Dark Energy in

The Context of Large Data Sets

Patrice M. OKOUMAUCT/SAAO/AIMS

December, 30th 2010 SKA Bursary conference/Stellenbosch

Outline

Paper I : A Fundamental uncertainty inThe BAO scale from Isocurvature

modes(based upon arXiv:1006.4687v1 : submitted )

Paper II : A Fundamental Uncertainty on The Cosmic Curvature From Dark

Energy ?

Paper IPaper I : A Fundamental uncertainty inThe BAO scale from Isocurvature modes

(based upon arXiv:1006.4687v1 : submitted )

• Initial conditions & Baryon Acoustic Oscillations (BAO) in brief

• Fisher formalism applied to galaxy power spectrum

• Current result( )s

• Summary

Work with C. Zunckel, S.M. Kasanda, K. Moodley, (UKZN) &

B.A. Bassett (UCT/SAAO/AIMS)

Motivation

--------> our current understanding of BAO relies on a set of restrictive assumptions about the initial conditions.

Question : Assuming more general initial conditions,

by how muchhow much could this assumption alter/bias

our understanding of DE via the BAO scale ?

T. Joseph W. Lazio, arxiv:0910.0632v1 (The SKA)

“ “ (...) The SKA will assemble a large enough sample of galaxies (...) The SKA will assemble a large enough sample of galaxies

to measure BAOs as a function redshift to constrain the to measure BAOs as a function redshift to constrain the

equation of state of dark energy (...) “equation of state of dark energy (...) “

space

Isocurvature (entropy) perturbations

Adiabatic (curvature) perturbations

Initial Conditions

space

“ Fingerprints ” on

Large Scale Structure

r0r0

r0

In adiabatic mode :

Eisenstein et al. (2005)

r0 = Standard ruler

Fisher formalism applied to galaxy power spectrum

If likelihood function of band powers of galaxy power spectrum is Gaussian :

(Tegmark et al. 1997; Seo & Eisenstein 2003)

Fisher formalism applied to galaxy power spectrum – cont'd

Minimum error on parameter =

+ Isocurvature parameters

Gaussian Likelihood

FoM alterations for stage III-IV -like survey parameters

1.5, 1.2

Bias in DE params. estimates for stage III-IV -like survey parameters

7σ (10σ) incorrect measurement of Wo and as much as

23σ (12σ) for Wa if ignoring isocurvature modes

Summary

• If one does not allow for general initial condition, one can get very large biases in dark enerrgy parameters for future BAOsurveys such as SKA

• BAO are a firm prediction of CDM models and one key-topic of the science programme for SKA ;

More at :arxiv.org/abs/1006.46871006.4687

Paper IIPaper II : A Fundamental Uncertainty on The Cosmic Curvature From Dark

Energy ?

• The cosmic curvature/dark energy dynamics degeneracy

• A Monte Carlo based exploration with the cosmomc code

• Current result( )s

• Summary & work in progress

Work with Y. Fantaye (SISSA, Italy) & B.A. Bassett (UCT/SAAO/AIMS)

Outline

The cosmic curvature/dark energy degeneracyD

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A Monte Carlo based exploration via the cosmomc codewith a general evolution for the dark energy

Current resut(s)

Okouma et al. in prep.

Summary & work in progress• Measuring Ωk accurately (σ < 0.003) will play an important role in the quest to hunt down the true origin of dark energy in the

coming decade.

If one allows for a completely general dark energydynamics, then distance measurements alone cannot constrain thecurvature of the universe.

Our preliminary results suggest that the CMB does give good constraints on curvature, even allowing up to 20 dark energyparameters; which gives confidence in our belief that the universe is close to being flat;

•Possible artificial cuts ? ; W_i = W(z = z_i) constraints

THANK YOU

Back up slides start here

Current resut(s)

The cosmic curvature/dark energy degeneracy

C. C

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10

A Monte Carlo based exploration with the cosmomc codein the context of a general evolution for the dark energy

a_i

W_i

Summary & future work

• Measuring accurately ( )

will play an important role in the quest to hunt down the true origin of dark energy in the coming decade. Baryon Acoustic Oscillations, in conjunction with complementary cosmic probesare likely to play a leading role ;

• Based on a Monte Carlo scheme, our current estimates

on are still inconclusive and further investigations are ongoing ;

• The present work may reinforce the growing recommendation that cosmic curvature must be included as a free parameter in all future studies of dark energy such SKA.