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Masahide Yamaguchi

Publications and source records attributed to Masahide Yamaguchi.

179 records · Page 10Linked to original sources
arXiv2001

Density Fluctuations and Primordial Black Hole Formation in Natural Double Inflation in Supergravity

We investigate the recently proposed natural double inflation model in supergravity. Chaotic inflation first takes place by virtue of the Nambu-Goldstone-like shift symmetry. During chaotic inflation, an initial value of second inflation (new inflation) is set, which is adequately far from the local maximum of the potential due to the small linear term in the Kähler potential. Then, primordial fluctuations within the present horizon scale may be produced during both inflations. Primordial fluctuations responsible for anisotropies of the cosmic microwave background radiation and the large scale structure are produced during chaotic inflation, while fluctuations on smaller scales are produced during new inflation. Because of the peculiar nature of new inflation, they can become as large as $10^{-1}$-$10^{-2}$, which may lead to the formation of primordial black holes.

hep-ph↗
arXiv2001

Natural Double Inflation in Supergravity

We propose a natural double inflation model in supergravity. In this model, chaotic inflation first takes place by virtue of the Nambu-Goldstone-like shift symmetry, which guarantees the absence of the exponential factor in the potential for the inflaton field. During chaotic inflation, an initial value of the second inflation (new inflation) is set. In this model, the initial value of new inflation can be adequately far from the local maximum of the potential for new inflation due to the small linear term of the inflaton in the Kähler potential. Therefore, the primordial fluctuations within the present horizon scale may be attributed to both inflations; that is, the first chaotic inflation produces the primordial fluctuations on the large cosmological scales while the second new inflation on the smaller scales. The successive decay of the inflaton for new inflation leads to a reheating temperature low enough to avoid the overproduction of gravitinos in a wide range of the gravitino mass.

hep-ph↗
arXiv2001

Natural Chaotic Inflation in Supergravity and Leptogenesis

We comprehensively investigate a chaotic inflation model proposed recently in the framework of supergravity. In this model, the form of Kähler potential is determined by a symmetry, that is, the Nambu-Goldstone-like shift symmetry, which guarantees the absence of the exponential factor in the potential for the inflaton field. Though we need the introduction of small parameters, the smallness of the parameters is justified also by symmetries. That is, the zero limit of the small parameters recovers symmetries, which is natural in the 't Hooft's sense. The leptogenesis scenario via the inflaton decay in this chaotic inflation model is also discussed. We find that the lepton asymmetry enough to explain the present baryon number density is produced for low reheating temperatures avoiding the overproduction of gravitinos.

hep-ph↗
arXiv2001

New inflation in supergravity with a chaotic initial condition

We propose a self-consistent scenario of new inflation in supergravity. Chaotic inflation first takes place around the Planck scale, which solves the longevity problem, namely, why the universe can live much beyond the Planck time, and also gives an adequate initial condition for new inflation. Then, new inflation lasts long enough to generate primordial fluctuations for the large scale structure, which generally has a tilted spectrum with the spectral index $n_{s} < 1$. The successive decay of the inflaton leads to the reheating temperature low enough to avoid the overproduction of gravitinos in a wide range of the gravitino mass.

hep-ph↗
arXiv2000

Topological Inflation in Supergravity

We investigate a topological inflation model in supergravity. By means of numerical simulations, it is confirmed that topological inflation can take place in supergravity. We also show that the condition for successful inflation depends not only on the vacuum-expectation value (VEV) of inflaton field but also on the form of its Kähler potential. In fact, it is found that the required VEV of the inflaton $ϕ$ can be as small as $<ϕ> \simeq 1 \times M_G$, where $M_{G}$ is the gravitational scale.

hep-ph↗
arXiv2000

Natural Chaotic Inflation in Supergravity

We propose a chaotic inflation model in supergravity. In the model the Kähler potential has a Nambu-Goldstone-like shift symmetry of the inflaton chiral multiplet which ensures the flatness of the inflaton potential beyond the Planck scale. We show that the chaotic inflation naturally takes place by introducing a small breaking term of the shift symmetry in the superpotential. This may open a new branch of model building for inflationary universe in the framework of supergravity.

hep-ph↗
arXiv2000

Kinetically Driven Quintessence

Recently, a novel class of models for inflation has been found in which the inflationary dynamics is driven solely by (non-canonical) kinetic terms rather than by potential terms. As an obvious extension, we show that a scalar field with non-canonical kinetic terms alone behaves like an energy component which is time-varying and has negative pressure presently, i.e. quintessence. We present a model which has a constant equation of state, that is, a ``kinetic'' counterpart of the Ratra-Peebles model of a quintessence field with a potential term. We make clear the structure of the phase plane and show that the quintessential solution is a late-time attractor. We also give a model for the ``phantom'' component which has an equation of state with $w=p/ρ<-1$.

astro-ph↗
arXiv2000

Evolution of a global string network in a matter dominated universe

We evolve the network of global strings in the matter-dominated universe by means of numerical simulations. The existence of the scaling solution is confirmed as in the radiation-dominated universe but the scaling parameter $ξ$ takes a slightly smaller value, $ξ\simeq 0.6 \pm 0.1$, which is defined as $ξ= ρ_{s} t^{2} / μ$ with $ρ_{s}$ the energy density of global strings and $μ$ the string tension per unit length. The change of $ξ$ from the radiation to the matter-dominated universe is consistent with that obtained by Albrecht and Turok by use of the one-scale model. We also study the loop distribution function and find that it can be well fitted with that predicted by the one-scale model, where the number density $n_{l}(t)$ of the loop with the length $l$ is given by $n_{l}(t) = ν/[t^2 (l + κt)^2]$ with $ν\sim 0.040$ and $κ\sim 0.48$. Thus, the evolution of the global string network in the matter-dominated universe can be well described by the one-scale model as in the radiation-dominated universe.

