Cosmic Acceleration: Ten Years On Josh Frieman Fermilab & University of Chicago McMaster Colloquium, Sept.
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Cosmic Acceleration: Ten Years On Josh Frieman Fermilab & University of Chicago McMaster Colloquium, Sept. 18, 2008 1 Components of the Universe 25% Dark Matter: Dominant in Galaxies & Clusters 70% Dark Energy: Dominates the Universe, causing Expansion to speed up 4% baryons Cosmic Microwave Background Radiation The Universe is filled with a bath of thermal radiation COBE map of the CMB temperature On large scales, the CMB temperature is nearly isotropic around us (the same in all directions): snapshot of the young Universe, t ~ 400,000 years T = 2.725 degrees above absolute zero Temperature fluctuations T/T~105 3 The Cosmological Principle • We are not priviledged observers at a special place in the Universe. • At any instant of time, the Universe should appear ISOTROPIC (averaged over large scales) to all observers. • A Universe that appears isotropic to all observers is HOMOGENEOUS the same at every location (averaged over large scales). 4 The only mode which preserves homogeneity and isotropy is overall expansion or contraction: Cosmic scale factor a(t) On average, galaxies are at rest in these expanding (comoving) coordinates, and they are not expanding--they are gravitationally bound. Wavelength of radiation scales with scale factor: a(t1 ) ~ a(t) a(t 2 ) Redshift of light: 1 z (t 2 ) a(t 2 ) (t1 ) a(t1 ) emitted at t1, observed at t2 6 <#> Distance between galaxies: d(t) a(t)r a(t1 ) where r fixed comoving distance Recession speed: d(t 2 ) d(t1 ) r[a(t 2 ) a(t1 )] t 2 t1 t 2 t1 d da dH(t) a dt dH0 for `small't 2 t1 Hubble’s Law (1929) d(t 2 ) a(t 2 ) 8 Modern Hubble Diagram Hubble Space Telescope Key Project Freedman etal Hubble parameter Cosmological Dynamics: Newton How does the scale factor of the Universe evolve? Consider a homogenous ball of matter and test particle: M m GM 4G Ý Ý d 2 d d d 3 Substitute d(t) a(t)r to find Ý 4G aÝ a 3 Friedmann equation 10 Size of the Universe Empty How much Dark Matter is there? crit In all these cases, Universe decelerates due to gravity Ý Ý 0 a 1980’s: Will the Universe expand forever or recollapse in a Big Crunch? Today Cosmic Time p = (w = 1) Size of the Universe 1998: discovered that the Universe started speeding up about 5 billion years ago Accelerating Empty Ý Ý 0 a Today Cosmic Time Cosmic Acceleration What can cause this? 1. The Universe is filled with stuff that gives rise to `gravitational repulsion’. We now call this Dark Energy 2. Einstein’s theory of General Relativity is wrong on cosmic distance scales. 3. We must drop the assumption of homogeneity/isotropy: Universe is only apparently accelerating, due to largescale structure. Cosmological Dynamics: Einstein Ý 4G aÝ 3pi i 2 a 3 i c Equation of st ate paramet:erw i pi / ic 2 Non- relat ivist ic matter : p ~ v 2 , w 0 Relatavistic part icles : p c 2 /3, w 1/3 For acceleration: w i 1/3 in dominant component : Dark Energy Ýi 3Hi (1 w i ) 0 r ~ a 4 DE ~ a3(1w) m ~ a3 The Cosmological Constant as Dark Energy 2 L • Einstein’s cosmological constant Einstein: Zel’dovich: (c 1) G g 8GT G g 8GT 8G(Tvac Tparticles) • Stress-energy of the Lorentz-invariant vacuum: g ( ) vac T vac 00 vac , pvac Tii , w 1 8G 8G vac 0.7 vac (0.003 eV)4 • Theory: vac ~ (1028 eV)4 16 The Cosmological Constant Problem the biggest embarassment for particle physics Quantum zero-point fluctuations: space is filled with virtual particles which continuously fluctuate into and out of the vacuum (via the Uncertainty principle). Vacuum energy density in Quantum Field Theory: 1 1 vac 8G V 2 kmax c(k 2 m 2 )1/ 2 d 3 k 0 4 Insert cutoff at kmax = M vac ~ M 1/ 2 19 112 4 Theory: M M Planck G 10 GeV vac ~ 10 eV M M SUSY ~ 1 T eV vac ~ 1048 eV4 Data: vac 1010 eV 4 Pauli Dark Energy Pre-History •Discovery of cosmic acceleration was not a complete surprise: it fit naturally into a pre-existing theoretical & observational framework favored by a number of theorists* in the mid-90’s: primordial inflation (1980) predicted 0=1, but cluster observations indicated m0.25; a smooth component of `missing energy’ with s0.75 was needed for galaxies and large-scale structure to form, this missing energy could not have been dynamically important until late times (redshifts z 1): CDM models predicted large-scale structure in good agreement with galaxy surveys (1990: APM) *JF, Hill, Stebbins, Waga ‘95; Krauss & Turner ‘95, Ostriker & Steinhardt ‘95 18 Tragic History of : a cautionary tale periodically invoked to solve cosmological crises, then dropped when they passed: 1916: Einstein: static Universe (`greatest blunder of my life’?) 