Studying the Dark Side of the Universe with. Tim Schrabback. Leiden Observatory Dresden, February 12th, 2009

Transcription

1 Studying the Dark Side of the Universe with Tim Schrabback Leiden Observatory Dresden, February 12th, 2009

2 Roadmap 1. Cosmology: Our changing view of the dark Universe 2. Seeing the invisible with gravitational lensing 3. Current surveys: GEMS, COSMOS, CFHTLS 4. Conclusions and Outlook

3 Our view of the Universe has changed... Early 1900s: Jacobus Cornelius Kapteyn: The Island Universe consists of the Milky Way only, with diameter 30,000 Ly and us close to the center.

4 The Universe is much bigger! 1925/1929: Edwin Hubble resolves Cepheid stars in NGC 6822 and Andromeda They are galaxies separate from the Milky Way (d=1.6/2.5mly) NGC6822 (Blanco Telescope CTIO) Andromeda Galaxy M31

5 An evolving Universe? 1917: To compensate gravity and obtain a static Universe, Albert Einstein introduces the cosmological constant Λ in the GR field equations. 1922: Alexander Friedman finds a solution to GR predicting an expanding universe. 1927: Georges Lemaître: The Universe must have started small: GR+homogeneity Big Bang!

6 An evolving Universe! 1929: Hubble discovers the expansion of the Universe. z=0.25 Redshift z=0.05 z= z=0 Redshift: z = λobs/λ0 1 = v/c+... Distance

7 What sets the rate? The expansion rate depends on the content of the Universe - So what are the ingredients?

8 The ingredients... Ordinary baryonic matter + (stars, gas, planets, lecture halls...)? Radiation + (mostly Cosmic Microwave Background)

9 There is more than meets the eye: Dark Matter (DM) In 1937 Fritz Zwicky reports first evidence for DM based on kinematics in the Coma galaxy cluster. Not accepted until Vera Rubin finds further evidence from galaxy rotations in Coma cluster (SDSS)

10 Anything else? Until 1998 most astronomers were happy with a DM-dominated expanding Universe. Only blemish: Observations pointed towards a low matter density of only 3 Ωtot~30% of the critical density crit 4m H /m Ωtot=1 would be required for flat space: Deviations unstable Fine-tuning problem.

11 How to check if there is anything else? Probe the expansion history! Extend Hubble's diagram of redshift versus distance to very large distances: Use type Ia supernovae as 1037W light bulbs Standard candles

12 A surprising result! The SNe are too faint! Riess et al. (1998) Perlmutter et al. (1999) Riess et al. (2007) z=λobs/λ0 1

13 Something speeds up the Universe! Let's call it Dark Energy (DE) How to accelerate the expansion? Look at the 2nd Friedman equation: 1 Cosmic scale factor: a= 1 z a 4π 3p = G ρ 2 a 3 c a t 0 =1 a nonrel. matter: p=0 0 a a 1 2 DE: 0 p ρ c a 3 1 p=w ρ c w DE 3 2

14 Maybe the vacuum has a non-zero energy density? 1st law of thermodynamics: d Q=d U p d V =0 d U =d ε V =d ρ c 2 V = p d V 2 If ρ = const p= ρ c w Vac = 1 This is equivalent to Einstein's cosmological constant Λ.

15 A Universe full of hypothetical Dark Matter and Dark Energy is very strange Maybe our theory is wrong? Supernovae could have redshift evolution Our theory of gravity could be wrong Urgently need independent tests!!

16 T he C M B : D a w n o f prec is io n c o s m o lo g y The temperature variations in the Cosmic Microwave Background are caused by density fluctuations at z= BOOMERANG/ MAXIMA: Space is flat! Ωtot 1 Red: hotter / blue: colder (BOOMERANG)

17 W M A P 5 yea r res ults (2008) CMB temperature fluctuations: -0.2mK (blue)to +0.2mK (red)

18 C M B P ow ers pec trum : B a ryo nic a c c o us tic o s c illa tio ns (DM Baryons+Photons) 1 compression= Sound horizon Curvature 1 compression + 1 expansion ΩB /ΩDM

