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Surveys of Dark Energy: Challenges and Prospects Ofer Lahav University College London Cosmology post WMAP/2dF/SDSS/… The Dark Energy Survey Photometric redshifts, and cross-talk with cosmic probes The future of the local universe

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“Evidence” for Dark Energy SN Ia CMB LSS – Baryonic Oscillations Cluster counts Weak Lensing Integrated Sachs Wolfe Physical effects: * Geometry * Growth of Structure

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The Chequered History of the Cosmological Constant The old CC problem: Theory exceeds observational limits on by 10 120 ! The new CC problem: Why are the amounts of Dark Matter and Dark Energy so similar?

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Globalisation and the New Cosmology How is the New Cosmology affected by Globalisation? Recall the Cold War era: Hot Dark Matter/top-down (East) vs. Cold Dark Matter/bottom-up (West) Is the agreement on the `concordance model’ a product of Globalisation? OL, astro-ph/0610713

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Matter and Dark Energy tell space how to curve: k = 1 - m - Curvature Matter Dark Energy(Vacuum)

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Matter and Dark Energy tell space how to curve: k = 1 - m - Curvature Matter Dark Energy(Vacuum) OR modified curvature k + = 1 - m

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The Universe is accelerating at present if q 0 = m /2 - < 0 e.g. For m = 0.3 and = 0.7 : k = 0 (the Universe is flat) and the Universe is accelerating (but only ‘recently’, z<0.7)

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Spherical Collapse d 2 r/dt 2 = -GM/r 2 + ( /3) r cf. Newton-Hooke force cf. Inflation For the mass of the Local Group (MW+M31) the forces are equal at r= 1.3 Mpc Dark Energy also affects the virialization radius the collapsed object (OL et al. 91; Maor & OL 05)

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Through the history of the expansion rate: H 2 (z) = H 2 0 [ M (1+z) 3 + DE (1+z) 3 (1+w) ] (flat Universe) matter dark energy (constant w) P = w Comoving distance r(z) = dz/H(z) Standard Candles d L (z) = (1+z) r(z) Standard Rulers d A (z) = (1+z) 1 r(z) The rate of growth of structure also determined by H(z) and by any modifications of gravity on large scales Probing Dark Matter & Dark Energy

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Baryon Wiggles as Standard Rulers

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DUNE: Dark UNiverse Explorer Mission baseline: 1.2m telescope FOV 0.5 deg 2 PSF FWHM 0.23’’ Pixels 0.11’’ GEO (or HEO) orbit Surveys (3-year initial programme): WL survey: 20,000 deg 2 in 1 red broad band, 35 galaxies/amin 2 with median z ~ 1, ground based complement for photo-z’s Near-IR survey (J,H). Deeper than possible from ground. Secures z > 1 photo-z’s SNe survey: 2 £ 60 deg 2, observed for 9 months each every 4 days in 6 bands, 10000 SNe out to z ~ 1.5, ground based spectroscopy

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Imaging Surveys Survey Sq. Degrees FiltersDepthDatesStatus CTIO751shallowpublished VIRMOS91moderatepublished COSMOS2 (space)1moderatecomplete DLS (NOAO) 364deepcomplete Subaru30?1?deep 2005? observing CFH Legacy 1705moderate 2004-2008 observing RCS2 (CFH) 8303shallow 2005-2007 approved VST/KIDS/ VISTA/VIKING 17004+5moderate 2007-2010? 50%approved DES (NOAO) 50004moderate 2008-2012? proposed Pan-STARRS ~10,000?5?moderate 2006-2012? ~funded LSST15,000?5?deep 2014-2024? proposed JDEM/SNAP 1000+ (space) 9deep 2013-2018? proposed VST/VISTA DUNE 5000? 2010-2015? moderate 4+5 proposed 20000? (space) 2+1? moderate 2012-2018? proposed Y. Mellier

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US Dark Energy Task Force Recommendations An immediate start of a near- term program (which we call Stage III) designed to advance our knowledge of dark energy and prepare for the ultimate “Stage IV” program, which consists of a combination of large survey telescopes and/or a space mission. cf. PPARC and ESO/ESA reports Advocate ‘a Figure of Merit’

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DETF FoM / 1/[ellipse area]

