State of the dark universe report

1
State of the (dark) universe report

• Uros Seljak
• Zurich/ICTP/Princeton
• Heidelberg, november 7, 2006

2
Outline

• Methods to investigate dark energy and dark
  matter SN, CMB, galaxy clustering, cluster
  counts, weak lensing, Lya forest
• Current constraints what have we learned so far,
  controversies
• 3) What can we expect in the future?

3
Context

• Conclusive evidence for acceleration of the
  Universe.
• Standard cosmological framework ? dark energy
  (70 of mass-energy).
• Possibility Dark Energy constant in space time
  (Einsteins L).
• Possibility Dark Energy varies with time (or
  redshift z or a (1z)-1).
• Impact of dark energy can be expressed in terms
  of equation of state
• w(a) p(a) / r(a) with w(a) -1 for L.
• Possibility GR or standard cosmological model
  incorrect.
• Whatever the possibility, exploration of the
  acceleration of the Universe
• will profoundly change our understanding of the
  composition and nature
• of the Universe.

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
How to test dark energy/matter?

• Classical tests redshift- luminosity distance
  relation (SN1A etc), redshift-angular diameter
  distance, redshift-Hubble parameter relation

6
Classical cosmological tests (in a new form)
Friedmanns (Einsteins) equation
7
How to test dark energy/matter?

• Classical tests redshift-distance relation (SN1A
  etc)
• Growth of structure CMB, Ly-alpha, weak lensing,
  clusters, galaxy clustering

8
Growth of structure by gravity

• Perturbations can be measured at different
  epochs
• CMB z1000
• 21cm z10-20 (?)
• Ly-alpha forest z2-4
• Weak lensing z0.3-2
• Galaxy clustering z0-1 (3?)
• Sensitive to dark energy, neutrinos

9
How to test dark energy/matter?

• Classical tests redshift-distance relation (SN1A
  etc)
• Growth of structure CMB, Ly-alpha, weak lensing,
  clusters, galaxy clustering
• Scale dependence of structure

10
Scale dependence of cosmological probes
WMAP
CBI
ACBAR
Lyman alpha forest
SDSS
Complementary in scales and redshift
11
Sound Waves from the Early Universe

• Before recombination
• Universe is ionized.
• Photons provide enormous pressure and restoring
  force.
• 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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This is how the Wilkinson Microwave Anisotropy
Probe (WMAP) sees the CMB
13
Determining Basic Parameters
Angular Diameter Distance w -1.8,..,-0.2 When
combined with measurement of matter density
constrains data to a line in Wm-w space
14
Determining Basic Parameters
Matter Density Wmh2 0.16,..,0.33
15
Determining Basic Parameters
Baryon Density Wbh2 0.015,0.017..0.031 also
measured through D/H
16
Current 3 year WMAP analysis/data situation
Current data favor the simplest scale invariant
model
17
Galaxy and quasar survey
400,000 galaxies with redshifts
18
Sloan Digital Sky Survey (SDSS)

• 2.5 m aperture
• 5 colors ugriz
• 6 CCDs per color, 2048x2048, 0.396/pixel
• Integration time 50 sec per color
• Typical seeing 1.5
• Limiting mag r23
• current 7000 deg2 of imaging data, 40 million
  galaxies
• 400,000 spectra (rlt17.77 main sample, 19.1
  QSO,LRG)

Image Credit Sloan Digital Sky Survey
19
Galaxy power spectrum shape analysis
Galaxy clustering traces dark matter on large
scales Current results redshift space power
spectrum analysis based on 200,000 galaxies
(Tegmark etal, Pope etal), comparable to 2dF
(Cole etal) Padmanabhan etal LRG power spectrum
analysis, 10 times larger volume, 2 million
galaxies Amplitude not useful (bias unknown)
Nonlinear scales
20
Are galaxy surveys consistent with each other?
Some claims that SDSS main sample gives more than
2 sigma larger value of W
Fixing h0.7
SDSS LRG photo 2dF SDSS main spectro
Bottom line no evidence for discrepancy, new
analyses improve upon SDSS main
21
Acoustic Oscillations in the Matter Power Spectrum

• Peaks are weak suppressed by a factor of the
  baryon fraction.
• Higher harmonics suffer from diffusion damping.
• Requires large surveys to detect!

