16 6 6 DOE Program

The Quest for Dark Energy

DOE Program Review

Roger BlandfordKIPAC

26 6 6 DOE Program

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%.

Vacuum energy, supersymmetric particles?, axions?

36 6 6 DOE Program

Synopsis

* Geometry* Kinematics

– Distances

* Dynamics– Vacuum energy– LCDM– Potential– Boundary conditions– Parametrized, generalizations

* Observational tests – Astronomical measurements– New telescopes

* Summary

Speed ~ Distance

46 6 6 DOE Program

Geometry

* Flat Universe (zero spatial curvature)

* Hypothesis * Inflation Theory* Microwave Backgound

– Good to 2 percent

0][

][

.3

4

2

3

3

22

=+

=−

adad

P

constaa

ρ

ρ&

nAlexander

56 6 6 DOE Program

Is the Universe Flat?

WMAP

* Microwave Background– Relic of Big Bang

Temperature fluctuationsFew parts per million

66 6 6 DOE Program

Kinematics

* Scale factor a(t)* a=1, now* Redshift =a 0

– 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)

Galileo the Scholastic. Speed ~ Distance

Speed ~ Distance

76 6 6 DOE Program

Distance

* Proper distance now

* Flat space– Distance additive

– Angular diameter distance = ad = proper size/angle subtended

– Luminosity distance =a-1d = (L/4F)1/2

€

d =dt

athen

now

∫

Measure d(a)

86 6 6 DOE Program

General Relativity

* General Relativity (Einstein 1915)– Singular “simple” theory of classical gravity

– G=8T

– 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!

96 6 6 DOE Program

Cosmological Constant/Vacuum Energy

* Einstein 1916– G+g=8T - Cosmological Constant

• Vacuum energy: P=-ρconstantW

* Friedmann 1922 €

€

˙ a 2

2−

4πρa2

3= const.

d[ρa3] = −Pd[a3]

B

Const. Measures curvature. Zero when flat

ρ ~ a-3 for matter

106 6 6 DOE Program

Historically, 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 - -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– CDM Universe

116 6 6 DOE Program

Simple World Models

* Static Universe• ρ• Einstein Universe• Unstable

only– ρ const– De Sitter Universe – a ~ exp t

* Matter only– ρ ~ a-3

– a ~ t2/3 – Einstein - De Sitter Universe– Deceleration

* Matter plus – Singular “simple” theory – a ~ (sinh t)2/3

– CDM universe– Deceleration -> acceleration

t

€

a(t) =Ω0

1− Ω0

⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 3

sinh2 / 3 3(1− Ω0)1/ 2 H0t

2

⎡

⎣ ⎢

⎤

⎦ ⎥

t0 =2cosh−1 Ω0

−1/ 2

3(1− Ω0)1/ 2 H0

126 6 6 DOE Program

CDM 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€

˙ ̇ φ + 4H ˙ φ + H 2(1+ 2q)φ = 4πδP

˙ ̇ φ +8coth[t]

3˙ φ +

4

3φ = 0

136 6 6 DOE Program

Boundary Conditions

* Kinematics:– Measure H0, 0 (or q0 ) now

– Predict d(a) for CDM

* Dynamics– Measure at arec

– Select “growing” mode

– Predict (a) in linear regime

– Correct for nonlinear effects on small scale

146 6 6 DOE Program

Equation of State for Scalar Field

* P=w ρ* Boyle’s law PV1+w ~ w

* w=w(a) = w0+wa(1-a)+…

* Measure wp

* Relate to scalar field theory

€

P =1

2˙ Φ 2 −

1

6∇Φ2 −V (Φ)

ρ =1

2˙ Φ 2 +

1

2∇Φ2 + V (Φ)

Φ''+3HΦ'−∇2Φ + dV /dΦ = 0

156 6 6 DOE Program

Jerk

* For CDM,

* Look at purely kinematic models – Adopt H0, q0

– j =1+j’a+j’’a2/2+…

€

j ≡˙ ̇ ̇ a a2

˙ a 3=1−

4π ˙ P

H 3=1

166 6 6 DOE Program

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

176 6 6 DOE Program

Type 1a supernovae

SDSS/HET: Sako, Romani, Zheng, Amin, Dai…

186 6 6 DOE Program

Supernova Acceleration Probe

* 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.

Spacecraft

Focal plane

196 6 6 DOE Program

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

206 6 6 DOE Program

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

=×= ρρρρρ 4.0B

B

MM

X-rays +Lensing

Nuclear PhysicsTegmark et al

Steve Allen

216 6 6 DOE Program

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…

226 6 6 DOE Program

* 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

236 6 6 DOE Program

Large Scale Structure II

* Find clusters of galaxies– X-ray

– Sunyaev-Zeldovich dips

– Optical galaxy counts

* Count clusters and compare with growth models.

246 6 6 DOE Program

Standard Model of the Universe* ρ = const

=0.7nJm-3 =6 x 10-28 kg m-3

Equivalent to:

• 0.4 mG, 40 K, 1meV, 100, 3THz

• m ~mSUSY2 /mP

• Extra dimensions…

• Anthropic arguments

* ρDM = 0.25nJm-3 Supersymmetric particle?

* ρ = 0.05nJm-3

* Flat spatial geometry

All contemporary data consistent with CDM to 10-20%

256 6 6 DOE Program

* MembersAndy Albrecht, DavisGary Bernstein, PennBob Cahn, LBNLWendy Freedman, OCIWJackie Hewitt, MITWayne Hu, ChicagoJohn Huth, HarvardMark Kamionkowski, CaltechRocky Kolb, Fermilab/ChicagoLloyd Knox, DavisJohn Mather, GSFCSuzanne Staggs, PrincetonNick Suntzeff, NOAO

* Agency Representatives

– DOE: Kathy Turner

– NASA: Michael Salamon

– NSF: Dana Lehr

Dark Energy Task ForceDark Energy Task ForceDark Energy Task ForceDark Energy Task Force

266 6 6 DOE Program

Dark Energy Task Force Charge*Dark Energy Task Force Charge*Dark Energy Task Force Charge*Dark Energy Task Force Charge*

1. Summarize existing program of funded projects

2. Summarize proposed and emergent approaches

3. Identify important steps, precursors, R&D, …

4. Identify areas of dark energy parameter space existing or

proposed projects fail to address

5. Prioritize approaches (not projects)

“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.”

* Fair range of interpretations of charge.† Optimum minimum (agencies); Optimum maximal (community)

276 6 6 DOE Program

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).

286 6 6 DOE Program

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.

296 6 6 DOE Program

1) 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 p aσ σ× ∝ p aσ σ−

306 6 6 DOE Program

Our 8D space: { }0 , , , , , , , lni a DE k m B sq w w n Pω ω∈ Ω ΩQ: Why 8D?

A: Correlations (in all 8D) are important. 2D illustration:

space only: In higher D:

-1

1

aw

DE-1

1

aw

DE

Combined Data1+Data2aw-1 1

Data1, Data2

Data1 Data2

aw-1 1

Data1+Data2

-1

1

aw

DE

aw-1 1

Data1+Data2

Data1+Data2

aw

316 6 6 DOE Program

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 CDM 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…