Creation of a non-singular universe

• Project Area: Why does the Universe accelerate? - Exhaustive study and challenge for the future -
• Project/Area Number: 18H04355
• Research Category: Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area)
• Allocation Type: Single-year Grants
• Research Institution: Rikkyo University
• Principal Investigator: 小林 努 立教大学, 理学部, 教授 (40580212)
• Project Period (FY): 2018-04-01 – 2020-03-31
• Project Status: Completed (Fiscal Year 2019)
• Budget Amount: ¥2,600,000 (Direct Cost: ¥2,000,000、Indirect Cost: ¥600,000); Fiscal Year 2019: ¥1,300,000 (Direct Cost: ¥1,000,000、Indirect Cost: ¥300,000); Fiscal Year 2018: ¥1,300,000 (Direct Cost: ¥1,000,000、Indirect Cost: ¥300,000)
• Keywords: インフレーション / 重力波 / 修正重力理論 / 宇宙論

Outline of Annual Research Achievements

インフレーションの代替シナリオの実現可能性ならびに観測的検証可能性を検討した。一般的な重力スカラー結合系でバウンス宇宙モデルを構築し、その背景上で密度揺らぎと重力波のふるまいを調べた。特にそれらの3点相関関数(非ガウス性)に着目した。その結果、まず、既存の研究で主張されていた禁止定理、すなわち、観測と整合的なバウンスシナリオの構築は不可能であるとする主張は、出発点となる理論を一般化することで回避可能であることを示した。また、そのようにして実現されたバウンスシナリオは、揺らぎの非ガウス性を見ることで通常のインフレーションシナリオと区別することが原理的には可能であることを示した。
インフレーション宇宙論それ自体の研究もおこなった。既存の研究で主張されていた、スローロールインフレーションの枠組では必ず原始重力波のスペクトルが右に傾く、という言説は、インフレーションと代替シナリオとを区別する重要な指標になり得る。この点を再検討するため、既存の研究の仮定を洗い直した。その結果、既存の研究の不備が見つかった。見落とされていた項をスローロール近似に取り入れ、これまでの多様なスローロールモデルを包含する一般的なモデルを構築した。それにもとづいて原始重力波のスペクトルを再評価したところ、スペクトルが左に傾く興味深いモデルが存在することが明らかになった。
以上の研究を通じて、インフレーションとその代替シナリオについての新たな知見が得られたことになる。