Linear regime matter power spectrum
22
A Standard Ruler

• The acoustic oscillation scale depends on the
  matter-to-radiation ratio (Wmh2) and the
  baryon-to-photon ratio (Wbh2).
• The CMB anisotropies measure these and fix the
  oscillation scale.
• In a redshift survey, we can measure this along
  and across the line of sight.
• Yields H(z) and DA(z)!

23
Baryonic wiggles
Best evidence SDSS LRG spectroscopic sample
(Eisenstein etal 2005), about 3.5 sigma
evidence SDSS LRG photometric sample
(Padmanabhan, Schlegel, US etal 2005) 2.5 sigma
evidence
24
To perturb or not to perturb dark energy

• Should one include perturbations in dark energy?
• For w-1 no perturbations
• For wgt-1 perturbations in a single scalar field
  model with canonical kinetic energy, speed of
  sound c
• Non-canonical fields may give speed of sound ltltc
  
• For wlt-1 (phantom model) one can formally adopt
  the same, but the model has instabilities
• For w crossing from lt-1 to gt-1 it has been argued
  that the perturbations diverge however, no
  self-consistent model based on Lagrangian exists
• There is a self-consistent ghost condensate model
  that gives wlt-1 (Creminelli etal 2006) and
  predicts no perturbations in DE sector

25
Weak Gravitational Lensing
Distortion of background images by foreground
matter

Unlensed Lensed
26
Weak Lensing Large-scale shear
Convergence Power Spectrum 1000 sq. deg.
to R 27 Huterer
27
Gravitational Lensing
Refregier et al. 2002

• Advantage directly measures mass
• Disadvantages
• Technically more difficult
• Only measures projected mass-distribution
• Intrinsic alignments?

Tereno et al. 2004
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Possible sources of systematic error

• PSF induced errors rounding (need to calibrate),
  ellipticity (use stars)
• Shear selection bias rounder objects can be
  preferentially selected
• Noise induced bias conversion from intensity to
  shear nonlinear
• Intrinsic correlations
• STEP2 project bottom line current acccuracy at
  5 level, plenty of work to do to reach 1 level,
  not clear 0.1 even possible

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Shear-intrinsic (GI) correlation
Hirata and US 2004

• Same field shearing is also tidally distorting,
  opposite sign
• What was is now , possibly an order of
  magnitude increase
• Cross-correlations between redshift bins does not
  eliminate it
• B-mode test useless (parity conservation)
• Vanishes in quadratic models

Lensing shear
Tidal stretch
30
Intrinsic correlations in SDSS
Mandelbaum, Hirata, Ishak, US etal 2005
300,000 spectroscopic galaxies No evidence for
II correlations Clear evidence for GI
correlations on all scales up to 60Mpc/h Gg
lensing not sensitive to GI
31
Implications for future surveys Mandelbaum etal
2005, Hirata and US 2004
Up to 30 for shallow survey at z0.5 10 for
deep survey at z1 current surveys underestimate
s8 More important for cross-redshift bins
32
Galaxy clustering power spectrum shape
Galaxy clustering traces dark matter on large
scales Current results redshift space power
spectrum analysis based on 200,000 galaxies
(Tegmark etal, Pope etal, 2dF (Cole
etal) Padmanabhan etal LRG photometric power
spectrum analysis, 10 times larger volume, 2
million galaxies LRG spectro analysis Tegmark
etal, Eisenstein etal, Percival etal Amplitude
not useful (bias)
Nonlinear scales
33
Galaxy bias determination

• Galaxies are biased tracers of dark matter the
  bias is believed to be scale independent on large
  scales (klt0.1-0.2/Mpc)
• If we can determine the bias we can use galaxy
  power spectrum to determine amplitude of dark
  matter spectrum s8
• High accuracy determination of s8 is important
  for dark energy constraints
• Weak lensing is the most direct method

34
galaxy-galaxy lensing

• dark matter around galaxies induces tangential
  distortion of background galaxies extremely
  small, 0.1
• Useful to have redshifts of foreground galaxies
  SDSS Express signal in terms of projected surface
  density and transverse r
• Signal as a function of galaxy luminosity, type

35
Galaxy-galaxy lensing measures galaxy-dark matter
correlations
Goal lensing determines halo masses (in fact,
full mass distribution, since galaxy of a given L
can be in halos of different mass) Halo mass
increases with galaxy luminosity SDSS gg 300,000
foreground galaxies, 20 million background,
S/N30, the strongest weak lensing signal to date
testing ground for future surveys such as
LSST,SNAP
Seljak etal 2004
36
dark matter corr function
On large scales galaxies trace dark matter
G-g lensing in combination with
autocorrelation analysis gives projected dark
matter corr. function Mandelbaum, US etal,
in prep
37
WMAP-LSS cross-correlation ISW

