KIAS cosmology Peculiar velocity: a window to the dark universe 张鹏杰 （ Zhang, Pengjie) 中国科学院上海天文台 Shanghai Astronomical Observatory Chinese Academy of Science Based on ZPJ, Michele Liguori, Rachel Bean & Scott Dodelson, 2007, PRL ZPJ & Xuelei Chen, 2008, PRD Bhuvnesh Jain & ZPJ, 2008, PRD ZPJ, Hume Feldman, Roman Juszkiewicz, Albert Stebbins, 2008, MNRAS ZPJ, 2008, arxiv: ZPJ, Rachel Bean, Michele Liguori, & Scott Dodelson, 2008, arxiv:0809.

KIAS cosmology The dark universe The visible world Dark matter? Dark energy? Modified gravity?

KIAS cosmology Windows to the dark universe 21cm Soon to detect

KIAS cosmology lensing SNe Ia BAO cluster abundance peculiar velocity CMB We are able to put everything together to reconstruct the elephant! the dark universe

KIAS cosmology The dark energy task force recommends four  probes of the expansion: SN and BAO  probes of structure growth: weak lensing and cluster abundance Figure of merit for stage IV space projects Peculiar velocity as the fifth!!

KIAS cosmology Matter distribution in our universe is inhomogeneous Gravitational attraction arising from inhomogeneity perturbs galaxies and causes deviation from the Hubble flow v r v r peculiar velocity v=Hr

KIAS cosmology What makes peculiar velocity special and important to probe the dark universe? At scales larger than galaxy clusters, only respond to gravity In linear regime, honest tracer of matter distribution Necessary for the complete phase-space description of the universe

KIAS cosmology  GREAT attractor(s), with far more mass than expected, must exist in order to pull the Milky way at ～ 600 km/s with respect to CMB  Such gigantic structures should be no coincidence, if we believe in the cosmological principle Great attractor Shapely concentration Early applications of peculiar velocity: (1) A brave new world with gigantic structures

KIAS cosmology Early applications of peculiar velocity: (2) road to the standard LCDM cosmology Largely based on peculiar velocity measurements of local and nearby galaxies, some cosmologists (e.g. Jim Peebles) argued that the the cosmological constant may exist and account for ~80% of the energy budget of the universe, in early 80s.

KIAS cosmology How to measure peculiar velocity? Traditional method v r Subtract the Hubble flow to obtain the peculiar velocity v=Hr Measure the recession velocity from the redshift Measure the distance through FP,TF,FJ,SN, etc. Error increases linearly with distance. Can not apply to cosmological distances

KIAS cosmology New probes of large scale peculiar velocity which do not rely on distance indicators These new probes of large scale peculiar velocity do not rely on subtracting the Hubble flow, so are applicable to z~1 Redshift distortion (bonus of BAO surveys) The kinetic Sunyaev Zel'dovich effect of galaxy clusters Type Ia supernovae at z<~0.5

KIAS cosmology Redshift distortion and cosmology Peacock et al Kaiser effect induced by large scale coherent infall Finger of God induced by small scale random motion

KIAS cosmology A sensitive measure of gravity Guzzo et al Acquaviva et al Spectroscopic redshift surveys Measure beta from the anisotropy Measure galaxy bias Obtain f Current measurements

KIAS cosmology BAO surveys (Refer to the WiggleZ talk) are reshaping the field of redshift distortion measurement! (1) the expansion from BAO and (2) the growth rate from redshift distortion Amendola, Quercellini &Giallongo 2004 BAO BAO+RD RD helps to improve dark energy constraints  However, the improvement is not significant for future big surveys  Because if smooth dark energy, BAO and RD basically probes the same H(z)

KIAS cosmology Strong tests on gravity Yun Wang 2007 See also Eric Linder 2007 DE and MG can have nearly degenerate H(z) But their structure growth rate can be very different

KIAS cosmology Testing the consistency relation through spectroscopic redshift surveys Acquaviva et al =0 in GR+smooth dark energy BAO +CMB Redshift distortion

KIAS cosmology Physics behind the consistency relation =

KIAS cosmology = ZPJ et al. 2007; Amendola et al Caldwell et al. 2007; Bertschinger& Zukin Also Uzan 2006 Hu & Sawicki 2007

KIAS cosmology Testing the (generalized) Poisson Equation = Gravitational lensing from peculiar velocity ? Galaxy redshifts to recover redshift information (2D ->3D)

