Quintom Bounce with a Galileon Model Chung-Yuan Christian University, Taiwan & Institute of High Energy Physics, Beijing Based on Collaborated with J. Evslin, Y. F. Cai, M. Z. Li, X. M. Zhang Speaker: Taotao Qiu

Outline Why Quintom bounce? Quintom bounce of galileon model ◦ Background ◦ Perturbation Conclusion outlook

WHY QUINTOM BOUNCE?

Problems/constraints from theoretical/observational aspects: (such as BBN,CMB(COBE), etc) Big Bang Standard Models of the Early Universe Big Bang Cosmology vs. Inflation Cosmology The age of galaxiesThe redshift of the galactic spectrum The He abundanceThe prediction of CMB temperature Flatness problemHorizon problem Singularity problem Monopole problem Structure formation problem Inflation

Problems/constraints from theoretical/observational aspects: (such as BBN,CMB(COBE), etc) Big Bang Standard Models of the Early Universe Big Bang Cosmology vs. Inflation Cosmology The age of galaxiesThe redshift of the galactic spectrum The He abundanceThe prediction of CMB temperature Flatness problemHorizon problem Singularity problem Monopole problem Structure formation problem Inflation

The Alternatives of Inflation Pre-big bang Scenario Ekpyrotic Scenario String gas/Hagedorn Scenario Non-local SFT Scenario Bouncing Scenario

Ekpyrotic Model The collision of two M branes in 5D gives rise to a nonsingular cyclic universe, and the description of effective field theory in 4D is 1 DE domination 2 decelerated expansion 3 turnaround 4 ekpyrotic contracting phase 5 before big crunch 6 a singular bounce in 4D 7 after big bang 8 radiation domination 9 matter domination J. Khoury, B. Ovrut, P. Steinhardt and N. Turok, Phys. Rev. D 64, (2001)

Ekpyrotic Model The collision of two M branes in 5D gives rise to a nonsingular cyclic universe, and the description of effective field theory in 4D is 1 DE domination 2 decelerated expansion 3 turnaround 4 ekpyrotic contracting phase 5 before big crunch 6 a singular bounce in 4D 7 after big bang 8 radiation domination 9 matter domination Failure of effective field theory description, uncertainty involved in perturbations.

Contraction:Expansion: Bouncing Point: Nearby: In order to connect this process to the observable universe (radiation dominant, matter dominant, etc), w goes to above -1 Y. Cai, T. Qiu, Y. Piao, M. Li and X. Zhang, JHEP 0710:071, 2007 (Non-singular) Bounce Cosmology expansioncontraction IR size with Low energy scale Singularity problem avoided! ( ) Formalism: So w crosses -1, namely Quintom bounce! If w>-1 at the beginning, w will cross twice.

Realization of a Quintom Bounce As for any kind of matter, which is (1) in 4D classical Einstein Gravity, (2)described by single simple component (either perfect fluid or single scalar field with lagrangian as ), and (3) coupled minimally to Gravity or other matter, its Equation of State can never cross the cosmological constant boundary (w=-1). Quintom realization: No-Go theorem To realize Quintom, one of the conditions should be violated i) Double field Quintom bounce: ii) Single field Quintom bounce with higher derivative term: Y. Cai, T. Qiu, R. Brandenberger, Y. Piao, X. Zhang, JCAP 0803:013,2008; Y. Cai, T. Qiu, J. Xia, X. Zhang, Phys.Rev.D79:021303,2009. (also known as Lee-Wick Bounce) Y. Cai, T. Qiu, R. Brandenberger, X. Zhang, Phys.Rev.D80:023511,2009; J. Karouby, T. Qiu, R. Brandenberger, Phys.Rev.D84:043505,2011. Bo Feng et al., Phys. Lett. B 607, 35 (2005); A. Vikman, Phys. Rev. D 71, (2005); Gong-Bo Zhao et al., Phys. Rev. D 72, (2005); J. Xia, Y. Cai, T. Qiu, G. Zhao and X. Zhang, Int.J.Mod.Phys.D17: ,2008.

