-term strings, axionic strings, domain walls and Q-balls in braneworld models of the Hanany-Witten type. For the D-term strings, we show that the strings are the daughter branes extended between mother branes. We show that the domain walls can be produced by conventional cosmological phase transitions. In this case, the formation of the domain walls is induced by the continuous deformation of the branes, which means that they are not created as daughter branes. First we consider classical configurations of the axionic strings and the domain walls, then we investigate the quantum effect of the brane dynamics. We also study brane Q-balls and show how they can be distinguished from conventional Q-balls.

hep-ph↗
arXiv2004

Formation of monopoles and domain walls after brane inflation

We study cosmological defect formation after brane inflation. The cosmological defects are corresponding to the branes that have less than three spacial dimensions in the uncompactified spacetime. Contrary to the previous arguments, production of monopoles and domain walls are not always negligible. Monopoles and domain walls are formed by the branes extended between mother branes.

hep-ph↗
arXiv2004

Brane Q-ball, branonium and brane Q-ball inflation

We study the stability of branonium. Contrary to the previous arguments, global structure of branonium is not stable against spatial fluctuations. We show that branonium decays into local objects, which looks like Q-balls in the effective action. Therefore, we consider the idea of brane Q-balls in this paper. Brane Q-balls are produced as the remnants of unstable branonium. Although the brane Q-balls are conceptually different from the conventional Q-balls, they look similar in some parametric regions. We show how to distinguish brane Q-balls from conventional Q-balls.

hep-ph↗
arXiv2004

String production after angled brane inflation

We describe string production after angled brane inflation. First, we point out that there was a discrepancy in previous discussions. The expected tension of the cosmic string calculated from the four-dimensional effective Lagrangian did not match the one obtained in the brane analysis. In the previous analysis, the cosmic string is assumed to correspond to the lower-dimensional daughter brane, which wraps the same compactified space as the original mother brane. In this case, however, the tension of the daughter brane cannot depend on the angle (θ). On the other hand, from the analysis of the effective Lagrangian for tachyon condensation, it is easy to see that the tension of the cosmic string must be proportional to θ, when θ<< 1. This is an obvious discrepancy that must be explained by consideration of the explicit brane dynamics. In this paper, we will solve this problem by introducing a simple idea. We calculate the tension of the string in the two cases, which matches precisely. The cosmological constraint for angled inflation is relaxed, because the expected tension of the cosmic string becomes smaller than the one obtained in previous arguments, by a factor of θ.

hep-ph↗
arXiv2003

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, D.H.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 this constraint. We show that the bound is drastically modified if there were multiple stages of inflation.

hep-ph↗
arXiv2003

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 moduli 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 avoided.

hep-ph↗
arXiv2003

Comment on the stability of the Yukawa couplings and the cosmological problems of intersecting brane models

In string theory, stabilization of moduli fields and their cosmological implications have been discussed by many authors. In this paper we do not consider conventional modulus, nor relative distance between two branes. We focus our attention to a relative position of three intersecting branes. Surprisingly, there had been no phenomonological argument on the stabilization of such moduli. We will show that the area of the corresponding triangle is not a free parameter, but an effective potential is generated from conventional loop corrections in the low energy effective theory. Of course, the stabilization does not induce any serious problem, because one is allowed to modify other parameters of the model to adjust the Yukawa couplings. Then the stabilization puts a constraint that is a different nature from the ones that have been discussed before. We also discuss cosmological problems and show a simple idea that can solve the problem.

hep-ph↗
arXiv2003

Incidental brane defects

In the models of brane construction, the isometry of a compactified space might be broken by branes. In four-dimensional effective Lagrangian, the breaking of the isometry is seen as the spontaneous breaking of the corresponding effective symmetry. Then it seems natural to expect that there are various kinds of defects that will be implemented by the spontaneous symmetry breaking. These defects are parametrized by the brane positions. In this paper we consider two kinds of such ``brane defects'', which are formed by the local fluctuations of the locations of branes along their transversal directions. The fluctuation of a brane position might leads to winding (or wraping) around a non-contractible circle of the compactified space. These ``primary'' brane defects are already discussed by several authors. On the other hand, if there are multiple branes in the compactified space and their configuration in a compactified space is determined by the potential that depends only on their relative positions, one might find incidental symmetry in the effective potential, which is spontaneously broken by branes. We examined the latter ``incidental'' symmetry breakings and stable defect configurations. We paid special attention to the difference between ``primary'' brane defects.

hep-th↗
arXiv2003

Topological hybrid inflation in brane world

In this paper we show that certain types of cosmological brane configurations may induce topological inflation. In generic models for topological inflation, there is a strict bound for the potential of the inflaton field. Naively, it seems that these constraints can be removed if inflation starts with a hybrid potential. However, it is difficult to combine topological inflation and hybrid inflation, because of the difference in their initial conditions. In this paper, we show some examples in which a novel type of topological inflation is realized with the unconventional hybrid potential.

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