Post on 20-Dec-2015
July 7, 2008 SLAC Annual Program Review Page 1
Future Dark Energy Surveys
R. Wechsler
Assistant Professor
KIPAC
July 7, 2008 SLAC Annual Program Review Page 2
Overview
* Future Dark Energy Surveys– Dark Energy Survey– LSST– SNAP
* Dark Energy Probes– Baryon Acoustic Oscillations– Galaxy Clusters– Weak Lensing– SN
* Theoretical Challenges– Simulations of Structure formation– The Galaxy-Dark Matter Connection
* Personnel– Wechsler group: postdoctoral fellows Busha, Gerke, student Hao-Yi Wu+ Blandford group, Allen group, Abel group + many others
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The Standard Cosmological Model
* Basic accounting of the Universe’s present contents is in place
* We know what dark matter does on a wide range of scales, but very little about what it is.
* We know what the baryons are, but understanding their behavior and evolution requires modeling complex interactions over a vast range of scales.
* We know very little about dark energy, except how much there is.
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The Dark Energy Challenge
* Universe is dominated by Dark Energy driving us into a period of accelerated expansion. – What is causing it?– Why is de ~ dm
– Why is de < 10120 planck?
* Its effect can be characterized through equation of state– w = /P w(a) = w0 + wa(1+a)
* Measuring this equation of state and its evolution can provide clues to its nature: cosmological constant? Vacuum energy? Modification of gravity?
* Goal is to get precise (few percent) determination of w and its change with time.
* Extremely puzzling physics question that can mostly be determined through astronomical surveys.
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Dark Energy Probes
* Expansion Rate– Supernovae: Standard Candles– Baryon Acoustic Oscillations: Standard Ruler
• The decoupling of photons and baryons in the early universe imprints a feature on the large-scale clustering of galaxies at a given scale.
• Systematics can come from non-linear bias, photo-z errors.
* Growth of Structure– Weak lensing
• focus for the next generation will be on the shear power spectrum, as probed by the shapes of background galaxies. E.g., DES will measure the shapes of ~ 3e8 galaxies, to determine the angular power spectrum in 4 redshift slices.
• Systematics can come from photo-z errors, PSF, calibration.
– Galaxy Clusters• The number of galaxy clusters is determined both by the geometry (volume)
and the growth of structure. Excellent probe of DE. • Primary systematic is calibration of the mass--observable relation.
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Upcoming Dark Energy Surveys
* The Dark Energy Survey– 525 nights using a new camera on the Blanco 4m telescope– 5000 sq. degrees of g, r, i, Z, Y imaging to r ~24.– Cluster counts, weak lensing, Baryon Acoustic Oscillations, SN – Overlap with VISTA imaging (better photoz’s); SPT SZ clusters– ~300 million galaxies
* Large Synoptic Survey Telescope– 10 year survey with new 8.4m telescope, 3.2Gpix camera– 20000 sq. degrees of g, r, i, z, Y imaging to r ~ 27.5.– image 1/4 of the sky every 3 nights.– 10 billion galaxies
* SNAP– Approx: 1000 sq degrees to 28th mag, with 0.15” seeing
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Theoretical Challenges
* Next generation DE probes will be systematics limited.* In order to meet the challenge of few % constraints on
dark energy parameters, need precision calibration of structure formation, e.g.– Dark matter and galaxy clustering– Dark matter halo mass function– Profiles and formation histories of dark matter halos
* Also need a precise calibration of how galaxies trace the dark matter distribution– Important for Weak Lensing, BAO, Clusters, Photo-z’s.– Requires a very large dynamic range for the simulations,
detailed and well-calibrated models
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simulation by A. Kravtsov
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
Dark Matter Simulations
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Mock Galaxy Catalogs
* In addition to pure dark matter properties, need to understand how galaxy formation affects the dark matter distribution, and also how galaxies trace the dark matter distribution.
* Want to be able to test all analysis methods on realistic mock galaxy distributions before data is available.
* Want to be able to marginalize over uncertainties in GF.* Major computational challenge: need to resolve better
than kpc over several Gpc.* Our group has developed many techniques for
constraining the galaxy--dark matter connection
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Mock Galaxy Catalogs
* Current work:– Have provided a 5000 sq. degree catalog to DES depth to DES science
working groups – Active engagement with photo-z, clusters, LSS, lensing.– These simulations have also provided the backbone for image
simulations for both DES and LSST.– Exploring uncertainties in the galaxy distribution; working on
improvements that will be possible as deeper data becomes available
Wechsler & Busha
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logMth
14.513.5 14.5logMth
Forecasting DE Constraints From Clusters
* Dark Energy constraints from clusters depend on: maximum redshift that clusters can be identified, minimum cluster mass that can be reliably detected, amount of scatter in mass observable relation, priors on mass-observable relation & scatter.
* Our work distinguished by close connection with the observers developing the tools to make the measurements.
Wu & WechslerIn preparation
zmax
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Summary
* The nature of dark energy is one of the foremost physics questions* Progress on determining the nature of dark energy over the next decade will
be led by large galaxy surveys:– Weak Lensing– Baryon Acoustic Oscillations– Galaxy Cluster Counts
* The constraints from these surveys will be systematics limited– Require precision characterization of structure formation– Require precise determination of the galaxy--dark matter connection
* Simulation and computation play an increasingly important role.– Surveys will probe several Gpc– Need to resolve better than kpc scales over this volume, for a wide
range of cosmological models– Need to accurately simulate galaxy formation or marginalize over
unknown baryonic physics