Transcript cosmic structure formation - Royal Observatory, Edinburgh

Using Baryon Acoustic Oscillations
to test Dark Energy
Will Percival
The University of Portsmouth
(including work as part of 2dFGRS and SDSS collaborations)
Baryon Acoustic Oscillations
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Image credit: Martin White
“Wavelength” of baryonic acoustic
oscillations is determined by the
comoving sound horizon at
recombination
varying the
baryon fraction
At early times can ignore dark energy,
so comoving sound horizon is given by
Sound speed cs
Gives the comoving sound horizon ~110h-1Mpc,
and BAO wavelength 0.06hMpc-1
BAO as a standard ruler
Changes in cosmological model
alter measured BAO scale
(∆dcomov) by:
BAO position (in a redshift slice)
therefore constrains some multiple
of
Radial direction
(evolution of Universe)
Angular direction
(line of sight)
If we are considering radial and angular
directions using randomly placed galaxy
pairs, we constrain (to 1st order)
Varying rs/DV
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
CMB
SDSS GALAXIES
Comparing CMB & BAO
CREDIT: WMAP & SDSS websites
z=0.2
SDSS LRGs
SDSS main galaxies + 2dFGRS
Comparing BAO at different redshifts
Tell us more about the
acceleration, rather than
just that we need it!
z=0.35
CREDIT: WMAP & SDSS websites
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 distanceredshift 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
CDM
SCDM
OCDM
Constraint including
observed peak
distance constrain
from CMB
rs/dA(cmb)=0.0104
Constraint fitting
rs/DV(z)
Constraint from
DV(0.35)/DV(0.2)
Cosmological constraints
Consider two simple models:
1. CDM
2. Flat, constant w
Constraint fitting rs/DV
Constraint including
distance to CMB
Constraint from
DV(0.35)/DV(0.2)
Cosmological constraints with SNLS
Consider two simple models:
–Lambda-CDM
–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, …