Baryon Acoustic Oscillations:overviewWill Sutherland (QMUL)
Talk overviewBaryon acoustic oscillations motivation.BAO theory overview.Review of current and planned BAO observations.
WMAP7 TT power spectrum: (Larson et al 2011)
Planck TT power spectrum: (Planck XV, 2013)
The CMB geometrical degeneracy CMB gives us acoustic angle * to < 0.1%, and m h2 to ~ 1%. This tells us angular distance to last scattering surface. But, this distance depends on many parameters, e.g. m, k, h, w (plus time-varying w ?).
Result: the geometrical degeneracy. Weakly broken by CMB lensing or flatness assumption. Strongly broken by independent low-z distances, e.g. SNe or BAOs.
WMAP7: allowed non-flat LambdaCDM models(Larson et al 2011)
Planck: flat LambdaCDM parameter likelihoods
Planck 2013, flat LambdaCDM :
(Supernovae Union-2 ; Amanullah et al 2010)
w = -1 assumed.
LambdaCDM + 1-param extensions
Planck only (red)Planck + BAO (blue)
(Planck coll XVI, 2013)
BAOs : analogue of CMB peaks in the matter power spectrum
Eisenstein, Seo & White, ApJ 2007Development of the BAO feature real space
2005: first observation of predicted BAO featureby SDSS and 2dFGRS(Eisenstein et al 2005)
BAO feature in BOSS DR9 data: ~ 6 sigma(Anderson et al 2012)
(Seo & Eisenstein 2005)Non-linearity smears out the BAO feature and gives a small shift(Seo et al 2008)
(Padmanabhan et al 2012)
(Seo et al 2010)
(Mehta et al 2012)Reconstruction un-does most of the effect of non-linearity(Seo et al 2010)
BAO observables: transverse and radial Spherical average gives rs / DV ,
BAOs : strengths and weaknessesBAO length scale calibrated by the CMB .+ Uses well-understood linear physics (unlike SNe). - CMB is very distant: hard to independently verify assumptions.
BAO length scale is very large, ~ 152 Mpc: + Ruler is robust against non-linearity, details of galaxy formation+ Observables very simple: galaxy positions and redshifts. - Huge volumes must be surveyed to get a precise measurement.- Cant measure BAO scale at z ~ 0
BAOs can probe both DA(z) and H(z); + no differentiation needed for H(z)+ enables consistency tests for flatness and homogeneity.
Precision from ideal BAO experiments:(Weinberg et al 2012)Right panel idealized: assumes matter+baryon densities known exactly
BAOs : present and futureWiggleZ (AAT): 0.4 < z < 0.9, complete. ~ 200k Emission line galaxies. Many papers recently.
BOSS (SDSS3): 0.2 < z < 0.65 ; in progress. > 1 million luminous red galaxies (LRGs); sky, complete 2014. Also at z ~ 2.5 with QSO absorbers. HetDEX: under construction. z ~ 2 Lyman-alpha emitters.
Large fibre-fed MOSs on 4-ms: start ~ 2018. USA: BigBOSS and DESpec have merged into MS-DESI. Passed CD-0 approval, telescope choice soon. ~ 3000 fibres ? WEAVE: 1000 fibres on WHT. 4MOST on VISTA: 2400 fibres, ESO decision coming soon.
AESOP for 4MOST (Australia ESO Positioner AAO)Independent tilting piezo-driven spines- developed from proven FMOS Echidna.AESOP has 2400 spines (1600 med-res, 800 high-res). Any point reachable by 3 7 spines (typical 5) flexible configuration
Fibre bundles - new wrap.Spectrographs on the yoke, under floor.Short fibre runs, gravity invariant.
BAOs : present and futureSubaru PFS (formerly WFMOS): 8m telescope, smaller FoV; mainly focused on galaxy evolution , also BAOs at z > 1.
Euclid (ESA): 1.2m, space. 0.7 < z < 2.0 Approved for 2020+. Near-IR slitless spectroscopy . Huge survey volume; but only H-alpha line detected. WFIRST (NASA): 1st ranked in US decadal survey ; not yet funded. Was 1.5m ; maybe 2.4m with free spy telescope .
SKA : potentially the ultimate BAO machine ?Depends on achievable mapping speed, FoV etc.
Cosmic expansion rate: da/dt
Cosmic expansion rate, relative to today
BOSS: Busca et al 2012Caveat: assumed flatness and standard rs
Good approximation at z < 0.5 :
The Neff / scale degeneracy :Nearly all our CMB + SNe + BAO observables are actually dimensionless (apart from baryon+photon densities) : redshift of matter-radiation equality CMB acoustic angle SNe give us distance ratios or H0 DL /c . BAOs also give distance ratios All these can give us robust values for s , w, E(z) etc. But: there are 3 dimensionful quantities in FRW cosmology ; Distances, times, densities.Two inter-relations : distance/time via c ,and Friedmann equation relates density + time, via G. This leaves one short, i.e. any number of dimensionless distance ratios cant determine overall scale.Usually, scales are (implicitly) anchored to the standard radiation density, Neff ~ 3.0 . But if we drop this, then there is one overall unknown scale factor.
Baryon and photon densities are determined in absolute units but these dont appear separately in Friedmann eq., only as contributions.
Rescaling total radiation, total matter and dark energy densities by a common factor leaves CMB, BAO and SNe observables (almost) unchanged; but changes dimensionful quantities e.g. H.
Potential source of confusion: use of h and s. These are unitless but they are not really dimensionless, since they involve arbitrary choice of H = 100 km/s/Mpc etc.
h becomes a derived parameter:Define as error inapproximation :This is exact (apart from non-linear shifts in rs )and fully dimensionless: all H and s cancelled. An easy route to mBAO ratio is :
This is all dimensionless, and nicely splits z-dependent effects: Zeroth-order term is just m-0.5 (strictly cb , without neutrinos)
Leading order z-dependence is E(2z/3)
The V is second-order in z, typically ~ z2 / 25 , almost negligible at z < 0.5
For WMAP baryon density, the above simplifies to the following , to 0.4 percent : An easy route to m
What BAOs really measure :
Standard rule-of-thumb is CMB measures m , and the sound horizon; then BAOs measure h ; this is only true assuming standard radiation density.
Really, CMB measures zeq , and then a low-redshift BAO ratio measures (almost) m. These two tell us H0 / (Xrad) , but not an overall scale.
Thus, measuring the absolute BAO length provides a strong test of standard early-universe cosmology, including the radiation content.
BAOs are a gold standard for cosmological standard rulers. Very well understood; observations huge in scope, but clean.
Most planned BAO surveys are targeting z > 0.7, to exploit the huge available volume and sensitivity to dark energy w.
However, there are still good cases for optimal low-z BAO surveys at z ~ 0.25 0.7 (e.g. extending BOSS to South and lower galactic latitude) : A direct test of cosmic acceleration with minimal assumptions
In conjunction with precision distance measurements, can provide a test of the CMB prediction rs ~ 152 Mpc, and/or a clean test for extra radiation Neff > 3.04 .
Thank you !