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- 1. 1What is the Dark Energy?David SpergelPrinceton University
- 2. 2One of the most challengingproblems in Physics Several cosmological observations demonstratedthat the expansion of the universe is accelerating What is causing this acceleration? How can we learn more about this acceleration,the Dark Energy it implies, and the questions itraises?
- 3. 3Outline A brief summary on the contents of the universe Evidence for the acceleration and the implied Dark Energy Supernovae type Ia observations (SNe Ia) Cosmic Microwave Background Radiation (CMB) Large-scale structure (LSS) (clusters of galaxies) What is the Dark Energy? Future Measurements
- 4. 4Contents of the universe(from current observations)Baryons (4%)Dark matter (23%)Dark energy: 73%Massive neutrinos: 0.1%Spatial curvature: very close to 0
- 5. 5A note on cosmologicalparameters The properties of the standard cosmologicalmodel are expressed in terms of variouscosmological parameters, for example: H0 is the Hubble expansion parameter today is the fraction of the matter energydensity in the critical density(G=c=1 units) is the fraction of the Dark Energydensity (here a cosmological constant) in thecritical densitycMM ρρ /≡Ωπρ83 2Hc ≡cρρ /ΛΛ ≡Ω
- 6. 6Evidence for cosmic acceleration:Supernovae type Ia
- 7. 7Evidence for cosmic acceleration:Supernovae type Ia Standard candles Their intrinsic luminosity is know Their apparent luminosity can be measured The ratio of the two can provide the luminosity-distance (dL) of the supernova The red shift z can be measured independentlyfrom spectroscopy Finally, one can obtain dL (z) or equivalently themagnitude(z) and draw a Hubble diagram
- 8. 8Evidence for cosmic acceleration:Supernovae type Ia
- 9. 9
- 10. 10Evidence from Cosmic MicrowaveBackground Radiation (CMB) CMB is an almost isotropic relic radiation ofT=2.725±0.002 K CMB is a strong pillar of the Big Bangcosmology It is a powerful tool to use in order toconstrain several cosmological parameters The CMB power spectrum is sensitive toseveral cosmological parameters
- 11. 11Anisotropy Probe (WMAP) sees theCMB
- 12. 12ADIABATIC DENSITY FLUCTUATIONS
- 13. 13ISOCURVATURE ENTROPY FLUCTUATIONS
- 14. 14Determining Basic ParametersBaryon DensityΩbh2= 0.015,0.017..0.031also measured through D/H
- 15. 15Determining Basic ParametersMatter DensityΩmh2= 0.16,..,0.33
- 16. 16Determining Basic ParametersAngular DiameterDistancew = -1.8,..,-0.2When combined withmeasurement of matterdensity constrains data to aline in Ωm-w space
- 17. 17Simple Model Fits CMB dataReadhead et al. astro/ph 0402359
- 18. 18Evolution from Initial Conditions IWMAP teamassembledDA leavePrincetonWMAP completes2 year ofobservations!WMAP at Cape
- 19. 19Evidence from large-scale structurein the universe (clusters of galaxies) Counting clusters of galaxies can infer the matter energydensity in the universe The matter energy density found is usually around ~0.3 thecritical density CMB best fit model has a total energy density of ~1, soanother ~0.7 is required but with a different EOS The same ~0.7 with a the same different EOS is requiredfrom combining supernovae data and CMB constraints
- 20. 20Cosmiccomplementarity:Supernovae,CMB,and Clusters
- 21. 21What is Dark Energy ?“ ‘Most embarrassing observationin physics’ – that’s the only quickthing I can say about dark energythat’s also true.”Edward Witten
- 22. 22What is the Dark Energy? Cosmological Constant Failure of General Relativity Quintessence Novel Property of MatterSimon Dedeo astro-ph/0411283
- 23. 23 Why is the total value measured fromcosmology so small compared to quantum fieldtheory calculations of vacuum energy? From cosmology: 0.7 critical density ~ 10-48GeV4 From QFT estimation at the Electro-Weak (EW)scales: (100 GeV)4 At EW scales ~56 orders difference, at Planckscales ~120 orders Is it a fantastic cancellation of a puzzling smallness? Why did it become dominant during the “present”epoch of cosmic evolution? Any earlier, would haveprevented structures to form in the universe (cosmiccoincidence)COSMOLOGICAL CONSTANT??