• Detection of a signal indicates time changing
  gravitational potential evidence of dark energy
  if the universe IS flat.
• Many existing analyses (Boughn and Crittenden,
  Nolta etal, Afshordi etal, Scranton etal,
  Padmanabhan etal)
• Results controversial, often non-reproducible and
  evidence is weak
• Future detections could be up to 6(10?) sigma,
  not clear if this probe can play any role in
  cosmological parameter determination

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WMAP-SDSS cross-correlation ISW N. Padmanabhan,
C. Hirata, US etal 2005

• 4000 degree overlap
• Unlike previous analyses we combine with
  auto-correlation bias determination (well known
  redshifts)

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• 2.5 sigma detection

Consistent with other probes
40
Counting Clusters of Galaxies
Sunyaev Zeldovich effect X-ray emission from
cluster gas Weak Lensing
Simulations
growth factor
41
Cosmic complementarity Supernovae, CMB, and
Clusters
42
Ly-alpha forest as a tracer of dark matter
Basic model neutral hydrogen (HI) is determined
by ionization balance between recombination of e
and p and HI ionization from UV photons (in
denser regions collisional ionization also plays
a role), this gives Recombination coefficient
depends on gas temperature Neutral hydrogen
traces overall gas distribution, which traces
dark matter on large scales, with additional
pressure effects on small scales (parametrized
with filtering scale kF) Fully specified within
the model, no bias issues
43
SDSS Lya power spectrum analysis McDonald, US
etal 2005

• Combined statistical power is better than 1 in
  amplitude, comparable to WMAP
• 2ltzlt4 in 11 bins
• ?2 129 for 104 d.o.f.
• A single model fits the data over a wide range of
  redshift and scale

Ly-alpha helps by reducing degeneracies between
dark energy and other parameters that Lya
determines well (amplitude, slope) Direct
search for dark energy at 2ltzlt4 reveals no
evidence for it
44
The amplitude controversy

• Some probes, Ly-alpha, weak lensing, SZ clusters
  prefer high amplitude (sigma_8gt0.85)
• Other probes, WMAP, X-ray cluster abundance,
  group abundance prefer low amplitude
  (sigma_8lt0.75)
• Statistical significance of discrepancy is
  2.5?-sigma or less
• For the moment assume this is a statistical
  fluctuation among different probes and not a sign
  of a systematic error in one or more probes

45
Putting it all together

• Dark matter fluctuations on 0.1-10Mpc scale
  amplitude, slope, running of the slope
• Growth of fluctuations between 2ltzlt4 from Lya
• Lya very powerful when combined with CMB or
  galaxy clustering for inflation (slope, running
  of the slope), not directly measuring dark energy
  unless DE is significant for zgt2
• still important because it is breaking
  degeneracies with other parameters and because it
  is determining amplitude at z3.

US etal 04, 06
46
Dark energy constraints complementarity of
tracers
US, Slosar, McDonald 2006
47
DE constraints degeneracies and dimension of
parameter space
48
Time evolution of equation of state w
Individual parameters very degenerate
49
Time evolution of equation of state

• w remarkably close to -1
• Best constraints at pivot z0.2-0.3, robust
  against adding more terms
• error at pivot the same as for constant w
• Perturbations switched off

50
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)
• (But DGP is already ruled out)

51
A look at neutrinos

• Neutrino mass is of great importance in particle
  physics (are masses degenerate? Is mass hierarchy
  inverted?) large next generation experiments
  proposed (KATRIN)
• Neutrino free streaming inhibits growth of
  structure on scales smaller than free streaming
  distance
• If neutrinos have mass they are dynamically
  important and suppress dark matter as well, 50
  suppression for 1eV mass
• For m0.1-1eV free-streaming scale is gt10Mpc
• Neutrinos are quasi-relativistic at z1000 CMB
  is also important, opposite sign

m0.15x3, 0.3x3, 0.6x3, 0.9x1 eV
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New limits on neutrino mass

• WMAP3SDSS LyaSDSS2dFSN 6p
• Together with SK and solar limits
• Lifting the degeneracy of neutrino mass

