1 What is the Dark Energy? David Spergel Princeton University

2 One of the most challenging problems in Physics Several cosmological observations demonstrated that 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 it raises?

3 Outline 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 Contents 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 A note on cosmological parameters The properties of the standard cosmological model are expressed in terms of various cosmological parameters, for example: H 0 is the Hubble expansion parameter today is the fraction of the matter energy density in the critical density (G=c=1 units) is the fraction of the Dark Energy density (here a cosmological constant) in the critical density

6 Evidence for cosmic acceleration: Supernovae type Ia

7 Standard candles Their intrinsic luminosity is know Their apparent luminosity can be measured The ratio of the two can provide the luminosity- distance (d L ) of the supernova The red shift z can be measured independently from spectroscopy Finally, one can obtain d L (z) or equivalently the magnitude(z) and draw a Hubble diagram

8 Evidence for cosmic acceleration: Supernovae type Ia

9

10 Evidence from Cosmic Microwave Background Radiation (CMB) CMB is an almost isotropic relic radiation of T=2.725±0.002 K CMB is a strong pillar of the Big Bang cosmology It is a powerful tool to use in order to constrain several cosmological parameters The CMB power spectrum is sensitive to several cosmological parameters

11 This is how the Wilkinson Microwave Anisotropy Probe (WMAP) sees the CMB

12 ADIABATIC DENSITY FLUCTUATIONS

13 ISOCURVATURE ENTROPY FLUCTUATIONS

14 Determining Basic Parameters Baryon Density  b h 2 = 0.015, also measured through D/H

15 Determining Basic Parameters Matter Density  m h 2 = 0.16,..,0.33

16 Determining Basic Parameters Angular Diameter Distance w = -1.8,..,-0.2 When combined with measurement of matter density constrains data to a line in  m -w space

17 Simple Model Fits CMB data Readhead et al. astro/ph

18 Evolution from Initial Conditions I WMAP team assembled DA leave Princeton WMAP completes 2 year of observations! WMAP at Cape

19 Evidence from large-scale structure in the universe (clusters of galaxies) Counting clusters of galaxies can infer the matter energy density in the universe The matter energy density found is usually around ~0.3 the critical density CMB best fit model has a total energy density of ~1, so another ~0.7 is required but with a different EOS The same ~0.7 with a the same different EOS is required from combining supernovae data and CMB constraints

20 Cosmic complementarity: Supernovae, CMB, and Clusters

21 What is Dark Energy ? “ ‘Most embarrassing observation in physics’ – that’s the only quick thing I can say about dark energy that’s also true.” Edward Witten

22 What is the Dark Energy? Cosmological Constant Failure of General Relativity Quintessence Novel Property of Matter Simon Dedeo astro-ph/

23 Why is the total value measured from cosmology so small compared to quantum field theory calculations of vacuum energy? From cosmology: 0.7 critical density ~ GeV 4 From QFT estimation at the Electro-Weak (EW) scales: (100 GeV) 4 At EW scales ~56 orders difference, at Planck scales ~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 have prevented structures to form in the universe (cosmic coincidence ) COSMOLOGICAL CONSTANT??

24 Anthropic Solution? Not useful to discuss creation science in any of its forms…. Dorothy… we are not in Kansas anymore …

25 Quintessence Introduced mostly to address the “why now?” problem Potential determines dark energy properties (w, sound speed) Scaling models (Wetterich; Peebles & Ratra ) V(  ) = exp  Most of the tracker models predicted w > -0.7  matter Zlatev and Steinhardt (1999)

26 Current Constraints Seljak et al. 2004

27 Looking for Quintessence Deviations from w = -1 BUT HOW BIG? Clustering of dark energy Variations in coupling constants (e.g.,  )  FF/M PL Current limits constrain  < If dark energy properties are time dependent, so are other basic physical parameters

28 Big Bang Cosmology Homogeneous, isotropic universe (flat universe)

29 Rulers and Standard Candles Luminosity Distance Angular Diameter Distance

30 Flat M.D. Universe D = 1500 Mpc for z > 0.5

31 Volume

32 Techniques Measure H(z) Luminosity Distance (Supernova) Angular diameter distance Growth rate of structure. Checks Einstein equations to first order in perturbation theory

33 What if GR is wrong? Friedman equation (measured through distance) and Growth rate equation are probing different parts of the theory For any distance measurement, there exists a w(z) that will fit it. However, the theory can not fit growth rate of structure Upcoming measurements can distinguish Dvali et al. DGP from GR (Ishak, Spergel, Upadye 2005)

34 Growth 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 A powerful cosmological probe of Dark Energy: Gravitational Lensing Abell 2218: A Galaxy Cluster Lens, Andrew Fruchter et al. (HST)

36 The binding of light

37 Gravitational Lensing by clusters of galaxies From MPA lensing group

38 Weak Gravitational Lensing Distortion of background images by foreground matter UnlensedLensed Credit: SNAP WL group

39 Gravitational Lensing Advantage: directly measures mass Disadvantages Technically more difficult Only measures projected mass- distribution Tereno et al Refregier et al. 2002

40 Baryon Oscillations C(  )   CMB Galaxy Survey Baryon oscillation scale 1o1o photo-z slices Selection function Limber Equation (weaker effect)

41 Baryon Oscillations as a Standard Ruler In a redshift survey, we can measure correlations along and across the line of sight. Yields H(z) and D A (z)! [Alcock-Paczynski Effect] Observer  r = (c/H)  z  r = D A 

42 Large Galaxy Redshift Surveys By performing large spectroscopic surveys, we can measure the acoustic oscillation standard ruler at a range of redshifts. Higher harmonics are at k~0.2h Mpc -1 ( =30 Mpc). Measuring 1% bandpowers in the peaks and troughs requires about 1 Gpc 3 of survey volume with number density ~10 -3 galaxy Mpc -3. ~1 million 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 Conclusions Cosmology provides lots of evidence for physics beyond the standard model. Upcoming observations can test ideas about the nature of the dark energy.