- 24. 24Anthropic Solution? Not useful to discuss creation sciencein any of its forms….Dorothy… we are not in Kansas anymore …
- 25. 25Quintessence Introduced mostly to addressthe “why now?” problem Potential determines darkenergy properties (w, soundspeed) Scaling models (Wetterich;Peebles & Ratra)V(φ) = exp(−φ)Most of the tracker modelspredicted w > -0.7ρmatterQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.Zlatev andSteinhardt(1999)
- 26. 26Current ConstraintsQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.Seljak et al.2004QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
- 27. 27Looking for Quintessence Deviations from w = -1 BUT HOW BIG? Clustering of dark energy Variations in coupling constants (e.g., α)λφFF/MPL Current limits constrain λ < 10-6If dark energy properties are time dependent, soare other basic physical parameters
- 28. 28Big Bang CosmologyHomogeneous,isotropic universe(flat universe)
- 29. 29Rulers and Standard CandlesLuminosityDistanceAngularDiameterDistance
- 30. 30Flat M.D. UniverseD = 1500 Mpc for z > 0.5
- 31. 31Volume
- 32. 32Techniques Measure H(z) Luminosity Distance (Supernova) Angular diameter distance Growth rate of structure.Checks Einstein equations to first order in perturbation theory
- 33. 33What if GR is wrong? Friedman equation (measured throughdistance) and Growth rate equation areprobing different parts of the theory For any distance measurement, there exists aw(z) that will fit it. However, the theory cannot fit growth rate of structure Upcoming measurements can distinguishDvali et al. DGP from GR (Ishak, Spergel,Upadye 2005)
- 34. 34Growth Rate of Structure Galaxy Surveys Need to measure biasNon-linear dynamicsGravitational LensingHalo ModelsBias is a function of galaxy properties,scale, etc….
- 35. 35A powerful cosmological probe of Dark Energy:Gravitational LensingAbell 2218: A Galaxy Cluster Lens, Andrew Fruchter et al. (HST)
- 36. 36The binding of light
- 37. 37Gravitational Lensing by clusters of galaxiesFrom MPA lensing group
- 38. 38Weak Gravitational LensingDistortion of background images by foreground matterUnlensed LensedCredit: SNAP WL group
- 39. 39Gravitational Lensing Advantage: directly measures mass DisadvantagesTechnically more difficult Only measures projected mass-distributionTereno et al. 2004Refregier et al. 2002
- 40. 40Baryon OscillationsC(θ)C(θ)θθCMBGalaxySurveyBaryon oscillation scale1ophoto-z slicesSelectionfunctionLimber Equation(weaker effect)
- 41. 41Baryon Oscillations as aStandard Ruler In a redshift survey, wecan measure correlationsalong and across the lineof sight. Yields H(z) and DA(z)![Alcock-Paczynski Effect]Observerδr = (c/H)δzδr = DAδθ
- 42. 42Large Galaxy Redshift Surveys By performing large spectroscopic surveys, we can measure theacoustic oscillation standard ruler at a range of redshifts. Higher harmonics are at k~0.2h Mpc-1(λ=30 Mpc). Measuring 1% bandpowers in the peaks and troughs requires about 1Gpc3of survey volume with number density ~10-3galaxy Mpc-3. ~1million galaxies! SDSS Luminous Red Galaxy Survey has done this at z=0.3! A number of studies of using this effect Blake & Glazebrook (2003), Hu & Haiman (2003), Linder (2003),Amendola et al. (2004) Seo & Eisenstein (2003), ApJ 598, 720 [source of next few figures]
- 43. 43Conclusions Cosmology provides lots of evidence forphysics beyond the standard model. Upcoming observations can test ideas aboutthe nature of the dark energy.

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