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					State of the (dark)
 universe report

     Uros Seljak

 Heidelberg, november 7, 2006
1) Methods to investigate dark energy and dark
   matter: SN, CMB, galaxy clustering, cluster
        counts, weak lensing, Lya forest

2) Current constraints: what have we learned so
                far, controversies

     3) What can we expect in the future?
1. Conclusive evidence for acceleration of the Universe.
   Standard cosmological framework  dark energy (70% of mass-energy).
2. Possibility: Dark Energy constant in space & time (Einstein’s L).
3. Possibility: Dark Energy varies with time (or redshift z or a = (1+z)-1).
4. 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.
5. Possibility: GR or standard cosmological model incorrect.
6. Whatever the possibility, exploration of the acceleration of the Universe
   will profoundly change our understanding of the composition and nature
   of the Universe.
         How to test dark
1) Classical tests: redshift- luminosity
   distance relation (SN1A etc), redshift-
   angular diameter distance, redshift-
   Hubble parameter relation
Classical cosmological tests (in a new form)

                          Friedmann’s (Einstein’s)
         How to test dark
1) Classical tests: redshift-distance
   relation (SN1A etc)…
2) Growth of structure: CMB, Ly-alpha,
   weak lensing, clusters, galaxy clustering
Growth of structure by gravity
                           Perturbations can
                           be measured at
                           different epochs:

                           1.CMB z=1000
                           2. 21cm z=10-20 (?)
                           3.Ly-alpha forest
                           4.Weak lensing
                           5.Galaxy clustering
                           z=0-1 (3?)
                           Sensitive to dark
                           energy, neutrinos…
         How to test dark
1) Classical tests: redshift-distance
   relation (SN1A etc)…
2) Growth of structure: CMB, Ly-alpha,
   weak lensing, clusters, galaxy clustering
3) Scale dependence of structure
Scale dependence of cosmological probes

                  WMAP                       z  1088

                                 CBI     ACBAR
                                       Lyman alpha forest

                                z 0            z 3

      Complementary in scales and redshift
                       Sound Waves from the
                          Early Universe
Before recombination:
 – Universe is ionized.                After recombination:
 – Photons provide enormous              – Universe is neutral.
   pressure and restoring force.         – Photons can travel freely past
 – Perturbations oscillate as              the baryons.
   acoustic waves.                       – Phase of oscillation at trec
                                           affects late-time amplitude.

                        Same Initial             Maximal Effect


                                                 Minimal Effect

This is how the Wilkinson Microwave
Anisotropy Probe (WMAP) sees the
                   Determining Basic
Angular Diameter
w = -1.8,..,-0.2
When combined with
measurement of matter
density constrains data to a
line in Wm-w space
                 Determining Basic

Matter Density
Wmh2 = 0.16,..,0.33
                Determining Basic

Baryon Density
Wbh2 = 0.015,0.017..0.031
also measured through D/H
Current 3 year WMAP analysis/data situation

Current data favor the simplest scale
invariant model
            400,000 galaxies with redshifts

Galaxy and quasar survey
     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 r~23
• current 7000 deg2 of
imaging data, 40 million
• 400,000 spectra
(r<17.77 main sample,
19.1 QSO,LRG)                 Image Credit: Sloan Digital Sky Survey
       Galaxy power spectrum: shape analysis
Galaxy clustering traces dark
matter on large scales                  Nonlinear
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
Amplitude not useful (bias
     Are galaxy surveys
 consistent with each other?
Some claims that SDSS main sample gives more
than 2 sigma larger value of W
    Fixing h=0.7
 SDSS LRG photo
SDSS main spectro

  Bottom line: no evidence for discrepancy,
  new analyses improve upon SDSS main
   Acoustic Oscillations in the
    Matter Power Spectrum
                                      • Peaks are weak;
                                        suppressed by a factor
                                        of the baryon fraction.
                                      • Higher harmonics
                                        suffer from diffusion
                                      • Requires large surveys
                                        to detect!

Linear regime matter power spectrum
            A Standard Ruler
• The acoustic oscillation scale
  depends on the matter-to-
  radiation ratio (Wmh2) and the
  baryon-to-photon ratio
• The CMB anisotropies
  measure these and fix the
  oscillation scale.               dr = DAdq    dr = (c/H)dz
• In a redshift survey, we can
  measure this along and across
  the line of sight.
• Yields H(z) and DA(z)!                   Observer
Baryonic wiggles
Best evidence: SDSS
LRG spectroscopic
sample (Eisenstein etal
2005), about 3.5 sigma
SDSS LRG photometric
sample (Padmanabhan,
Schlegel, US etal 2005):
2.5 sigma evidence
     To perturb or not to perturb dark
• Should one include perturbations in dark energy?
• For w=-1 no perturbations
• For w>-1 perturbations in a single scalar field model with canonical
  kinetic energy, speed of sound c
• Non-canonical fields may give speed of sound <<c
• For w<-1 (phantom model) one can formally adopt the same, but the
  model has instabilities
• For w crossing from <-1 to >-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 w<-1
  (Creminelli etal 2006) and predicts no perturbations in DE sector
    Weak Gravitational Lensing