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The Dark Energy Survey Study Dark Energy using 4 complementary techniques: I. Cluster Counts II. Weak Lensing III. Baryon Acoustic Oscillations IV. Supernovae Two multi-band surveys 5000 deg 2 g, r, i, z 40 deg 2 repeat (SNe) Build new 3 deg 2 camera and data management system Survey 2010-2015 (525 nights) Response to NOAO AO Blanco 4-meter at CTIO 300,000,000 photometric redshifts

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The DES Collaboration Fermilab: J. Annis, H. T. Diehl, S. Dodelson, J. Estrada, B. Flaugher, J. Frieman, S. Kent, H. Lin, P. Limon, K. W. Merritt, J. Peoples, V. Scarpine, A. Stebbins, C. Stoughton, D. Tucker, W. Wester University of Illinois at Urbana-Champaign: C. Beldica, R. Brunner, I. Karliner, J. Mohr, R. Plante, P. Ricker, M. Selen, J. Thaler University of Chicago: J. Carlstrom, S. Dodelson, J. Frieman, M. Gladders, W. Hu, S. Kent, R. Kessler, E. Sheldon, R. Wechsler Lawrence Berkeley National Lab: N. Roe, C. Bebek, M. Levi, S. Perlmutter University of Michigan: R. Bernstein, B. Bigelow, M. Campbell, D. Gerdes, A. Evrard, W. Lorenzon, T. McKay, M. Schubnell, G. Tarle, M. Tecchio NOAO/CTIO: T. Abbott, C. Miller, C. Smith, N. Suntzeff, A. Walker CSIC/Institut d'Estudis Espacials de Catalunya (Barcelona): F. Castander, P. Fosalba, E. Gaztañaga, J. Miralda-Escude Institut de Fisica d'Altes Energies (Barcelona): E. Fernández, M. Martínez CIEMAT (Madrid): C. Mana, M. Molla, E. Sanchez, J. Garcia-Bellido University College London: O. Lahav, D. Brooks, P. Doel, M. Barlow, S. Bridle, S. Viti, J. Weller University of Cambridge: G. Efstathiou, R. McMahon, W. Sutherland University of Edinburgh: J. Peacock University of Portsmouth: R. Crittenden, R. Nichol, R. Maartnes, W. Percival University of Sussex: A. Liddle, K. Romer plus postdocs and students

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The Dark Energy Survey UK Consortium (I) PPARC funding: O. Lahav (PI), P. Doel, M. Barlow, S. Bridle, S. Viti, J. Weller (UCL), R. Nichol (Portsmouth), G. Efstathiou, R. McMahon, W. Sutherland (Cambridge) J. Peacock (Edinburgh) Submitted a proposal to PPARC requesting £ 1.7M for the DES optical design. In March 2006, PPARC Council announced that it “will seek participation in DES”. PPARC already approved £220K for current R&D. (II) SRIF3 funding: R. Nichol, R. Crittenden, R. Maartens, W. Percival (ICG Portsmouth) K. Romer, A. Liddle (Sussex) Funding the optical glass blanks for the UCL DES optical work These scientists will work together through the UK DES Consortium. Other DES proposals are under consideration by US and Spanish funding agencies.

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The Dark Energy Survey Camera: DECam DECam will replace the prime focus cage 4m Blanco telescope

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Supernovae Ia Geometric Probe of Dark Energy Repeat observations of 40 deg 2, using 10% of survey time ~1900 well-measured SN Ia lightcurves, 0.25 < z < 0.75 Larger sample, improved z-band response compared to ESSENCE, SNLS; address issues they raise Improved photometric precision via in- situ photometric response measurements SDSS

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Observer Dark matter halos Background sources Statistical measure of shear pattern, ~1% distortion Radial distances depend on geometry of Universe Foreground mass distribution depends on growth of structure A. Taylor

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DES Forecasts: Power of Multiple Techniques Ma, Weller, Huterer, etal Assumptions: Clusters: SPT-selected, 8 =0.75, z max =1.5, WL mass calibration (no clustering self-calibration) Mass-observable power-law w/ Lognormal spread BAO: l max =300 WL: l max =1000 (no bispectrum or galaxy-shear) Statistical+photo-z systematic errors only Spatial curvature, galaxy bias marginalized Planck CMB prior w(z) =w 0 +w a (1–a) 68% CL

