Cosmic Acceleration: Ten Years On

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Cosmic Acceleration Ten Years On

• Josh Frieman
• Fermilab
• University of Chicago

McMaster Colloquium, Sept. 18, 2008
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Components of the Universe
25 Dark Matter Dominant in Galaxies
Clusters 70 Dark Energy Dominates the
Universe, causing Expansion to speed up 4
baryons

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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/T10?5
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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).

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The only mode which preserves homogeneity and
isotropy is overall expansion or
contraction Cosmic scale factor
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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 Redshift of light emitted at t1,
observed at t2

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Distance between galaxies where
fixed comoving distance Recession speed
Hubbles Law (1929)

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Modern Hubble Diagram Hubble Space
Telescope Key Project
Hubble parameter
Freedman etal
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Cosmological Dynamics Newton
How does the scale factor of the Universe
evolve? Consider a homogenous
ball of matter and test particle



Substitute
to find

M
m
d
Friedmann equation
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1980s Will the Universe expand forever or
recollapse in a Big Crunch? How much Dark
Matter is there?
Empty
Size of the Universe
In all these cases, Universe decelerates due to
gravity
Today
Cosmic Time
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p ?? (w ?1)
Accelerating
Empty
Size of the Universe
1998 discovered that the Universe started
speeding up about 5 billion years ago
Today
Cosmic Time
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Cosmic Acceleration

• What can cause this?
• The Universe is filled with stuff that gives
• rise to gravitational repulsion. We now
  call this
• Dark Energy
• Einsteins 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 large-scale structure.

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Cosmological Dynamics Einstein



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The Cosmological Constant as Dark Energy

• Einsteins cosmological constant
• Stress-energy of the Lorentz-invariant vacuum
• Theory

Einstein Zeldovich
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The Cosmological Constant Problemthe 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 Insert cutoff
at kmax M ? Theory Data

Pauli
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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-90s
• ?primordial inflation (1980) predicted
  ?01, but cluster
• observations indicated ?m?0.25 a
  smooth component
• of missing energy with ?s?0.75 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

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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 its different
now?
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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
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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
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Nearby SN 1994D (Ia)
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Supernova Ia Theory

• Standard model
• SNe Ia are thermonuclear
• explosions of CO 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

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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
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SN Ia Spectral Homogeneity
from SDSS Supernova Survey
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Luminosity
?m15
15 days
Time
Empirical Correlation Brighter SNe Ia decline
more slowly Phillips 1993
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• Type Ia SN
• Peak Brightness
• as calibrated
• Standard Candle
• Peak brightness
• correlates with
• decline rate
• Variety of algorithms for modeling these
  correlations
• After correction,
• ? 0.15 mag
• (7 distance error)

Luminosity
Time
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Correction for Brightness-Decline relation
reduces scatter in nearby SN Ia Hubble
Diagram Distance modulus (log measure of
distance) Riess etal 96

• Distance modulus

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Discovery DataHigh-z SN Team
V

• 10 of 16 shown transformed to SN rest-frame
• Riess etal
• Schmidt etal

B1
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Acceleration Discovery by 2 Teams from
High-redshift SNe Ia Apply same Brightness-Decli
ne relation at High-z SNe at z0.5 are 0.25
mag fainter than in an open Universe with same
value of ?m
?? 0.7 ?? 0. ?m 1.
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Vacuum energy density
Density of matter
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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
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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.

• After recombination
• Universe is neutral.
• Photons can travel freely past the baryons.
• Phase of oscillation at trec affects late-time
  amplitude.

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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
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Acoustic Oscillations in the CMB
Temperature map of the cosmic microwave backgroun
d 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 photon-baryon fluid can travel just
  before recombination sound horizon s cstls

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Einstein space can be globally curved
Geometry of three-dimensional space
Kgt0
Klt0
K0
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?
s
K0
Klt0
Kgt0
CMB Maps
Pryke
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Angular positions of acoustic peaks probe
spatial curvature of the Universe
Hu
1/?
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Microwave Background AnisotropyProbes Spatial
Curvature
DASI (2001) Pryke et al

• Boomerang (2001) Netterfield et al

Data indicates nearly flat geometry
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Current CMB Results

• CMB plays important complementary role in
  constraining cosmological parameters Planck

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SN circa 1998
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More Popular Phenomenological Model w
constant, Spatially flat geometry
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SDSS 2.5 meter telescope
SDSS-I 2000-5 Apache Point Observatory
SDSS-II
2005-8 New Mexico
SDSS-III 2008-14
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SDSS Galaxy Distribution
Luminous Red Galaxies
SDSS Galaxy Distribution
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Large-scale Correlations of SDSS Luminous Red
Galaxies
Baryon Acoustic Oscillations seen in Large-scale
Structure mean distance to galaxies at z0.35
Redshift-space Correlation Function
Eisenstein, etal 2005
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SN circa 1998
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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
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SNLS Rolling Search
Early light curves
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SNLS 1st Year Results
Astier et al. 2006 Using 72 SNe from SNLS 40
Low-z
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SNLS 1st Year Results
w ?1 or Flat
?m??1, and w constant
BAO
SNLS
Om 0.263 0.042 (stat) 0.032 (sys) w -1.02
0.09 (stat) 0.054 (sys)
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60 ESSENCE SNe 72 SNLS SNe
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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
  light-curve shape relations?
• These effects reflected in differences between SN
  distance estimators applied to the same data.

