Xin Zhang, Phys.Lett.B 683 (2010) 81 Heal the world: Avoiding the cosmic doomsday in the holographic dark energy model Xin Zhang Northeastern University Cosmic acceleration and dark energy Dark energy was found by SNIa observations in 1998. It causes a cosmic acceleration. Riess Perlmutter Dark Energy The main characteristic of DE: its equation of state has a negative value. Today, the universe is dominated by dark energy! Sith Our universe is governed by dark side! Master Yoda: beware of the dark side… Concordance model Dark energy: 73% Dark matter: 23% Normal matter: 4% 和谐宇宙 WMAP 三个代表 “Three Represents” What is Dark Energy？ Cosmological constant? cc is hard to understand in QFT A difference of 120 orders of magnitude cc1: Why so small? cc2: Why now? The cosmological constant problem Steven Weinberg, Rev. Mod. Phys. 61 (1989) 1 Susy, sugra, super string Anthropic principle Adjustment mechanism Changing gravity Quantum cosmology Steven Weinberg Phenomenological models At theoretical level, the cc is an obstacle. We are frustrated! At phenomenological level, we can propose some dynamical models of dark energy to interpret the observational data. Dynamical DE In terms of equation of state, dynamical DE can be classified: • Quintessence • Phantom • Quintom -1<w<1 w<-1 w crosses -1 W<-1? The Phantom Menace? 《星战前传：幽灵的威胁》 (Star Wars: The Phantom Menace) 福克斯影片公司(Fox, USA)1999年出品 主 演：埃文-麦克格莱格(Ewan McGregore) 利亚姆-尼森(Liam Neeson) 娜塔丽-珀特曼(Natalie Portman) 导 演：乔治-卢卡斯(George Lucas) 类 型：科幻片 R.R.Caldwell, PLB 545(2002)23 astro-ph/9908168 Weak energy condition is violated “a wrong sign” the big rip R.R.Caldwell, M.Kamionkowski and N.N.Weinberg, Phys.Rev.Lett.91(2003)071301 Due to such a supernegative equation of state, the phantom component leads to a “big rip” singularity at a finite time, at which all bound objects will be torn apart. Cosmic Doomsday Observations: quintom? big rip?! WMAP 5-year result J.-Q. Xia, H. Li, G.-B. Zhao and X. M. Zhang, PRD 2008 CPL parametrization Observations: quintom? big rip?! cc is still favored other possibilities still cannot be ruled out WMAP 7-year result How to avoid the big bang and big rip singularities? The Phantom Bounce: A New Oscillating Cosmology M.G. Brown, K. Freese, & W.H. Kinney, astro-ph/0405353 This version of cyclic cosmology was proposed in the light of such a modified Friedmann equation Derived from loop quantum cosmology or braneworld scenario. Phantom plays an essential role in this model. The universe can bounce and turn around. Both nonsingular. Problems See: XZ, EPJC(2009)59:755-759; EPJC(2009)60:661-667 1. The entropy of the universe increases from cycle to cycle due to the second law of thermodynamics, so that extrapolation into the past will lead back to an initial singularity. 2. Black holes cannot disappear due to the Hawking area theorem, so they eventually will occupy the entire horizon volume during contracting phase so that calculations break down. Dark Energy, Quantum Gravity? CC/DE problem is essentially a problem of Quantum Gravity A completely successful quantum theory of gravity is still not yet available It is more realistic to consider some consequences of some fundamental principles of Quantum Gravity Holographic Principle is commonly believed as a fundamental principle of Quantum Gravity So, what can the Holographic Principle do for DE? Holographic Principle? Effective Field Theory, Black Holes, and the Cosmological Constant A.Cohen et al. PRL 1999 (hep-th/9803132) considerations involving black holes suggests that the underlying theory of nature is not a local quantum field theory quantum field theory breaks down in large volume due to degrees of freedom vastly overcounted Holographic Principle: UV/IR duality an effective field theory with such a UV/IR relation is a good description of nature implication for DE? A model of holographic dark energy M. Li, PLB 2004 (hep-th/0403127) Let dark energy density be the one saturating that bound where L is the IR cutoff of the theory, the radius of a certain kind of horizon. S. Hsu pointed out that taking Hubble horizon as IR cutoff cannot obtain acceleration. Miao Li proposed that, instead of Hubble horizon, one can take the event horizon as the IR cutoff then the holographic density indeed behaves as a dark energy. Holographic dark energy: cosmological evolution definition equation of state equation of motion Holographic dark energy: cosmological evolution HDE satisfies: EoS c=1: w tends towards a c.c. c>1: w>-1; c<1: w crosses -1. Zhang & Wu, PRD2005 (astro-ph/0506310) Observational constraints : quintomlike? XZ & F.Q. Wu, PRD07 XZ & F.Q. Wu, PRD05 Observational constraints on holographic dark energy models a comparison M. Li, X.D. Li, S. Wang & X. Zhang, JCAP2009 (arXiv:0904.0928) It is necessary to compare the three holographic dark energy models. We constrain the holographic dark energy models by using the latest observational data, and then make a comparison for them by using the proper model selection criterion. Model selection criterion: Bayesian evidence (BE) although HDE performs a little poorer than LCDM, it performs better than ADE and RDE. Therefore, among three holographic models, HDE is more favored by the observational data Observational constraints : quintomlike? Holographic dark energy (HDE) 1σ 2σ M. Li, X.D. Li, S. Wang & X. Zhang, JCAP2009 (arXiv:0904.0928) Observational constraints : cosmic doomsday in the holographic dark energy scenario XZ, PLB2010 Physical motivation: why must eliminate the big-rip in the holographic dark energy model? Quantum gravity theory is not available, so a local effective field theory with a UV/IR duality Holographic dark energy model is nothing but an effective field theory with the IR cutoff the event horizon far future: dR eh dt c 1 so c<1 leads to big rip Prior to the big rip, the UV cutoff Λ will first exceed the Planck mass M P l , and then the IR scale R eh will become smaller than the Planck length l Pl Such a super-Planck phenomenon breaks down the theoretical foundation of the model How to erase the big-rip singularity? In the high energy regime, some mechanism may work, which leads to some high-energy correction to c to realize natural anticipation: the steady state (de Sitter) finale will emerge extra-dimension mechanism (braneworld scenario)? Why braneworld? holography: a fundamental principle of quantum gravity extra dimensions: another important aspect of spacetime in quantum gravity in high energy (phantom) regime, the extra-dimension effects are expected to play a significant role might provide positive correction to c Randall-Sundrum: Cosmological evolution at late times is a stable late-time solution Finale of the universe: de Sitter (steady state) spacetime de Sitter space-time! the universe begins with an inflation and also ends with another inflation Another braneworld scenario: turnaround? Shtanov-Sahni: the negative sign arises from a second time-like dimension negative: so the big rip could not be replaced by steady state (dS) turnaround? No! impossible! a cyclic universe has no a future event horizon Jingfei Zhang, XZ & Hongya Liu, EPJC52(2007)693 The whole story: From past to future Holographic Ricci dark energy in a braneworld: also Steady-State future braneworld: a modified (effective) Friedmann equation The future evolution: Big rip is avoided de Sitter future is expected C.J. Feng & X. Zhang, PLB2009 Alternative way of avoiding the big rip: Interacting holographic dark energy b<0 means that dark energy decays to matter; b>0 will lead to unphysical consequences, since matter density will become negative in the far future Probing interaction and spatial curvature Miao Li, Xiao-Dong Li, Shuang Wang, Yi Wang, & XZ, JCAP2009 Now, c<1only in about 1 sigma range. Good news?--- The likelihood of phantom decreases. Probing interaction and spatial curvature b is fairly small, in order of 0.01----observations do not favor an interacting model even a small b could avoid the big rip the sufficient and necessary conditions of avoiding big rip satisfied: Probing interaction and spatial curvature both interaction and curvature involved, the parameter space is amplified significantly this result can be used to solve the cosmic age problem caused by QSO APM 08279+5255 at z=3.91 Cosmic age problem solved QSO APM 08279+5255 at z=3.91: 2-3 Gyr Jinglei Cui& XZ, PLB2010 Further support to the IHDE: avoid the age problem caused by the old quasar SUMMARY Holographic viewpoint of dark energy Dark energy comes from quantum zero-point energy IR problem instead of UV problem Dynamical dark energy Can be connected to observational data How to avoid the cosmic doomsday? necessary! extra dimension dS future interaction between DE & DM Extra dimension OR Interaction? space-time is fundamentally higher dimensional in quantum gravity theory event horizon is a global concept, how to imagine local interaction? Thank You! Questions?!

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# What is Dark Energy？