July 7, 2008SLAC Annual Program ReviewPage 1 Future Dark Energy Surveys R. Wechsler Assistant Professor KIPAC

July 7, 2008 SLAC Annual Program Review Page 1

Future Dark Energy Surveys

R. Wechsler

Assistant Professor

KIPAC


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


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.


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.


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.


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


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


simulation by A. Kravtsov

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are needed to see this picture.

Dark Matter Simulations


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


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


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


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

Documents

July 7, 2008SLAC Annual Program ReviewPage 1 Future Dark Energy Surveys R. Wechsler Assistant Professor KIPAC