astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...

, dark flow is a controversial hypothesis to explain certain non-random measurements of peculiar velocity of

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. Clusters consist of galax ...

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 a galaxy is from the Earth, the faste ...

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 model Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fu ...

s, the motion of galaxy clusters with respect to the

cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...

should be randomly distributed in all directions. However, analyzing the three-year

Wilkinson Microwave Anisotropy Probe The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...

(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 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 88 modern constellations by area, largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one ...

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 average price of goods and services in terms of money. This increase is measured using a price index, typically a consumer price index (CPI). When the general price level rises, each unit of curre ...

. Telescopes cannot see events earlier than about 380,000 years after the

Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...

, when the universe became transparent (the

); this corresponds to the

particle horizon The particle horizon (also called the cosmological horizon, the comoving horizon (in Scott 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 o ...

at a distance of about 46 billion (4.6×10)

light year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...

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

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

Astrophysical Journal Letters ''The Astrophysical Journal'' (''ApJ'') is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler. The journal discontinued its print edition and b ...

''. 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 Atrio-Barandela ''et al.'' claim to have found support for its existence using both Planck and WMAP data. 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.

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, 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, 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 original authors released a statement in return, claiming that the criticism is largely invalid. 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 of the cosmic microwave background (CMB) to be as important as they are. Some have suggested 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 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 The European Space Agency (ESA) is a 23-member International organization, international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 ...

'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

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
{{Webarchive, url=https://web.archive.org/web/20110610075051/http://sciencenews.org/index/generic/activity/view/id/36959/title/Galaxies_on_the_move , date=2011-06-10 ''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 Great Attractor Exotic matter Celestial mechanics Dark concepts in astrophysics Physical cosmological concepts

Location

Criticisms

See also

References

External links