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Neutrino as dark matter

• Initial conditions set by inflation (or something
  similar)
• Neutrino free streaming erases structure on
  scales smaller than free streaming distance
• For neutrino to be dark matter it must have short
  free streaming length low temperature or high
  mass
• We can put lower limit on mass given T model
• One possibility to postulate a sterile neutrino
  that is created through mixing from active
  neutrinos. This is natural in a 3 right handed
  neutrinos setting, two are used to generate mass
  for LH, 3rd can be dark matter. To act like CDM
  need high mass, gtkeV. To suppress its abundance
  need small mixing angle, Qlt0.001, never
  thermalized

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Sterile neutrino as dark matter

• A sterile neutrino in keV range could be the dark
  matter and could also explain baryogenesis,
  pulsar kicks, seems very natural as we need
  sterile neutrinos anyways (Dodelson and Widrow,
  Asaka, Shaposhnikov, Kusenko, Dolgov and Hansen)
• However, a massive neutrino decays and in keV
  range its radiative decays can be searched for in
  X-rays. If the same mixing process is responsible
  for sterile neutrino generation and decay then
  the physics is understood (almost, most of the
  production happens at 100MeV scale and is close
  or above QCD phase transition)
• Strongest limits come from X-ray background and
  COMA/Virgo cluster X-rays and our own galaxy,
  absence of signal gives mlt3.5-8keV (Abazajian
  2005, Boyarsky etal 2005)

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Sterile neutrino as dark matter

• To proceed we need to specify the model assume
  no generation of sterile neutrinos above GeV, no
  lepton asymmetry enhancements, only production
  through mixing
• First approximation production independent of
  momentum
• calculations in Abazajian (2005) give more
  accurate momentum distribution 10 weaker mass
  constraints relative to previous calculations
  which assume momentum distribution is the same as
  active
• The limits for this model can be easily modified
  to other models (mirror, thermal, entropy
  injection from massive steriles etc)

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Results and implications

• Combined with the 6keV (COMA), 8-9keV (Virgo,
  X-ray background) upper limit from radiative
  decays THIS model is excluded
• How do the constraints change with possible
  entropy injection that dilutes sterile neutrinos
  relative to CMB photons/active neutrinos?
• T is decreased relative to CMB, neutrinos are
  colder
• Dilution requires larger mixing angle for same
  matter density, so decay rate higher, which
  makes X-ray constraints tighter
• This does not open up the window
• To solve the model need to generate neutrinos
  with additional interactions at high energies
  above GeV

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Future Dark Energy Prospects
Based on dark energy task force
65
Future as seen by the dark side of the universe
task force

• Members
• Andy Albrecht, Davis
• Gary Bernstein, Penn
• Bob Cahn, LBNL
• Wendy Freedman, OCIW
• Jackie Hewitt, MIT
• Wayne Hu, Chicago
• John Huth, Harvard
• Mark Kamionkowski, Caltech
• Rocky Kolb, Fermilab/Chicago
• Lloyd Knox, Davis
• John Mather, GSFC
• Suzanne Staggs, Princeton
• Nick Suntzeff, NOAO

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Goals and Methodology

• The goal of dark-energy science is to determine
  the very nature of the dark
• energy that causes the Universe to accelerate
  and seems to comprise
• most of the mass-energy of the Universe.
• Toward this goal, our observational program must
• Determine as well as possible whether the
  accelerated expansion is
• consistent with being due to a cosmological
  constant.
• If it is not due to a constant, probe the
  underlying dynamics by
• measuring as well as possible the time evolution
  of dark energy, for
• example by measuring w(a) our parameterization
  is w(a) w0 wa(1 - a).
• Search for a possible failure of GR through
  comparison of cosmic
• expansion with growth of structure.
• Goals of dark-energy observational program
  through measurement of
• expansion history of Universe dL(z) , dA(z) ,
  V(z), and through measurement
• of growth rate of structure. All described by
  w(a). If failure of GR, possible
• difference in w(a) inferred from different types
  of data.

