Baryon Acoustic Oscillations: overview Will Sutherland (QMUL)

Talk overview 1.Baryon acoustic oscillations – motivation. 2.BAO theory overview. 3.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 θ * to < 0.1%, and Ω m h 2 to ~ 1%. This tells us angular distance to last scattering surface. But, this distance depends on many parameters, e.g. Ω m, Ω 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 2007 Development of the BAO feature – real space

2005: first observation of predicted BAO feature by 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 r s / D V,

BAOs : strengths and weaknesses §BAO 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. - Can’t measure BAO scale at “ z ~ 0 ” §BAOs can probe both D A (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 future §WiggleZ (AAT): 0.4 < z < 0.9, complete. ~ 200k Emission line galaxies. Many papers recently. §BOSS (SDSS3): 0.2 < z < 0.65 ; in progress. l > 1 million luminous red galaxies (LRGs); ¼ sky, complete l Also at z ~ 2.5 with QSO absorbers. §HetDEX: under construction. z ~ 2 Lyman-alpha emitters. §Large fibre-fed MOSs on 4-m’s: start ~ l USA: BigBOSS and DESpec have merged into MS-DESI. Passed CD-0 approval, telescope choice soon. ~ 3000 fibres ? l WEAVE: 1000 fibres on WHT. l 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 future §Subaru PFS (formerly WFMOS): l 8m telescope, smaller FoV; mainly focused on galaxy evolution, also BAOs at z > 1. §Euclid (ESA): 1.2m, space. 0.7 < z < 2.0 l Approved for Near-IR slitless spectroscopy. l Huge survey volume; but only H-alpha line detected. §WFIRST (NASA): l 1 st ranked in US decadal survey ; not yet funded. l Was 1.5m ; maybe 2.4m with “free” spy telescope. §SKA : potentially the ultimate BAO machine ? l Depends on achievable mapping speed, FoV etc.

Cosmic expansion rate: da/dt

Cosmic expansion rate, relative to today

BOSS: Busca et al 2012 Caveat: assumed flatness and standard r s

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) : l redshift of matter-radiation equality l CMB acoustic angle l SNe give us distance ratios or H 0 D L /c. l BAOs also give distance ratios  All these can give us robust values for Ω’s, w, E(z) etc. §But: there are 3 dimensionful quantities in FRW cosmology ; l Distances, times, densities. l Two inter-relations : distance/time via c,and Friedmann equation relates density + time, via G. l This leaves one short, i.e. any number of dimensionless distance ratios can’t determine overall scale. l 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.

Explanation : §Baryon and photon densities are determined in absolute units… but these don’t 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 ω’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 ε as error in approximation : This is exact (apart from non-linear shifts in r s ) and fully dimensionless: all H and ω’s cancelled. An easy route to Ω m BAO ratio is :

This is all dimensionless, and nicely splits z-dependent effects: Zeroth-order term is just Ω m -0.5 (strictly Ω cb, without neutrinos) Leading order z-dependence is E(2z/3) The ε V is second-order in z, typically ~ z 2 / 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 Ω m

What BAOs really measure :  Standard rule-of-thumb is “CMB measures ω m, and the sound horizon; then BAOs measure h ” ; this is only true assuming standard radiation density.  Really, CMB measures z eq, and then a low-redshift BAO ratio measures (almost) Ω m. These two tell us H 0 / √( X rad ), but not an overall scale. §Thus, measuring the absolute BAO length provides a strong test of standard early-universe cosmology, including the radiation content.

Conclusions : §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) : l A direct test of cosmic acceleration with minimal assumptions l In conjunction with precision distance measurements, can provide a test of the CMB prediction r s ~ 152 Mpc, and/or a clean test for extra radiation Neff > 3.04.

Thank you !