Consistently large cosmic flows on scales of 100 h−1 Mpc: a challenge for the standard ΛCDM cosmology

By R. Watkins, H. Feldman, M. Hudson. 2008

314 citations recorded by the source corpus.

The bulk flow, i.e. the dipole moment of the peculiar velocity field, is a sensitive probe of matter density fluctuations on very large scales. However, the peculiar velocity surveys for which the bulk flow has been calculated have non-uniform spatial distributions of tracers, so that the bulk flow estimated does not correspond to that of a simple volume such as a sphere. Thus bulk flow estimates are generally not strictly comparable between surveys, even those whose effective depths are similar. In addition, the sparseness of typical surveys can lead to aliasing of small-scale power into what is meant to be a probe of the largest scales. Here we introduce a new method of calculating bulk flow moments where velocities are weighted to give an optimal estimate of the bulk flow of an idealized survey, with the variance of the difference between the estimate and the actual flow being minimized. These 'minimum variance' estimates can be designed to estimate the bulk flow on a particular scale with minimal sensitivity to small-scale power, and are comparable between surveys. We compile all major peculiar velocity surveys and apply this new method to them. We find that most surveys we studied are highly consistent with each other. Taken together the data suggest that the bulk flow within a Gaussian window of radius 50 h -1 Mpc is 407 ± 81krn s -1 toward l = 287° ± 9°, b = 8° ± 6°. The large-scale bulk motion is consistent with predictions from the local density field. This indicates that there are significant density fluctuations on very large scales. A flow of this amplitude on such a large scale is not expected in the WMAP5 (Wilkinson Microwave Anisotropy Probe) normalized A cold dark matter cosmology, for which the predicted one-dimensional rms velocity is ∼110 km s -1 . The large amplitude of the observed bulk flow favours the upper values of the WMAP5 Ω m h 2 -σ 8 error-ellipse, but even the point at the top of the WMAP5 95 per cent confidence ellipse predicts a bulk flow which is too low compared to that observed at >98 per cent confidence level.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/cbb96ceab8c1544a112cb6db0d4393a7c1a31d11

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In Astronomy and Astrophysics

1. M. Hudson
2. R. Watkins
3. H. Feldman
4. B. Famaey
5. H. Ebeling
6. Roya Mohayaee
7. Stephen J. Turnbull
8. S. Sarkar

In Nuclear and High Energy Physics

1. Stephen J. Turnbull
2. M. Hudson
3. R. Watkins
4. H. Feldman
5. G. Lavaux
6. B. Famaey
7. J. Carrick
8. S. McGaugh

Citations in the corpus

• The 2M++ galaxy redshift catalogue (2011, background): “Recently, several intriguing measurements (Kashlinsky et al. 2008; Watkins et al. 2009; Lavaux et al. 2010; Kashlinsky et al. 2010; Feldman et al. 2010) of the mean or “bulk” flow on scales larger than 100h−1 Mpc suggest a high velocity of our local ∼ 100 h−1 Mpc volume with respect to the Cosmic…”
• Probing the anisotropic local Universe and beyond with SNe Ia data (2010, result): “…C© 2011 RAS Downloaded from https://academic.oup.com/mnras/article-abstract/414/1/264/1089369 by guest on 26 July 2018 Our results are consistent with Watkins et al. (2009) who estimate a bulk flow of Vbulk = 416 ± 78 km s−1 towards b = 60◦(+6, −6), = 282◦(+11, −11) at a scale of 100 h−1 Mpc.”
• Cosmic flows in the nearby universe from Type Ia supernovae (2011, methodology): “When all SN data are removed from the ‘Composite’ data set, the two surveys become completely independent and can be compared using the same formalism described in Section 3 of this paper and section 5.1 of Watkins et al. (2009).”
• Cosmic flows on 100 h−1 Mpc scales: standardized minimum variance bulk flow, shear and octupole moments (2009, methodology): “This evidence comes directly from estimates of the bulk flow (BF) on large scales (Feldman & Watkins 2008; Watkins et al. 2009).”
• Probing cosmic isotropy with a new X-ray galaxy cluster sample through the LX–T scaling relation (2020, background): “…Tiwari & Nusser 2016; Bengaly et al. 2018, 2019; Colin et al. 2019), gamma-ray bursts (Řípa & Shafieloo 2017; Andrade et al. 2019), peculiar velocities of galaxy clusters (Kashlinsky et al. 2008, 2010, 2011; Watkins et al. 2009; Atrio-Barandela et al. 2015) and of SNIa (Appleby et al. 2015) etc.”
• Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions (2011, background): “This bulk flow challenge appears not only in SZ studies but also in galaxy studies [484].”
• Evidence for anisotropy of cosmic acceleration (2018, background): “This is reasonable as the directions of the reported bulk flows (Hudson et al. 2004; Watkins et al. 2009; Lavaux et al. 2010; Feldman et al. 2010; Colin et al. 2011; Feindt et al. 2013; Magoulas et al. 2016) are all within ∼40◦ of each other and of the CMB dipole.”
• A NEW MEASUREMENT OF THE BULK FLOW OF X-RAY LUMINOUS CLUSTERS OF GALAXIES (2009, result): “The flow’s axis is also consistent with earlier measurements of the local cluster dipole (Kocevski et al. 2004) as well as with independent measurements of bulk flows on smaller scales by Watkins et al. (2009).”
• Cosmological parameters from the comparison of peculiar velocities with predictions from the 2M++ density field (2014, methodology): “As previous studies in this vein have found hints of excess power on large scales (cf. Watkins et al. 2009), there has been interest in performing such analyses using peculiar velocity surveys (e.g. Turnbull et al. 2012; Hong et al. 2014).”