What have we learnt from WMAP?

Robert CrittendenInstitute of Cosmology and Gravitation, Portsmouth, UK

Over 100 papers in the past year just with ‘WMAP’ in the title! What is qualitatively new in WMAP?

Outline• Introduction to WMAP• Polarization on the largest scales• Correlations with large scale structure• Large scale power deficit• Topology of the universe• Evidence of non-Gaussianity

Wilkinson Microwave Anisotropy Probe

David Wilkinson

CMB Pioneer

WMAP satellite

Launched June 2001

Reached L2 August 2001

WMAP science team

• Charles Bennett, PI NASA GoddardG. Hinshaw, M. Limon, A. Kogut, E. Wollack

• L. Page, N. Jarosik, PrincetonD. Spergel, D. Wilkinson

• E. Wright UCLA• G. Tucker Brown• S. Meyer Chicago• M. Halpern UBC

Many thanks to them for their hard work as well as images and data products used in this talk!

what’s so great about WMAP?

Mostly quantitative:

• More pixels: nearly 1,000,000 independent pixels

• More frequencies: five frequency channels better foreground removal, even the through galaxy

• Full sky coverage tight calibration from the dipole (0.5%) compare and unify other CMB observations cosmic variance limited to l=350

• Better systematic error control

five frequency maps

23, 33, 41, 61 and 93 GHz

derived maps

Dust map Synchrotron map

Free-free map CMB map

Power spectrum of WMAP

Cosmic variance limited to l=350

S/N > 1 for l<650

Features:

• Doppler peak well characterized

• missing small scale power?

• glitches at l=40, 210

Remarkably consistent with earlier data, apart from 10% calibration issue.

Hinshaw et al.

cosmological parameters

The power spectrum has confirmed earlier best fit models, with smaller error barsBottom line: very close to flat

significant dark matter (25%) dominated by dark energy (70%)

adiabatic, n=1

Most papers have focused on using spectrum to constraint variants of the lambda CDM model

But the full sky nature of WMAP has allowed us to discover a number of things which we could not have known otherwise…

Large scale CMB polarization

On the last scattering surface, polarisation is generated effectively by the fluid velocity gradient

Thus, we do not expect it to be large for modes outside the horizon (l < 100)

Any polarization we see on these scales must have been generated by later scattering!

WMAP is the first experiment to see the polarization on the largest scales (though it was seen in earlier measurements by DASI)

Temperature-polarization cross correlation

WMAP’s polarization sensitivity is poor, making direct detection very difficult, but large sky coverage means finding correlation with temperature is relatively easy.

Advantage is that we know the polarization we see is cosmological! WMAP saw both reionization and polarization from last scattering!

First calculations of T-P by Coulson, RC and Turok, 1994

Kogut et al. 2003

Reionization

Polarization on very large scales means some fraction of the light was recently rescattered

The amplitude indicates 1/6 photons scattered, which can only be done if the universe reionized fairly early (z = 20 10)Optical depth,

= 0.17 0.04

This will be discussed in detail in talks by J. Ostriker, M. Kaplinghat

Polarization from last scattering

WMAP also measured the polarization from the largest modes at last scattering

The sign of the cross correlation tells us something about the direction of the velocity flows at that time.

The sign was consistent with that predicted by adiabatic fluctuations, but not isocurvature.

Inflow

Radial

OutflowTangential

Correlation with large scale structure

The large sky coverage of WMAP means that one is able to detect even weak correlations with other surveys.

There are a number of reasons why such correlations might exist:

• The integrated Sachs-Wolfe effect• The Sunyaev-Zeldovich effect in clusters• Unremoved foreground sources

integrated Sachs-Wolfe effect

while most cmb anisotropies arise on the last scattering surface, some may be induced by passing through a time varying gravitational potential:

dT

T2 linear regime – integrated Sachs-Wolfe

(ISW)

non-linear regime – Rees-Sciama effect

when does the linear potential change?

22 4 aG Poisson’s equation

• constant during matter domination• decays after curvature or dark energy come to dominate (z~1)

induces an additional, uncorrelated layer of large scale anisotropies

two independent maps

Integrated Sachs-Wolfe mapMostly large angular features

Early time map (z > 4)Mostly from last scattering surface

Observed map is total of these, and has features of both (3 degree resolution)

compare with large scale structure

potential depth changes as cmb photons pass through

time dependent gravitational potential

observer

density of galaxies traces the potential depth

ISW fluctuations are correlated with the galaxy distribution!

since the decay happens slowly, we need to see galaxies at high redshifts (z~1)

active galaxies (quasars, radio, or hard x-ray sources) possibility of accidental correlations means full sky needed

how do we trace the matter?

X-rays from active galaxies

HEAO-1 x-ray satelliteGalaxy and virtually all visible structures cleaned out

Radio galaxies

NRAO VLA Sky Survey (NVSS)

ISW correlations detected!

Correlations seen with both at the 2.5 – 3.0 sigma level

Also seen to some extent in galaxy surveys: SDSS, 2MASS, APM

Amplitudes are largely consistent with dark energy model and argue against any pure dark matter model. (See E. Copeland talk.)

