Naoya Kitajima

Publications (7)0 Total impact

• Article: Gravitational waves from a curvaton model with blue spectrum

  Masahiro Kawasaki, Naoya Kitajima, Shuichiro Yokoyama
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  ABSTRACT: We investigate the gravitational wave background induced by the first order scalar perturbations in the curvaton models. We consider the quadratic and axion-like curvaton potential which can generate the blue-tilted power spectrum of curvature perturbations on small scales and derive the maximal amount of gravitational wave background today. We find the power spectrum of the induced gravitational wave background has a characteristic peak at the frequency corresponding to the scale reentering the horizon at the curvaton decay, in the case where the curvaton does not dominate the energy density of the Universe. We also find the enhancement of the amount of the gravitational waves in the case where the curvaton dominates the energy density of the Universe. Such induced gravitational waves would be detectable by the future space-based gravitational wave detectors or pulsar timing observations.
  05/2013;
• Article: Heavy gravitino in hybrid inflation

  Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama, Tsutomu T. Yanagida
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  ABSTRACT: It is known that supersymmetric hybrid inflation model may require severe tunings on the initial condition for large gravitino mass of order 100 - 1000 TeV due to the constant term in the superpotential. We propose a modified hybrid inflation model, where the constant term is suppressed during inflation and generated after inflation by replacing a constant term with dynamical field. In this modified model, successful inflation consistent with large gravitino mass takes place without severe tunings on the initial condition. Constraint from cosmic strings is also relaxed.
  01/2013;
• Article: Smooth hybrid inflation in a supersymmetric axion model

  Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama
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  ABSTRACT: We show that the smooth hybrid inflation is naturally realized in a framework of supersymmetric axion model. Identifying the Peccei-Quinn scalar fields as a part of the infaton sector, successful inflation takes place reproducing the amplitude and spectral index of the curvature perturbation observed by WMAP. A relatively large axion isocurvature perturbation and its non-Gaussianity are predicted in our model. The saxion coherent oscillation has a large amplitude and dominates the Universe. The subsequent decay of the saxion produces huge amount of entropy, which dilutes unwanted relics. Winos, the lightest supersymmetric particles in this scenario, are produced non-thermally in the decay and account for dark matter.
  11/2012;
• Article: Primordial black hole formation from an axion-like curvaton model

  Masahiro Kawasaki, Naoya Kitajima, Tsutomu T. Yanagida
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  ABSTRACT: We argue that the existence of the cold dark matter is explained by primordial black holes.We show that a significant number of primordial black holes can be formed in an axion-like curvaton model, in which the highly blue-tilted power spectrum of primordial curvature perturbations is achieved.It is found that the produced black holes with masses $\sim 10^{20} -10^{38} \mathrm{g}$ account for the present cold dark matter.We also argue the possibility of forming the primordial black holes with mass $\sim 10^5 M_{\odot}$ as seeds of the supermassive black holes.
  07/2012;
• Source

  Available from: ArXiv

  Article: Revisiting the cosmological coherent oscillation

  Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama
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  ABSTRACT: It is often the case that scalar fields are produced in the early Universe in the form of coherent oscillation. These scalar fields may have huge abundances and affect the evolution of the Universe. In particular, if the lifetime is long enough, they may cause cosmological disasters. We revisit the issue of coherent oscillation of the scalar field when it couples with another oscillating scalar field, and find a situation that the abundance, or the amplitude of the oscillation, is significantly reduced by a variant type of the adiabatic suppression mechanism. As a concrete example, it is applied to the saxion, a flat direction in the supersymmetric axion model, and we show that the cosmological saxion problem is solved in a particular setup.
  12/2011;
• Source

  Available from: ArXiv

  Article: Cosmological aspects of inflation in a supersymmetric axion model

  Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama
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  ABSTRACT: We show that the hybrid inflation is naturally realized in the framework of a supersymmetric axion model, which is consistent with the WMAP observation if the Peccei-Quinn symmetry breaking scale is around 10^{15}GeV. By solving the post inflationary scalar dynamics, it is found that the scalar partner of the axion, saxion, oscillates with large amplitude and its decay produces a huge entropy and dilutes the axion. As a result, the axion coherent oscillation can be the dominant component of the dark matter in the Universe. Cosmological gravitino and axino problems are solved.
  04/2011;
• Source

  Available from: ArXiv

  Article: Inflation from a Supersymmetric Axion Model

  Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama
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  ABSTRACT: We show that a supersymmetric axion model naturally induces a hybrid inflation with the waterfall field identified as a Peccei-Quinn scalar. The Peccei-Quinn scale is predicted to be around 10^{15}GeV for reproducing the large-scale density perturbation of the Universe. After the built-in late-time entropy-production process, the axion becomes a dark matter candidate. Several cosmological implications are discussed.
  08/2010;