The Quest for Dark Energy

1. The Quest for Dark Energy DOE Program Review Roger Blandford KIPAC DOE Program

2. Recent Progress in Big Bang Cosmology • The Universe is: • R > 7 Hubble radii • Acceleration ~0.6 v2/d • Matter is only 28% of the mass energy; • baryon matter only 4.5%. Flat Accelerating Lightweight => Dark energy, dark matter Vacuum energy, supersymmetric particles?, axions? DOE Program

3. Synopsis Speed ~ Distance • Geometry • Kinematics • Distances • Dynamics • Vacuum energy • LCDM • Potential • Boundary conditions • Parametrized, generalizations • Observational tests • Astronomical measurements • New telescopes • Summary DOE Program

4. Geometry • Flat Universe (zero spatial curvature) • Hypothesis • Inflation Theory • Microwave Backgound • Good to 2 percent S q=p n Alexander DOE Program

5. Is the Universe Flat? WMAP • Microwave Background • Relic of Big Bang Temperature fluctuations Few parts per million DOE Program

6. Galileo the Scholastic. Speed ~ Distance Kinematics • Scale factor a(t) • a=1, now • Redshift l=a l0 • z=1/a-1 • Hubble constant H0=d ln a/dt, 0.07 Gyr-1 now • Deceleration parameter q0=-a’’a/a’2 =0.6, now • MWB a=0.00092 • Quasars a=0.12 • Reionization 0.05 < a < 0.1 • a is good independent variable • No good chronometers - can’t measure t(a) Speed ~ Distance DOE Program

7. Distance • Proper distance now • Flat space • Distance additive • Angular diameter distance = ad = proper size/angle subtended • Luminosity distance =a-1d = (L/4pF)1/2 Measure d(a) DOE Program

8. General Relativity • General Relativity (Einstein 1915) • Singular “simple” theory of classical gravity • G=8pT • Many, more elaborate alternatives • Scalar tensor, bimetric, extra dimensions, PPN… • Experimental Program • Classical tests • Redshift, Mercury. Light deflection • Modern tests • Shapiro delay, gravitational radiation, EP, inverse square law... GR/AE vindicated at level from 10-2 to 10-4! DOE Program

9. Cosmological Constant/Vacuum Energy B • Einstein 1916 • G+Lg=8pT - Cosmological Constant • Vacuum energy: P=-r = constant; W=0 • Friedmann 1922 Const. Measures curvature. Zero when flat r ~ a-3 for matter DOE Program

10. Historically, L was taken very seriously • Lemaitre 1927 • Basic equations, relativistic growth of perturbations • Eddington 1933 • The universe is much bigger than particles; therefore there must a cosmological lengthscale - L-1/2 • “I would as soon think of reverting to Newtonian theory as of dropping the cosmical constant” • “To drop the cosmical constant would knock the bottom out of space” • Bondi 1948 • LCDM Universe DOE Program

11. Simple World Models • Static Universe • L + r • Einstein Universe • Unstable • L only • r const • De Sitter Universe • a ~ exp t • Matter only • r ~ a-3 • a ~ t2/3 • Einstein - De Sitter Universe • Deceleration • Matter plus L • Singular “simple” theory • a ~ (sinh t)2/3 • LCDM universe • Deceleration -> acceleration t DOE Program

12. LCDM Dynamics • Perturbations “grow” • Gravity vs expansion • Two modes • Linear perturbations evolve with time according to: • Extend into nonlinear phase using simulations • Many uncertainties on short scales • Major test of departures from GR DOE Program

13. Boundary Conditions • Kinematics: • Measure H0,W0 (or q0 ) now • Predict d(a) for LCDM • Dynamics • Measure f at arec • Select “growing” mode • Predict f(a) in linear regime • Correct for nonlinear effects on small scale DOE Program

14. Equation of State for Scalar Field • P=w r • Boyle’s law PV1+w ~ w • w=w(a) = w0+wa(1-a)+… • Measure wp • Relate to scalar field theory DOE Program

15. Jerk • For CDM, • Look at purely kinematic models • Adopt H0, q0 • j =1+j’a+j’’a2/2+… DOE Program

16. Distance Measurement • Angular Diameter Distance • Density fluctuations at recombination • H0d(0)=3.4 • Baryon Oscillations • Observe vestigial relic of acoustic oscillation scale at recombination imprinted on galaxy correlation function • Distance from “there” to recombination • Luminosity Distance • Type Ia Supernovae • Surprisingly good standard candles • One parameter empirical lumiinosity DOE Program

17. Type 1a supernovae SDSS/HET: Sako, Romani, Zheng, Amin, Dai… DOE Program

18. SNAP is designed to study dark energy by measuring the rate of expansion of the Universe using supernovae and through determining the distortion of the images of distant galaxies. It is complementary to LSST, emphasizing small over large scale structure SNAP is a collaboration with LBL. KIPAC will be responsible for the Observatory Control Unit and the strong lensing science At present the timescale for SNAP is set by NASA and is unacceptably long. Supernova Acceleration Probe Spacecraft Focal plane DOE Program