KIAS cosmology Weak lensing Cosmic shear DES, LSST, SNAP, Euclid, SKA, etc. Cosmic magnification SKA Cosmic microwave and 21cm backgrounds Large scale peculiar velocities (bulk flows) Galaxy redshift distortion from spectroscopic redshift surveys Stage III: LAMOST, BOSS, WFMOS, etc. Stage IV: ADEPT, Euclid, HSHS, SKA, etc.  Other methods (KSZ, SNe Ia.... )

KIAS cosmology A discriminating probe of gravity No dependence on galaxy bias No dependence on the shape and amplitude of the matter power spectrum, in the linear regime Scale independent in LCDM and QCDM, whose amplitude is completely fixed by the expansion rate Contains smoking guns of modifications in gravity and particle physics Changes in the amplitude Violation of the scale independence Poisson equation! Linear density growth rate galaxy-galaxy lensing redshift distortion f

KIAS cosmology LCDM f(R) DGP MOND ZPJ, Liguori, Bean & Dodelson 2007, PRL E G will be measured to 1% level accuracy within two decades Promising to detect one percent level deviation from general relativity+canonical dark energy model (if systematics can be controlled)!

KIAS cosmology One can further construct an estimator of η≡-Φ/Ψ Lensing: Φ-Ψ; Peculiar velocity: Ψ ZPJ et al. 2008b Velocity measurement forecasted for SKA ?

KIAS cosmology ZPJ et al. 2008b eta can be measured to 10% accuracy. Errors in eta is larger than errors in E_G Even so, eta can have stronger discriminating power, in some cases. η of DGP differs significantly from that of LCDM. (E G of DGP is very close to that of LCDM.) eta and E_G are complementary DGP with high Omeag_m SKA forecast DGP MOND TeVeS dark energy with anisotropic stress

KIAS cosmology Layers of assumptions/approximations e.g. Matsubara 2007 e.g. Tegmark et al. 2002,2004 Scoccimarro 2004 ZPJ et al. 2007, ZPJ 2008 deterministic bias e.g. Peacock et al. 2001; Guzzo et al. 2008; Amendola et al Linder 2007; Wang 2007 More uncertainties: Linear evolution Light cone distant observer assumption..... F: Lorentz or Gaussian scale independent galaxy bias e.g. Acquaviva et al. 2008

KIAS cosmology On real data Tegmark et al on 2dF Tegmark et al. 2004, on SDSS One can measure the gg,gv,vv power spectra simultaneously. errors (vv)>errors(gv)>errors(gg)

KIAS cosmology Forecast for future surveys the Square Kilometer Array (SKA) as an example Future surveys can detect (1) stochasticity in galaxy bias (2) scale dependence in galaxy bias We are no longer able to use the usual Kaiser formula. At such stage, more detailed check against current RD model and/or more accurate RD modeling are required Other velocity probes? ZPJ 2008 SKA, ADEPT, HSHS, Euclid

KIAS cosmology CMB photon free electron scattered CMB photon v p : bulk velocity scattering probability The kinetic Sunyaev Zel'dovich effect of galaxy clusters Recently, the South Pole Telescope (SPT) has for the first time discovered clusters, through the thermal SZ effect!

KIAS cosmology Measuring velocity from KSZ Allows statistical measurement of v p (v p power spectrum) Measure v p of individual clusters Requires other measurements to infer M g –Thermal SZ to have M g T –X-ray to have T ZPJ et al. 2008, MNRAS Haehnelt & Tegmark 1996; Kashlinsky & Atrio-Barandela 2000; Aghanim et al. 2001; Atrio-Barandela et al. 2004; Holder 2004 Statistical errors>systematical errors

KIAS cosmology SNe Ia as cosmic speed censors at intermediate redshift ~0.5 ZPJ & Chen, 2008, PRD Peculiar velocity causes fluctuations in SNe Ia flux Already allow velocity measurement at z<0.1 At z>0.1, lensing dominates over velocity Measure the 3D power spectrum of SNe Ia flux, in which noise can be significantly suppressed signal (velocity) Noise (lensing) z=0.5

KIAS cosmology If 3 or more independent LSS variables can be measured, modified gravity models can be unambiguously discriminated from DE/DM Weak lensing Peculiar velocity Bhuvnesh Jain & ZPJ, 2008, PRD Peculiar velocity is indispensable for ultimate discrimination between dark energy and modified gravity One necessary condition for DE to mimic MG