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

“Evidence” for Dark Energy

SN Ia CMB LSS – Baryonic

Oscillations Cluster counts Weak Lensing Integrated Sachs Wolfe

Physical effects: * Geometry * Growth of Structure

The Chequered History of theCosmological Constant

The old CC problem:Theory exceeds observational limits on by 10120 !

The new CC problem:Why are the amounts of Dark Matter and Dark Energy so similar?

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

Matter and Dark Energy tell space how to curve:

k = 1 -m - Curvature Matter Dark Energy(Vacuum)

Matter and Dark Energy tell space how to curve:

k = 1 -m - Curvature Matter Dark Energy(Vacuum)

OR modified curvature

k + = 1 -m

The Universe is accelerating at present if

q0 = 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)

Spherical Collapse

d2r/dt2 = -GM/r2 + (/3) r

cf. Newton-Hooke forcecf. Inflation

For the mass of the Local Group (MW+M31)the forces are equal at r= 1.3 Mpc

Dark Energy also affects the virializationradius the collapsed object (OL et al. 91; Maor & OL 05)

Through the history of the expansion rate:

H2(z) = H20 [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 dL(z) = (1+z) r(z) Standard Rulers dA(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

Baryon Wiggles as Standard Rulers

DUNE: Dark UNiverse Explorer

Mission baseline: • 1.2m telescope • FOV 0.5 deg2

• PSF FWHM 0.23’’• Pixels 0.11’’ • GEO (or HEO) orbit

Surveys (3-year initial programme):• WL survey: 20,000 deg2 in 1 red broad band, 35 galaxies/amin2 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 deg2, observed for 9 months each every 4 days in 6 bands, 10000 SNe out to z ~ 1.5, ground based spectroscopy

Imaging SurveysSurvey Sq. Degrees Filters Depth Dates Status

CTIO 75 1 shallow published

VIRMOS 9 1 moderate published

COSMOS 2 (space) 1 moderate complete

DLS (NOAO) 36 4 deep complete

Subaru 30? 1? deep 2005? observing

CFH Legacy 170 5 moderate 2004-2008 observing

RCS2 (CFH) 830 3 shallow 2005-2007 approvedVST/KIDS/

VISTA/VIKING 1700 4+5 moderate 2007-2010? 50%approved

DES (NOAO) 5000 4 moderate 2008-2012? proposedPan-

STARRS ~10,000? 5? moderate 2006-2012? ~funded

LSST 15,000? 5? deep 2014-2024? proposed

JDEM/SNAP1000+ (space)

9 deep 2013-2018? proposed

VST/VISTA

DUNE

5000? 2010-2015?moderate 4+5 proposed

20000? (space) 2+1? moderate 2012-2018? proposed

Y. Mellier

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’

DETF FoM / 1/[ellipse area]

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 deg2 g, r, i, z 40 deg2 repeat (SNe)

• Build new 3 deg2 camera and data management system Survey 2010-2015 (525 nights) Response to NOAO AO

Blanco 4-meter at CTIO

300,000,000 photometric redshifts

The DES CollaborationFermilab: 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. WesterUniversity of Illinois at Urbana-Champaign: C. Beldica, R. Brunner, I. Karliner, J. Mohr, R. Plante, P. Ricker, M. Selen, J. ThalerUniversity of Chicago: J. Carlstrom, S. Dodelson, J. Frieman, M. Gladders, W. Hu, S. Kent, R. Kessler, E. Sheldon, R. WechslerLawrence Berkeley National Lab: N. Roe, C. Bebek, M. Levi, S. PerlmutterUniversity of Michigan: R. Bernstein, B. Bigelow, M. Campbell, D. Gerdes, A. Evrard, W. Lorenzon, T. McKay, M. Schubnell, G. Tarle, M. TecchioNOAO/CTIO: T. Abbott, C. Miller, C. Smith, N. Suntzeff, A. WalkerCSIC/Institut d'Estudis Espacials de Catalunya (Barcelona): F. Castander, P. Fosalba, E. Gaztañaga, J. Miralda-EscudeInstitut de Fisica d'Altes Energies (Barcelona): E. Fernández, M. MartínezCIEMAT (Madrid): C. Mana, M. Molla, E. Sanchez, J. Garcia-BellidoUniversity 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. PercivalUniversity of Sussex: A. Liddle, K. Romer

plus postdocs and students

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.