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DES – Figure of Merit

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Photo-z – WL – BAO - SNIa cross talk Approximately, for a photo-z slice: ( w/ w) = a ( z/ z) = a ( z /z) N s -1/2 => the target accuracy in w and photo-z scatter z dictate the number of required spectroscopic redshifts N s =10 5 -10 6

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Photometric redshifts Probe strong spectral features (e.g. 4000 break) z=3.7z=0.1

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ANNz - Artificial Neural Network Output: redshift Input: magnitudes Collister & Lahav 2004 http://www.star.ucl.ac.uk/~lahav/annz.html z = f(m,w)

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*Training on ~13,000 2SLAQ *Generating with ANNz Photo-z for ~1,000,000 LRGs MegaZ-LRG z = 0.046 Collister, Lahav, Blake et al., astro-ph/0607630

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Excess Power on Gpc Scale? Blake et al. 06 Padmanabhan et al. 06

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DES and VDES DES (griz)DES+VISTA(JK) VISTA J (<21) and K (<19) would improve photo-z by a factor of 2 for z> 1 F. Abdalla, M. Banerji, OL, H. Lin, et al.

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* 4-5 complementary probes * Survey strategy delivers substantial DE science after 2 years * Relatively modest (~ $20-30M), low-risk, near-term project with high discovery potential * Synergy with SPT and VISTA on the DETF Stage III timescale * Scientific and technical precursor to the more ambitious Stage IV Dark Energy projects to follow: LSST and JDEM DES and a Dark Energy Programme

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The Future of the Local Universe m =0.3 LCDM a = 1 (t= 13.5 Gyr) OCDM a = 1 (t= 11.3 Gyr) LCDM a = 6 (t= 42.4 Gyr) OCDM a = 6 (t= 89.2 Gyr) Hoffman, Dover, Yepes, OL

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Some Outstanding Questions: * Vacuum energy (cosmological constant, w= -1.000 after all?) * Dynamical scalar field? * Modified gravity? * Why / m = 3 ? * Non-zero Neutrino mass < 1eV ? * The exact value of the spectral index: n < 1 ? * Excess power on large scales? * Is the curvature zero exactly ?

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Extra Slides

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Expected performance of DECAM, Blanco, and CTIO site Blanco Effective Aperture/ f number @ prime focus4 m/ 2.7 Blanco Primary Mirror - 80% encircled energy0.25 arcsec Optical Corrector Field of View2.2 deg. Corrector Wavelength Sensitivity<350-1000 nm FiltersSDSS g, r, i, z (400-1000 nm) Effective Area of CCD Focal Plane3.0 sq. deg. Image CCD pixel format/ total # pixels2K X 4K/ 520 Mpix Guide, Focus & Wavefront Sensor CCD pixel format2K X 2K Pixel Size0.27 arcsec/ 15 μm Readout Speed/Noise requirement250 kpix/sec/ 10 e Survey Area SPT overlap SDSS stripe 82 Connection region 5,000 sq. deg. total RA -60 to 105, DEC -30 to -65 RA -75 to -60, DEC -45 to -65 RA -50 to 50, Dec -1 to 1 RA 20 to 50, Dec -30 to -1 Survey Time/Duration525/5 (nights/years) Median Site Seeing Sept. – Feb.0.65 arcsec Median Delivered Seeing with Mosaic II on the Blanco0.9-1.0 arcsec (V band) Limiting Magnitude: 10 in 1.5” aperture assuming 0.9” seeing g=24.6, r=24.1, i=24.3, z=23.9 Limiting Magnitude: 5 for point sources assuming 0.9” seeing g=26.1,r=25.6, i=25.8, z=25.4

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DeCam Optical Lay Out C1 C2 C3 Filter C4 C5 978mm 1870mm

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Sources of uncertainties Cosmological (parameters and priors) Astrophysical (e.g. cluster M-T, biasing) Instrumental (e.g. “seeing”)

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MegaZ-LRG Angular power spectra Blake, Collister, Bridle & Lahav, astro-ph/0605303

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Blanco Telescope 4m diameter equatorial mount telescope. Located at altitude of 2200m at Cerro Tololo Inter-American Observatory (CTIO), Chile (Lat. 30 o 10’ S, Long. 70 o 49’ W).

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