Cosmic shear

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Cosmic shear
Current status and prospects
Henk Hoekstra Department of Physics and
Astronomy University of Victoria
Cosmo 06 Lake Tahoe
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Cosmic shear outline

• Why cosmic shear?
• How do we measure it?
• What have we done so far?
• What would we like to do?
• What do we need to do?

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What is gravitational lensing?
As predicted by Einstein, rays of light are
deflected by massive objects in the universe.
The angle of deflection is a direct measure of
mass!
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What is gravitational lensing?
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What is weak gravitational lensing?
Weak lensing
Strong lensing
A measurement of the ellipticity of a galaxy
provides an unbiased but noisy measurement of the
shear
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What is cosmic shear?
Cosmic shear is the lensing of distant galaxies
by the distribution of matter in the universe it
is the most common lensing phenomenon.
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What is weak lensing?
The underlying assumption is that the position
angles are random in the absence of lensing. At
some level intrinsic alignments will complicate
things (can be dealt with using photometric
redshifts?).
no lensing
lensing
Averaged shape
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What do we measure from the data?

• To quantify the cosmic shear signal we use the
  ellipticity correlation functions. The results
• do not depend on survey geometry.
• provide a measure of the residual systematics.

r
no lensing
r
lensing
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Why cosmic shear?
The large scale distribution of matter in the
universe depends on the underlying cosmology it
is the result of how density fluctuations grow.
The complication is that most of the matter is
invisible One can use the distribution of
galaxies as a proxy for the large scale mass
distribution, but this can yield biased results!
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Relation between light and people...
NO!
Can we use light to infer the distribution of
people?
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Why cosmic shear?
The study of the distribution of galaxies has
provided important results on cosmological
parameters, but it is not clear how much better
one can do. This is why we try to study the
statistical properties of the matter distribution
through weak lensing by large scale structure
a.k.a. cosmic shear
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Why cosmic shear?
Current observational constraints on the
properties of dark energy are crude at best.
Progress depends critically on how well various
cosmological probes can be understood Do we
understand what we are doing? DETF comments
The WL technique is also an emerging technique.
Its eventual accuracy will also be limited by
systematic errors that are difficult to predict.
If the systematic errors are at or below the
level asserted by the proponents, it is likely to
be the most powerful individual Stage-IV
technique and also the most powerful component in
a multi-technique program.
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Why cosmic shear?
Although weak lensing has many interesting
applications, cosmic shear is the most demanding
and has been driving most technological
advances in the past decade.

• How do we measure this signal?
• How do we interpret this signal?

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How do we measure this?

• The weak lensing signal is small
• We need to measure the shapes of many galaxies.
• We need to remove systematic signals at a high
  level of accuracy.

Only recently we have been able to overcome both
obstacles, although we still need various
improvements to deal with the next generation of
surveys
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Build a big camera

• 1 square degree field of view
• 350 megapixels

Megacam
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Put it on a good telescope
Such as the CFHT or VST, LSST, SNAP, etc
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and take a lot of data!
CFHTLS RCS2
Thats when the fun starts
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Dealing with systematics

• The induced lensing signal is small and
  observational distortions are typically larger
  than the lensing signal.
• The observed shapes of galaxies need to be
  corrected for
• PSF anisotropy
• Circularisation by seeing
• Camera shear

Various correction techniques have been developed
and tested extensively. In particular the Kaiser
et al. (1995) approach is widely used. This
method works fine for current data sets, but we
need improved methods for upcoming large surveys
shapelets?
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Dealing with systematics the PSF
The first step in the correction for the PSF is
to identify a suitable sample of stars
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Dealing with systematics the PSF
The shape parameters from the stars allow us to
quantify the PSF anisotropy variation and the
effect of seeing
Old Megacam PSF anisotropy pattern
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Dealing with systematics the PSF
Weak lensing is rather unique in the sense that
we can study systematics very well. Several
diagnostic tools can be used. However, knowing
systematics are present doesnt mean we know how
to deal with them But we can readily simulate
weak lensing surveys. The Shear Testing Programme
(STEP) is aiming to improve our techniques this
way.
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Dealing with systematics tests
The lensing signal should be curl-free. We can
project the correlation functions into one that
measures the divergence and one that measures
the curl E-B mode decomposition. We can also
look for correlations between the corrected
galaxy shapes and the PSF anisotropy.
E-mode (curl-free)
B-mode (curl)
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Dealing with systematics improvements

• There are two concerns with PSF correction
• Correct PSF model?
• Correct correction?

Jarvis Jain recently have shown how to get a
(near?) perfect PSF model. Much work is
devoted to improve the correction itself
(shapelets, lensreg, etc.)
Once we have solved these problems
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What does the signal mean?
The statistical properties of the (dark) matter
distribution depend on the cosmology. The power
spectrum (the variance as a function of scale)
contains a wealth of information.
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What does the signal mean?
The cosmic shear signal is mainly a measurement
of the variance in the density fluctuations.
Same lensing signal
To first order lensing measures a combination of
the amount of matter Wm and the normalisation of
the power spectrum s8.
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What does the signal mean?
We want to know the convergence (or projected
mass) power spectrum
But we can measure
Shear correlation function at separation Q
Aperture mass (Map) variance at scale Q
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What does the signal mean?
The lensing signal depends on the (non-linear)
matter power spectrum this gives extra
sensitivity to the parameter estimation and can
break degeneracies We need improved estimates
for the power spectrum in the non-linear regime
(now use Peacock Dodds and Smith et al. (2003)
recipes). The lensing signal depends on the
redshift distribution of the (faint) source
galaxies this allows us to measure how structure
grows in the universe We need better estimates
for the source redshift distribution, e.g. from
photometric redshift surveys.
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What have we done so far?
Since the first detections reported in spring
2000, many cosmic shear measurements have been
published.
Status in 2001
Courtesy Y. Mellier
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some old surveys

• Red-sequence Cluster Survey (RCS)
• Data taken with CFHT and CTIO.
• 53 square degrees analysed.
• Measured gt 2x106 galaxy shapes down to R24.
• VIRMOS-DESCART
• Data taken with CFHT
• 11 square degrees analysed
• Measured gt 8x105 galaxy shapes down to IAB24.5
• CTIO survey
• Data taken with CTIO
• 80 square degrees down to R23

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Aperture mass variance
ltzgt0.58
ltzgt0.9
Best fit RCS model
Improved weak lensing signal from VIRMOS
vanishing B-mode on all scales!
Weak lensing signal from RCS
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CTIO survey
Jarvis et al. (2006)
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Does the signal make sense?
YES!

• Consistency of different measurements
• Vanishing B-modes
• Redshift dependence (!?)
• Shape of the signal
• all support the cosmological origin of the
  observed weak lensing signal.

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CFHT Legacy Survey
The Canada-France-Hawaii Telescope Legacy
Survey is a five year project, with three major
components

• Very Wide Survey solar system
• 600 square degrees
• around ecliptic (strip)
• Wide Survey weak lensing
• 140 square degrees (3 fields)
• 5 filters (u,g,r,i,z)
• ilt24.5
• Deep Survey type Ia supernovae
• 4 square degrees (4 fields)
• repeated observations in 5 filters
• expect 1000 supernovae!

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CFHTLS first results
Hoekstra et al (2006) first results based on 30
square degrees
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CFHTLS the next step

• Since the first results several things have
  happened
• Image quality has improved
• More data have been taken (100 sq. deg in
  g,r,i)
• 30 sq. deg. of u,g,r,i,z

• Reduced systematics
• Can probe larger scales
• Improve estimates of cosmological parameters
  using photo-zs

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CFHTLS the next step
Improvements in image quality
August 2005 L3 flip Spacer tilt
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CFHTLS the next step
Improvements in PSF anisotropy
before
after
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CFHTLS moving forward
We have recently analysed 50 sq. deg of i
data We have more than 100 sq. deg of g,r,i data
on disk, and about 30 sq. deg. of ugriz data.
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CFHTLS moving forward
Fu et al. (in prep)
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CFHTLS first results
But what about the source redshifts?
Hoekstra et al. (2006)
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But
We think the observed lensing signal is fairly
accurate. But how about the interpretation. Do we
know? Were getting there! The published
results were based on the HDF photometric
redshift distribution. The redshift distribution
obtained by Ilbert et al. (2006) based on CFHTLS
data suggests a higher mean redshift!
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CFHTLS-Deep redshifts
Knowledge of the source redshifts is currently
the dominant limiting factor for cosmic shear
surveys. Redshift information will allow for a
proper interpretation of the signal, improve
accuracy on w and help with intrinsic
alignments. A lot of work is needed to improve
the current situation!
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Updated results
Jon Benjamin (in prep)
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Details, details, details
The devil is in the details. The accuracy of
first cosmic shear surveys is mostly limited by
statistical noise (small areas). For the next
generation of surveys many subtle effects need to
be taken into account, such as Non-linear
covariance, complex survey areas, intrinsic
alignments, effect of baryons on matter power
spectrum,
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What to do next?
The study of dark energy is an important
application of cosmic shear. The CFHTLS will
provide the first constraints from a ground based
survey. But a useful measurement of the dark
energy parameters requires much better
precision. The several hundred million dollar
question is Can we reach the 1 precision from
the ground or should we go to space?
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Advantages of space

• much smaller PSF
• optical NIR bands
• higher source density
• higher source redshift
• better photo-zs
• large reduction in systematics

SuperNova Acceleration Probe
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Conclusions
The measurement of the cosmic shear signal using
CFHTLS data is progressing well. The accuracy of
weak lensing measurements continues to
improve. We can expect a dramatic improvement in
the accuracy of cosmological parameters thanks to
redshift information and better control of
systematics.
BUT
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Much to learn, we still have!
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There is some(?) work left

• Source redshift distribution
• photometric redshift from the survey data
• deep (photometric) redshift surveys
• Non-linear power spectrum
• large numerical simulations
• good initial conditions
• Intrinsic alignments of galaxies
• can be dealt with using photometric redshifts
• Observational systematic effects
• Improved correction schemes
• Detailed simulations