Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 7 August 2012 (MN LATEX style file v2.2) Galaxy clustering as dark energy probe: baryon acoustic oscillations and beyond Yun Wang⋆ Homer L. Dodge Department of Physics & Astronomy, Univ. of Oklahoma, 440 W Brooks St., Norman, OK 73019, U.S.A. 7 August 2012 ABSTRACT Galaxy clustering is one of the most powerful probes of the true nature of the observed cosmic acceleration. It contains baryon acoustic oscillations (BAO) that are cosmological standard rulers calibrated by the cosmic microwave background anisotropy data. The BAO allows us to measure the cosmic expansion history directly. Beyond the BAO, the full shape of galaxy clustering (either in the measured galaxy power spectrum or the galaxy correlation function) provides significantly more cosmological information, and in particular, allows us to test deviations from general relativity via the redshift space distortions. Here we introduce the basic ideas and analysis techniques for using galaxy clustering data to constrain dark energy and test gravity. We examine the critical issues, current status, as well as future prospects. s Since the 1980s, galaxy redshift surveys have been used to sk sH(z), s⊥ , (1) map the large scale structure in the universe, and constrain cosmo- ∝ ∝ DA(z) logical parameters. Galaxy redshift surveys are powerful as dark where s is the sound horizon scale at the drag epoch, and the angu- energy probe, since they can allow us to measure the cosmic ex- lar diameter distance DA(z) = r(z)/(1 + z), with r(z) denoting pansion history H(z) through the measurement of baryon acous- the comoving distance given by tic oscillations (BAO) in the galaxy distribution, and the growth −1 −1/2 1/2 history of cosmic large scale structure f (z) through independent r(z)= cH Ωk sinn[ Ωk Γ(z)], (2) g 0 | | | | measurements of redshift-space distortions and the bias factor be- z dz′ tween the distribution of galaxies and that of matter (Wang 2008). Γ(z)= ′ , E(z)= H(z)/H0 Z0 E(z ) where sinn(x) = sin(x), x, sinh(x) for Ωk < 0, Ωk = 0, and Ωk > 0 respectively. Thus comparing the observed BAO scales 1 BARYON ACOUSTIC OSCILLATIONS AS STANDARD with the expected values gives H(z) in the radial direction, and RULER DA(z) in the transverse direction. The use of BAO as a cosmological standard ruler is a relatively new method for probing dark energy (Blake & Glazebrook 2003; Calibration of the BAO Scale Seo & Eisenstein 2003), but it has already yielded impressive CMB data give us the comoving sound horizon at photon- observational results (Eisenstein et al. 2005). decoupling epoch (Eisenstein & Hu 1998; Page 2003) t∗ ∞ cs dt −1 cs Measuring H(z) and DA(z) from BAO r (z ) = = H dz s ∗ a 0 E(z) At the last scattering of CMB photons, the acoustic oscilla- Z0 Zz∗ a∗ tions in the photon-baryon fluid became frozen, and imprinted their −1 da = cH0 , (3) signatures on both the CMB (the acoustic peaks in the CMB angu- 4 2 Z0 3(1 + Rb) a E (z) lar power spectrum) and the matter distribution (the baryon acoustic p oscillations in the galaxy power spectrum). Because baryons com- where a is the cosmic scale factor, a∗ = 1/(1 + z∗), and prise only a small fraction of matter, and the matter power spectrum 4 2 2 4 has evolved significantly since last scattering of photons, BAO are a E (z) = Ωm(a + aeq) + Ωka + ΩX X(z)a , (4) much smaller in amplitude than the CMB acoustic peaks, and are where the dark energy density function , washed out on small scales. X(z) ρX (z)/ρX (0) and the cosmic scale factor at the epoch of matter≡ and radiation BAO in the observed galaxy power spectrum have the char- equality is given by acteristic scale determined by the comoving sound horizon at the drag epoch (shortly after photon-decoupling), which is precisely −4 Ωrad 1 4 2 TCMB measured by the CMB anisotropy data (Page 2003; Spergel et al. aeq = = , zeq = 2.5 10 Ωmh .(5) Ωm 1+ zeq × 2.7K 2007; Komatsu et al. 2009). The observed BAO scales appear as slightly preferred redshift separation sk and angular separation s⊥: We have assumed three massless neutrino species, so that the radi- c 0000 RAS 2 Yun Wang ation energy density today is (Kolb & Turner 1990) π2 ρ0 = g0T 4 , rad 30 ∗ CMB 4/3 0 7 4 g∗ =2+ 2 3 (6) 8 × × × 11 The sound speed cs and the baryon/photon ratio Rb are given by 2 2 2 2 δp c δργ /3 c c cs = = (7) ≡ δρ ≃ δργ + δρb 3(1+ρ ˙b/ρ˙γ ) 3(1 + Rb) −4 3ρb 2 TCMB Rb Rb a, Rb = 31500Ωb h .