1929: 1st `age crisis’: Universe younger than Earth 1967: apparent clustering of quasars at fixed redshift 1974: inferred distances using galaxy brightness 1995: 2nd ‘age crisis’: Universe younger than stars 1998: Supernovae 2000: Cosmic Microwave Background and Galaxy Surveys Why do we think it’s different now? Discovery Evidence for Acceleration • 1998: Type Ia Supernovae Supernova Cosmology Project High-z Supernova Team • 2000-1: First CMB Acoustic Peak DASI, Boomerang, Maxima Independent, robust lines of evidence for the first time 20 Discovery Evidence for Acceleration • 1998: Type Ia Supernovae Supernova Cosmology Project High-z Supernova Team • 2000-1: First CMB Acoustic Peak DASI, Boomerang, Maxima Independent, robust lines of evidence for the first time 21 Nearby SN 1994D (Ia) 22 Supernova Ia Theory “Standard model”: SNe Ia are thermonuclear explosions of C+O white dwarf stars. Evolution to criticality: Accretion from a binary companion leads to growth of the WD to the critical Chandrasekhar mass, M ~ 1.4 Msun After ~1000 years of slow thermonuclear “cooking”, a violent explosion is triggered at or near the center; complete incineration within less than two seconds, no compact remnant Type Ia Supernovae General properties: • • • • Homogeneous class* of events, only small (correlated) variations Rise time: ~ 15 – 20 days Decay time: months Bright: MB ~ – 19.5 at peak No hydrogen in the spectra • Early spectra: Si, Ca, Mg, ...(absorption) • Late spectra: Fe, Ni,…(emission) • Very high velocities (~10,000 km/s) SN Ia found in all types of galaxies, including ellipticals • Progenitor systems must have long lifetimes *luminosity, color, spectra at max. light 24 SN Ia Spectral Homogeneity from SDSS Supernova Survey Luminosity m15 15 days Time Empirical Correlation: Brighter SNe Ia decline more slowly Phillips 1993 Peak brightness correlates with decline rate Variety of algorithms for modeling these correlations After correction, ~ 0.15 mag (~7% distance error) Luminosity Type Ia SN Peak Brightness as calibrated Standard Candle Time 27 Distance modulus Distance modulus Correction for Brightness-Decline relation reduces scatter in nearby SN Ia Hubble Diagram (log measure of distance) m M 5log 5log H 0 Riess etal 96 28 Discovery Data: High-z SN Team V 10 of 16 shown; transformed to SN rest-frame B+1 Riess etal Schmidt etal 29 Acceleration Discovery by 2 Teams from High-redshift SNe Ia Apply same Brightness-Decline relation at High-z SNe at z~0.5 are ~0.25 mag fainter than in an open Universe with same value of m = 0.7 = 0. m = 1. 30 Density of matter 31 Vacuum energy density Discovery Evidence for Acceleration • 1998: Type Ia Supernovae Supernova Cosmology Project High-z Supernova Team • 2000-1: First CMB Acoustic Peak DASI, Boomerang, Maxima Independent, robust lines of evidence for the first time 32 CMB: Sound Waves in the Early Universe Before recombination: Universe is ionized. Photons provide enormous pressure and restoring force. Photon-baryon perturbations oscillate as acoustic waves. Recombination & Last scattering z ~ 1000 ~400,000 years Time Neutral Today Ionized After recombination: Universe is neutral. Photons can travel freely past the baryons. Phase of oscillation at trec affects late-time amplitude. Sound Waves Each initial overdensity (in dark matter & gas) is an overpressure that launches a spherical sound wave. This wave travels outwards at 57% of the speed of light. Pressure-providing photons decouple at recombination. CMB travels to us from these spheres. Eisenstein Acoustic Oscillations in the CMB Temperature map of the cosmic microwave background radiation Although there are fluctuations on all scales, there is a characteristic angular scale, ~ 1 degree on the sky, set by the distance sound waves in the photonbaryon fluid can travel just before recombination: sound horizon s~ cstls Einstein: space can be globally curved Geometry of three-dimensional