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The Quest for Dark Energy
DOE Program Review Roger Blandford KIPAC 6 6 6 DOE Program
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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? 6 6 6 DOE Program
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Synopsis Geometry Kinematics Dynamics Observational tests Summary
Speed ~ Distance Geometry Kinematics Distances Dynamics Vacuum energy LCDM Potential Boundary conditions Parametrized, generalizations Observational tests Astronomical measurements New telescopes Summary 6 6 6 DOE Program
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Geometry S q=p Flat Universe (zero spatial curvature) Hypothesis
Inflation Theory Microwave Backgound Good to 2 percent S q=p Alexander n 6 6 6 DOE Program
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Is the Universe Flat? Microwave Background Relic of Big Bang WMAP
Temperature fluctuations Few parts per million WMAP Freeze the action when everyone tells the same time Long thought that it was negatively curved Universe 1/3 million yr young Cf 14 billion old Size of full moon Expand to present size Do Lobachevsky’s expt. Flat to 2% Easiest for students Ironic that Columbus’ sailors thought that the Earth was flat And we thought the universe was curved 6 6 6 DOE Program
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Kinematics Scale factor a(t) a=1, now Redshift l=a l0
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= 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 6 6 6 DOE Program
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Measure d(a) 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) 6 6 6 DOE Program
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GR/AE vindicated at level from 10-2 to 10-4!
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! 6 6 6 DOE Program
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Cosmological Constant/Vacuum Energy
Einstein 1916 G+Lg=8pT - Cosmological Constant Vacuum energy: P=-r = constant; W=0 Friedmann 1922 B Const. Measures curvature. Zero when flat r ~ a-3 for matter 6 6 6 DOE Program
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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 6 6 6 DOE Program
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Simple World Models t Static Universe L only Matter only Matter plus L
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 6 6 6 DOE Program
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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 6 6 6 DOE Program
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Boundary Conditions Kinematics: Dynamics 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 6 6 6 DOE Program
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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 6 6 6 DOE Program
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Jerk For CDM, Look at purely kinematic models Adopt H0, q0
j =1+j’a+j’’a2/2+… 6 6 6 DOE Program
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Distance Measurement Angular Diameter Distance Luminosity 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 6 6 6 DOE Program
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Type 1a supernovae SDSS/HET: Sako, Romani, Zheng, Amin, Dai… 6 6 6
DOE Program
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Supernova Acceleration Probe
Spacecraft 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. 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 Focal plane 6 6 6 DOE Program
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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 6 6 6 DOE Program
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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 6 6 6 DOE Program
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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… 6 6 6 DOE Program
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Deep, Wide, Fast 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 6 6 6 DOE Program
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Large Scale Structure II
Find clusters of galaxies X-ray Sunyaev-Zeldovich dips Optical galaxy counts Count clusters and compare with growth models. 6 6 6 DOE Program
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Standard Model of the Universe
rL = const =0.7nJm-3 =6 x 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 All contemporary data consistent with LCDM to 10-20% 6 6 6 DOE Program
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Dark Energy Task Force Agency Representatives DOE: Kathy Turner
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 6 6 6 DOE Program
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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) 6 6 6 DOE Program
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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). 6 6 6 DOE Program
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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. 6 6 6 DOE Program
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Data can be combined by adding the Fisher matrices
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 6 6 6 DOE Program
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A: Correlations (in all 8D) are important. 2D illustration:
Our 8D space: Q: Why 8D? A: Correlations (in all 8D) are important. 2D illustration: space only: In higher D: -1 1 1 Data1, Data2 Data1 Data2 -1 -1 1 Combined Data1+Data2 1 Data1+Data2 Data1+Data2 Data1+Data2 -1 -1 1 -1 1 6 6 6 DOE Program
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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… 6 6 6 DOE Program
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