19. Baryon Oscillations • Observed in SDSS, 2DF at low redshift • Proposals for large surveys - WFMOS… • ISW effects can complicate • How accurate can this be? • Very promising! Eisenstein et al DOE Program

20. Large Scale Structure I • Growth of Potential • Newtonian physics in Universe expanding at rate given by a(t) • Measure CMB fluctuation spectrum • Clusters of galaxies • Growth of structure • Count clusters of galaxies • Compare with CMB Nuclear Physics Tegmark et al X-rays +Lensing Steve Allen DOE Program

21. Weak Gravitational Lensing • Monitor growth of structure by measuring potential wells using weak lensing • Combines kinematics, dynamics • Emphasizes large scales where growth is linear • Beat down the systematics • Use colors to get distances of sources and lenses • Tomography • Also observe supernovae, baryon oscillations… DOE Program

22. 8.4 m, 3 mirror, 3 lens • 3.3Gpx camera, 10s exposures, 2 s readout • 10sq deg FOV; half sky in 4d • 20 PB/yr data archive, little compression possibility • Dep. Director - Kahn, System Engineer - Althouse • Recent recruits include Burke, Perl, Schindler • Rehab CEH • 14M$ NSF grant to project over 3.5yr Deep, Wide, Fast DOE Program

23. Large Scale Structure II • Find clusters of galaxies • X-ray • Sunyaev-Zeldovich dips • Optical galaxy counts • Count clusters and compare with growth models. DOE Program

24. Standard Model of the Universe All contemporary data consistent with LCDM to 10-20% • rL = const =0.7nJm-3 =6 x 10-28 kg m-3 Equivalent to: • 0.4 mG, 40 K, 1meV, 100m, 3THz • mL ~mSUSY2 /mP • Extra dimensions… • Anthropic arguments • rDM = 0.25nJm-3 Supersymmetric particle? • rB = 0.05nJm-3 • Flat spatial geometry DOE Program

25. Members Andy Albrecht, Davis Gary Bernstein, Penn Bob Cahn, LBNL Wendy Freedman, OCIW Jackie Hewitt, MIT Wayne Hu, Chicago John Huth, Harvard Mark Kamionkowski, Caltech Rocky Kolb, Fermilab/Chicago Lloyd Knox, Davis John Mather, GSFC Suzanne Staggs, Princeton Nick Suntzeff, NOAO Agency Representatives DOE: Kathy Turner NASA: Michael Salamon NSF: Dana Lehr Dark Energy Task Force DOE Program

26. Dark Energy Task Force Charge* “The DETF is asked to advise the agencies on the optimum† near and intermediate-term programs to investigate dark energy and, in cooperation with agency efforts, to advance the justification, specification and optimization of LST and JDEM.” • Summarize existing program of funded projects • Summarize proposed and emergent approaches • Identify important steps, precursors, R&D, … • Identify areas of dark energy parameter space existing or • proposed projects fail to address • 5. Prioritize approaches (not projects) * Fair range of interpretations of charge. † Optimum  minimum (agencies); Optimummaximal (community) DOE Program

27. Four Stages of Investigation • Stage I represents what is now known; • Stage II represents the anticipated state of knowledge upon completion of ongoing projects that are relevant to dark-energy; • Stage III comprises near-term, medium-cost, currently proposed projects; • Stage IV comprises a Large Survey Telescope (LST), and/or the Square Kilometer Array (SKA), and/or a Joint Dark Energy (Space) Mission (JDEM). DOE Program

28. Recommendation IV • IV. We recommend that the dark energy program include a combination of techniques from one or more Stage IV projects designed to achieve, in combination, at least a factor of ten gain over Stage II in the DETF figure of merit, based on critical appraisals of likely statistical and systematic uncertainties. Because JDEM, LST, and SKA all offer promising avenues to greatly improved understanding of dark energy, we recommend continued research and development investments to optimize the programs and to address remaining technical questions and systematic-error risks. DOE Program

29. Bottom Line • The task: • Want to compare constraints from different simulated data sets on dark energy • These comparisons need to include combinations of different simulated data • Our approach: • For each data set, construct a probability distribution in 8D cosmic parameter space using the Fisher matrix method. • Data can be combined by adding the Fisher matrices • Marginalize out non-DE parameters to construct figure of merit area in space DOE Program

30. 1 -1 Our 8D space: Q: Why 8D? A: Correlations (in all 8D) are important. 2D illustration: space only: In higher D: 1 Data1, Data2 Data1 Data2 -1 -1 1 Combined Data1+Data2 1 Data1+Data2 Data1+Data2 Data1+Data2 -1 -1 1 -1 1 DOE Program

31. Summary • Universe is flat, accelerating and lightweight • Unidentified “Dark Matter and Dark Energy” • Simplest view is “particles and vacuum energy” • Good approach is to test LCDM predictions kinematically and dynamically to understand behavior of dark sector and seek failures of classical GR. • Very promising projects to choose between; LSST, SNAP, CMB, SKA… DOE Program