Many of the earliest astronomers believed that stars were affixed to a sphere above the Earth. All the stars, therefore, were thought to be the same distance from us. Since they didn’t appear to be moving, that made some intuitive sense. But once it became clear that not only the planets and Sun, but also the “fixed” stars, were located at varying but nonetheless vast distances from the Earth, astronomers were faced with another huge problem: how to tell whether they were looking at a nearby dim object or a faraway bright one. Without a point of reference, how can you tell whether you’re seeing a tiny penlight 10 meters away or a bright spotlight from a distance of a kilometer?
Over time, excellent methods were developed for judging celestial distances—even the almost unfathomable distances between galaxies. But each measurement breakthrough brings new sets of problems. As technological advances have uncovered ever more distant galaxies, it’s become increasingly difficult to determine whether what appear to be groups of galaxies are actually clumped together in space or only aligned by chance along our line of sight. Yet it is precisely these galaxies, located near the edge of the observable universe, which could solve deep mysteries concerning the nature and origin of the cosmos.
One promising technique for finding and studying galaxy clusters is explained by Jon Voisey, an astronomy graduate and math teacher, on his blog, The Angry Astronomer. The method relies on the Sunyaev-Zel’dovich (SZ) effect, which he describes as “essentially reverse Compton scattering.” Compton scattering occurs when short-wavelength, high-energy light (like x-rays or gamma rays) interacts with matter and shifts to lower energies and longer wavelengths; this phenomenon may partially explain why Earth’s sky appears blue. Like most physical processes, Compton scattering can also happen in reverse, and this is precisely what occurs in the SZ effect: Light interacting with the hot plasma that surrounds galaxy clusters has its energy increased rather than attenuated. Using the SZ effect, astronomers can discover and study very distant and dim galaxy clusters based on how these objects amplify intervening light from the very early universe. Zak Staniszewski of Case Western Reserve University and his colleagues located three new galaxy clusters using this technique, publishing their results last year in The Astrophysical Journal.
There are many other methods of galaxy-cluster detection. In September, a team led by Stefano Andreon at Italy’s Osservatorio Astronomico di Brera published their confirmation of the most distant galaxy cluster ever found. Megan Argo, a post-doctoral fellow at the Curtin Institute of Radio Astronomy, blogged that the cluster was more than 10 billion light-years away. Andreon’s team first found its galaxies in 2006 and posited their vast distance from us based on their color. Subsequent measurements confirmed the immense distance to the galaxies, but the question of whether they formed a cohesive gravitationally bound object was unanswered: The galaxies could have been aligned by chance to deceptively appear as a cluster. But using x-ray imagery from the Chandra telescope, the astronomers were able to observe the signature of gas heated to scorching temperatures between the galaxies—a sure sign that the galaxies formed a genuine cluster. The team is now working on reconfirming their discovery by measuring the SZ effect.
All this spurs the question of what exactly distant galaxy clusters can tell us about the origins of the universe. What makes such painstaking work worthwhile? Sometimes it can reveal things totally unexpected and of potentially profound importance. Greg Fish, a blogger for Discovery.com and BusinessWeek, tells of a strange new finding called “dark flow.” In 2008, a team led by Alexander Kashlinsky of NASA’s Goddard Space Flight Center found evidence that many galaxy clusters were moving toward a single small section of the vast horizon of our visible universe. Last month, new observations by the team confirmed these unexpected findings and extended them to 1,400 galaxy clusters as far as 3 billion light-years from Earth, again using the SZ effect to help identify and measure the clusters. The velocity of the clusters is as high as a thousand kilometers per second, suggesting that some immense, unknown force is acting upon the galaxies like flotsam caught in an invisible current. This is the dark flow.
Kashlinsky’s team doesn’t speculate on what might be causing the dark flow, but others have weighed in with their own ideas. The simplest explanation, Fish says, is some massive object or body outside of our visible universe, gravitationally tugging the galaxies toward it. But such a huge discrepancy in what, on the largest scales, otherwise appears to be a very spatially homogenous universe would require a rethinking of some of the fundamental principles of cosmology. Notably, it would require a reconsideration of the prevailing notion that our surroundings, on cosmic scales, are strictly average. If we occupy a cosmological position that is somehow “special,” this uniqueness may taint our extrapolations about the conditions in other distant regions and eras of the cosmos. In other words, it would become more difficult to pin down the correct large-scale structure and behavior of the universe.
A potentially more harmonious explanation has been proposed by Laura Mersini-Houghton and Richard Holman, suggesting that another universe interfered with ours during the big bang, causing both the dark flow and also two huge “holes,” or voids, in the universe. One of these holes may have already been observed, lending credence to this explanation. It could gain even further support if astronomers can find another similarly sized hole—an endeavor that could be aided by looking for phenomena like the SZ effect.
But these findings—and their implications—are by no means settled. Other astronomers dispute that the dark flow is anything more than a statistically insignificant anomaly. The difficulty and expense of making these observations, as well as the logical leaps needed to extrapolate the accompanying vast scales of distance and time, mean that these results and any related forthcoming findings will necessarily be controversial. Visit ResearchBlogging.org to watch and participate in this conversation as it unfolds.
Originally published December 2, 2009