19 Gravitational collapse leads to the structures we see today Galaxy distribution measured by the SDSS Redshift Survey.

20 Late-time integrated Sachs-Wolfe effect In a DE-dominated Universe photons have a net energy gain when falling into and leaving (decaying) potential wells. Test for DE! WMAP CMB signal stacked at the positions of clusters and voids traced by SDSS luminous red galaxies (Granett et al. 2008, also: Scranton et al. 2003, Ho et al. 2008, Giannantonio et al. 2008)

21 Good accounting? + =

22 Progress? WMAP5 We don't know what 96% of the Universe are...

23 What is Dark Matter? We don't know, but we know a few things: It cannot be in compact objects of ~ stellar size It must be non-baryonic (interacts only via gravity, maybe weak interaction) It should mostly consist of fairly stable heavy particles (cold or non-relativistic at time of CMB) Neutrinos contribute, but are sub-dominant Requires physics beyond the Standard Model: Several WIMP candidates, e.g. neutralino

24 What is Dark Matter? The prospects are good: Several experiments try direct detection: DAMA, EGRET, CDMS, XENON, CRESST, ZEPLIN, DRIFT, YOU might soon find hints with LHC!! Gamma-ray observatories try to detect a possible annihilation signal Gamma-ray all-sky map from FERMI (GLAST)

25 What is Dark Energy? We have no clue! So far the observations are consistent with vacuum energy (w =-1), but then why is the QFT estimate too large by a factor 10120? It could also be a dynamic quantity with w (a), e.g. a scalar field ( quintessence ) Alternatively GR could be wrong, incomplete, or incorrectly applied (e.g. Wiltshire 2007). In any case: NEW PHYSICS!

26 How to make progress with DE? There are several probes available, each with their own advantages and disadvantages. Distances w (a) Standard candles (SNe) Standard rulers (BAO+CMB) Growth of structure w (a) + Test of GR Clustering of matter Counting galaxy clusters

27 ΛCDM OCDM Spot the difference? z=3 z=1 Kaufmann et al. z=0 The cosmological parameters influence the evolution of matter clustering. A measurement of the clustering as function of scale and redshift can be used as cosmological probe.

28 Look at the bright side? We can simulate the dark matter distribution quite well. Unfortunately we don't observe dark matter, but galaxies... Galaxy formation is a complex non-linear process (DM, gas, star formation, heating, cooling, AGN feedback, magnetic fields, merging, cosmic rays, ), which is still subject to immense investigation.

29 Light...

30 Light density

31 Gravitational lensing Credit: NASA Gravitational light deflection is a natural consequence of GR! Strong lensing: The deflection angle is a direct measure of mass! Lensing by stars: Einstein (1936, notes 1912), also Lodge (1919) and Chwolson (1924). Consider extragalactic lenses: Zwicky (1937) First lens: "Twin Quasar" Q (Walsh et al. 1979)

32 HST/ACS image of the galaxy cluster RXJ (NASA/ESA/STScI/AIfA, PI: Erben, Image: Schrabback)

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34 Weak lensing No multiple images Cannot use deflection angles... But: Galaxy images are coherently distorted ( sheared ) by tidal gravitational field Can be measured statistically!

35 Simulation of a lensing cluster Credit: Erben No lens With cluster lens

36 RXJ : Lensing vs. X-ray mass The X-ray temperature and luminosity of hot intra-cluster gas provides a mass estimate assuming spherical symmetry and hydrostatic equilibrium. Use lensing for calibration! S+W Lensing X-Ray Light Mass (<Radius) Radius Bradac, Schrabback, et al. 2008

37 A direct proof for DM: The Bullet Cluster Cluster collision: The lensing mass (blue) coincides with the stars and not the trailing gas (red) which contains most of the baryons. Clowe et al. 2006, Bradac et al Limits for DM self-interaction cross-section: σ/m < 1.25 cm2/g (Randall et al. 2008)

38 SL is rare, but WL everywhere: Cosmic Shear Ωm=0.3 ΩΛ=0 Shear is the projected tidal gravitational field. Measure statistics cosmological constraints independent of relation to luminous matter. Ωm=1 ΩΛ=0 Ray-tracing through N-body simulations (Jain, Seljak, & White 2000). Left: Magnification (similar to projected mass) Right: Shear field.

