The DoE/NASA Joint DarkEnergy Mission

November 2005Kim Griest

• JDEM Science Definition Team (SDT):Cochairs: Chuck Bennett and Kim Griest

• Members: Albrecht, Baltay, Barish, Bernstein, Blandford,Caldwell, Cheng, Deustra, Donahue, Eisenstein, Ellis, Freedman,Frieman, Gardener, Glazebrook, Kirshner, Lauer, Levi, Linder, Lupton,Moos, Morse, Perlmutter, Rausher, Riess, Seiffert, Wheeler, Wright

• Charge: advice to DoE/NASA on whatJDEM should do; AO will be written basedupon our report

• Discover and characterize the nature of thedark energy with a space mission

• JDEM idea extremely highly rated by NAS,DoE, NASA, etc: great support everywhere

• Very little current money and current totalcost cap is $600M, but may cost more

• Date of launch, even of AO not clear(money)

• Started out as SNAP (Super NovaAcceleration Probe) led by DoE’s LBL, butwhen NASA joined required a competition

• NASA: “Beyond Einstein Probes” fewhundred thousand dollars, but now severalmillion from NASA for “JDEM MissionConcept Design” (SNAP, DESTINY, JEDI,ADEPT)

• DoE spent substantial amount on LBL, etc.effort (all on SNAP)

What Science will JDEM do?

• Depends upon what method is chosen!• Top contenders are SNIa and weak lensing

=> wide field imager and spectrograph• Baryon oscillations and X-ray clusters also

being studied and might compete• Key: able to do substantially better than

from the ground

How to compare differentexperiments?

• Need to know what we are looking for; i.e. what isDE and how to characterize it!

• Help from theorists? No compelling models• Comparison depends on goal:

– Cosmo-illogical constant or not?• But is w= -1.0 +- 0.05 good enough? -1+- .01? -1+- 0.001?

Again theoretical guidance is lacking– Is w changing in time?

• Constraint in w, w’ plane?• w’ vs. w_a? To what accuracy is w’ needed? Consider big

change at early redshift?– Is GR correct?

• My opinion: eventually need both geometry (SNor BAO) and growth of structure (WL or ?)methods to test GR, but will both be needed fromspace? Will it be too expensive to do all fromspace?

• With no specific absolute target goal, can onlycompare and rank different experiments andmethods– But different methods/experiments are sensitive to

different redshift range => different types of DE• To compare power of experiments, JDEM SDT is

creating set of fiducial models with behaviors thathopefully span the range of reasonablepossibilities

• We specify priors (e.g. value and error in H0;value of Omega_tot) and model parameters andask each proposer to calculate how well theirexperiment measures the appropriate parameters(e.g. w, w_a)

• Besides Lambda, and simple quintessence, willhave models with DE contribution at higherredshift and models which do not follow GR

• Wide Field camera in space has many purposes:Should JDEM do ancillary science?– Community support better, but– DoE vs. NASA– Cost for longer mission is substantial (Guest observers)– Is JDEM a wide field camera at all?

Most developed JDEM missionconcept is SNAP

• 2 meter aperature wide field imager in L2 orbit• 1/3 square degree for SNIa and weak lensing• 6 optical (350-1000nm, 0.1 arcsec/pixel) and 3 IR

(900-1700nm, 0.17 arcsec/pixel) filter bands:(total around 600 M pixel)

• Spectrograph (R=100) for SN spectra• Very stable PSF, good control of systematics

SNAP Surveys

• Deep 15 sq deg survey every 4 days for SNIasearch: in 2 years get 2000 SNIa with excellentspectra and lightcurves out to z=1.7

• Wide weak lensing survey: perhaps 1000 squaredegrees to AB 27.7, or 10000 sq deg to 26.7?

• Goal is w to 4% and w’ to 10% (is this goodenough?)

•2 meter mirror, HgCdTe arrays, L2 orbit,•SNIa and SNII out to z=1.7•Measure w to 0.05, w’ to 0.2

JEDIYang, Crotts, Garnevich, Priedhosky, Habib, Heitmann, Kutyrev,

Moseley, Squires, Tegmark, Wright

• 2 meter, wide field (1 sq deg) plus slitspectroscopy (1’’ by 5’’ micro-shutters) forall objects in field

• NIR .8-4 micron, ~250 2Kx2K HgCdTe• 3 years: 20000 SNIa over 36 deg sq to

z=1.7, redshift survey to z=~4, weak lensing• L2 orbit, dark energy density vs time to 10%

or better

ADEPTC.Bennett, etc.

• Still being designed: 1 meter mirror, baryonacoustic oscillations? Etc.???

Conclusions• Dark Energy problem is important enough

and has enough support that a space missionwill be flown as long as substantialimprovement over ground is convincinglyproved.

• Time scale is uncertain, cost cap uncertain,method uncertain, but probably will includeSNIa and/or weak lensing.