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The Universe Is Not Expanding Uniformly

Partial map of the Local Group of galaxies.  Credit:  Planet Quest

Partial map of the Local Group of galaxies. Credit: Planet Quest

A few weeks ago, researchers announced the discovery of a “dark flow” of invisible matter tugging at distant galaxy clusters at the edge of the universe. Now comes more evidence of unseen and unknown forces in the cosmos, but this time its closer to home. A group of researchers have discovered that our particular part of the Universe — out to a distance of 400 million light years — is not expanding uniformly in all directions as expected. To be exact, the expansion is faster in one half of the sky than in the other. “It’s as if, in addition to the expansion, our ‘neighbourhood’ in the Universe has an extra kick in a certain direction,” says Mike Hudson from the University of Waterloo in Ontario, Canada. “We expected the expansion to become more uniform on increasingly larger scales, but that’s not what we found.” If confirmed, their findings will result in a new understanding of the origin of structure in the universe and possible revisions to the standard cosmological model.

Hudson and two other scientists have been conducting research on large-scale cosmic flows and the general expansion of the universe. This expansion increases the distances between galaxies steadily with time, and is called the Hubble flow. Deviations of the velocity of galaxies from the overall Hubble flow is called the “peculiar velocity.” By examining the peculiar velocities of clusters and superclusters scientists can obtain estimates of local mass concentrations that may be responsible for causing any deviations from the Hubble flow.

In particular, these researchers were attempting to address a longstanding question about the origin of the approximately 600 km/s peculiar velocity of the Local Group of galaxies, with respect to the Cosmic Microwave Background.

Using several different surveys they discovered that about 50% of the Local Group’s motion is faster than anticipated. To produce this motion, they believe there must be large unseen and unknown structures in the universe. They write, “The large value of the residual motion implies that there are significant velocities generated by very-large scale structures,” and the structures lie beyond the Local Group.

Brian McNamara, a University Research Chair in UW’s department of physics and astronomy, says Hudson is finding that much of the matter in the nearby universe moves as an ensemble with a surprisingly high speed. “If the work he and others are doing is confirmed, it will require a major revision in the way we think the universe came into being and how it evolved.”

Hudson and his colleagues have submitted a paper to the Royal Astronomical Society, and a preprint version is available here.

Sources: arXiv, University of Waterloo


Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • yoogih October 12, 2008, 1:20 PM

    don’t bother yourselves with such things as this could be just a simulated reality, and our Universe altogether with what we experience as our individual reality is just a some sort of “videogame”. We not gonna get to the bottom of it, as we don’t expect the characters in the latest computer videogames to understand what, who or how they been created (by computer programmers). I would like to hear more opinions about this… with attempts to explain weird things such as why light speed is limited at its current 300,000km/s and other…

  • robbb October 12, 2008, 4:20 PM

    I’ll just quote Spock on this one: ‘Fascinating.’ (Deadpan look on face.)

    Super Nerd

  • Rey October 12, 2008, 4:49 PM

    Yoogih, no one will believe in you, becuz our brains are designed not to understand the “real reality” 😀 …our brains will start to hurt if we brood on the real reality.

    Whoever built our reality has designed our “platform” really well, perhaps there “were” several versions that existed before that “crashed” or had several “bugs” 😀 .

    For all we know, our “Universe” is still in its “beta” stages 😀

  • Damian October 12, 2008, 5:05 PM

    Hills and Valleys in the fabric of the universe, the bubble may be expanding however localized parts flow the 4D landscape of matter and energy.

    Very interesting.

    Perhaps future interstellar travelers, may also follow the (currents) of space time. :)

    Sustained on the warm flows of matter.


  • Stephen R. Deens October 12, 2008, 8:25 PM

    I have been saying for years to my fellow astromers that the “The Universe Is Not Expanding Uniformly” and that we need to reconsider the BIG BANG theory and expansion models to incorporate more thinking along these lines.

    this evidence (which is still in debate) is an excellent first step in that direction.

    cosmoloigy is changing every day, advancing years in just a few months, so is it really a surprise that we found somehting like this?


    we should have been expecting it, and we should expect to see even more diversity and NON uniformity in the years to come.

    Hubble would feel uncomfortable with these results and Hoyle would be delighted.

    The tapestry of the cosmological body needs non unformity to maintain healthy evolution and growth.

  • Ronin October 12, 2008, 9:02 PM


    It seems that one of the most evident mechanisms in the Universe is overlooked.. Gravity.
    I think Galaxies are subject to the same gravitational influences as our planets do revolving around Sol (Doh..), and the Universe doesn’t _seem_ to be expanding uniformly because Galaxies are influencing each other.
    Also Black Holes will influence the trajectories of everything that isn’t captured in them, even Galaxies.
    I just think that the Universe just became messier when getting older, and this finding doesn’t surprise me one bit..