39 Why weak lensing? Weak lensing provides a direct measurement of the projected (dark) matter distribution. The physics is well understood: GR The applications are numerous: Statistical properties of the matter (cosmological parameters). Relation between galaxies and DM (galaxy biasing). Properties of DM halos in galaxies and clusters (test of CDM and law of gravity).

40 Why cosmic shear? Dark Energy Task Force comments: The WL technique is also an emerging technique. Its eventual accuracy will also be limited by systematic errors that are difficult to predict. If the systematic errors are at or below the level asserted by the proponents, it is likely to be the most powerful individual Stage-IV technique and also the most powerful component in a multi-technique program. Progress in Dark Energy constraints depends critically on our understanding of the cosmological probes, their systematic errors, and involved physics (WL: only GR). Do we understand what we are doing?

41 What are the assumptions? Underlying assumption: Galaxy position angles are random in the absence of lensing. At some level intrinsic alignments will complicate things (can probably be dealt with using redshifts). No lensing Lensing

42 Basics of weak lensing slightly more formal Linearize lens-mapping: Define complex ellipticity with 2nd-order brightness moments: Ellipticity is an unbiased estimator for shear:

43 What do we need to do? We only need to measure: Shapes Redshifts

44 Photometric redshifts Galaxies are typically too faint for spectroscopic redshifts. Instead determine photometric redshifts from multi-color data. Only the most recent lensing surveys have photo-zs! Source: Bridle

45 Measuring shapes The weak lensing shear is small (γ 0.01) compared to the intrinsic shape noise (σe 0.3) Average over many galaxies (millions!) Remove instrumental signals at high accuracy Great'08 challenge

46 Systematics Instrumental systematics from the pointspread-function (PSF) often exceed the cosmological signal by a factor 10. Observed shapes need to be corrected for: PSF anisotropy PSF circularization Camera distortion HST/ACS PSF (TinyTim, ACS Instrument handbook) Various correction techniques have been developed. In particular the Kaiser et al. (1995) approach is widely used. This method works fine for current data sets, but we need improved methods for upcoming large surveys.

47 Checking for systematics Lensing provides internal checks for (PSF-) systematics: E-/B-mode decomposition (lensing creates only E-modes) Star-galaxy correlation Only problem: if non-zero, they don't tell you what the origin is... Calibration biases can be identified using image simulations, e.g. STEP (Heymans et al. 2006; Massey et al. 2007), GREAT'08 Current accuracies: 1-2%

48 What we really measure: shear-shear correlations Correlate galaxy ellipticities as function of separation. These are estimators for the shear correlation functions. Shear 2pt-correlations: power spectrum Convergence/shear power spectrum: redshifts cosmology geometry

49 Shear power spectrum Non-linear structure growth Refregier et al. (2003)

50 What we measure to 1st order To 1st order the shear signal measures the variance of the matter density fluctuations combination of the power spectrum normalization σ8 and Ωm. Similar lensing signal: Little bit of matter, large fluctuations Lot of matter, small fluctuations This degeneracy can partially be broken by using 3rd order shear statistics or tomography.

51 Weak lensing tomography Split galaxies into redshift bins Dark energy Increases distances Suppresses structure growth at low z Only the most recent surveys have photo-zs! Hu (1999)

52 Results a decade ago... This page was left blank intentionally...

53 Results: Status in 2007 Since its first detection in 2000, several cosmic shear measurements have been published: Lensing Xray clusters Wmap3 Hetterscheidt et al. 2007: σ8 for Ωm=0.3

54 WL: From Ground vs. Space Ground-based Resolution 0.8'' 5-30 gals / (')2 Large field, e.g. MEGACAM 1deg2 State-of-the-art: CFHTLS-Wide: 170deg2 ugriz zm 0.8 CFHT CFHT MEGACAM (Terapix) Space-based (HST/ACS) Resolution gals / (')2 Reduces PSF effects by (0.8/0.11)2=50 Small field: 3.3'x3.3' GEMS: 0.25deg2 (78 tiles) COSMOS: 1.64deg2 (580 tiles), zm 1.3 HST/GEMS HST (NASA/ESA)

55 Weak lensing with HST/ACS: Challenges... PSF ellipticity

56 Challenges... PSF variation Few stars in galaxy fields no polynomial interpolation. New: template fitting (Schrabback et al. 2007), PCA (Schrabback et al. in prep.)