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MLCS Light Curve Fitter
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SALT 2 Light Curve Fitter same data
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MLCS vs. SALT constraints
ESSENCESNLSLow-z MLCS
ESSENCESNLSLow-z SALT
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Spectroscopic follow-up telescopes
R. Miquel, M. Molla
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Frieman, et al (2008) Sako, et al (2008)
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B. Dilday
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SDSS SN Photometry Holtzman et al. (2007)
submitted Scene modeling
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Light Curve Fitting with MLCS2k2 and SALT-II
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Preliminary Cosmology Results
w open
Kessler, et al. 2008
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Carnegie Supernova Project Nearby Optical NIR
Light Curves
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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
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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)
• Density pressure

V(j)
j
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Scalar Field Dark Energy
aka quintessence
General features meff lt 3H0 10-33 eV (w lt
0) (Potential gt Kinetic Energy) V m2?2
?crit 10-10 eV4 ? 1028 eV MPlanck
V(j)
(103 eV)4
j
1028 eV
Ultra-light particle Dark Energy hardly
clusters, nearly smooth Equation of state
usually, w gt ?1 and evolves in time Hierarchy
problem Why m/? 10?61? Weak coupling
Quartic self-coupling ?? lt 10?122
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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(1w)
a(t)
Today
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Scalar Field Models Coincidence
Dynamics models (Freezing models)
Mass scale models (Thawing models)
V
V
e.g., e? or ?n
?
?
MPl
Runaway potentials DE/matter ratio
constant (Tracker Solution)
Pseudo-Nambu Goldstone Boson Low mass protected
by symmetry (Cf. axion) JF, Hill, Stebbins,
Waga V(?) M41cos(?/f) f MPlanck M
0.001 eV m?
Ratra Peebles Caldwell, Steinhardt,etal
Albrecht etal,
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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
Cross-over from 4D to 5D gravity at
scale
Features effective scalar-tensor gravity--gt
lunar laser ranging and growth of large-scale
structure Issues does a consistent model exist?



Dvali, Gabadadze, Porrati
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Could a Very Large Void Be Mimicking Dark Energy?
Hubble parameter smaller at distances gt 1 Gpc?
Anthropocentric Universe We dont observe cosmic
acceleration directly. Apparent acceleration
due to pattern of peculiar velocities from
large-scale structure.
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What is causing cosmic acceleration?

• Dark Energy
• Gravity
• Key Experimental Questions for the Future
• Is DE observationally distinguishable from a
  cosmological
• constant, for which w 1?
• Can we distinguish between modified gravity and
  dark energy?
• Combine distance with structure-growth
  probes
• Does dark energy evolve ww(z)?

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Clusters and Dark Energy
Number of clusters above observable mass threshold

• Requirements
• Understand formation of dark matter halos
• Cleanly select massive dark matter halos (galaxy
  clusters) over a range of redshifts
• Redshift estimates for each cluster
• Observable proxy O that can be used as cluster
  mass estimate
• p(OM,z)
• Primary systematic
• Uncertainty in bias scatter of mass-observable
  relation

Dark Energy equation of state

Mohr
Volume Growth (geometry)
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Clusters form hierarchically
z 7
z 5
z 3
dark matter
time
z 0.5
z 0
z 1
Kravtsov
5 Mpc
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Cluster Mass Estimates

• 4 Techniques for Cluster Mass Estimation
• Optical galaxy concentration
• Weak Lensing
• Sunyaev-Zeldovich effect (SZE)
• X-ray
• Cross-compare these techniques to reduce
  systematic errors
• Additional cross-checks
• shape of mass function cluster
• correlations

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Statistical Weak Lensing by SDSS Galaxy Clusters
Mean Tangential Shear Profile in
Optical Richness (Ngal) Bins to 30
h-1Mpc Sheldon, Johnston, etal
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Cluster Mass-Observable Relation

• SDSS Weak Lensing by stacked Clusters
• insensitive to projection effects
• Calibrate mass-observable relations

Johnston, Sheldon, etal 07
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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

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Weak lensing shear and mass
Jain
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Lensing Tomography
zl1
zl2
z1
lensing mass
z2

• Shear at z1 and z2 given by integral of growth
  function distances over lensing mass
  distribution.

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Weak Lensing Tomography

• Measure shapes for millions of source galaxies
  in photo-z bins
• Shear-shear galaxy-shear correlations probe
  distances growth rate of perturbations

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The Dark Energy Survey

• Study Dark Energy using
• 4 complementary techniques
• 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

Blanco 4-meter at CTIO
in systematics in cosmological parameter
degeneracies geometricstructure growth test
Dark Energy vs. Gravity
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The DES Instrument DECam
F8 Mirror
Filters Shutter
3556 mm
CCD Read out
Hexapod
Optical Lenses
1575 mm
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10-m South Pole Telescope (SPT)

• Sunyaev-Zeldovich 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
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Large Synoptic Survey Telescope

• 8.4m ground based telescope with 10 sq. degree
  field
• 5000 Gbytes/night of data
• Real-time analysis
• Celestial Cinematography

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Conclusions

• In 2008, evidence for cosmic acceleration is much
  stronger and more robust than it was in 1998.
• On the other hand, were 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 DESSPT, PanSTARRS,
  SDSS-III BOSS, followed by LSST, JDEM, and Euclid