(8) ≡ 4ργ ≡ 2.7K −4 −3 We have used ργ a and ρb a . ∝ ∝ COBE four year data give TCMB = 2.728 0.004 K (95% C.L.) (Fixsen et al. 1996). The data from WMAP± 5 year obser- vations give the redshift and the sound horizon at the photon- decoupling epoch z∗ = 1090.51 0.95, rs(z∗) = 146.8 1.8 Mpc, (9) ± ± assuming TCMB = 2.725 (Komatsu et al. 2009). The BAO scale measured in galaxy redshift surveys correspond to the sound hori- zon scale at the drag epoch (Hu & Sugiyama 1996). The drag epoch occurs when the photon pressure (or “Comp- ton drag”) can no longer prevent gravitational instability in the Figure 1. The large-scale redshift-space correlation function of the SDSS baryons. Thus there is no reason for the photon-decoupling epoch, LRG sample measured by Eisenstein et al. (2005). The error bars are from the diagonal elements of the mock-catalog covariance matrix (the points , to be the same as the drag epoch, . The scattering in the pho- z∗ zd are correlated). Note that the vertical axis mixes logarithmic and linear ton/baryon fluid leads to an exchange of momentum, with momen- scalings. The inset shows an expanded view with a linear vertical axis. 2 tum densities for photons and baryons given by (Hu & Sugiyama The models are Ωmh = 0.12 (top), 0.13 (middle), and 0.14 (bottom), 2 1996): all with Ωbh = 0.024 and n = 0.98 and with a mild non-linear prescription folded in. The featureless smooth line shows a pure CDM 4 2 (ργ + pγ ) Vγ = ργ Vγ for photons model (Ω h = 0.105), which lacks the acoustic peak. The bump at 3 m 100h−1 Mpc scale is statistically significant. (ρb + pb) Vb ρbVb for baryons, (10) ≃ where Vγ and Vb are the photon and baryon bulk velocities. As a consequence of momentum conservation, the rate of change of the −1 2 BAO OBSERVATIONAL RESULTS baryon velocity due to Compton drag is scaled by a factor of Rb compared with the photon case, which means that (Hu & Sugiyama The power of BAO as a standard ruler resides in the fact that the 1996) BAO scale can in principle be measured in both radial and trans- τ˙ verse directions, with the radial measurement giving H(z) directly, τ˙d = (11) Rb and the transverse measurement giving DA(z). However, there are only a few published papers on measuring the BAO scale from the where τb and τ are the Compton optical depths for baryons and photons respectively. Since the epoch of photon decoupling is de- existing galaxy redshift survey data, and most of them extract a spherically averaged BAO scale (Eisenstein et al. 2005; Hutsi 2006; fined by τ(z∗) = 1, and the drag epoch is defined by τd(zd) = 1, Percival et al. 2007). z∗ = zd only if Rb = 1. We live in a universe with a low Eisenstein et al. (2005) and Hutsi (2006) found roughly con- baryon density, Rb(z∗) < 1 (see Eq.[8]), thus τd(zd) = 1 requires sistent spherically averaged correlation functions using SDSS data, zd < z∗, i.e., the drag epoch occurs after photon decoupling (Hu & Sugiyama 1996). with about the same BAO scale. Fig.1 shows the galaxy correlation function ξ(s) measured from the SDSS data by Eisenstein et al. The redshift of the drag epoch zd is well approximated by (Eisenstein & Hu 1998) (2005). This BAO scale measurement is usually quoted in the form of 2 0.251 1291(Ωm h ) 2 b2 (12) 1/3 2 1/2 zd = 2 0.828 1+ b1(Ωbh ) , 1 + 0.659(Ωm h ) 2 czm ΩmH0 ABAO r (zm) where ≡ H(zm) czm −0.35 nS b = 0.313(Ω h2)−0.419 1 + 0.607(Ω h2)0.674 , (13) = 0.469 0.017 (16) 1 m m 0.98 ± 2 0.223 b2 = 0.238(Ωm h ) . (14) where zm = 0.35, and nS denotes the power-law index of the pri- Using this fitting formula for zd, Komatsu et al. (2009) found that mordial matter power spectrum. Note that ABAO essentially mea- from the WMAP 5 year observations sures the product of a volume-averaged distance −1 2 1/3 s rs(zd) = 153.3 2.0 Mpc, zd = 1020.5 1.6 (15) dV cH (z) DA(z) , (17) ≡ ± ± ∝ c 0000 RAS, MNRAS 000, 000–000 Galaxy Clustering as Dark Energy Probe: BAO and Beyond 3 Figure 3. Contour plots of the redshift-space two-point correlation func- tion measured from the SDSS LRG sample by Okumura et al. (2008). The right half of the figure shows their measurement, and the left half shows the corresponding analytical formula derived by Matsubara (2004) using a linear perturbation theory (Matsubara 2004).