  • Greg October 13, 2008, 1:04 AM

    These findings could turn out to be a statistical fluke but that appears unlikely. As for the effect being generated by one of the superclusters, this was my first thought as well. But the distance is so vast (3% of the visible universe) that more than one super cluster would be involved. That would put this on par with dark flow and require supermassive structures which we aren’t seeing. Dark flow if verified certainly makes for a much more interesting universe as opposed to the boring uniformity once considered sacrosanct. But it is potentially explainable via tinkering with the inflation period of big bang theory. So the big bang is by no means dead.

  • Thorkil2 October 13, 2008, 9:51 AM

    Frankly, I have pretty serious problems with the assignment of undefinables as “causes” for observed phenomena. “Dark matter” and “Dark Flow” are meaningless supposition until you can establish not only whether they exist, but precisely what they are. Such irregularities of motion are not to be unexpected on a range of scales. They may be the result of gravitational attraction or they may be an artifact of an early state of turbulance, or they may be both. This arbitrary assignment of cause to something that remains in itself undefinable is hardly serious science.

  • Thorkil2 October 13, 2008, 9:52 AM

    (addition to previous): Shades of phlogiston and the Ether…..

  • mike October 13, 2008, 1:52 PM

    I would put my money on Jeff’s explanation; it makes the most sense to me. MOK

  • Jon Hanford October 13, 2008, 2:25 PM

    Perhaps this is a signal of a “baby universe” inflating and being pinched off from our own universe, as hypothesized in Andre Linde’s work with spontaneous inflation and multiverse theory. It also brings to mind a possible influence of the “Great Attractor” discovered behind the Milky Way in Centaurus some years ago. This also may seem to be a more plausible explanation of this “dark flow”. But more research will be needed to confirm and explain this latest cosmological curveball thrown our way.

  • docatomic October 13, 2008, 5:09 PM

    This “dark flow” stuff – is it lumpy? Does it have big pointy chunks in it; flotsam that could take out an entire galaxy – that would be *undetectable*? Is one of those ‘lumps’ bearing at unimaginable speed upon our very galaxy right now?

    Oh great: another thing to worry about. Thanks.

  • Peter October 13, 2008, 8:15 PM

    Quantum Flux,

    You are kidding right? About the red/blue shift applicable to the rotation of the earth? I mean, a few thousand km an hour compared to percentages of light speed?
    Come now, let’s elevate.

  • Jarod October 14, 2008, 12:30 AM

    Ronin hit the nail on the head!

  • Jon Hanford October 14, 2008, 9:44 AM

    After reading both papers on this finding, I still find no reference ruling in or out possible effects of the “Great Attractor” located in the constellations Centaurus and Hydra at a distance close to this new “dark flow” anomaly( see the Wiki page for the “Great Attractor”). It is located behind the Milky Way in the so-called Zone of Avoidance. Alternatively, might a more distant, undiscovered supercluster exist even further out behind the GA? Until these questions are answered, I see no reason to invoke new astrophysics to explain their result. Already the Local Group and Virgo Cluster show kinematic movement towards the GA along with several other galaxy clusters. Maybe astronomers need to search for infalling galaxy clusters located on the far side of this apparent anomaly. Wikipedia entry mentions “The first indications of a deviation from uniform expansion of the universe were reported in 1973 and again in 1978. The location of the Great Attractor was finally determined in 1986, and is situated at a distance of somewhere between 150 million and 250 million light years (the latter being the most recent estimate) from the Milky Way, in the direction of the Hydra and Centaurus constellations. That region of space is dominated by the Norma cluster (ACO 3627),[1][2] a massive cluster of galaxies, containing a preponderance of large, old galaxies, many of which are colliding with their neighbours, and/or radiating large amounts of radio waves.

    Attempts to further study the Great Attractor and other phenomena are hampered due to line of sight obstruction by its location in the zone of avoidance (the part of the night sky obscured by the Milky Way galaxy). Ronin and Jarod both seem to be on the right track.

  • Jon Hanford October 14, 2008, 10:06 AM

    The two published papers referenced in the above article can by found at the arXiv.org site as arXiv:0809.3734v1 and arXiv:0809.3733v1.

  • JL October 14, 2008, 10:09 AM

    Rey, we are one fo the bugs.

  • Uclock October 15, 2008, 12:39 PM

    The question I would like to ask is whether the ‘faster’ expansion is in an area where large amounts of visable matter are present.
    If so, as I have always suspected, it is matter causing the expansion as it generates spacetime.

  • Lucy Haye Ph. D. November 3, 2008, 3:26 PM

    Too many fantasies. The Universe is eternal, has not starting point and no end. Consequently, has not expansion but it is in perpetual evolution. It is flat and the space is Euclidian. Gravitation is not Attraction but Pushing by Gravitons. Forget FANTASIA. Go back to the common sense of Galileo Galilee, Newton and Carezani with his Autodynamics for short: Fundamental Basis for a New Relativistic Mechanics.

    Lucy Haye

  • Newton only January 9, 2009, 2:25 AM

    Anyone bothered to measure the speed of starlight? ANYONE? Few people understand photons and light.