Research Progress Status

令和元年度が最終年度であるため、記入しない。

Strategy for Future Research Activity

令和元年度が最終年度であるため、記入しない。

Research Products

• [Int'l Joint Research] ポーツマス大学(英国)
• [Journal Article] Primordial non-Gaussianities of scalar and tensor perturbations in general bounce cosmology: Evading the no-go theorem (2020)
  • Author(s): Shingo Akama, Shin'chi Hirano, and Tsutomu Kobayashi
  • Journal Title: Physical Review D Volume: 101 Issue: 4 Pages: 43529-43529
  • DOI: 10.1103/physrevd.101.043529
  • Peer Reviewed / Open Access
• [Journal Article] Revisiting slow-roll dynamics and the tensor tilt in general single-field inflation (2020)
  • Author(s): Yosuke Mishima and Tsutomu Kobayashi
  • Journal Title: Physical Review D Volume: 101 Issue: 4 Pages: 43536-43536
  • DOI: 10.1103/physrevd.101.043536
  • Peer Reviewed
• [Journal Article] Relativistic stars in a cubic Galileon universe (2020)
  • Author(s): Hiromu Ogawa, Tsutomu Kobayashi, and Kazuya Koyama
  • Journal Title: Physical Review D Volume: 101 Issue: 2 Pages: 24026-24026
  • DOI: 10.1103/physrevd.101.024026
  • Peer Reviewed / Int'l Joint Research
• [Journal Article] Generalized 2D dilaton gravity and kinetic gravity braiding (2019)
  • Author(s): Kazufumi Takahashi and Tsutomu Kobayashi
  • Journal Title: Classical and Quantum Gravity Volume: 36 Issue: 9 Pages: 095003-095003
  • DOI: 10.1088/1361-6382/ab1355
  • Peer Reviewed
• [Journal Article] Horndeski theory and beyond: a review (2019)
  • Author(s): Tsutomu Kobayashi
  • Journal Title: Reports on Progress in Physics Volume: 82 Issue: 8 Pages: 1-27
  • DOI: 10.1088/1361-6633/ab2429
  • Peer Reviewed
• [Journal Article] Constraining degenerate higher-order scalar-tensor theories with linear growth of matter density fluctuations (2019)
  • Author(s): Shin'ichiro Hirano, Tsutomu Kobayashi, Daisuke Yamauchi, Shuichiro Yokoyama
  • Journal Title: Physical Review D Volume: 99 Issue: 10 Pages: 104051-104051
  • DOI: 10.1103/physrevd.99.104051
  • Peer Reviewed
• [Journal Article] Screening mechanism in degenerate higher-order scalar-tensor theories evading gravitational wave constraints (2019)
  • Author(s): Sinh'ichi Hirano, Tsutomu Kobayashi, Daisuke Yamauchi
  • Journal Title: Physical Review D Volume: 99 Issue: 10 Pages: 104073-104073
  • DOI: 10.1103/physrevd.99.104073
  • Peer Reviewed
• [Journal Article] General theory of cosmological perturbations in open and closed universes from the Horndeski action (2019)
  • Author(s): Akama Shingo、Kobayashi Tsutomu
  • Journal Title: Physical Review D Volume: 99 Issue: 4 Pages: 1-13
  • DOI: 10.1103/physrevd.99.043522
  • Peer Reviewed / Open Access
• [Journal Article] Extended cuscuton: formulation (2018)
  • Author(s): Iyonaga Aya、Takahashi Kazufumi、Kobayashi Tsutomu
  • Journal Title: Journal of Cosmology and Astroparticle Physics Volume: 2018 Issue: 12 Pages: 1-23
  • DOI: 10.1088/1475-7516/2018/12/002
  • Peer Reviewed
• [Journal Article] Parity-violating gravity and GW170817 (2018)
  • Author(s): Nishizawa Atsushi、Kobayashi Tsutomu
  • Journal Title: Physical Review D Volume: 98 Issue: 12 Pages: 1-5
  • DOI: 10.1103/physrevd.98.124018
  • NAID: 120006623784
  • Peer Reviewed
• [Journal Article] Self-anisotropizing inflationary universe in Horndeski theory and beyond (2018)
  • Author(s): Tahara Hiroaki W. H.、Nishi Sakine、Kobayashi Tsutomu、Yokoyama Jun'ichi
  • Journal Title: Journal of Cosmology and Astroparticle Physics Volume: 2018 Issue: 07 Pages: 1-13
  • DOI: 10.1088/1475-7516/2018/07/058
  • Peer Reviewed
• [Journal Article] Relativistic stars in degenerate higher-order scalar-tensor theories after GW170817 (2018)
  • Author(s): Kobayashi Tsutomu、Hiramatsu Takashi
  • Journal Title: Physical Review D Volume: 97 Issue: 10 Pages: 1-11
  • DOI: 10.1103/physrevd.97.104012
  • Peer Reviewed
• [Presentation] Revisiting slow-roll dynamics and the tensor tilt in general single-field inflation (2020)
  • Author(s): Tsutomu Kobayashi
  • Organizer: Cosmic Acceleration
  • Int'l Joint Research
• [Presentation] On the screening mechanism in DHOST theories evading gravitational wave constraints (2019)
  • Author(s): Tsutomu Kobayashi
  • Organizer: 日本物理学会秋季大会
• [Presentation] On the screening mechanism in DHOST theories evading gravitational wave constraints (2019)
  • Author(s): Tsutomu Kobayashi
  • Organizer: COSMO 19
  • Int'l Joint Research
• [Presentation] Vainshtein mechanism and its breaking in scalar-tensor theories (2019)
  • Author(s): Tsutomu Kobayashi
  • Organizer: 6th Korea-Japan workshop on dark energy
  • Int'l Joint Research / Invited
• [Presentation] One-loop matter power spectrum beyond Horndeski (2019)
  • Author(s): 小林努
  • Organizer: 日本物理学会年次大会
• [Presentation] Extended Cuscuton (2019)
  • Author(s): Tsutomu Kobayashi
  • Organizer: Why does the Universe accelerate? -Exhaustive study and challenge of the future
  • Int'l Joint Research
• [Presentation] Relativistic stars in degenerate higher-order scalar-tensor theories after GW170817 (2018)
  • Author(s): 小林努
  • Organizer: 日本物理学会秋季大会
• [Presentation] Parity-violating gravity and GW170817 (2018)
  • Author(s): 小林努
  • Organizer: 第 7 回観測的宇宙論ワークショップ
• [Presentation] 一般相対論とその拡張 (2018)
  • Author(s): 小林努
  • Organizer: 第 31 回理論懇シンポジウム
  • Invited
• [Presentation] Scalar-tensor theories after GW170817 and relativistic stars in DHOST (2018)
  • Author(s): Tsutomu Kobayashi
  • Organizer: MG15
  • Int'l Joint Research
• [Presentation] Scalar-tensor theories after GW170817 and relativistic stars in DHOST (2018)
  • Author(s): Tsutomu Kobayashi
  • Organizer: MOGRA2018
  • Int'l Joint Research
• [Presentation] Relativistic stars in degenerate higher-order scalar-tensor theories after GW170817 (2018)
  • Author(s): Tsutomu Kobayashi
  • Organizer: Essential next steps for gravity and cosmology
  • Int'l Joint Research