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Goals and Methodology

• To quantify progress in measuring properties of
  dark energy we define
• dark energy figure-of-merit from combination of
  uncertainties in w0 and wa.
• Use of statistical (Fisher-matrix) techniques
• incorporating CMB and H0 information to predict
  future performance.
• Our considerations follow developments in Stages
  
• What is known now (12/31/05).
• Anticipated state upon completion of ongoing
  projects.
• Near-term, medium-cost, currently proposed
  projects.
• Large-Survey Telescope (LST) and/or Square
  Kilometer Array (SKA),
• and/or Joint Dark Energy (Space) Mission (JDEM).
• Dark-energy science has far-reaching implications
  for other fields of
• physics ? discoveries in other fields may point
  the way to understanding
• nature of dark energy (e.g., evidence for
  modification of GR).

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Fifteen Findings

• Four observational techniques dominate future
  proposals
• Baryon Acoustic Oscillations (BAO) large-scale
  surveys measure features in distribution of
  galaxies. BAO dA(z) and H(z).
• Cluster (CL) surveys measure spatial distribution
  of galaxy clusters. CL dA(z), H(z), growth of
  structure.
• Supernovae (SN) surveys measure flux and redshift
  of Type Ia SNe. SN dL(z).
• Weak Lensing (WL) surveys measure distortion of
  background images due to garavitational lensing.
  WL dA(z), growth of structure.
• Different techniques have different strengths and
  weaknesses and sensitive in different ways to
  dark energy and other cosmo. parameters.
• Each of the four techniques can be pursued by
  multiple observational approaches (radio,
  visible, NIR, x-ray observations), and a single
  experiment can study dark energy with multiple
  techniques. Not all missions necessarily cover
  all techniques in principle different
  combinations of projects can accomplish the same
  overall goals.

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Techniques

• Four techniques at different levels of maturity
• BAO only recently established. Less affected by
  astrophysical uncertainties than other
  techniques.
• CL least developed. Eventual accuracy very
  difficult to predict. Application to the study of
  dark energy would have to be built upon a strong
  case that systematics due to non-linear
  astrophysical processes are under control.
• SN presently most powerful and best proven
  technique. If photo-zs are used, the power of
  the supernova technique depends critically on
  accuracy achieved for photo-zs. If
  spectroscopically measured redshifts are used,
  the power as reflected in the figure-of-merit is
  much better known, with the outcome depending on
  the ultimate systematic uncertainties.
• WL also emerging technique. Eventual accuracy
  will be limited by systematic errors that are
  difficult to predict. If the systematic errors
  are at or below the level proposed by the
  proponents, it is likely to be the most powerful
  individual technique and also the most powerful
  component in a multi-technique program.

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Systematics, Systematics, Systematics
A sample WL fiducial model
StatisticalSystematics
Statistical
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Fifteen Findings

• A program that includes multiple techniques at
  Stage IV can provide an order-of-magnitude
  increase in our figure-of-merit. This would be a
  major advance in our understanding of dark
  energy.
• No single technique is sufficiently powerful and
  well established that it is guaranteed to address
  the order-of-magnitude increase in our
  figure-of-merit alone. Combinations of the
  principal techniques have substantially more
  statistical power, much more ability to
  discriminate among dark energy models, and more
  robustness to systematic errors than any single
  technique. Also, the case for multiple techniques
  is supported by the critical need for
  confirmation of results from any single method.

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w(a) w0 wa(1-a)
wa

• The ability to exclude L is better than
• it appears

• There is some z where limits on
• Dw are better than limits on Dw0

• Call this zp (p pivot) corresponding
• to Dwp

0
w0
-1
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wp
w(a) w0 wa(1-a)
Our figure of merit s (wp) ? s (wa)
-1.0
wa
0
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The Power of Two (or Three, or Four)
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Fifteen Findings

• Results on structure growth, obtainable from weak
  lensing or cluster observations, are essential
  program components in order to check for a
  possible failure of general relativity.

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Fifteen Findings

• In our modeling we assume constraints on H0 from
  current data and constraints on other
  cosmological parameters expected to come from
  measurement of CMB temperature and polarization
  anisotropies.
• These data, though insensitive to w(a) on their
  own, contribute to our knowledge of w(a) when
  combined with any of the dark energy techniques
  we have considered.
• Different techniques most sensitive to different
  cosmo. parameters.
• Increased precision in a particular cosmological
  parameter may benefit one or more techniques.
  Increased precision in a single technique is
  valuable for the important procedure of comparing
  dark energy results from different techniques.
• Since different techniques have different
  dependences on cosmological parameters, increased
  precision in a particular cosmological parameter
  tends to not improve the figure-of-merit from a
  multi-technique program significantly. Indeed, a
  multi-technique program would itself provide
  powerful new constraints on cosmological
  parameters.

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Fifteen Findings

• In our modeling we do not assume a spatially flat
  Universe. Setting the spatial curvature of the
  Universe to zero greatly helps the SN technique,
  but has little impact on the other techniques.
  When combining techniques, setting the spatial
  curvature of the Universe to zero makes little
  difference because the curvature is one of the
  parameters well determined by a multi-technique
  approach.
• Experiments with very large number of objects
  will rely on photometrically determined
  redshifts. The ultimate precision that can be
  attained for photo-zs is likely to determine the
  power of such measurements.

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Systematics

• Our inability to forecast reliably systematic
  error levels is the biggest impediment to judging
  the future capabilities of the techniques. We
  need
• BAO Theoretical investigations of how far into
  the non-linear regime the data can be modeled
  with sufficient reliability and further
  understanding of galaxy bias on the galaxy power
  spectrum.
• CL Combined lensing and Sunyaev-Zeldovich and/or
  X-ray observations of large numbers of galaxy
  clusters to constrain the relationship between
  galaxy cluster mass and observables.
• SN Detailed spectroscopic and photometric
  observations of about 500 nearby supernovae to
  study the variety of peak explosion magnitudes
  and any associated observational signatures of
  effects of evolution, metallicity, or reddening,
  as well as improvements in the system of
  photometric calibrations.
• WL Spectroscopic observations and narrow-band
  imaging of tens to hundreds of thousands of
  galaxies out to high redshifts and faint
  magnitudes in order to calibrate the photometric
  redshift technique and understand its
  limitations. It is also necessary to establish
  how well corrections can be made for the
  intrinsic shapes and alignments of galaxies,
  removal of the effects of optics (and from the
  ground) the atmosphere and to characterize the
  anisotropies in the point-spread function.

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Future Probes

• Four types of next-generation projects have been
  considered
• an optical Large Survey Telescope (LST), using
  one or more of the four techniques
• an optical/NIR JDEM satellite, using one or more
  of four techniques
• an x-ray JDEM satellite, which would study dark
  energy by the cluster technique
• a Square Kilometer Array, which could probe dark
  energy by weak lensing and/or the BAO technique
  through a hemisphere-scale survey of 21-cm
  emission
• Each of these projects is in the 0.3-1B range,
  but dark energy is not the only (in some cases
  not even the primary) science that would be done
  by these projects.
• Each of these projects considered (LST, JDEM, and
  SKA) offers compelling potential for advancing
  our knowledge of dark energy as part of a
  multi-technique program. The technical
  capabilities needed to execute LST and JDEM are
  largely in hand.

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Findings

• The Stage IV experiments have different risk
  profiles
• SKA would likely have very low systematic errors,
  but needs technical advances to reduce its cost.
  The performance of SKA would depend on the
  number of galaxies it could detect, which is
  uncertain.
• Optical/NIR JDEM can mitigate systematics because
  it will likely obtain a wider spectrum of
  diagnostic data for SN, CL, and WL than possible
  from ground, incurring the usual risks of a space
  mission.
• LST would have higher systematic-error risk, but
  can in many respects match the statistical power
  of JDEM if systematic errors, especially those
  due to photo-z measurements, are small. An LST
  Stage IV program can be effective only if photo-z
  uncertainties on very large samples of galaxies
  can be made smaller than what has been achieved
  to date.
• A mix of techniques is essential for a fully
  effective Stage IV program. No unique mix of
  techniques is optimal (aside from doing them
  all), but the absence of weak lensing would be
  the most damaging provided this technique proves
  as effective as projections suggest. Combining
  all information can lead to a factor of 3
  improvement on w, w each.

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Conclusions

• Dark energy remarkably similar to cosmological
  constant, w-1.04/- 0.06, no
  evidence for w evolution or modified gravity
• Best constraints achieved by combining multiple
  techniques this is also needed to test
  robustness of the results against systematics.
• Dark matter best described as cold and
  collisionless no evidence for warm dark matter
  (sterile neutrinos)
• Neutrinos not yet detected cosmologically, but
  getting really close to limits from mixing
  experiments unlikely to be degenerate and
  inverted hieararchy is mildly disfavored (at one
  sigma)
• Future prospects many planned space and ground
  based missions, this will lead to a factor of
  several improvements in dark energy parameters
  like w, w.