Galileon Theories Galileon Models: Lagrangian with higher derivative operator, but the equation of motion remains second order, so the model can have w cross -1 without ghost mode. Basically 5 kinds of Galileon model: But can be generalized… Usually, both of the two cases have more than two DYNAMICAL degrees of freedom, which will contain ghost modes. Problem with Quintom bounce: Recently: a kind of Galileon theory has been proposed! A.Nicolis et al., Phys.Rev.D79:064036,2009; C. Deffayet et al., Phys.Rev.D79:084003,2009. C. Deffayet et al., arXiv: [hep-th]

Cosmological Applications of Galileon Theories Galileon as dark energy models:  R. Gannouji,M. Sami, Phys.Rev.D82:024011,2010.  A. De Felice, S. Tsujikawa, Phys.Rev.Lett.105:111301,2010.  C. Deffayet,O. Pujolas,I. Sawicki, A. Vikman, JCAP 1010:026,2010. Galileon as inflation and slow expanstion models:  P. Creminelli, A. Nicolis, E. Trincherini, JCAP 1011:021,2010.  T. Kobayashi,M. Yamaguchi,J. Yokoyama, Phys.Rev.Lett.105:231302,2010.  C. Burrage,C. de Rham,D. Seery,A. Tolley, JCAP 1101:014,2011.  K. Kamada, T. Kobayashi, M. Yamaguchi, J. Yokoyama, Phys.Rev.D83:083515,2011.  Z. Liu, J. Zhang, Y. Piao, arXiv: [astro-ph.CO] Observational constraints on Galileon models:  S. Nesseris,A. De Felice, S. Tsujikawa, Phys.Rev.D82:124054,2010  A. Ali,R. Gannouji, M. Sami, Phys.Rev.D82:103015,2010. Galileon as spherically symmetric models:  D. Mota, M. Sandstad,T. Zlosnik, JHEP 1012:051,2010. … … … Can Galileon be used as bounce models???

QUINTOM BOUNCE WITH A GALILEON MODEL BACKGROUND PERTURBATION

Our New Bounce Model with Galileon The action: Stress energy tensor: From which we get energy density and pressure: which was also used in arXiv: for “Galileon Genesis”. where

Considering, and thus is monotonic increasing, so the first term in H, is always larger than 0. Solution for Bounce to Happen So we get one property of the field: evolve as a monotonic function! In order to have bounce, H must reach 0, so negative branch is chosen. Reality of square root: From the Friedmann Equation we get the Hubble parameter: where

Asymptotic solution of Our Model with Equation of motion: Hubble parameter: In contracting phase: Analysis of the asymptotic behavior when Terms in EoM has different orders of t inconsistent ! EoM becomes: inconsistent ! consistent ! The only consistent solution has a radiation dominant behavior! iii) I.I. II. i) ii)

Numerical Plots of Our Model (1) Plots of Hubble parameter and scale factor in our model: Parameter choice: Bounce can happen naturally in our model around t=30. Reheating?

Plots of field and EoS w in our model: behaves as a monotonic function, and the equation of state is approximately 1/3 (radiation-dominant like) in contracting phase, and cross -1 before bounce in our model. Numerical Plots of Our Model (2)

QUINTOM BOUNCE WITH A GALILEON MODEL BACKGROUND PERTURBATION

y Perturbation Theory  Theoretical aspects: stability must be guaranteed!  Observational aspects: should obtain a (nearly) scale- invariant power spectrum and small tensor-to-scalar ratio Why perturbations? Primordial perturbations provide seeds for structure formation and explains why our current universe is not complete isotropic. Two constraints for linear perturbations:

Perturbations of Our Bounce Model Perturbed metric in ADM form: Perturbed action: Gauge: uniform lapse function inverse shift vector Constraint equations: Solution:

is positive definite: no ghost instability! is model dependent: have to be checked numerically. Up to second order  Ghost instability:  Gradient instability: In our model, There are two kinds of instabilities at linear level: Stability of Perturbation of Our Model

Numeric plots for and Both and are positive all over the bouncing process, and we have which also behaves like radiation!

Spectrum of Perturbation of Our Model Blue spectrum inconsistent with observational data! Equation of motion: set In radiation dominant phase: Effective mass Solution: like a massless scalar field!

