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Dark energy by david spergel


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  • Obviously a puzzling and far reaching question that has many unexpected consequences
  • The acoustic oscillations are also quantitatively useful, because they can form a standard ruler.
  • Transcript

    • 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
    • 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 MatterSimon 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 biasNon-linear dynamicsGravitational LensingHalo ModelsBias 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 DisadvantagesTechnically 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.