Dark flow

Not to be confused with dark energy, dark fluid, dark matter, or dark wave.
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In astrophysics, dark flow is a controversial hypothesis to explain certain non-random measurements of peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble's law plus a possible small velocity flowing in a common direction. Very large scale correlated flow, called bulk flow is proposed in this model to be related to certain models of inflationary cosmology.

According to standard cosmological models, the motion of galaxy clusters with respect to the cosmic microwave background should be randomly distributed in all directions. However, analyzing the three-year Wilkinson Microwave Anisotropy Probe (WMAP) data using the kinematic Sunyaev–Zeldovich effect, a team of astronomers led by Alexander Kashlinsky found evidence of a "surprisingly coherent" 600–1000 km/s[1][2] flow of clusters toward a 20-degree patch of sky between the constellations of Centaurus and Vela.

The researchers had suggested that the motion may be a remnant of the influence of no-longer-visible regions of the universe prior to inflation. Telescopes cannot see events earlier than about 380,000 years after the Big Bang, when the universe became transparent (the cosmic microwave background); this corresponds to the particle horizon at a distance of about 46 billion (4.6×1010) light years. Since the matter causing the net motion in this proposal is outside this range, it would in a certain sense be outside our visible universe; however, it would still be in our past light cone.

The results appeared in the October 20, 2008, issue of Astrophysical Journal Letters.[1][2][3][4]

In 2013, data from the Planck space telescope showed no evidence of "dark flow" on that sort of scale, discounting the claims of evidence for either gravitational effects reaching beyond the visible universe or existence of a multiverse.[5] However, in 2015 Atrio-Barandela et al. claim to have found support for its existence using both Planck and WMAP data.[6] The paper stated that a more complete analysis was in preparation to exploit the full Planck cluster sample to further build evidence; however, the team have published no further papers on the topic.

  1. ^ a b A. Kashlinsky; Fernando Atrio-Barandela; D. Kocevski; H. Ebeling (2008). "A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications". Astrophys. J. 686 (2): 49–52. arXiv:0809.3734. Bibcode:2008ApJ...686L..49K. CiteSeerX 10.1.1.1013.233. doi:10.1086/592947. S2CID 16335692. Retrieved 2010-07-15.
  2. ^ a b A. Kashlinsky; Fernando Atrio-Barandela; D. Kocevski; H. Ebeling (2009). "A measurement of large-scale peculiar velocities of clusters of galaxies: technical details" (PDF). Astrophys. J. 691 (2): 1479–1493. arXiv:0809.3733. Bibcode:2009ApJ...691.1479K. doi:10.1088/0004-637X/691/2/1479. S2CID 11185723. Archived from the original (PDF) on 2018-11-23. Retrieved 2010-07-15.
  3. ^ "Scientists Detect Cosmic 'Dark Flow' Across Billions of Light Years" (Press release). Goddard Space Center (Nasa.gov). 2008-09-23. Retrieved 2012-11-11.
  4. ^ "Galaxy Clusters Trace Huge Cosmic Flow" (Press release). University of Hawai`i (Ifa.hawaii.edu). 2008-09-24. Retrieved 2012-11-11.
  5. ^ Cite error: The named reference newsci-dn23340 was invoked but never defined (see the help page).
  6. ^ Atrio-Barandela, Fernando; Kashlinsky, A.; Ebeling, H.; Fixsen, D. J.; Kocevski, D. (2015). "Probing the Dark Flow Signal in WMAP 9 -Year and Planck Cosmic Microwave Background Maps". Astrophys. J. 810 (2): 143. arXiv:1411.4180. Bibcode:2015ApJ...810..143A. doi:10.1088/0004-637X/810/2/143. S2CID 119257373.