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Northeastern University Xin Zhang Heal the world: Avoiding the cosmic doomsday in the holographic dark energy model Xin Zhang, Phys.Lett.B 683 (2010) 81

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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. - PowerPoint PPT Presentation

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Page 1: Northeastern University

Northeastern University

Xin Zhang

Heal the world:

Avoiding the cosmic doomsday in the holographic dark energy model

Xin Zhang, Phys.Lett.B 683 (2010) 81

Page 2: Northeastern University
Page 3: Northeastern University

Cosmic acceleration and dark energy

Dark energy was found by SNIa observations in 1998. It causes a cosmic acceleration.

Riess Perlmutter

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Our universe is governed by dark side!

Dark EnergyThe main characteristic of DE: its equation of state has a negative value.Today, the universe is dominated by dark energy!

Sith

Page 5: Northeastern University

Master Yoda:beware of the dark side…

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和谐宇宙Concordance model

WMAP

Dark energy: 73%Dark matter: 23%Normal matter: 4%

三个代表“Three Represents”

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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?

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Steven Weinberg

The cosmological constant problemSteven Weinberg, Rev. Mod. Phys. 61 (1989) 1

Susy, sugra, super string

Anthropic principle

Adjustment mechanism

Changing gravity

Quantum cosmology

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Phenomenological models

Dynamical DE

• Quintessence -1<w<1

• Phantom w<-1

• Quintom w crosses -1

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.

In terms of equation of state, dynamical DE can be classified:

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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”

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the big ripDue 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

R.R.Caldwell, M.Kamionkowski and N.N.Weinberg, Phys.Rev.Lett.91(2003)071301

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J.-Q. Xia, H. Li, G.-B. Zhao and X. M. Zhang, PRD 2008WMAP 5-year result

Observations: quintom? big rip?!

CPL parametrization

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Observations: quintom? big rip?!

WMAP 7-year result

cc is still favored

other possibilities still cannot be

ruled out

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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.

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.

Problems See: XZ, EPJC(2009)59:755-759; EPJC(2009)60:661-667

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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?

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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?

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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.

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Holographic dark energy: cosmological evolution

definition

equation of state

equation of motion

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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)

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Observational constraints : quintomlike?

XZ & F.Q. Wu, PRD05

XZ & F.Q. Wu, PRD07

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Observational constraints on holographic dark energy models

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)

M. Li, X.D. Li, S. Wang & X. Zhang, JCAP2009 (arXiv:0904.0928)a comparison

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

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Holographic dark energy (HDE)

M. Li, X.D. Li, S. Wang & X. Zhang, JCAP2009 (arXiv:0904.0928)

Observational constraints : quintomlike?

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Observational constraints : cosmic doomsday in the holographic dark energy scenario

XZ, PLB2010

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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: so c<1 leads to big rip

Prior to the big rip, the UV cutoff Λ will first exceed the Planck mass , and

then the IR scale will become smaller than the Planck length

Such a super-Planck phenomenon breaks down the theoretical foundation of the

model

1ehdRc

dt

PlM

ehR Pll

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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)?

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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:

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Cosmological evolution at late times

is a stable late-time solution

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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

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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 horizonJingfei Zhang, XZ & Hongya Liu, EPJC52(2007)693

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The whole story: From past to future

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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

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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

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Probing interaction and spatial curvature

Now, c<1only in about 1 sigma range.

Good news?--- The likelihood of phantom decreases.

Miao Li, Xiao-Dong Li, Shuang Wang, Yi Wang, & XZ, JCAP2009

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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:

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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

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Cosmic age problem solved

Jinglei Cui& XZ, PLB2010

QSO APM 08279+5255 at z=3.91: 2-3 Gyr

Further support to the IHDE: avoid the age problem caused by the old quasar

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SUMMARY Holographic viewpoint of dark energy

How to avoid the cosmic doomsday?

Extra dimension OR Interaction?

Dark energy comes from quantum zero-point energy IR problem instead of UV problem Dynamical dark energy Can be connected to observational data

necessary! extra dimension dS future interaction between DE & DM

space-time is fundamentally higher dimensional in quantum gravity theory event horizon is a global concept, how to imagine local interaction?

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Questions?!