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Measuring dark energy from galaxy surveys. Carlton Baugh Durham University London 21 st March 2012. Galaxies to cosmological parameters. Connecting dark matter to linear theory. Linear perturbation theory. Evolved matter distribution: MXXL. Angulo et al. 2012.

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Measuring dark energy from galaxy surveys

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Measuring dark energy from galaxy surveys

Carlton Baugh

Durham University

London 21st March 2012

Galaxies to cosmological parameters

Connecting dark matter to linear theory

Linear perturbation theory

Evolved matter distribution: MXXL

Angulo et al. 2012

Connecting galaxies to mass



Orsi et al. 2010

A hybrid tool to study dark energy

Galaxy formation simulation

N-body simulation

Bias and nonlinear effects

Angulo et al. 2008

Scale dependent biases

Dark matter

Linear theory

Angulo et al. 2008

Scale dependent RSD

Linear theory

Jennings et al. 2011

Scale dependent RSD

Jennings et al. 2011

The need for improved RSD models

Using linear theory leads to systematic errors similar to difference in signal

Jennings et al. 2011

Measuring DE using galaxy pairs

Marinoni & Buzzi 2010

Peculiar motions distort apparent

distribution of pair angles:

Jennings et al. 2012

A probe of dark energy

  • Peculiar velocities give distortion in pair angle distribution

  • Probe of Hubble expansion

  • Distortion sensitive to nature of dark energy

  • Distinguish models with same BAO and halo mass function

  • Requires simulations

    Jennings et al. 2012

Why build a mock catalogue?

A synthetic galaxy can catalogue be used to:

  • Test estimators: e.g. Galaxy clustering, photo-z, group-finders

  • Devise new statistics: e.g. pair distortions

  • Uncover systematic effects (we already know the right answer)

  • Estimate errors

  • Generate predictions for different dark energy cosmologies

What is a mock catalogue?



Combine with galaxy

formation model

N-body simulation

A priori calculation

of baryon physics

Millennium, Horizon, MICE, MXXL


Springel et al. 2005, Teyssier et al. 2009;

Fosalba et al 2008; Angulo et al. 2012

Cole et al. ;Hatton etal; de Lucia et al; Monaco +

Constructing observer’s past lightcone


Build galaxy catalogue on observer’s past



e.g. Blaizot et al 2005; Fosalba et al.

Kitzbichler & White; Merson et al.

Apply survey “window function”

  • Apply angular footprint

  • Apply scanning mask

  • Incorporate sampling

  • Redshift completeness (synergy with OU-SIM)

  • Steps 1, 2, 3 generic

  • Step 4 is survey specific

Synthetic images

Merson et al. 2012

Merson et al. 2012

Merson et al. 2012


  • Hybrid tool: N-body and galaxy formation sims

  • Linear theory not accurate enough for Euclid on large-scales

  • Scale dependent: nonlinearities, bias, RSD

  • Mock catalogues will help refine estimators

  • Devise new tests

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