Talk detail

Participant	Motohashi, Hayato
Institution	Yukawa Institute for Theoretical Physics - Kitashirakawa Oiwakecho - Sakyo-ku - Kyoto - Japan

Session	DE1	Accepted		Order	Time
Talk	Oral abstract	Title	Constructing ghost-free theories with higher derivatives
Coauthors
Abstract	Scalar-tensor theories serve models for inflation and dark energy. Many efforts have been made recently for constructing the most general scalar-tensor theories with higher-order derivatives in their Lagrangian. Since higher-derivative theories are typically associated with Ostrogradsky ghost which causes unbounded Hamiltonian, it is important to clarify how to evade it. In this talk, I will explain construction of healthy degenerate theories with higher-order derivatives which circumvent Ostrogradsky ghost. ----- References: Motohashi, Suyama, Phys. Rev. D91 (2015) 8, 085009, [arXiv:1411.3721]. Motohashi, Noui, Suyama, Yamaguchi, Langlois, JCAP 1607 (2016) 07, 033, [arXiv:1603.09355]. Motohashi, Suyama, Yamaguchi, arXiv:1711.08125.
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Session	AT1	Accepted		Order	Time
Talk	Oral abstract	Title	Constant-roll inflation
Coauthors
Abstract	We propose a phenomenological class of inflationary models in which the assumption of inflaton slow-roll is replaced by more general, constant-roll condition. We derive general exact solution for the inflaton potential and dynamics, and show that there exists parameter region that satisfies the latest observational constraint on the scalar spectral index and the tensor-to-scalar ratio. We also consider its generalization to the case of f(R) gravity, for which we provide a simple constant-roll condition defined in the original, Jordan frame. The f(R) constant roll inflation also allows us to obtain exact solutions for the scalaron potential in the Einstein frame, the function f(R) in the parametric form and inflationary dynamics, which satisfies the observationally constraint. ----- References: Motohashi, Starobinsky, Yokoyama, JCAP 1509, 018 (2015), [arXiv:1411.5021]. Motohashi, Starobinsky, Europhys. Lett. 117, 39001 (2017), [arXiv:1702.05847]. Motohashi, Starobinsky, Eur.Phys.J. C77, 538 (2017), [arXiv:1704.08188].
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Session	NS1	Accepted		Order	Time
Talk	Oral abstract	Title	Primordial Black Holes and Slow-Roll Violation
Coauthors
Abstract	We show that for primordial black holes (PBHs) to be the dark matter in single-field inflation, the slow-roll approximation must be violated by at least O(1) in order to enhance the curvature power spectrum within the required number of efolds between CMB scales and PBH mass scales. Power spectrum predictions which rely on the inflaton remaining on the slow-roll attractor can fail dramatically leading to qualitatively incorrect conclusions in models like an inflection potential and misestimate the mass scale in a running mass model. We show that an optimized temporal evaluation of the Hubble slow-roll parameters to second order remains a good description for a wide range of PBH formation models where up to a 10^7 amplification of power occurs in 10 efolds or more. ----- References: Motohashi, Hu, Phys.Rev. D 92, 043501 (2015), [arXiv:1503.04810]. Motohashi, Hu, Phys.Rev. D 96, 023502 (2017), [arXiv:1704.01128]. Motohashi, Hu, Phys.Rev. D 96, 063503 (2017), [arXiv:1706.06784].
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Session	BH2	Accepted		Order	Time
Talk	Oral abstract	Title	General Relativity solutions in modified gravity
Coauthors
Abstract	Since recent gravitational wave observations constrain deviation from General Relativity (GR) both on strong-field regime and cosmological scales with high accuracy, it is important to identify theories of modified gravity that intrinsically possess the same solutions as in GR among a huge number of theories. We clarify three conditions for a wide class of single-/multi-field scalar-tensor theories to allow GR solutions, i.e., solutions with the metric satisfying the Einstein equations in GR and the constant profile of the scalar fields. Our analysis applies in the presence of matter component, and any spacetime geometry including cosmological background as well as spacetime around black hole and neutron star, for the latter of which these conditions provide a necessary condition for no-hair theorem. The three conditions classify general theories of modified gravity into three classes, each of which possesses i) unique GR solutions (i.e., no-hair cases), ii) only hairy solutions (except the cases that GR solutions are realized by cancellation between singular coupling functions in the Euler-Lagrange equations), and iii) both GR and hairy solutions, for the last of which one of the two solutions may be selected dynamically. ----- Reference: Motohashi, Minamitsuji, arXiv1804.01731.
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