57 Challenges... CTE degradation Non-linear effect, creates spurious ellipticity. Apply parametric model for stars and galaxies! CTE trails of hot pixels

58 Challenges... Masking

59 GEMS E-/B-mode decomposition Schrabback et al. 2007

60 Cosmological parameters from GEMS Photo-z distribution from GOODS-MUSIC (Grazian et al. 2006): zm=1.46±0.12 Assume flat ΛCDM, non-linear PS: Smith et al Covariance from Gaussian shear field realizations 0.12 Schrabback et al Result GEMS: 8 m =0.25 = =0.81±0.04 WMAP5: Worried? No: Sampling variance! CDFS under-dense, also Phleps et al Hartlap, Schrabback, et al. 2009: Ray-tracing Chance 5% in WMAP5 cosmology, 15%-20% incl. field selection

61 COSMOS E-/B-mode decomposition Massey et al Schrabback et al. in prep.

62 Preliminary constraints from COSMOS Photo-zs from COSMOS-30 (Ilbert et al. 2008): zm=1.2, 45% individual tomography! Flat ΛCDM, Smith et al. 2003, Gaussian covariance 1.5 Schrabback et al. in prep σ8 1.0 Preliminary results m=0.25 Our analysis: 8=0.79±0.06 8=0.81±0.04 WMAP5: 0.06 =0.96±0.04 Massey et al 07: Full analysis with ray-tracing covariance, w constraints and test for intrinsic alignments in progress Ωm

63 COSMOS: Dark Matter maps Massey et al. 2007b Schrabback et al. in prep.

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65 The latest from CFHTLS Fu et al. 2008: Included area: 57deg2 Measure shear out to 4deg! z-distribution from CFHTLSDEEP No tomography yet 0.64 σ (Ω /0.25) =0.79± m

66 CFHTLS: Simplified tomography Wide only Wide+Deep tomography Semboloni et al. 2006

67 CFHTLS: Next steps The survey has just been completed (170deg2 ugriz)! A large team (~25 person) from Bonn, Edinburgh, Leiden, Naples, Oxford, Paris, Vancouver, and Waterloo is currently working hard to improve the analysis and fix residual systematics, also combining with the full photo-z information... Stay tuned!

68 Conclusions and Outlook Weak lensing is the most direct tool available to study Dark Matter on scales from galaxies to large-scale structure Weak lensing has the potential to become the most powerful probe for Dark Energy. COSMOS provides a first demonstration for lensing tomography and a proof of concept for future space-based missions. The upcoming CFHTLS analysis will utilize the full redshift information and provide substantially improved constraints. For the next years several large ground-based lensing surveys are scheduled to start: PanSTARRS, VST/KIDS, DES, LSST The ultimate accuracy for DE studies might however only be reachable from space. NASA+ESA plan a joint mission in 2017+! There are still several issues which require improvement (shape measurements, photo-zs, intrinsic alignments), but so far no show-stopper has been identified!

69 Thank you very much for your kind attention! I would also like to thank my collaborators: Elisabetta Semboloni, Patrick Simon, Jan Hartlap, Benjamin Joachimi, Martin Kilbinger, Peter Schneider, Thomas Erben, Catherine Heymans, Phil Marshall, Hendrik Hildebrandt, Henk Hoekstra, Konrad Kuijken, Ludovic van Waerbeke, Chris Fassnacht, Eric Morganson, Marusa Bradac, Marco Hetterscheidt, Joan-Marc Miralles, Tim Eifler, Jörg Dietrich, Robert Fosbury, Wolfram Freudling, Norbert Pirzkal, & the CFHTLS lensing team. When thinking about places for your next Post-Doc, PhD, etc., check: