Baryon Acoustic Oscillations: overview

1. Baryon Acoustic Oscillations:overview Will Sutherland (QMUL)

2. Talk overview • Baryon acoustic oscillations – motivation. • BAO theory overview. • Review of current and planned BAO observations.

3. WMAP7 TT power spectrum: (Larson et al 2011)

4. Planck TT power spectrum: (Planck XV, 2013)

5. 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.

6. WMAP7: allowed non-flat LambdaCDM models (Larson et al 2011)

7. Planck: flat LambdaCDM parameter likelihoods

8. Planck 2013, flat LambdaCDM :

9. (Supernovae Union-2 ; Amanullah et al 2010) w = -1 assumed.

10. LambdaCDM + 1-param extensions Planck only (red) Planck + BAO (blue) (Planck coll XVI, 2013)

11. BAOs : analogue of CMB peaks in the matter power spectrum

12. Development of the BAO feature – real space Eisenstein, Seo & White, ApJ 2007

14. 2005: first observation of predicted BAO feature by SDSS and 2dFGRS (Eisenstein et al 2005)

15. BAO feature in BOSS DR9 data: ~ 6 sigma (Anderson et al 2012)

16. Non-linearity smears out the BAO feature … and gives a small shift (Seo et al 2008) (Seo & Eisenstein 2005)

17. (Padmanabhan et al 2012)

18. (Seo et al 2010)

19. “Reconstruction” un-does most of the effect of non-linearity (Seo et al 2010) (Mehta et al 2012)

20. BAO observables: transverse and radial Spherical average gives rs / DV ,

21. 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 neededfor H(z) + enables consistency tests for flatness and homogeneity.

22. Precision from ideal BAO experiments: (Weinberg et al 2012) Right panel idealized: assumes matter+baryon densities known exactly

23. 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 DESpechave 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.

24. 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

25. Fibre bundles - new wrap. Spectrographs on the yoke, under floor. Short fibre runs, gravity invariant.

26. 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.

28. Cosmic expansion rate: da/dt

29. Cosmic expansion rate, relative to today

30. BOSS: Busca et al 2012 Caveat: assumed flatness and standard rs

31. Good approximation at z < 0.5 :

32. 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.

33. 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.

34. An easy route to Ωm h becomes a derived parameter: Define ε as error in approximation : BAO ratio is : This is exact (apart from non-linear shifts in rs ) and fully dimensionless: all H and ω’s cancelled.

35. An easy route to Ωm For WMAP baryon density, the above simplifies to the following , to 0.4 percent : • 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

36. 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.

37. 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 .

38. Thank you !