Baryon Acoustic Oscillations: overview
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Transcript of Baryon Acoustic Oscillations: overview
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 Ωm h2 to ~ 1%.
• This tells us angular distance to last scattering surface.
• • But, this distance depends on many
parameters, e.g. Ωm, Ωk, 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 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 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 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 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. > 1 million luminous red galaxies (LRGs); ¼ 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-m’s: 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 future
Subaru 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 Ω’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 can’t 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 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 inapproximation :
This is exact (apart from non-linear shifts in rs )and fully dimensionless: all H and ω’s cancelled.
An easy route to Ωm
BAO ratio is :
This is all dimensionless, and nicely splits z-dependent effects: •Zeroth-order term is just Ωm
-0.5 (strictly Ωcb , without neutrinos)
•Leading order z-dependence is E(2z/3)
•The εV 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 Ωm
What BAOs really measure :
Standard rule-of-thumb is “CMB measures ωm , 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) Ωm. 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 !