张鹏杰中国科学院上海天文台

二十一世纪第二个十年的宇宙学 1

The origin of “cosmic acceleration”: void?

backreaction? modified gravity?

dark energy? or simply Lambda?

张鹏杰中国科学院上海天文台


The standard cosmology

Six parameters describe (almost) everything (cosmological observations).


宇宙学探针

膨胀历史和几何– 一型超新星– 重子声波振荡– 强引力透镜– 星系团 SZ+X-ray

– 、、– 年龄（球状星团、星系）– 、、– 标准警报、脉泽测距、 real

time cosmology 、、

大尺度结构– 弱引力透镜– 红移畸变– ISW 效应– 星系团计数– 星系成团性– 星系和星系团速度– 热 SZ 和运动学 SZ 效应– 、、、

宇宙初始条件：微波背景辐射、、中微子背景、引力波背景、、

星系动力学（ kpc-Mpc 尺度）、、、


Initial condition of the universe

Physical principlesThe observable

universe

Copernican principle implies that

the observable universeis a fair sample of

THE universe

?

?

Uncertainties in modern cosmology

?Nonlinearity in GR,Nonlinearity in matter fluidGastrophysics

?

Gaussian, adiabatic, nearly scale invariant primordial fluctuations

GR+SM +Cold dark matter+Cosmological constant

Dynamical dark energy

PMG? B-mode?

Standard Cosmology 3

21


Unexpected dimming of type Ia supernovae was first detected in 1998

Riess et al. 2005

Riess et al. 1998Perlmutter et al. 1999


What causes the Supernova dimming?

Violations of the Copernican Principle:– What if we live at or near the center of a gigantic v

oid?

Cosmic acceleration– The cosmological constant or dynamical dark en

ergy– Alternatives:

• GR backreaction(?)

• Modified gravity

Astrophysics:– Intrinsic evolution (?); Dust extinction. Compton dimming. [Not suf

ficient].....

32

1


Low density region

（ void)

(1) LTB: "cosmic acceleration" without dark energyLemaitre-Tolman-Bondi model:•The universe has a center•We live at or near the center•Matter distribution is isotropic with respect to and only to the center (us)

In this radially nhomogeneous universe, supernovae can appear dimmer than in a FRW universe, without cosmic acceleration.

Gravitationalpotential

distance

background universe(higher density)

compensating shell


哥白尼原理的宇宙学检验• 宇宙微波背景黑体谱检验

– 排除了许多 void模型• 星系团运动学 SZ效应检验

– 排除了许多 void模型• 背景运动学 SZ效应功率谱检验

– 排除了 void模型，证实了哥白尼原理• 除非微调，破坏掉宇宙极早期的均匀性和物质 -辐射的强耦合

M(r), E(r),t_b(r)

r


Testing the Copernican principle with free electrons as messenger

Goodman 1995; Caldwell & Stebbins, 2008; Garcia-Bellideo & Haughbolle 2008; ZPJ & Stebbins, 2010

e

Compton scatteringsallow us to sit atdistant universe andjudge whether CP holds

consequence 1:

CMB spectrum willbe non-blackbody

T3

T2

T1


2008

Void size

void density

rules out manyvoid modelscapable of replacing darkenergy, but not allof them. Furthermore, ICSinduces non-blackbody too.


Galaxy clusters (a bunch of electrons) as moving mirrors

Dust (matter) frame

Violation of the Copernican principle

Violation of CP causes relative motion between CMB and the matter comoving frame

Causes a large cluster kSZ effect

prediction

observations

CMB frame

In a homogeneous universe, no motion between the two

Goodman 1995

rules out manyvoid modelscapable of replacing darkenergy, but not allof them


The kSZ power spectrum measurement rules out void modelsCopernican principle is confirmed

Cosmic acceleration is REAL!

Background universe

Compensating shell

void

The Hubble bubble model

at l=3000, a few arcminute

13 uk^2 (SPT)->8 uk^2(ACT)->6.5 uk^2(SPT)

ZPJ & Stebbins 2010


So, cosmic acceleration is REAL!

But what causes it?– Mirage of GR backreaction?

– Infrared corrections to GR?

– Cosmological constant/dark energy?


(2) GR backreaction on the background expansion

( )uv uv abT G g

( )FRWuv uv abT G g

22

2

1( ) ( )

2FRW FRW uv uv

uv uv ab ab uv ab ab abab ab

G GT G g h G g h h

g g

L

effuvT

Exact equation

Usual approximation

Backreaction

A century old topic, still highly controversial!


(3) Modified gravity and the cosmic acceleration f(R)

DGP

该领域研究在过去十年出现了长足进展– New/old models: f(T), cascading gravity, Galileo gravity, Ein

stein-Ather, etc. Refer to the review article by Jain & Khoury, 2010– N-body codes/simulations! (e.g. Oyaizu 08; Fabian et al. 08-

11; Chan & Scoccimarro 09; Zhao et al. 01)– Fundamental features/impacts:

• Gravity is environmental depenent• Screening mechanisms required• Violation of the Birkhoff theorem• Violation of equivalence principle (apparent or real)


广义相对论的宇宙学检验宇宙膨胀历史 +宇宙大尺度结构 + 太阳系检验

– 具体模型的检验– 参数拟合– Smoking guns

SN Ia, BAO,CMB

Jain & Khoury 2010

二十一世纪第二个十年的宇宙学 17Song et al. 2006

DGP is disfavored comparing to LCDM

Zhu & Alcaniz 2004

Testing specific models: expansion and DGP


CMB and DGP

Fang et al. 2008


Redshift distortion and DGP

Guzzo et al. 2008

Blake et al. 2010

f=dlnD/dlna


参数化引力的宇宙学数据拟合

Daniel et al. 2010

Gong-Bo Zhao et al. 2010

一般 2 个参量，分别描述物质弯曲时空的总能力和弯曲时间、空间的相对能力


The first E_G measurement: confirmed GR at ~10-40 Mpc/h scales

Reyes et al. 2010

2galaxy-lensing ( )

galaxy-velocityGE

ZPJ et al. 2007

BigBOSS can improveby a factor of 10

检验宇宙学尺度上的广义泊松方程


The origin of “cosmic acceleration”:

Void?

Backreaction?

Modified gravity?

Dynamical dark energy?

or simply Lambda?

X

??

X?

??

√?

√


lensing

SNe Ia

BAOcluster abundance

peculiar velocity

CMB In O(10) years, hopefully we are able to put everything together to reconstruct the elephant.

the dark universe


Systematics, Systematics, Systematics

statistical error

•Theory•The dark degeneracies!•Unbiased parametrizations

•Calculation•Nonlinearity and gastrophysics•N-body and hydro simulations

•Measurement•SN Ia: evolution? sub-types? Dust extinction? ..•Cluster: mass-flux scaling relation...•Weak lensing: PSF, intrinsic alignment, photo-z...•Redshift distortion: high order coupling, velocity bias, distant observer....•BAO: nonlinearity, scale dependent bias•.....

张鹏杰 中国科学院上海天文台

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张鹏杰中国科学院上海天文台