Distortion of background images by foreground matter

      Unlensed                         Lensed
   Weak Lensing: Large-scale shear


1000 sq. deg.
to R ~ 27

     Gravitational Lensing            Refregier et al. 2002

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

         Tereno et al. 2004
    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
Intrinsic correlations
      in SDSS
 Mandelbaum, Hirata, Ishak, US etal 2005

 300,000 spectroscopic
 No evidence for II
 Clear evidence for GI
 correlations on all scales up
 to 60Mpc/h
 Gg lensing not sensitive to GI
        Implications for future surveys
        Mandelbaum etal 2005, Hirata and US 2004

Up to 30% for
shallow survey at
10% for deep
survey at z=1:
current surveys
underestimate s8
More important for
cross-redshift bins
     Galaxy bias determination
                        Pgg (k )
             b (k ) =

                        Pdm (k )
•Galaxies are biased tracers of dark matter;
 the bias is believed to be scale independent
        on large scales (k<0.1-0.2/Mpc)
  •If we can determine the bias we can use
     galaxy power spectrum to determine
    amplitude of dark matter spectrum s   8

     •High accuracy determination of s is

    important for dark energy constraints
  •Weak lensing is the most direct method
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…
 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
SDSS gg: 300,000 foreground
galaxies, 20 million background,
S/N=30, the strongest weak
lensing signal to date
testing ground for future surveys
such as LSST,SNAP                   Seljak etal 2004
        dark matter corr function
  On large scales
galaxies trace dark
   G-g lensing in
combination with
autocorrelation analysis
gives projected dark
matter corr. function
Mandelbaum, US etal, in prep
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
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)
•2.5 sigma detection

                                                                 QuickTime™ an d a
                                                             TIFF (LZW) decomp resso r
                                                          are need ed to see this picture.

               QuickTime™ an d a
           TIFF (LZW) decomp resso r
        are need ed to see this picture.

                                           Consistent with other probes
                                     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
                      r HI  r gas

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
   SDSS Lya power spectrum analysis
                         McDonald, US etal 2005
                                         • Combined statistical
                                           power is better than 1%
                                           in amplitude, comparable
                                           to WMAP
                                         • 2<z<4 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 2<z<4 reveals no evidence for it
  The amplitude controversy
• Some probes, Ly-alpha, weak lensing, SZ clusters
  prefer high amplitude (sigma_8>0.85)
• Other probes, WMAP, X-ray cluster abundance,
  group abundance… prefer low amplitude
• 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
Putting it all together                      Dark matter fluctuations on
         US etal 04, 06                      0.1-10Mpc scale: amplitude,
                                             slope, running of the slope
                                             Growth of fluctuations
                                             between 2<z<4 from Lya
                                             Lya very powerful when
                                             combined with CMB or galaxy
                                             clustering for inflation (slope,
                                             running of the slope), not
                QuickTime™ and a
       TIFF (Uncompressed) decompre ssor
          are neede d to see this picture.
                                             directly measuring dark energy
                                             unless DE is significant for z>2
                                              still important because it is
                                             breaking degeneracies with
                                             other parameters and because it
                                             is determining amplitude at
Dark energy constraints:
complementarity of tracers

                     US, Slosar, McDonald 2006
DE constraints: degeneracies and
 dimension of parameter space
Time evolution of equation of state w

Individual parameters very degenerate
         Time evolution of equation of state

•  w remarkably close to -
• Best constraints at
  pivot z=0.2-0.3, robust
  against adding more
• error at pivot the same
  as for constant w
• Perturbations switched
      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)
                        look at
                     Agreat importance in neutrinos
    Neutrino mass is of
    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 m=0.1-1eV free-streaming scale is
•   Neutrinos are quasi-relativistic at
    z=1000: CMB is also important, opposite

                                        m=0.15x3, 0.3x3, 0.6x3, 0.9x1 eV
New limits on neutrino mass
 • WMAP3+SDSS Lya+SDSS+2dF+SN 6p:

 • Together with SK and solar limits:

 • Lifting the degeneracy of neutrino mass
          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, >keV. To suppress its
  abundance need small mixing angle, Q<0.001, never thermalized
         Sterile neutrino as dark
• 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 m<3.5-
  8keV (Abazajian 2005, Boyarsky etal 2005)
            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
      Future as seen by the dark side
•   Membersof the universe task force
    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
 Four techniques at different levels of maturity:
a.  BAO only recently established. Less affected by astrophysical
    uncertainties than other techniques.
b.  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.
c.  SN presently most powerful and best proven technique. If photo-z’s
    are used, the power of the supernova technique depends critically on
    accuracy achieved for photo-z’s. 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.
d.  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.
 Our inability to forecast reliably systematic error levels is the biggest
 impediment to judging the future capabilities of the techniques. We need
a.   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.
b.   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.
c.   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.
d.   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.
                        Future Probes
    Four types of next-generation projects have been considered:
   a.    an optical Large Survey Telescope (LST), using one or more of the
         four techniques
   b.    an optical/NIR JDEM satellite, using one or more of four techniques
   c.    an x-ray JDEM satellite, which would study dark energy by the cluster
   d.    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.
13. 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.
The Stage IV experiments have different risk profiles:
   a.     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.
   b.     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.
   c.     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.
• 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’.

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