Tracker and scaling solutions in DHOST theories

Noemi Frusciante (Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Lisboa, Portugal) ; Ryotaro Kase (Department of Physics, Faculty of Science, Tokyo University of Science, Tokyo, Japan) ; Kazuya Koyama (Institute of Cosmology & Gravitation, University of Portsmouth, Portsmouth, United Kingdom) ; Shinji Tsujikawa (Department of Physics, Faculty of Science, Tokyo University of Science, Tokyo, Japan) ; Daniele Vernieri (Centro de Astrofísica e Gravitação – CENTRA, Departamento de Física, Instituto Superior Técnico – IST, Universidade de Lisboa – UL, Lisboa, Portugal)

In quadratic-order degenerate higher-order scalar–tensor (DHOST) theories compatible with gravitational-wave constraints, we derive the most general Lagrangian allowing for tracker solutions characterized by ϕ˙/Hp=constant, where ϕ˙ is the time derivative of a scalar field ϕ, H is the Hubble expansion rate, and p is a constant. While the tracker is present up to the cubic-order Horndeski Lagrangian L=c2Xc3X(p1)/(2p)ϕ, where c2,c3 are constants and X is the kinetic energy of ϕ, the DHOST interaction breaks this structure for p1. Even in the latter case, however, there exists an approximate tracker solution in the early cosmological epoch with the nearly constant field equation of state wϕ=12pH˙/(3H2). The scaling solution, which corresponds to p=1, is the unique case in which all the terms in the field density ρϕ and the pressure Pϕ obey the scaling relation ρϕPϕH2. Extending the analysis to the coupled DHOST theories with the field-dependent coupling Q(ϕ) between the scalar field and matter, we show that the scaling solution exists for Q(ϕ)=1/(μ1ϕ+μ2), where μ1 and μ2 are constants. For the constant Q, i.e., μ1=0, we derive fixed points of the dynamical system by using the general Lagrangian with scaling solutions. This result can be applied to the model construction of late-time cosmic acceleration preceded by the scaling ϕ-matter-dominated epoch.

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      "surname": "Tsujikawa", 
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      "source": "Elsevier", 
      "value": "In quadratic-order degenerate higher-order scalar\u2013tensor (DHOST) theories compatible with gravitational-wave constraints, we derive the most general Lagrangian allowing for tracker solutions characterized by <math><mover><mrow><mi>\u03d5</mi></mrow><mrow><mo>\u02d9</mo></mrow></mover><mo>/</mo><msup><mrow><mi>H</mi></mrow><mrow><mi>p</mi></mrow></msup><mo>=</mo><mrow><mi>constant</mi></mrow></math>, where <math><mover><mrow><mi>\u03d5</mi></mrow><mrow><mo>\u02d9</mo></mrow></mover></math> is the time derivative of a scalar field \u03d5, H is the Hubble expansion rate, and p is a constant. While the tracker is present up to the cubic-order Horndeski Lagrangian <math><mi>L</mi><mo>=</mo><msub><mrow><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msub><mi>X</mi><mo>\u2212</mo><msub><mrow><mi>c</mi></mrow><mrow><mn>3</mn></mrow></msub><msup><mrow><mi>X</mi></mrow><mrow><mo>(</mo><mi>p</mi><mo>\u2212</mo><mn>1</mn><mo>)</mo><mo>/</mo><mo>(</mo><mn>2</mn><mi>p</mi><mo>)</mo></mrow></msup><mo>\u25a1</mo><mi>\u03d5</mi></math>, where <math><msub><mrow><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>,</mo><msub><mrow><mi>c</mi></mrow><mrow><mn>3</mn></mrow></msub></math> are constants and X is the kinetic energy of \u03d5, the DHOST interaction breaks this structure for <math><mi>p</mi><mo>\u2260</mo><mn>1</mn></math>. Even in the latter case, however, there exists an approximate tracker solution in the early cosmological epoch with the nearly constant field equation of state <math><msub><mrow><mi>w</mi></mrow><mrow><mi>\u03d5</mi></mrow></msub><mo>=</mo><mo>\u2212</mo><mn>1</mn><mo>\u2212</mo><mn>2</mn><mi>p</mi><mover><mrow><mi>H</mi></mrow><mrow><mo>\u02d9</mo></mrow></mover><mo>/</mo><mo>(</mo><mn>3</mn><msup><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>)</mo></math>. The scaling solution, which corresponds to <math><mi>p</mi><mo>=</mo><mn>1</mn></math>, is the unique case in which all the terms in the field density <math><msub><mrow><mi>\u03c1</mi></mrow><mrow><mi>\u03d5</mi></mrow></msub></math> and the pressure <math><msub><mrow><mi>P</mi></mrow><mrow><mi>\u03d5</mi></mrow></msub></math> obey the scaling relation <math><msub><mrow><mi>\u03c1</mi></mrow><mrow><mi>\u03d5</mi></mrow></msub><mo>\u221d</mo><msub><mrow><mi>P</mi></mrow><mrow><mi>\u03d5</mi></mrow></msub><mo>\u221d</mo><msup><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msup></math>. Extending the analysis to the coupled DHOST theories with the field-dependent coupling <math><mi>Q</mi><mo>(</mo><mi>\u03d5</mi><mo>)</mo></math> between the scalar field and matter, we show that the scaling solution exists for <math><mi>Q</mi><mo>(</mo><mi>\u03d5</mi><mo>)</mo><mo>=</mo><mn>1</mn><mo>/</mo><mo>(</mo><msub><mrow><mi>\u03bc</mi></mrow><mrow><mn>1</mn></mrow></msub><mi>\u03d5</mi><mo>+</mo><msub><mrow><mi>\u03bc</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>)</mo></math>, where <math><msub><mrow><mi>\u03bc</mi></mrow><mrow><mn>1</mn></mrow></msub></math> and <math><msub><mrow><mi>\u03bc</mi></mrow><mrow><mn>2</mn></mrow></msub></math> are constants. For the constant Q, i.e., <math><msub><mrow><mi>\u03bc</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>=</mo><mn>0</mn></math>, we derive fixed points of the dynamical system by using the general Lagrangian with scaling solutions. This result can be applied to the model construction of late-time cosmic acceleration preceded by the scaling \u03d5-matter-dominated epoch."
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Published on:
17 January 2019
Publisher:
Elsevier
Published in:
Physics Letters B , Volume 790 C (2019)

Pages 167-175
DOI:
https://doi.org/10.1016/j.physletb.2019.01.009
Copyrights:
The Author(s)
Licence:
CC-BY-3.0

Fulltext files: