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Dark Energy
David Spergel
Princeton University & APC
Moriond Meeting: Blois
May 22 2008
What is the Dark Energy?
 We
don’t know
What Can We Measure?
Growth Rate
Distance Measures
Homogenous GR (FRW)
What Can We Measure?
Growth Rate
Distance Measures
Distance Measures
Angular Diameter
BAO
Luminosity
SN
SUPERNOVA
•Proven record of success
•Standardizable Candles
•Systematics are a concern: dust, evolution, …
•Have we reached the systematic limit?
•Most effective at low redshift
Distance Measures
Angular Diameter
BAO
Luminosity
SN
BAO
•Proven record of success (SDSS)
•Standard Rulers (Calibrated from the CMB)
•Cosmic Variance Limited (Need Large Volume)
•Most effective at z ~ 1- 2
Radius = sound speed x time
150 Mpc
5/8/2008
STScI
Bennett / ADEPT
ADEPT-9
0.23 Myrs
CDM
z=1440
Photon-Baryon
Fluid
Each initial over-density/over-pressure
launches a spherical sound wave
Over-pressure causes the wave to travel
outward at the sound speed
(57% speed of light)
The photon-baryon fluid remains coupled
until z = 1090, so the CMB calibrates
the baryon fluctuations
5/8/2008
STScI
Bennett / ADEPT
ADEPT-10
23.4 Myrs
z=79
CDM
At z = 1090:
Photon pressure decouples
CMB travels to us
Sound speed plummets
Wave stalls at a radius of 150 Mpc
For z < 1090…
Baryons
Gravity couples dark matter to baryons
Photons
Dark matter + baryon shells and centers
seed galaxy formation
The universe has a super-position of these
shells
5/8/2008
STScI
Bennett / ADEPT
ADEPT-11
474.5 Myrs
z=10
Baryons
Photons
5/8/2008
STScI
BAO generate a 1% bump in the galaxy
correlation function at 150 Mpc
CDM
Bennett / ADEPT
ADEPT-12
CDM with baryons is a good fit:
c2 = 16.1 with 17 dof.
0.75 Gpc3
3816 deg2
Pure CDM rejected at
Dc2 = 11.7 (3.4s)
Ratio of the distances to z =0.35
and z = 1090 to 4% accuracy
150
Mpc
(h-1=1.4)
Absolute distance to z = 0.35
determined to 5% accuracy
Measuring Growth
Calibrate to CMB
Measure Linear Amptitude
Count Peaks
Measure Velocity
Measuring Linear Amplitude
•Weak Lensing
• LSST: Highest FOM
•Systematics:
•Distortions in Telescopes
•Shear-Galaxy Alignment Effect
•Can be controlled by dividing
galaxies into redshift slices
•Essential to know redshift distribution
Measuring Growth
Calibrate to CMB
Measure Linear Amptitude
Count Peaks
Measuring Linear Amplitude
•Galaxy Redshift Surveys
• Need to Measure bias:
•CMB Lensing, Internal Measures
•Systematics:
•Scale-dependent bias
Measure Velocity
Measuring Growth
Calibrate to CMB
Measure Linear Amptitude
Count Peaks
Measure Velocity
Count Peaks:
•Need to relate observable (X-ray Properties,
Lensing Signal, SZ signal) to Mass
•Systematics:
•Time-evolution in Mass/Observable Relation
Measuring Growth
Calibrate to CMB
Measure Linear Amptitude
Count Peaks
Redshift Space Distortions
•Systematics:
•“Finger of God Effects”
•Galaxy Bias (Less Sensitive)
Measure Velocity
Current Limits
Vikhlinin et al. (2008)
Vikhlinin et al.
(2008)
Self-Consistency
Distance Indicators measure H(a)
CMB measures matter density, WmH2
Redshift distortion measures growth rate of structure
Acquaviva et al. (2008)
Future Limits

Upcoming Large Redshift
Surveys:



Lensing Surveys




SDSS III
ADEPT
LSST
Pan-STARRS
BAO: ADEPT
SN


SNAP
LSST
Future Experiments

Major Projects


Ground: DES, Pan-Starrs,
LSST
Space:
• JDEM: ADEPT, DESTINY,
SNAP
• ESA: DUNE, SPACE

Hopefully have multiple
methods


Complementary Approach
Use space for space-only
measurements
Conclusions
 Observational


Probes of Dark Energy
Is the dark energy a constant with time?
Does GR fail at large scales?
 GR
+ cosmological constant consistent
with current astronomical data
 Upcoming experiments should test GR at
1% level and constrain w(z) at 1% level