Modern astronomy offers a curious mixture of humility and bravado. Earth is not special, we say, as Copernicus asserted and Galileo confirmed. Rather it is, in the scheme of things, a tiny speck in an unremarkable location. Yet, from our modest perch, we make sweeping statements about the entire observable universe—from here to billions of light years away. Although we are the smallest of the small, we speak with authority about the largest of the large.
How can we be so bold? The key is to make use of our typicality to assume that the universe is homogenous—pretty much the same throughout. But now, the discovery of a phenomenon called dark flow is challenging our assumption that the universe doesn’t allow one place to be any more “special” than any other. Astronomers call this assumption the Copernican Principle. Thus when Edwin Hubble discovered in 1929 that all galaxies, except our nearest neighbors, seem to be moving away from us, astronomers used the Copernican Principle to infer that space is expanding in a uniform way—at the same rate in every cosmic locale. Big Bang growth marches at the same pace everywhere.
In my own calculations related to cosmology, I’ve generally followed the accepted method of using the assumption of homogeneity to greatly simplify the equations. Otherwise the procedure would be much trickier—like applying a recipe to a dish that requires different ingredients for every morsel. If it does turn out that the universe has local differences, cooking up cosmological solutions will be a tall order indeed!
Astonishingly, a newly identified phenomenon called dark flow could slash through cosmic uniformity, casting the Copernican Principle into doubt. Dark flow represents the movement of hundreds of galaxy clusters at about two million miles per hour in the direction of a patch of sky between the constellations Centaurus and Vela. Like the cloaked duo of dark matter and dark energy, dark flow is another masked marauder challenging long-held cosmological assumptions. It is “dark” in the sense of having mysterious origins—origins that may lie beyond our cosmic horizon, or perhaps even in another universe.
Discovered in 2008 by Alexander “Sasha” Kashlinsky of NASA’s Goddard Space Flight Center, dark flow is a streak of irregularity in a universe that is otherwise as uniform as a perfect, rising loaf of bread. Kashlinsky and his research team discovered dark flow by cleverly analyzing data collected by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite during the 2000s. WMAP’s main purpose was to map out the cosmic microwave background (CMB) radiation released some 380,000 years after the Big Bang, when electrons and protons first cooled down enough to form hydrogen atoms. This “baby picture of the universe” has offered cosmologists a unique look back in time and given them new insight into the universe’s birth, growth, and structure formation.
Kashlinsky’s group put the detailed map to different use, though. when they examined the motion of galaxy clusters through the background radiation. The galaxy clusters are filled with hot gas that scatters light from the background radiation, shifting the spectral lines that define the “fingerprint” of that light. Because the amount of shifting depends on the clusters’ speeds relative to the CMB, this acts as a kind of speedometer, telling us how fast they are moving.
Soon after Kashlinsky and his collaborators published their results, they were confronted by a sharp challenge to their claims. In an article posted on his website, “Dark Flow Detected – Not!”, UCLA astronomer Edward (Ned) Wright pointed out several errors and inconsistencies in their paper’s statistical analysis and argued that these placed their conclusions in doubt. Undaunted by Wright’s allegations, Kashlinsky posted a detailed rebuttal and gathered further evidence for dark flow.
In 2010, Kashlinsky and his team published a follow-up paper with results that were even more startling. Not only did they confirm dark flow, they found its parade of clusters to be far more extensive that they had previously thought. Remarkably, from a survey of more than 1000 clusters, they provided evidence that dark flow extends out as far as 2.5 billion light years away. With such a large scale, it slashes through a significant chunk of the observable universe.
In recent decades, cosmology has become an increasingly rigorous science. Claims in the field, particularly ones of such a revolutionary nature as dark flow, must be sifted by sophisticated statistical tests and verified by independent analyses. Interestingly, while no other teams have found dark flow to the extent mapped out by Kashlinsky’s group, some groups have found a less potent, but still notable, movement in roughly the same direction. For example, researchers Richard Watkins of Willamette University, Hume Feldman of the University of Kansas, and Michael Hudson of the University of Waterloo have noted a significant flow of galaxies, but at much lower rate than Kashlinsky’s team found. Kashlinsky’s next goal is to analyze data from the European Space Agency’s Planck satellite, hoping it will offer proof positive of dark flow and reveal its extent.
If dark flow were conclusively established what would it mean? The Copernican Principle, at least for the observable universe, would be cast into doubt. There would be something special about a particular segment of space. Space would have a gaping irregularity—a fissure through its firmament. How could astronomy explain such a rift?
Kashlinsky and others have speculated that the inflationary universe model could provide the answer. According to many versions of inflation, the observable universe grew from a fluctuation in a primordial energy field. Beyond our “bubble” could be countless other universes that grew from other fluctuations in the great cosmic bath called the multiverse. Perhaps dark flow represents the result of a gravitational tug from mass housed in another universe—or at least a region beyond the observable universe. This interaction would have happened very early in cosmic history, long before the universe grew to its present-day size. Nevertheless it could have left the relic of irregularity, much like geological processes long ago produced today’s mountain chains.
In coming years, we’ll see if dark flow positions itself in the pantheon of bona fide cosmic mysteries, such as dark matter and dark energy, or if further analysis will reveal dark flow to have been an illusion. If dark flow does stand up to scrutiny, though, we may have to reevaluate the assumptions we’ve made about our universe. Perhaps the universe isn’t as uniform as we thought; perhaps what we see from our perch here on Earth isn’t necessarily what you get in distant corners of the cosmos.
Editor's picks for further reading
Ars Technica: Supernova Research Challenges Cosmic "Dark Flow" Mystery
The case against dark flow.
Science Friday: Does The Universe Have a "Dark Flow?"
Host Ira Flatow talks with theoretical physicist Michael Turner about dark flow.
Scientific American: In Our Expanding Universe, Earth Is Nothing Special
John Rennie asks: What do "Fight Club" and the Copernican principal have in common?