The origin of “cosmic acceleration”: void. backreaction 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 二十一世纪第二个十年的宇宙学

The standard cosmology Six parameters describe (almost) everything (cosmological observations). 二十一世纪第二个十年的宇宙学

宇宙学探针 膨胀历史和几何 大尺度结构 宇宙初始条件：微波背景辐射、、中微子背景、引力波背景、、 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 、、、 星系动力学 （kpc-Mpc尺度）、、、 二十一世纪第二个十年的宇宙学

Uncertainties in modern cosmology 3 ? Dynamical dark energy Standard Cosmology Gaussian, adiabatic, nearly scale invariant primordial fluctuations GR+SM +Cold dark matter+Cosmological constant The observable universe Physical principles Initial condition of the universe ? 2 Nonlinearity in GR, Nonlinearity in matter fluid Gastrophysics ? 1 ? Copernican principle implies that the observable universe is a fair sample of THE universe PMG? B-mode? 二十一世纪第二个十年的宇宙学

Unexpected dimming of type Ia supernovae was first detected in 1998 Riess et al. 1998 Perlmutter et al. 1999 Riess et al. 2005 二十一世纪第二个十年的宇宙学

What causes the Supernova dimming? 1 Violations of the Copernican Principle: What if we live at or near the center of a gigantic void? Cosmic acceleration The cosmological constant or dynamical dark energy Alternatives: GR backreaction(?) Modified gravity Astrophysics: Intrinsic evolution (?); Dust extinction. Compton dimming. [Not sufficient]..... 2 3 二十一世纪第二个十年的宇宙学

(1) LTB: "cosmic acceleration" without dark energy Lemaitre-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. compensating shell Low density region （void) background universe (higher density) Gravitational potential distance 二十一世纪第二个十年的宇宙学

哥白尼原理的宇宙学检验 宇宙微波背景黑体谱检验 星系团运动学SZ效应检验 背景运动学SZ效应功率谱检验 排除了许多void模型 r M(r), E(r),t_b(r) 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 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 r 二十一世纪第二个十年的宇宙学

Testing the Copernican principle with free electrons as messenger Goodman 1995; Caldwell & Stebbins, 2008; Garcia-Bellideo & Haughbolle 2008; ZPJ & Stebbins, 2010 Compton scatterings allow us to sit at distant universe and judge whether CP holds consequence 1: CMB spectrum will be non-blackbody T1 e T2 T3 二十一世纪第二个十年的宇宙学

rules out many void models capable of replacing dark 2008 rules out many void models capable of replacing dark energy, but not all of them. Furthermore, ICS induces non-blackbody too. void density Void size 二十一世纪第二个十年的宇宙学

Galaxy clusters (a bunch of electrons) as moving mirrors CMB frame Violation of the Copernican principle prediction Dust (matter) frame Violation of CP causes relative motion between CMB and the matter comoving frame Causes a large cluster kSZ effect In a homogeneous universe, no motion between the two rules out many void models capable of replacing dark energy, but not all of them observations Goodman 1995 二十一世纪第二个十年的宇宙学

The kSZ power spectrum measurement rules out void models Copernican principle is confirmed Cosmic acceleration is REAL! 13 uk^2 (SPT)->8 uk^2(ACT)->6.5 uk^2(SPT) The Hubble bubble model Background universe Compensating shell void at l=3000, a few arcminute 二十一世纪第二个十年的宇宙学 ZPJ & Stebbins 2010

So, cosmic acceleration is REAL! But what causes it? Mirage of GR backreaction? Infrared corrections to GR? Cosmological constant/dark energy? 二十一世纪第二个十年的宇宙学

(2) GR backreaction on the background expansion Exact equation Backreaction Usual approximation A century old topic, still highly controversial! 二十一世纪第二个十年的宇宙学

(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, Einstein-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) 二十一世纪第二个十年的宇宙学

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

Testing specific models: expansion and DGP DGP is disfavored comparing to LCDM Zhu & Alcaniz 2004 Song et al. 2006 二十一世纪第二个十年的宇宙学

CMB and DGP Fang et al. 2008 二十一世纪第二个十年的宇宙学

Redshift distortion and DGP f=dlnD/dlna Blake et al. 2010 Guzzo et al. 2008 二十一世纪第二个十年的宇宙学

参数化引力的宇宙学数据拟合 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 Gong-Bo Zhao et al. 2010 Daniel 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 二十一世纪第二个十年的宇宙学

The first E_G measurement: confirmed GR at ~10-40 Mpc/h scales 检验宇宙学尺度上的 广义泊松方程 ZPJ et al. 2007 BigBOSS can improve by a factor of 10 Reyes et al. 2010 The first E_G measurement: confirmed GR at ~10-40 Mpc/h scales 二十一世纪第二个十年的宇宙学

√ The origin of “cosmic acceleration”: Void? Backreaction? Modified gravity? Dynamical dark energy? or simply Lambda? X X? ?? ?? √? 二十一世纪第二个十年的宇宙学

In O(10) years, hopefully we are able to put everything together to reconstruct the elephant. CMB lensing peculiar velocity the dark universe SNe Ia BAO cluster abundance 二十一世纪第二个十年的宇宙学

Systematics, Systematics, Systematics 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 ..... statistical error 二十一世纪第二个十年的宇宙学