1. Princeton University & APC
Dark Energy
David Spergel
Princeton University & APC
Moriond Meeting: Blois
May

2. What is the Dark Energy?
We don’t know

3. Homogenous GR (FRW)

5. SUPERNOVA
Proven record of success
Standardizable Candles
Systematics are a concern: dust, evolution, …
Have we reached the systematic limit?
Most effective at low redshift

6. BAO
Proven record of success (SDSS)
Standard Rulers (Calibrated from the CMB)
Cosmic Variance Limited (Need Large Volume)
Most effective at z ~ 1- 2

9. BAO Evolution Radius = sound speed x time 150 Mpc 5/8/2008 STScI
Bennett / ADEPT
ADEPT-9 9

10. Each initial over-density/over-pressure
0.23 Myrs z=1440
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
Photon-Baryon
Fluid
CDM
5/8/ STScI
Bennett / ADEPT
ADEPT-10 10

11. Photon pressure decouples CMB travels to us Sound speed plummets
At z = 1090:
Photon pressure decouples
CMB travels to us
Sound speed plummets
Wave stalls at a radius of 150 Mpc
For z < 1090…
Gravity couples dark matter to baryons
Dark matter + baryon shells and centers seed galaxy formation
The universe has a super-position of these shells
CDM
Baryons
Photons
23.4 Myrs z=79
5/8/ STScI
Bennett / ADEPT
ADEPT-11

12. BAO Evolution 474.5 Myrs z=10 BAO generate a 1% bump in the galaxy
CDM
Photons
474.5 Myrs z=10
Baryons
BAO generate a 1% bump in the galaxy
correlation function at 150 Mpc
5/8/ STScI
Bennett / ADEPT
ADEPT-12

13. 2005 BAO Discovery in z = 0.35 SDSS
CDM with baryons is a good fit: c2 = 16.1 with 17 dof.
Pure CDM rejected at
Dc2 = 11.7 (3.4s)
Ratio of the distances to z =0.35
and z = 1090 to 4% accuracy
Absolute distance to z = 0.35
determined to 5% accuracy
0.75 Gpc3
3816 deg2
150
Mpc
(h-1=1.4) 13

14. 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

15. Measuring Linear Amplitude
Galaxy Redshift Surveys
Need to Measure bias:
CMB Lensing, Internal Measures
Systematics:
Scale-dependent bias

16. Count Peaks:
Need to relate observable (X-ray Properties, Lensing Signal, SZ signal) to Mass
Systematics:
Time-evolution in Mass/Observable Relation

17. Redshift Space Distortions
Systematics:
“Finger of God Effects”
Galaxy Bias (Less Sensitive)

18. Current Limits
Vikhlinin et al. (2008)
Vikhlinin et al. (2008)

19. Self-Consistency Distance Indicators measure H(a)
CMB measures matter density, WmH2
Redshift distortion measures growth rate of structure
Acquaviva et al. (2008)

20. Future Limits Upcoming Large Redshift Surveys: Lensing Surveys
SDSS III
ADEPT
Lensing Surveys
LSST
Pan-STARRS
BAO: ADEPT SN
SNAP

21. Future Experiments Major Projects Hopefully have multiple methods
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

22. 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