Using Baryon Acoustic Oscillations to test Dark Energy Will Percival The University of Portsmouth...
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Using Baryon Acoustic Oscillationsto test Dark Energy
Will Percival
The University of Portsmouth
(including work as part of 2dFGRS and SDSS collaborations)
Baryon Acoustic Oscillations
“Wavelength” of baryonic acoustic oscillations is determined by the comoving sound horizon at recombination
At early times can ignore dark energy, so comoving sound horizon is given by
Sound speed cs
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
varying thebaryon fraction
Gives the comoving sound horizon ~110h-1Mpc, and BAO wavelength 0.06hMpc-1
Image credit: Martin White
BAO as a standard ruler
If we are considering radial and angular directions using randomly placed galaxy pairs, we constrain (to 1st order)
BAO position (in a redshift slice) therefore constrains some multiple of
Varying rs/DV
Changes in cosmological model alter measured BAO scale (∆dcomov) by:
Radial direction
(evolution of Universe)
Angular direction
(line of sight)
Extracting BAO from P(k)
fit data with a 2-component model comprising a smooth spline (node separation 0.05hMpc-1), and the sinusoidal (in the transfer function) multiplicative BAO component usually applied to a CDM model. The ability of this model to fit linear CDM power spectra is good.
Percival et al., 2007, astro-ph/0608635 & astro-ph/0705.3323
The SDSS DR5 sample
Main sample galaxies
Type-I LRGs
Type-II LRGs
After various selection cuts, the DR5 sample gives 51251 LRGs and 462791 main galaxies (factor ~2 larger than previously analysed)
BAO from all the SDSS DR5 galaxies
Compared with WMAP 3-year best fit linear CDM cosmological model. N.B. not a fit to the data, but a prediction from WMAP.
Interesting features:
1. Overall P(k) shape
2. Observed baryon acoustic oscillations (BAO)
Percival et al., 2007, ApJ, 657, 645
Matter density from SDSS BAO
When combined with, and marginalised over the WMAP 3-year peak position, For flat CDM cosmologies
Percival et al., 2007, ApJ, 657, 51
Comparing BAO at different redshifts
CREDIT: WMAP & SDSS websites
SDSS LRGs
SDSS main galaxies + 2dFGRS
Tell us more about the acceleration, rather than just that we need it!
z=0.35z=0.2
Combining the SDSS and 2dFGRS
Work for astro-ph/0705.3323 in collaboration with: Shaun Cole, Dan Eisenstein, Bob Nichol, John Peacock, Adrian Pope, Alex Szalay
BAO from the 2dFGRS + SDSS
BAO detected at low redshift 0<z<0.3 (effective redshift 0.2)
BAO detected at high redshift 0.15<z<0.5 (effective redshift 0.35)
BAO from combined sample (detected over the whole redshift range 0<z<0.5)
Percival et al., 2007, MNRAS, astro-ph/0705.3323
Galaxy distances needed for analysis
Galaxy redshifts need to be converted to distances before BAO can be measured
Not a problem for small numbers of parameters, but time consuming for more
Solve problem by parameterizing distance-redshift relation by smooth fit with small number of modes: can then be used to constrain multiple sets of models
For SDSS+2dFGRS analysis, choose two modes at z=0.2 and z=0.35, for fit to DV
This forms an intermediate link between the cosmological models to be tested and data
BAO distance scale constraints
Constraint fromDV(0.35)/DV(0.2)
Constraint fitting rs/DV(z)
Constraint including observed peak distance constrain from CMB rs/dA(cmb)=0.0104
SCDMSCDM
OCDMOCDMCDMCDM
Cosmological constraints
Constraint fromDV(0.35)/DV(0.2)
Constraint fitting rs/DV
Constraint including distance to CMB
Consider two simple models:1. CDM2. Flat, constant w
Discrepancy with CDM?
LRG BAO on too small scales: further away than expected, so more acceleration between z=0.2 and 0.35
Discrepancy is 2.4
Can increase BAO damping and reduce significance of result, but then match with data becomes worse
conclusions
BAO offer an attractive method for DE studies– Good reasons to believe that systematics are of low amplitude
– Physics is well known and can be modeled today
SDSS+2dFGRS measures BAO and rs/DV at z=0.2, z=0.35
– Constraint DV(0.35)/DV(0.2) = 1.812 is higher than predicted by LambdaCDM+WMAP+SNLS
DV(0.35)/DV(0.2) = 1.67 (2.4 discrepancy). Needs more acceleration at low redshift
– But, can reduce significance slightly by adjusting BAO fit
Many future BAO experiments are planned– BOSS, DES, PanSTARRS, WFMOS, ADEPT, SPACE, HetDEX, SKA, …