Yu-ichi Takamizu

Collision of domain walls and reheating of the brane universe

We study particle production at the collision of two domain walls in 5-dimensional Minkowski spacetime. This may provide the reheating mechanism of an ekpyrotic (or cyclic) brane universe, in which two BPS branes collide and evolve into a hot big bang universe. We evaluate a production rate of particles confined to the domain wall. The energy density of created particles is given as {rho}{approx_equal}20g{sup 4}N{sub b}m{sub {eta}}{sup 4} where g is a coupling constant of particles to a domain-wall scalar field, N{sub b} is the number of bounces at the collision and m{sub {eta}} is the fundamental mass scale of the domain wall. It does not depend on the width d of the domain wall, although the typical energy scale of created particles is given by {omega}{approx}1/d. The reheating temperature is evaluated as T{sub R}{approx_equal}0.88gN{sub b}{sup 1/4}. In order to have the baryogenesis at the electro-weak energy scale, the fundamental mass scale is constrained as m{sub {eta}} > or approx. 1.1x10{sup 7} GeV for g{approx}10{sup -5}.

Localized features in non-Gaussianity from heavy physics

We discuss the possibility that we could obtain some hints of the heavy physics during inflation by analyzing local features of the primordial bispectrum. A heavy scalar field could leave large signatures in the primordial spectra through the parametric resonance between its background oscillation and the fluctuations. Since the duration of the heavy-mode oscillations is finite, the effect of the resonance is localized in momentum space. In this paper, we show that the bispectrum is amplified when such a resonance occurs, and that the peak amplitude of the feature can be (101−2), or as large as (105) depending on the type of interactions. In particular, the resonance can give large contributions in finitely squeezed configurations, while the bispectrum cannot be large in the exact squeezed limit. We also find that there is a relation between the scales at which the features appear in the bispectrum and the power spectrum, and that the feature in the bispectrum can be much larger than that in the power spectrum. If correlated features are observed at characteristic scales in the primordial spectra, it will indicate the presence of heavy degrees of freedom. By analyzing these features, we may be able to obtain some information on the physics behind inflation.

Fermions on colliding branes

We study the behaviour of five-dimensional fermions localized on branes, which we describe by domain walls, when two parallel branes collide in a five-dimensional Minkowski background spacetime. We find that most fermions are localized on both branes as a whole even after collision. However, how much fermions are localized on which brane depends sensitively on the incident velocity and the coupling constants unless the fermions exist on both branes.

Hybrid compactifications and brane gravity in six dimensions

We consider a six-dimensional axisymmetric Einstein–Maxwell model of warped braneworlds. The bulk is bounded by two branes, one of which is a conical 3-brane and the other is a 4-brane wrapped around the axis of symmetry. The latter brane is assumed to be our universe. If the tension of the 3-brane is fine-tuned, it folds the internal two-dimensional space in a narrow cone, making sufficiently small the Kaluza–Klein circle of the 4-brane. An arbitrary energy–momentum tensor can be accommodated on this ring-like 4-brane. We study linear perturbations sourced by matter on the brane, and show that weak gravity is apparently described by a four-dimensional scalar–tensor theory. The extra scalar degree of freedom can be interpreted as the fluctuation of the internal space volume (or that of the circumference of the ring), the effect of which turns out to be suppressed at long distances. Consequently, four-dimensional Einstein gravity is reproduced on the brane. We point out that as in the Randall–Sundrum model, the brane bending mode is crucial for recovering the four-dimensional tensor structure in this setup.

Nonlinear superhorizon perturbations of non-canonical scalar field

We develop a theory of non-linear cosmological perturbations at superhorizon scales for a scalar field with a Lagrangian of the form P(X,), where X = ????? and is the scalar field. We employ the ADM formalism and the spatial gradient expansion approach to obtain general solutions valid up to the second order in the gradient expansion. This formulation can be applied to, for example, DBI inflation models to investigate superhorizon evolution of non-Gaussianities. With slight modification, we also obtain general solutions valid up to the same order for a perfect fluid with a general equation of state P = P(?).

Bulk scalar emission from a rotating black hole pierced by a tense brane

We study the emission of scalar fields into the bulk from a six-dimensional rotating black hole pierced by a 3-brane. We determine the angular eigenvalues in the presence of finite brane tension by using the continued fraction method. The radial equation is integrated numerically, giving the absorption probability (graybody factor) in a wider frequency range than in the preexisting literature. We then compute the power and angular momentum emission spectra for different values of the rotation parameter and brane tension, and compare their relative behavior in detail. As is expected from the earlier result for a nonrotating black hole, the finite brane tension suppresses the emission rates. As the rotation parameter increases, the power spectra are reduced at low frequencies due to the smaller Hawking temperature and are enhanced at high frequencies due to superradiance. The angular momentum spectra are enhanced over the whole frequency range as the rotation parameter increases. The spectra and the amounts of energy and angular momentum radiated away into the bulk are thus determined by the interplay of these effects.

Thermal equilibrium of string gas in the Hagedorn universe

The thermal equilibrium of string gas is necessary to activate the Brandenberger-Vafa mechanism, which makes our observed 4-dimensional universe enlarge. Nevertheless, the thermal equilibrium is not realized in the original setup, a problem that remains as a critical defect. We study thermal equilibrium in the Hagedorn universe, and explore possibilities for avoiding the issue aforementioned flaw. We employ a minimal modification of the original setup, introducing a dilaton potential. Two types of potential are investigated: exponential and double-well potentials. For the first type, the basic evolutions of universe and dilaton are such that both the radius of the universe and the dilaton asymptotically grow in over a short time, or that the radius converges to a constant value while the dilaton rolls down toward the weak coupling limit. For the second type, in addition to the above solutions, there is another solution in which the dilaton is stabilized at a minimum of potential and the radius grows in proportion to t. Thermal equilibrium is realized for both cases during the initial phase. These simple setups provide possible resolutions of the difficulty.

Application of beyond

We focus on the evolution of curvature perturbation on superhorizon scales by adopting the spatial gradient expansion and show that the nonlinear theory, called the beyond

\delta N

\delta N

formalism: Varying sound speed

-formalism as the next-leading order in the expansion. As one application of our formalism for a single scalar field, we investigate the case of varying sound speed. In our formalism, we can deal with the time evolution in contrast to