Transcript What is Dark Energy？

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?!