The Dark Energy Survey Camera: DECam

DECam will replace the prime focus cage

4m Blanco telescope

Supernovae Ia

• Geometric Probe of Dark Energy

• Repeat observations of 40 deg2 , 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

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

Weak Lensing: Cosmic Shear

A. Taylor

DES Forecasts: Power of Multiple Techniques

Ma, Weller, Huterer, etal

Assumptions:Clusters: SPT-selected,8=0.75, zmax=1.5,WL mass calibration(no clustering self-calibration)

Mass-observable power-law w/Lognormal spread

BAO: lmax=300WL: lmax=1000(no bispectrum or galaxy-shear)

Statistical+photo-z systematic errors only

Spatial curvature, galaxy biasmarginalizedPlanck CMB prior

w(z) =w0+wa(1–a) 68% CL

DES – Figure of Merit

Photo-z – WL – BAO - SNIa cross talk

• Approximately, for a photo-z slice:

(w/ w) = a (z/ z) = a (z/z) Ns-1/2

=> the target accuracy in w

and photo-z scatter z dictate the number of required spectroscopic redshifts

Ns =105-106

Photometric redshifts

Probe strong spectral features (e.g. 4000 break)

z=3.7z=0.1

ANNz - Artificial Neural Network

Output:redshift

Input:magnitudes

Collister & Lahav 2004http://www.star.ucl.ac.uk/~lahav/annz.html

z = f(m,w)

*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

Excess Power on Gpc Scale?

Blake et al. 06 Padmanabhan et al. 06

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> 1F. Abdalla, M. Banerji, OL, H. Lin, et al.

• * 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

The Future of the Local Universem =0.3

LCDMa = 1 (t= 13.5 Gyr)

OCDMa = 1 (t= 11.3 Gyr)

LCDMa = 6 (t= 42.4 Gyr)

OCDMa = 6 (t= 89.2 Gyr)

Hoffman, Dover, Yepes, OL

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 ?

Extra Slides

Expected performance of DECAM, Blanco, and CTIO site

Blanco Effective Aperture/ f number @ prime focus 4 m/ 2.7

Blanco Primary Mirror - 80% encircled energy 0.25 arcsec

Optical Corrector Field of View 2.2 deg.

Corrector Wavelength Sensitivity <350-1000 nm

Filters SDSS g, r, i, z (400-1000 nm)

Effective Area of CCD Focal Plane 3.0 sq. deg.

Image CCD pixel format/ total # pixels 2K X 4K/ 520 Mpix

Guide, Focus & Wavefront Sensor CCD pixel format 2K X 2K

Pixel Size 0.27 arcsec/ 15 μm

Readout Speed/Noise requirement 250 kpix/sec/ 10 e

Survey Area SPT overlap SDSS stripe 82 Connection region

5,000 sq. deg. totalRA -60 to 105, DEC -30 to -65RA -75 to -60 , DEC -45 to -65RA -50 to 50, Dec -1 to 1RA 20 to 50, Dec -30 to -1

Survey Time/Duration 525/5 (nights/years)

Median Site Seeing Sept. – Feb. 0.65 arcsec

Median Delivered Seeing with Mosaic II on the Blanco 0.9-1.0 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

DeCamOptical Lay Out

C1

C2 C3

Filter

C4C5

978mm

1870mm

Sources of uncertainties

• Cosmological (parameters and priors)

• Astrophysical (e.g. cluster M-T, biasing)

• Instrumental (e.g. “seeing”)

MegaZ-LRG Angular power spectra

Blake, Collister, Bridle & Lahav, astro-ph/0605303

Blanco Telescope

• 4m diameter equatorial mount telescope.

• Located at altitude of 2200m at Cerro Tololo Inter-American Observatory (CTIO) , Chile (Lat. 30o 10’ S, Long. 70 o 49’ W).