S. Boughn & RC, Nature2004

SZ cross correlation

Hot gases in clusters can upscatter CMB photons, also producing a correlation on smaller angular scales.

Evidence for this is growing, but still somewhat contradictory

Apparently detected with some surveys, not seen in others, seems to depend on the method used to trace clusters.

This will lead to a constraint on the size and temperature of hot gas in clusters (Compton y-parameter) see also S. Majumdar talk.

Are there missing large scale correlations?The WMAP papers reported

a deficit of large scale power to that expected in cosmological constant dominated models

One statistic showed that this was likely only at a level of 1 chance in 700.

(Posterior statistic?)

Missing power also observed for COBE.

Difficult to measure given cosmic variance.

Spergel et al.

Why has this received so much attention?

Other glitches are more statistically significant, but this is at a very interesting scale, the present horizon, and is not constrained on larger scales

Difficult to produce by additional effects because it requires cancelling large scale power

Some proposed solutions:1) Running spectra tilt2) Some minimum k cutoff3) Related to curvature scale4) High frequency oscillations in spectrum5) SZ from local supercluster6) Related to topology of the universe

Is the deficit significant?

Efstathiou argues that the WMAP analysis suffers from a number of problems:

• Low estimates of power spectrum given the uncertainties in the masking

• Biases from frequentist statistics• Decided on the test based on seeing the data

He argues that the discrepancy is more like 1 chance in 10 or 20 and is consistent with lambda CDM.

Can only be improved by better subtraction of the galaxy.

Remains a tempting target for theorists.

Could we be seeing the effects of a finite universe?

Based on the WMAP low l power spectrum, it has been suggested that our space could be dodecahedral (shaped like a soccer ball)

This model is slightly closed and positively curved, = 1.013

No indications have been found using correlation of patches

Luminet, Weeks, Riazuelo, Lehoucq and Uzan, 2003.

More general searches have given only upper limits

So far general searches for topology have only placed upper limits

Focus has been on looking for matched ‘circles in the sky’ or symmetries in the temperature patterns

While the dodecahedral model hasn’t been specifically excluded, evidence is against most models with a topology scale less than 24 GPc (Cornish et al, de Oliveira-Costa et al., Bond, Pogosyan & Souradeep)

Future tests – looking for statistical isotropy (Hajian & Souradeep)

Non-Gaussianity in WMAP?

Initial analyses indicated that the WMAP results were consistent with Gaussianity:1) Three point tests are consistent up to known point source contribution (Komatsu et al., Gaztanaga & Wagg)2) Apparent non-Gaussianities in COBE bispectrum do not appear in WMAP (Magueijo & Madeiros)3) Topological tests (Minkowski functionals, genus) are also consistent (Komatsu et al., Colley & Gott)

So far the limits are not sufficient to endanger the levels of non-Gaussianity that might be predicted by inflation

Some trouble on the horizon?Some recent analyses have pointed to possible

inconsistencies:1) Evidence that north ecliptic hemisphere has less

large scale power than southern (Eriksen et al.) 2) A wavelet analysis shows evidence for non-

Gaussianity in the southern Galactic hemisphere (Vielva et al.)

3) Asymmetry between some genus statistics for north and south Galactic hemispheres (Park)

4) Some strange alignments seen in the quadrupole and octopole moments (Tegmark et al.)

5) Multipole vector analysis indicates unexpected alignments at low l (Copi et al.)

6) Evidence for some strange phase correlations at l=16 (Coles et al.) and at very high l (Chiang et al.)

Is it significant?

Most authors argue against foreground being responsible, but its not impossible

Possibly a problem with a posteriori statistics, but many seem to be pointing to similar problems

Could it be similar to COBE problems, where some of the data was contaminated? This seems unlikely for the large scale problems.

The jury is still out, and more investigation is needed!

Conclusions

WMAP has not only improved our understanding on a quantitative level, but also in qualitatively new ways thanks to its all sky coverage

1) Large scale polarization data shows that the CMB was significantly rescattered – new physics of reionization?

2) The velocity flows after last scattering were consistent with adiabatic fluctuations

3) Evidence that some fluctuations were produced recently due to ISW, consistent with predictions of lambda CDM

4) Interesting hints at a lack of power on large scales, but its still consistent with the standard picture

Conclusions continued…

5) One interesting explanation for it, a ‘soccerball’ universe has not been ruled out, but most small topological universes are unlikely.

6) The fluctuations appear largely Gaussian as would be expected by inflation, but some interesting aspects of the data still puzzle us, particularly on large scales

Future:Two year data should be out soonWMAP Polarization auto-correlation (E-E) Small scale temperature and polarization experimentsPlanck is just 3.5 years away!

large scale correlations

The anisotropies created by the ISW effect are primarily on large scales and are largely uncorrelated with those produced earlier

On small scales, positive and negative ISW effects will tend to cancel out. However, on larger scales photons receive fewer kicks of larger amplitude

The early and late power is fairly weakly correlated, so the power spectra add directly:

iswl

earlyl

totall CCC

0.172.073.0 nh

WMAP best fit scale invariant spectrum

08.0/ 222 earlyiswCCCHighest correlations are for the quadrupole, but it is still very weak