Probing the anisotropic local Universe and beyond with SNe Ia data

Mon. Not. R. Astron. Soc. 414, 264–271 (2011) doi:10.1111/j.1365-2966.2011.18402.x Probing the anisotropic local Universe and beyond with SNe Ia data Jacques Colin,1 Roya Mohayaee,1 Subir Sarkar2 and Arman Shafieloo2,3 1 UPMC, CNRS, Institut d’Astrophysique de Paris, 98 bis Bd. Arago, Paris 75014, France 2 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP 3 Institute for the Early Universe, Ewha Womans University, Seoul 120-750, South Korea Accepted 2011 January 20. Received 2011 January 20; in original form 2010 December 11 ABSTRACT Key words: cosmic background radiation – cosmological parameters – cosmology: theory – dark energy – large-scale sctructure of Universe. 1 I N T RO D U C T I O N Modern cosmology is founded on the cosmological principle which assumes that the Universe is homogeneous and isotropic. The local Universe is, however, observed to be anisotropic and inhomogeneous, exhibiting the ‘cosmic web’ of voids and superclusters. This presumably causes the Local Group of galaxies move towards a preferred direction  = 276◦ ± 3◦ , b = 30◦ ± 2◦ at 627 ± 22 km s−1 , as inferred from the dipole anisotropy of the cosmic microwave background (CMB) radiation (Kogut et al. 1993). On the other hand, the overall isotropy of the CMB (barring the dipole anisotropy due to our local motion) provides strong evidence for an isotropic universe on very large scales.1 Where does the transition between these two regimes occur? While high-quality data  E-mail: 1 The WMAP observations of anomalies in large-angle anisotropies in the CMB, e.g. the hemispherical asymmetry (Eriksen et al. 2004) and the unexpected quadrupole–octupole alignment (de Oliveira-Costa et al. 2004), have led many to question whether the CMB is indeed statistically isotropic (e.g. Copi et al. 2007). However, others have argued that these anomalies may simply be due to the manner in which the galactic foreground was masked exist at low redshifts and the CMB provides reliable information at very high redshifts, the data are rather sparse and mainly of poor quality on the intermediate scales of interest. Here the SNe Ia Hubble diagram is the key source of information, and so we use the comprehensive Union 2 catalogue (Amanullah et al. 2010) to investigate these important questions. SNe Ia data have been examined previously to test the isotropy of the Universe (Kolatt & Lahav 2001; Bonvin, Durrer & Kunz 2006; Gordon, Land & Slosar 2007; Schwarz & Weinhorst 2007; Gupta, Saini & Laskar 2008; Koivisto & Mota 2008a,b; Blomqvist, Enander & Mortsell 2008; Cooray, Holz & Caldwell 2010; Gupta & Saini 2010; Koivisto et al. 2011) and to determine whether the Universe accelerates differently in different directions. Recently, a marginal (1σ ) detection of anisotropy has been reported on spatial scales where dark energy becomes important (Cooke & LyndenBell 2010; Antoniou & Perivolaropoulos 2010). Clearly, betterquality and more complete surveys are needed before any firm conclusions can be drawn. However, although these detections are not significant by themselves, a puzzling and perhaps accidental (Pontzen & Peiris 2010). We look to forthcoming observations by Planck to resolve this contentious issue.  C 2011 The Authors C 2011 RAS Monthly Notices of the Royal Astronomical Society  The question of the transition to global isotropy from our anisotropic local universe is studied using the Union 2 catalogue of Type Ia supernovae (SNe Ia). We construct a ‘residual’ statistic sensitive to systematic shifts in their brightness in different directions and use this to search in different redshift slices for a preferred direction on the sky in which the SNe Ia are brighter or fainter relative to the standard cold dark matter (CDM) cosmology. At low redshift (z < 0.05), we find that an isotropic model such as CDM is barely consistent with the SNe Ia data at 2σ –3σ . A maximum-likelihood analysis of peculiar velocities confirms this finding – there is a bulk flow of 260 km s−1 extending out to z ∼ 0.06, which disagrees with CDM at 1σ –2σ . Since the Shapley concentration is believed to be largely responsible for this bulk flow, we make a detailed study of the infall region: the SNe Ia falling away from the Local Group towards Shapley are indeed significantly dimmer than those falling towards us on to Shapley. Convergence to the CMB rest frame must occur well beyond Shapley (z > 0.06) so this low-redshift bulk flow will systematically bias any reconstruction of the expansion history of the Universe. At higher redshifts z > 0.15 the agreement between the SNe Ia data and the CDM model does improve, however, the sparseness and low quality of the data mean that the latter cannot be singled out as the preferred cosmological model. Probing the anisotropic Universe with SNe Ia 265 feature is the alignment of the detected anisotropy with the CMB dipole direction. In this work, we demonstrate that the alignment at low redshift is due to the attraction of huge structures such as the Shapley supercluster. At high redshift, the alignment seems to become statistically insignificant but given the sparse and poorquality data, no strong conclusions can be drawn. On small scales, the CMB dipole and the bulk flow are aligned, which is unsurprising as the common source of both these motions is very likely the anisotropic distribution of matter in the local Universe. However, a bulk flow much larger than expected has been found extending out to at least 120 Mpc (Watkins, Feldman & Hudson J 2009; Lavaux et al. 2010),2 which is a ∼2σ –3σ fluctuation in a cold dark matter (CDM) model since convergence to the CMB rest frame ought to occur at much smaller scales in this model. At low redshifts, we study the bulk flow using two different methods: first by a method of ‘smoothing and residuals’ (Sections 3 and 4) and secondly by a maximum likelihood analysis (Section 5). We show that these two methods are in good agreement at small redshifts (z < 0.05) and confirm that there is indeed a discrepancy between the CDM model prediction for the bulk flow and the SNe Ia observations. The Shapley concentration at z  0.035–0.055 is believed to be the main source of our large bulk motion. We study the infall region around Shapley (Section 6) and demonstrate that SNe Ia beyond Shapley are systematically brighter than expected in an isotropic universe (as they are falling towards Shapley), while SNe Ia between the Local Group and Shapley are systematically dimmer (as they are also falling towards Shapley, but away from us). This result is obtained using our smoothing and residuals scheme. At high redshift, the χ 2 statistic cannot be used since the bulk flow becomes small relative to the Hubble expansion rate. Using the method of smoothing and residuals for z > 0.05 we find that an isotropic model such as CDM is consistent with the data. However, the poor quality of the data means that aniso (...truncated)