In astrophysics, dark flow is a theoretical non-random component of the

peculiar velocity Peculiar motion or peculiar velocity refers to the velocity of an object relative to a ''rest frame'' — usually a frame in which the average velocity of some objects is zero. Galactic astronomy In galactic astronomy, peculiar motion refers to ...

galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-lar ...

s. The actual measured velocity is the sum of the velocity predicted by

Hubble's Law Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving a ...

plus a possible small and unexplained (or ''dark'') velocity flowing in a common direction. According to standard cosmological models, the motion of galaxy clusters with respect to the

cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

should be randomly distributed in all directions. However, analyzing the three-year Wilkinson Microwave Anisotropy Probe (WMAP) data using the kinematic Sunyaev–Zeldovich effect, astronomers Alexander Kashlinsky, Fernando Atrio-Barandela, D. Kocevski and H. Ebeling found evidence of a "surprisingly coherent" 600–1000 km/s flow of clusters toward a 20-degree patch of sky between the constellations of

Centaurus Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. ...

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 In economics, inflation is an increase in the general price level of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a reduct ...

. Telescopes cannot see events earlier than about 380,000 years after the Big Bang, when the universe became transparent (the

); this corresponds to the

particle horizon The particle horizon (also called the cosmological horizon, the comoving horizon (in Dodelson's text), or the cosmic light horizon) is the maximum distance from which light from particles could have traveled to the observer in the age of the univers ...

at a distance of about 46 billion (4.6×10) 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 The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these obj ...

; however, it would still be in our past

light cone In special and general relativity, a light cone (or "null cone") is the path that a flash of light, emanating from a single event (localized to a single point in space and a single moment in time) and traveling in all directions, would take thro ...

. The results appeared in the October 20, 2008, issue of ''

Astrophysical Journal Letters ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and ...

''. 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. However, in 2015 Kashlinsky ''et al.'' claim to have found support for its existence using both Planck and WMAP data.

Location

The dark flow was determined to be flowing in the direction of the

and Hydra constellations.NASA/Goddard Space Flight Center (March 11, 2010)
"Mysterious cosmic 'dark flow' tracked deeper into universe"
''ScienceDaily''. Retrieved 2010-09-28. This corresponds with the direction of the

Great Attractor The Great Attractor is a gravitational anomaly in intergalactic space and the apparent central gravitational point of the Laniakea Supercluster. The observed anomalies suggest a localized concentration of mass millions of times more massive than ...

, which is a gravitational mystery originally discovered in 1973. However, the source of the Great Attractor's attraction was thought to originate from a massive cluster of galaxies called the

Norma Cluster The Norma Cluster (ACO 3627 or Abell 3627) is a rich cluster of galaxies located near the center of the Great Attractor; it is about distant. Although it is both nearby and bright, it is difficult to observe because it is located in the Zone of ...

, located about 250 million light-years away from Earth. In a study from March 2010, Kashlinsky extended his work from 2008, by using the 5-year WMAP results rather than the 3-year results, and doubling the number of galaxy clusters observed from 700. The team also sorted the cluster catalog into four "slices" representing different distance ranges. They then examined the preferred flow direction for the clusters within each slice. The report concluded that while the size and exact position of this direction display some variation, the overall trends among the slices exhibit remarkable agreement. "We detect motion along this axis, but right now our data cannot state as strongly as we'd like whether the clusters are coming or going," Kashlinsky said. The team has so far catalogued the effect as far out as 2.5 billion light-years, and hopes to expand its catalog out further still to twice the current distance. Nasa-dark flow-large

Criticisms

Astrophysicist Ned Wright posted an online response to the study arguing that its methods are flawed. The authors of the "dark flow" study released a statement in return, refuting three of Wright's five arguments and identifying the remaining two as a typo and a technicality that do not affect the measurements and their interpretation. A more recent statistical work done by Ryan Keisler claims to rule out the possibility that the dark flow is a physical phenomenon because Kashlinsky et al. did not consider the primary

anisotropies Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

of the cosmic microwave background (CMB) to be as important as they are. NASA's Goddard Space Flight Center considered that this could be the effect of a sibling universe or a region of space-time fundamentally different from the observable universe. Data on more than 1,000 galaxy clusters have been measured, including some as distant as 3 billion light-years. Alexander Kashlinsky claims these measurements show the universe's steady flow is clearly not a statistical fluke. Kashlinsky said: "At this point we don't have enough information to see what it is, or to constrain it. We can only say with certainty that somewhere very far away the world is very different than what we see locally. Whether it's 'another universe' or a different fabric of space-time we don't know."

Laura Mersini-Houghton Laura Mersini-Houghton (''née'' Mersini) is an Albanian-American cosmologist and theoretical physicist, and professor at the University of North Carolina at Chapel Hill. She is a proponent of the multiverse hypothesis and the author of a theor ...

and Rich Holman observe that some anisotropy is predicted both by theories involving interaction with another universe, or when the frame of reference of the CMB does not coincide with that of the universe's expansion. In 2013, data from the European Space Agency's Planck satellite was claimed to show no statistically significant evidence of existence of dark flow. However, another analysis by a member of the Planck collaboration, Fernando Atrio-Barandela, suggested the data were consistent with the earlier findings from WMAP. Popular media continued to be interested in the idea, with Mersini-Houghton claiming the Planck results support existence of a multiverse.

References

{{Reflist

External links

Mysterious New 'Dark Flow' Discovered in Space
''Space.com''

''New Scientist'', 10 June 2008
Galaxies on the move: Scientists detect a mysterious flow of galactic clusters
''Science News'', Vol.174, p. 12, Oct. 25, 2008.

''New Scientist'', 23 January 2009
New Proof Unknown Structures Tug at Our Universe
''National Geographic'', 22 March 2010
The peculiar velocity field: constraining the tilt of the Universe
* Tsagas, C.G. (2011
"Peculiar motions, accelerated expansion, and the cosmological axis"
''Physical Review D'' 84:063503 Physical cosmology Great Attractor Exotic matter Celestial mechanics Dark concepts in astrophysics

Location

Criticisms

See also

References

External links