Mechanism of Getting Scale Invariant Power Spectrum D. Lyth and D. Wands, Phys.Lett.B524:5-14,2002. An alternative: Curvaton Mechanism Curvaton: a light scalar field other than inflaton to produce curvature perturbation. The simplest curvaton model: Curvature perturbation: For Gaussian part: where The equation of motion: with Solution: Power spectrum:

Curvaton Mechanism in Our Model Our curvaton action: The general solution: Our model: due to the background, in order to have scale invariant power spectrum, curvaton have to couple kinetically to the Galileon field The equation of motion: In radiation dominant phase:

Scale Invariant Power Spectrum from Curvaton growing constant decaying Superhorizon solution: There are two cases of getting scale invariant power spectrum: Subhorizon solution: From matching condition: and are independent of k!  q=2:  q=-4:

Back reaction of the Curvaton Field The energy density of : In contracting phase where the universe is radiation-like: From the equation of motion: In order for not to destroy the background evolution: one needs Question: will the growth of energy density of destroy the process of bounce? In our case which can produce scale invariant power spectrum:  q=2: Safe from back reaction of  q=-4: Needs severe fine-tuning.

Tensor Perturbation of Our Model Perturbed metric: Perturbed action (up to second order): Expand the tensor perturbation: Equation of motion:

Tensor Perturbation of Our Model In radiation dominant phase: Solution: Tensor spectrum: Spectrun index: In observable region we have, namely the spectrum is severely suppressed, so the tensor-to-scalar ratio like a massless scalar field! Blue spectrum! WMAP data predicts quite small r, so is consistent with our model! D. Larson et al. [WMAP collaboration], arXiv: [astro-ph.CO].

Conclusion Bounce needs equation of state w cross -1, namely Quintom Bounce can be in form of galileon, where there are only two dynamical degrees of freedom and ghost can be eliminated. Quintom bounce in Galileon form: ◦ Background behavior: Radiation-dominant like. ◦ Perturbation1: free from instability but cannot provide scale invariant power spectrum ◦ Perturbation2: The way of providing scale-invariant power spectrum is curvaton. In our model there are two cases. ◦ Perturbation3: The back reaction is small in one case, but the other case needs fine tuning. ◦ Perturbation4: The tensor spectrum is blue and the tensor- to-scalar ratio is small.

Outlook Final state of reheating Nongaussianities

Reheating Through reheating, inflaton decay to matter and radiation after inflation  In galileon cosmology, reheating can help avoid divergence L. Levasseur, R. Brandenberger, A. Davis, arXiv:  In bouncing cosmology, reheating is also important and maybe different from normal inflation T. Qiu, K. Yang, JCAP 1011:012,2010. Y. Cai, R. Brandenberger, X. Zhang, arXiv: What is the reheating process like of our Galileon bounce model??? Reheating is important in inflationary scenario! Reheating mechanisms: The motivation of reheating in our bounce model:  Normal: L. Kofman, A. Linde, A. Starobinsky, Phys.Rev.Lett.73: ,1994.  Geometric: B. Bassett, S. Liberati, Phys.Rev.D58:021302,1998.  Curvaton: Bo Feng, Ming-zhe Li, Phys.Lett.B564: ,2003. …… Our bounce model “Galileon genesis”, P. Creminelli et al.,

Non-gaussianities WMAP-7 data: Planck data: E. Komatsu et al., arXiv: Planck collaboration, astro-ph/  For canonical single scalar field inflation: X. Chen, M. Huang, S. Kachru, G. Shiu, JCAP 0701:002,2007.  For bounce cosmology: new shape with sizable amplitude and Y. Cai, W. Xue, R. Brandenberger, X. Zhang, JCAP 0905:011,2009. Non-Gaussianities is important for 1) meeting the more and more accurate observational data and 2) distinguishing the over models for early universe Definition: Observational constraints: Theoretical results: What will the non-Gaussianities behave like for our Galileon bounce model???

THANKS FOR ATTENTION! 谢谢！

Galileon genesis Our bounce model “Galileon genesis”,