Baryon Acoustic Oscillations: overview

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Baryon Acoustic Oscillations: overview. Will Sutherland (QMUL). Talk overview. Baryon 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). - PowerPoint PPT Presentation


  • 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.

  • Explanation :

    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.

  • 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) : 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 !

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