hep-ph↗
arXiv1999

Extended Open Inflation

We present a new type of one-field models for open inflation utilizing a nonminimally coupled scalar field with polynomial potentials in which the Coleman-de Luccia instanton does exist and slow-roll inflation after the bubble nucleation is realized.

hep-ph↗
arXiv1999

Initial condition for new inflation in supergravity

We study the initial value problem of new inflation models in the scheme of supergravity. We find that an inflaton generally acquires a mass of the order of the Hubble parameter via gravitationally suppressed interactions with particles in the thermal bath of the universe. This additional mass term stabilizes dynamically the inflaton at the local maximum of the potential, which offers the initial condition of new inflation.

hep-ph↗
arXiv1999

Scaling Property of the global string in the radiation dominated universe

We investigate the evolution of the global string network in the radiation dominated universe by use of numerical simulations in 3+1 dimensions. We find that the global string network settles down to the scaling regime where the energy density of global strings, $ρ_{s}$, is given by $ρ_{s} = ξμ/ t^2$ with $μ$ the string tension per unit length and the scaling parameter, $ξ\sim (0.9-1.3)$, irrespective of the cosmic time. We also find that the loop distribution function can be fitted with that predicted by the so-called one scale model. Concretely, the number density, $n_{l}(t)$, of the loop with the length, $l$, is given by $n_{l}(t) = ν/[t^{3/2} (l + κt)^{5/2}]$ where $ν\sim 0.0865$ and $κ$ is related with the Nambu-Goldstone(NG) boson radiation power from global strings, $P$, as $P = κμ$ with $κ\sim 0.535$. Therefore, the loop production function also scales and the typical scale of produced loops is nearly the horizon distance. Thus, the evolution of the global string network in the radiation dominated universe can be well described by the one scale model in contrast with that of the local string network.

hep-ph↗
arXiv1999

Evolution of Axionic Strings and Spectrum of Axions Radiated from Them

Cosmological evolution of axionic strings is investigated by numerically solving field equations of a complex scalar field in 3+1 dimensions. It is shown that the global strings relax to the scaling solution with a significantly smaller number density than the case of local strings. The power spectrum of axions radiated from them is calculated from the simulation data, which is found to be highly peaked around the Hubble scale, and a more accurate constraint on the Peccei-Quinn breaking scale is obtained.

hep-ph↗
arXiv1999

Cosmological Moduli Problem and Oscillating Inflation in Gauge-Mediated Supersymmetry Breaking

We investigate cosmological moduli problem in the gauge-mediated supersymmetry breaking (GMSB). A mini-inflation (oscillating inflation) takes place when a scalar field corresponding to the flat direction in GMSB oscillates along the logarithmic potential induced by the gauge-mediation mechanism. It is shown that this oscillating inflation can sufficiently dilute the relic abundance of the string moduli for some ranges of parameters in the GMSB models.

hep-ph↗
arXiv1998

Numerical analysis of formation and evolution of global strings in 2+1 dimensions

We simulate the formation and the evolution of global strings taking into account the expansion of the universe and the concomitant change of the effective potential, that is, the change from the restoration stage of the global {\it U}(1)-symmetry to the broken stage. Starting from the thermal equilibrium state, we run simulations for 100 sets of different initial conditions in 2+1 dimensions. As a result, we find deviation from scale invariance. This is because the energy loss mechanism is pair annihilation rather than intercommutation, so that a string and an anti-string attract each other under a logarithmic potential.

hep-ph↗
arXiv1998

Probability Distribution Function of the Coarse-grained Scalar Field at Finite Temperature

We present a formalism to calculate the probability distribution function of a scalar field coarse-grained over some spatial scales with a Gaussian filter at finite temperature. As an application, we investigate the role of subcritical fluctuations in the electroweak phase transition in the minimal standard model. It is concluded that the universe was in a mixed state of true and false vacua already at the critical temperature.

hep-ph↗
arXiv1997

Numerical approach to the onset of the electroweak phase transition

We investigate whether the universe was homogeneously in the false vacuum state at the critical temperature of a weakly first-order phase transition such as the electroweak phase transition in terms of a series of numerical simulations of a phenomenological Langevin equation, whose noise term is derived from the effective action but the dissipative term is set so that the fluctuation-dissipation relation is met. The correlation function of the noise terms given by a non-equilibrium field theory has a distinct feature if it originates from interactions with a boson or with a fermion. The spatial correlation function of noises from a massless boson damps with a power-law, while the fermionic noises always damp exponentially above the inverse-temperature scale. In the simulation with one-loop effective potential of the Higgs field, the latter turns out to be more effective to disturb the homogeneous field configuration. Since noises of the both types are present in the electroweak phase transition, our results suggest that conventional picture of a phase transition, namely, nucleation of critical bubbles in a homogeneous background does not apply or the one-loop approximation breaks down in the standard model.

hep-ph↗