space K>0 K<0 K=0 36 s K>0 K=0 CMB Maps K<0 Pryke Angular positions of acoustic peaks probe spatial curvature of the Universe Hu ~1/ Microwave Background Anisotropy Probes Spatial Curvature Boomerang (2001) Netterfield et al DASI (2001) Pryke et al Data indicates nearly flat geometry Current CMB Results CMB plays important complementary role in constraining cosmological parameters: Planck <#> SN circa 1998 More Popular Phenomenological Model w = constant, Spatially flat geometry <#> SDSS 2.5 meter telescope Apache Point Observatory New Mexico SDSS-I: 2000-5 SDSS-II: 2005-8 SDSS-III: 2008-14 SDSS Galaxy Distribution Luminous Red Galaxies QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. SDSS Galaxy Distribution Large-scale Correlations of SDSS Luminous Red Galaxies Redshiftspace Correlation Function Acoustic series in P(k) becomes a single peak in (r) (r) (x )(x r ) Pure CDM model has no peak Warning: Correlated Error Bars Baryon Acoustic Oscillations seen in Large-scale Structure: mean distance to galaxies at z~0.35 Eisenstein, etal 2005 SN circa 1998 46 47 Supernova Legacy Survey (2003-2008) 5 year survey, goal: 500 distant SNe Ia to measure w Uses CFHT Megacam 36 CCDs, good blue response 4 filters griz for good k-corrections and color measurement Spectroscopic follow-up on 8-10m Megaprime 48 SNLS Rolling Search Early light curves SNLS 1st Year Results First-Year SNLS Hubble Diagram Astier et al. 2006 Using 72 SNe from SNLS +40 Low-z 50 SNLS 1st Year Results w = 1 or Flat: m+=1, and w = constant BAO SNLS Ωm = 0.263 ± 0.042 (stat) ± 0.032 (sys) w = -1.02 ± 0.09 (stat) ± 0.054 (sys) 52 60 ESSENCE SNe 72 SNLS SNe 53 54 Systematic Errors • Host galaxy dust extinction/reddening: partially degenerate with variations in SN colors • Small, heterogeneous, possibly unrepresentative sample at low-z used for training light-curve fitters (MLCS) and anchoring the Hubble diagram • Possible evolution in SN population: host galaxies, metallicity, dust, etc, could change lightcurve shape relations? • These effects reflected in differences between SN distance estimators applied to the same data. 55 MLCS Light Curve Fitter 56 SALT 2 Light Curve Fitter same data 57 MLCS vs. SALT constraints ESSENCE+SNLS+Low-z : SALT MLCS 58 59 Spectroscopic follow-up telescopes R. Miquel, M. Molla Frieman, et al (2008); Sako, et al (2008) B. Dilday SDSS SN Photometry: Holtzman et al. (2007) submitted ``Scene modeling” Light Curve Fitting with MLCS2k2 and SALT-II 64 Preliminary Cosmology Results w open Kessler, et al. 2008 Carnegie Supernova Project Nearby Optical+ NIR Light Curves 66 Light Scalar Fields as Dark Energy Perhaps the Universe is not yet in its ground state. The `true’ vacuum energy could be zero (for reasons yet unknown). Transient vacuum energy can exist if there is a field that takes a cosmologically long time to reach its ground state. This was the reasoning behind inflation. For this reasoning to apply now, we must postulate the existence of an extremely light scalar field, since the dynamical evolution of such a field is governed by 1 t~ , t 1/H0 m H0 m Scalar Field Dark Energy (inspired by inflation) If Dark Energy is due to a scalar field, j, slowly evolving in a potential, V(j) (ignoring matter density): j 3Hj V ' V(j) Density & pressure: 2 j V (j ) 1 2 P 12 j 2 V (j ) j 68 Scalar Field Dark Energy aka quintessence General features: V(j) meff < 3H0 ~ 10-33 eV (w < 0) (Potential > Kinetic Energy) (10–3 eV)4 V ~ m2j2 ~ crit ~ 10-10 eV4 j~ 1028 eV ~ MPlanck j 1028 eV Ultra-light particle: Dark Energy hardly clusters, nearly smooth Equation of state: usually, w > 1 and evolves in time Hierarchy problem: Why m/ ~ 1061? Weak coupling: Quartic self-coupling < 10122 The Coincidence Problem Why do we live at the `special’ epoch when the dark energy density is comparable to the matter energy density? matter ~ a-3 DE~ a-3(1+w) a(t) Today Scalar Field Models & Coincidence V `Dynamics’ models (Freezing models) e.g., e– or V –n MPl j Runaway potentials DE/matter ratio constant (Tracker Solution) Ratra & Peebles; Caldwell, Steinhardt,etal; Albrecht etal,… `Mass scale’ models (Thawing models) j Pseudo-Nambu Goldstone Boson Low mass protected by symmetry (Cf. axion) JF, Hill, Stebbins, Waga V(j) = M4[1+cos(j/f)] f ~ MPlanck M ~ 0.001 eV ~ m IR-Modified Gravity Models At large distances, gravity can leak off 3-brane into the bulk, infinite 5th dimension Acceleration without vacuum energy on the brane, driven by brane curvature term: SM 3 5 d 5 X g5 1/ 2 R5 M 2 Pl d 4 xg 1/ 2 R Cross-over from 4D to 5D gravity at scale: rc M Pl / M 5 1 H 2 M 5 ~ 1 GeV rc ~ H 2 H0 rc 3M Pl 2 3 Features: effective scalar-tensor gravity--> lunar laser ranging and growth of large-scale structure Issues: does a consistent model exist? Dvali, Gabadadze, Porrati Could a Very Large Void Be Mimicking Dark Energy? Hubble parameter smaller at distances > 1 Gpc? Anthropocentric Universe We don’t observe cosmic acceleration directly. Apparent acceleration due to pattern of peculiar velocities from largescale structure. Required by SNe Ruled out CMB spectrum Caldwell & Stebbins 73 ‘08 What is causing cosmic acceleration? Dark Energy: G 8G[T (matt er) T (dark energy)] DE equation of state : w Tii /T00 1/3 Gravity: G f (g ) 8GT (matt er) Key Experimental Questions for the Future: 1. Is DE observationally distinguishable from a cosmological constant, for which w =—1? 2. Can we distinguish between modified gravity and dark energy? Combine distance with structure-growth probes 3. Does dark energy evolve: w=w(z)? 74 Clusters and Dark Energy Number of clusters above observable mass threshold •Requirements 1.Understand formation of dark matter halos 2.Cleanly select massive dark matter halos (galaxy clusters) over a range of redshifts 3.Redshift estimates for each cluster 4.Observable proxy O that can be used as cluster mass estimate: p(O|M,z) Primary systematic: Uncertainty in bias & scatter of mass-observable relation Dark Energy equation of state dN(z) dV n (z) dzd dzd Volume (geometry) Growth Mohr 75 Clusters form hierarchically z=7 dark matter z=5 z=3 time z=1 Kravtsov z = 0.5 z=0 5 Mpc 76 Cluster Mass Estimates 4 Techniques for Cluster Mass Estimation: • Optical galaxy concentration • Weak Lensing • Sunyaev-Zel’dovich effect (SZE) • X-ray • Cross-compare these techniques to reduce systematic errors • Additional cross-checks: shape of mass function; cluster correlations 77 Statistical Weak Lensing by SDSS Galaxy Clusters Mean Tangential Shear Profile in Optical Richness (Ngal) Bins to 30 h-1Mpc Sheldon, Johnston, etal 78 Cluster Mass-Observable Relation • SDSS Weak Lensing by stacked Clusters • insensitive to projection effects • Calibrate massobservable relations Johnston, Sheldon, etal 07 79 Background sources Dark matter halos Observer Statistical measure of shear pattern, ~1% distortion Radial distances depend on geometry of Universe Foreground mass distribution depends on growth of structure 80 Weak lensing: shear and mass Jain 81 Lensing Tomography zl1 zl2 lensing mass z1 z2 Shear at z1 and z2 given by integral of growth function & distances over lensing mass distribution. 82 The Dark Energy Survey • Study Dark Energy using 4 complementary* techniques: Blanco 4-meter at CTIO I. Cluster Counts II. Weak Lensing III. Baryon Acoustic Oscillations IV. Supernovae • Two multiband surveys: 5000 deg2 g, r, i, z,Y smaller area repeat (SNe) • Build new 3 deg2 camera and Data management sytem Survey 2011-2016 (525 nights) Response to NOAO AO *in systematics & in cosmological parameter degeneracies *geometric+structure growth: test Dark Energy vs. Gravity 84 The DES Instrument: DECam F8 Mirror Filters Shutter 3556 mm CCD Read out Hexapod Optical Lenses 1575 mm 10-m South Pole Telescope (SPT) Sunyaev-Zel’dovich effect (SZE) - Compton upscattering of CMB photons by hot gas in clusters - nearly independent of redshift: - can probe to high redshift - need ancillary redshift measurement from DES DES survey area encompasses 4000 sq. deg. SPT SZE Survey Survey; SPT taking data now 86 Large Synoptic Survey Telescope 8.4m ground based telescope with 10 sq. degree field 5000 Gbytes/night of data Real-time analysis “Celestial Cinematography” Conclusions •In 2008, evidence for cosmic acceleration is much stronger and more robust than it was in 1998. •On the other hand, we’re no closer to physical understanding of the underlying cause. •Excellent prospects for increasing the precision on Dark Energy parameters from a sequence of increasingly complex and ambitious experiments over the next 5-15 years: DES+SPT, PanSTARRS, SDSS-III BOSS, followed by LSST, JDEM, and Euclid 88