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

3. 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
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DOE Program

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

5. 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
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DOE Program

6. 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
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DOE Program

7. 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)
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DOE Program

8. 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!
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DOE Program

9. 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
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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
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11. 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
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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
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DOE Program

13. 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
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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
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DOE Program

15. Jerk For CDM, Look at purely kinematic models Adopt H0, q0
j =1+j’a+j’’a2/2+…
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16. 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
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17. Type 1a supernovae SDSS/HET: Sako, Romani, Zheng, Amin, Dai… 6 6 6
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18. 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
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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
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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
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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…
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DOE Program

22. 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
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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.
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DOE Program

24. 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%
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25. 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
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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)
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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).
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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.
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DOE Program

29. 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
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30. 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
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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…
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DOE Program